<table><caption>**Table 3.1.** Performance summary of the EuPRAXIA design. Further information on the foreseen machine components and properties can be found in the respective sub-system chapters of Part 4, with detailed lists of performance parameters shown in Chapter 9.</caption><thead><tr><th>Quantity</th><th>Baseline Value</th></tr></thead><tbody><tr><td colspan="2"><strong>Laser systems</strong></td></tr><tr><td>Wavelength</td><td>800 nm</td></tr><tr><td>Energy on target</td><td>5–100 J</td></tr><tr><td>Pulse duration</td><td>≥20–60 fs</td></tr><tr><td>Repetition rate</td><td>20–100 Hz</td></tr><tr><td colspan="2"><strong>High-energy electron beam from beam-driven plasma accelerator (PWFA)</strong></td></tr><tr><td>Energy</td><td>1.0–5.0 GeV</td></tr><tr><td>Charge</td><td>30–40 pC</td></tr><tr><td>Bunch duration</td><td>~13 fs</td></tr><tr><td>Energy spread</td><td>0.4–1.1 %</td></tr><tr><td>Normalised emittance</td><td>0.7–1.2 mm mrad</td></tr><tr><td colspan="2"><strong>High-energy electron beam from laser-driven plasma accelerator (LWFA)</strong></td></tr><tr><td>Energy</td><td>5.0–6.0 GeV</td></tr><tr><td>Charge</td><td>23–30 pC</td></tr><tr><td>Bunch duration</td><td>3–11 fs</td></tr><tr><td>Energy spread</td><td>0.1–0.9 %</td></tr><tr><td>Normalised emittance</td><td>0.1–1.4 mm mrad</td></tr><tr><td colspan="2"><strong>Free-electron laser</strong></td></tr><tr><td>Radiation wavelength</td><td>0.19–35.9 nm</td></tr><tr><td>Pulse duration</td><td>0.4–15 fs</td></tr><tr><td>Saturation length</td><td>16–126 m</td></tr><tr><td>Photons per pulse</td><td>1.9 × 10<sup>9</sup>–7.2 × 10<sup>11</sup></td></tr><tr><td>Brightness</td><td>2 × 10<sup>28</sup>–4.8 × 10<sup>32</sup> photons/[mm<sup>2</sup>mrad<sup>2</sup>s(0.1%BW)]</td></tr><tr><td colspan="2"><strong>Betatron source</strong></td></tr><tr><td>Photon energy</td><td>0.6–110 keV</td></tr><tr><td>Source size</td><td>1.4–2.4 µm</td></tr><tr><td>Photons per pulse</td><td>2 × 10<sup>8</sup>–4 × 10<sup>10</sup></td></tr><tr><td>Peak X-ray brightness</td><td>2 × 10<sup>21</sup>–1 × 10<sup>26</sup> photons/(mm<sup>2</sup>mrad<sup>2</sup>s[0.1%BW])</td></tr><tr><td colspan="2"><strong>Inverse Compton source</strong></td></tr><tr><td>Photon energy</td><td>≥100 MeV</td></tr><tr><td>Pulse duration</td><td>~30 fs</td></tr><tr><td>Divergence</td><td>&lt;1 mrad</td></tr><tr><td colspan="2"><strong>Low-energy positron source</strong></td></tr><tr><td>Positron energy</td><td>0.5–10 MeV (tunable)</td></tr><tr><td>Beam duration</td><td>20–90 ps</td></tr><tr><td>Positrons per shot</td><td>≥1 × 10<sup>6</sup></td></tr><tr><td colspan="2"><strong>High-energy positron source</strong></td></tr><tr><td>Positron energy</td><td>≥1.0 GeV (tunable)</td></tr><tr><td>Beam duration</td><td>≤10 fs</td></tr><tr><td>Positrons per shot</td><td>~1 × 10<sup>7</sup></td></tr></tbody></table>

- The collaboration operates the detector and shares in data analysis, common publication, and knowledge gain.

We propose that the EuPRAXIA project would be organised in a similar way:

## Model of EuPRAXIA

- The connected plasma accelerator groups work together in the EuPRAXIA collaboration, which is organised in “technical clusters” (equivalent of working groups). Coordinators of the clusters organise the distributed work and prepare progress reports.

- The EuPRAXIA collaboration appoints the EuPRAXIA project management team. This team organises agreement on the deliverables of the various institutes and tracks the overall progress in clusters and local groups.
<figure><img src="image_28.png" /><figcaption>Fig. 3.1. EuPRAXIA facility concept.</figcaption></figure>

- Local project groups in the various countries carry out the R&D work, perform local prototyping and tests, and construct the final components. For this, any available local facility or workshop will be used.

- In addition, EuPRAXIA defines four additional excellence and prototyping sites (at most one per country) that will be upgraded in capability and will focus on aspects of particular importance and difficulty.

- The collaboration assembles the delivered components in the location of the two EuPRAXIA construction sites. Each construction site focuses on a particular technological path such that complementarity is assured.

- The collaboration operates the EuPRAXIA facility and shares in data analysis, common publication, and knowledge gain.

This EuPRAXIA implementation model is visualised in Figure 3.1. More details of the governance model can be found in Chapter 5.2. Here, we describe the model of the EuPRAXIA infrastructure in more detail. It is noted that the consortium found during the CDR work that two complementary and equally promising technological paths can be defined for EuPRAXIA, namely, one relying on a beam-driven plasma accelerator and the other one on a laser-driven plasma accelerator approach. Instead of combining both in one construction site, the consortium proposes to exploit the two paths in two construction sites. Both construction sites shall be located at existing facilities that are upgraded into one leg of the EuPRAXIA infrastructure. Local project groups will coordinate the work per site, in close collaboration with the work of the clusters and institutes around Europe.

The definition of two construction sites maximises the use of existing equipment as one existing site might have important laser infrastructure, and the other existing site might be more focused on RF technology. This minimises the need for duplicating existing infrastructure. In addition, the scientific output is significantly increased,
<figure><img src="image_29.png" /><figcaption>**Fig. 3.2.** Overview of the proposed technical clusters for EuPRAXIA. More detailed descriptions of each cluster can be found in this chapter.</figcaption></figure>

and the integration of the European research landscape is much improved. The EuPRAXIA concept avoids competition among institutes inside Europe and brings all institutes together with a common scientific goal and shared benefits. Competition is instead defined between two technological paths. This technical competition will produce the fastest possible progress (“competition is good for business”).

### 3.3.1 EuPRAXIA Clusters for Organising Europe-Wide Work

The EuPRAXIA clusters will organise the Europe-wide work similar to working packages in the EuPRAXIA design study. Clusters have been defined along areas of expertise, such that scientists with overlapping expertise are bundled and work together towards the common EuPRAXIA goals. The clusters that were defined are shown in Figure 3.2.

Various institutes of EuPRAXIA have expertise in one or several EuPRAXIA clusters. The mapping of clusters to available competencies in institutes around Europe and Russia is shown in Table 3.2. It is seen that the present EuPRAXIA consortium members cover all the competencies required for the EuPRAXIA technical clusters. A final assignment of institutes to clusters must be done during the preparatory phase of EuPRAXIA and will depend on available project resources.
# I. Theory & Simulation

**Keywords:** start-to-end simulations, performance predictions, evaluation of measured hardware performance, code benchmarking, development of new theoretical concepts, high-performance computing, machine learning and data science, new codes and new approaches

The mission of the “Theory & Simulation” cluster is to study the beam physics and performance throughout the EuPRAXIA machine using theoretical and computational tools. This includes activities on four different topics:

(1) **Development and study of new theoretical concepts:** These will comprise energy dechirping and external injection concepts, electron injection and acceleration control mechanisms (using e.g. magnetic fields, multiple lasers, and shaped plasma targets), radiation generation techniques in plasma channels from the X-ray to mid-IR regime, and thermodynamics studies of plasma in capillaries.

(2) **Start-to-end simulations & theoretical modelling of the beamlines foreseen for the EuPRAXIA facility:** Activities additionally flowing into the start-to-end simulations of LWFA-based, PWFA-based, and hybrid designs will include investigations of the FEL performance, detailed error and tolerance studies, the integration of different types of codes across beamline components (e.g. between RF and plasma components), and the development of a theoretical machine model.

(3) **Evaluation of the performance of the machine and its components:** Benchmarking between different simulation codes will need to be carried out and the measured performance hardware evaluated against theoretical results.

(4) **Development of new codes and new computational approaches:** Topics of interest are code performance improvements, the design of new, robust, and realistic modelling and simulation tools, high-performance computing (exa-scale computing) developments (using CPU-, GPU-, and cloud-based systems) as well as the integration of machine learning and other data science methods. Areas of particular importance are the integration of plasma density non-uniformities as well as realistic laser pulse and electron beam profiles into simulations.

The “Theory & Simulation” cluster will be active throughout the whole life cycle of the EuPRAXIA facility. Its most essential work will be carried out during the technical design phase by assessing and optimising machine performance through the activities described above. During the implementation and operation phases, the cluster will take on a more supportive role in evaluating and improving the facility. Strong interaction with most of the other clusters involved in the development of specific components will be required to coordinate simulation and modelling tasks. Further cooperation is planned with the “Layout & Implementation” cluster for the development of data analysis and storage capabilities at the EuPRAXIA sites.

# II. Laser Technology

**Keywords:** laser drivers, photo-injector laser, other laser systems, laser diagnostics, optical components, diodes, thermal management, laser transport line, compressor, kHz laser technology (with industry link)

The cluster on laser technologies will deal with all scientific and technical issues related to the EuPRAXIA laser design and, in particular, the transition from the current EuPRAXIA laser conceptual design to the final technical design and construction. The cluster EuP-LASTECH will primarily tackle several aspects of the
<table><caption>**Table 3.2.** List of institutes with interests in particular EuPRAXIA cluster topics. An explanation for each of the institute acronyms can be found in the “List of EuPRAXIA Partner and Associated Partner Institutes” at the end of the report.</caption><thead><tr><th></th><th>Theory & Simulation</th><th>Laser Technology</th><th>Plasma Components & Systems</th><th>RF Technology</th><th>Magnets & Other Beamline Components</th><th>Diagnostics</th><th>Applications</th><th>Transformative Innovation Paths</th><th>Training, Outreach & Dissemination</th><th>Layout & Implementation</th></tr></thead><tbody><tr><td>CEA</td><td>•</td><td></td><td>•</td><td></td><td>•</td><td>•</td><td></td><td></td><td></td><td></td></tr><tr><td>CNR</td><td>•</td><td>•</td><td>•</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td></td></tr><tr><td>CNRS</td><td>•</td><td>•</td><td>•</td><td></td><td></td><td>•</td><td>•</td><td>•</td><td></td><td></td></tr><tr><td>DESY</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>ELI</td><td>•</td><td></td><td>•</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td></td></tr><tr><td>ENEA</td><td>•</td><td></td><td></td><td></td><td></td><td></td><td>•</td><td></td><td></td><td></td></tr><tr><td>FBH</td><td></td><td>•</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HIJ</td><td></td><td></td><td></td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td></td><td></td></tr><tr><td>HUJI</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HZDR</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td></td><td>•</td><td></td><td></td></tr><tr><td>IAP-RAS</td><td></td><td>•</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ICL</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td>•</td><td>•</td><td></td><td></td></tr><tr><td>INFN</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>IST</td><td>•</td><td></td><td>•</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>JIHT</td><td>•</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>KIT</td><td></td><td></td><td></td><td></td><td></td><td>•</td><td>•</td><td></td><td></td><td>•</td></tr><tr><td>LMU</td><td></td><td></td><td>•</td><td></td><td></td><td>•</td><td>•</td><td>•</td><td></td><td></td></tr><tr><td>QUB</td><td></td><td></td><td>•</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td></td></tr><tr><td>SOLEIL</td><td></td><td></td><td></td><td></td><td>•</td><td>•</td><td>•</td><td></td><td></td><td></td></tr><tr><td>STFC</td><td></td><td></td><td></td><td></td><td></td><td>•</td><td>•</td><td></td><td></td><td>•</td></tr><tr><td>UHH</td><td>•</td><td></td><td>•</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ULIV</td><td></td><td></td><td></td><td></td><td></td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td></tr><tr><td>UMAN</td><td></td><td></td><td></td><td></td><td>•</td><td>•</td><td></td><td></td><td></td><td></td></tr><tr><td>UOXF</td><td>•</td><td></td><td>•</td><td>•</td><td>•</td><td>•</td><td>•</td><td></td><td></td><td></td></tr><tr><td>URLS</td><td>•</td><td></td><td>•</td><td>•</td><td>•</td><td>•</td><td></td><td></td><td></td><td>•</td></tr><tr><td>URTV</td><td></td><td></td><td>•</td><td></td><td>•</td><td>•</td><td></td><td></td><td></td><td></td></tr><tr><td>USTRATH</td><td>•</td><td></td><td>•</td><td></td><td>•</td><td>•</td><td>•</td><td></td><td></td><td></td></tr><tr><td>WIGNER</td><td>•</td><td></td><td></td><td></td><td>•</td><td>•</td><td></td><td>•</td><td>•</td><td>•</td></tr><tr><td>YORK</td><td></td><td></td><td>•</td><td></td><td></td><td>•</td><td></td><td></td><td></td><td></td></tr></tbody></table>
laser design, as emerged during the design study phase, through a number of developments and prototyping activities to address and solve critical issues. An initial set of such topics is outlined below, divided into five subject areas concerning the main blocks of the entire laser system, from the amplifiers to the final focusing on the plasma, including manipulations of the initial temporal and spectral properties.

## Main Scientific and Technical Issues:

- Amplifier configuration:

* Prototyping of Ti:Sa amplifiers

* Building a test amplifier to test thermal load and cooling

- Pumping technology:

* Scaled 100 Hz repetition rate, high-energy pumping

* Addressing 100 Hz pump lasers developments

- Optical compressor technology:

* Thermal management of compressor gratings

* Running high-average-power illumination tests at existing facilities to make assessments on LIDT, thermal load, cooling, and lifetime

- Pointing stability:

* Stability (pointing and more) and active control

* Building tools and running tests at existing facilities; defining routes for active stabilisation

- Temporal and spatial shaping:

* Synchronisation

* Developing efficient pulse train, temporal contrast, AO control and measurements

* 3D pulse shaping (formation of triangular, cylindrical, ellipsoidal, etc. pulses)

- kHz repetition rate and beyond:

* Lasing material development and characterisation

* Architecture design and modelling

* Wavelength scaling of applications

The implementation of *development* and *prototyping* activities will be established mainly at participating institutes and will involve a number of leading companies with known expertise in the field. Interested companies include full system manufacturers (AT and Thales) and component manufacturers (Safran, VCS, VacuumFAB, Horiba-JobinYvon, ISP, TRUMPF, Imagine Optic, Dynamic Optics, etc.).

## Higher Repetition-Rate Developments

The current layout of the EuPRAXIA laser foresees a 20 Hz configuration using arrays of Ti:Sa pumping systems like the DIPOLE or P60. This introduces a significant complexity that could be avoided using pumping systems running natively at 20 Hz, eventually reaching the 100 Hz of the P1 configuration. More generally, >kHz repetition rate is foreseen as a milestone for future laser driver developments. Such higher repetition rates will require major technology developments beyond the simple evolution of current established architectures.

Totally alternative systems are those based on direct diode pumping of different gain media. Yb:YAG and Yb:CaF2 are being explored and are both candidates as gain media for such systems; however, there are only limited experimental data available on how these approach at high average powers. On the other hand, Tm:YLF is also being considered as a gain material that offers a significant lifetime advantage over the well-established Yb doped materials traditionally used for diode pumped fiber and bulk systems.

The cluster EuP-LASTECH will establish initiatives to support developments of direct-pumping, high-average-power systems in a two-stage approach, starting with a
conceptual design development, supported by existing programmes at participating partners (CNR, CNRS, FBH, LLNL, STFC, etc.), followed by a downselection and a technical design of the chosen architecture.

### III. Plasma Components & Systems

**Keywords:** plasma sources, plasma injectors, plasma-accelerator stages, plasma mirrors, plasma lenses

This cluster brings together experts from partner institutes to develop plasma components and systems requested for the implementation of the EuPRAXIA accelerator and beamlines, in particular: laser-driven plasma injectors (LPIs), laser- and beam-driven plasma accelerators (LPAs & BPAs), and interface components such as plasma mirrors and passive or active plasma lenses. Particular efforts will be oriented towards the development of components providing high-quality, stable electron beams at a high repetition rate. The contribution of identified partners is planned as follows:

#### Development of LPIs

1. Development of optimised electron injection schemes in tailored density profiles for the EuPRAXIA accelerator baseline: modelling through fluid, PIC and Monte Carlo simulations, design and construction of relevant gas cells, and experimental testing of stable injection schemes; numerical and experimental investigation of optical injection with spatio-temporally controlled laser pulses.

2. Investigation of physics implications for operation at a higher repetition rate, >10 Hz; development of high-repetition-rate LWFA targets; investigation of strategies for post-acceleration removal and recovery of unused wake energy.

3. Development of LWFA-integrated targets optimised for betatron radiation and imaging applications (aiming high-quality hard X-ray imaging (>50 keV) and high magnification (>20× or sub-micron resolution) with low noise and low dose).

#### Development of LPAs / BPAs

1. Optimisation of plasma parameters and the development of gas-filled discharge capillaries to reach long plasma channel lengths, typically several tens of centimetres.

2. Development of novel long low-density plasma channels suitable for multi-GeV accelerator stages operating at high repetition rates.

3. Guiding of high-intensity laser beams in curved plasma channels: multi-stage capillaries built and tested in guiding experiments; numerical and experimental investigation of curved hydrodynamic optical-field-ionised (HOFI) plasma channels.

4. Development of all-optical, multi-component, large-volume plasma sources based on (selective) tunneling ionisation (in collaboration with the “Transformative Innovation Paths” cluster).

#### Development of plasma components for laser or electron beam transport and shaping

1. Development of active plasma lenses and passive plasma lenses for electron beam shaping, plasma lenses for laser focusing; theoretical modeling of plasma channel lenses using a short capillary discharge (2–3 mm) to create a plasma lens, used to increase the intensity of the impinging beam to increase the laser intensity.

2. Investigation of energy dechirping concepts, including multiple plasma stages.

3. Plasma mirrors, adaptive mirrors, feedback loops.

#### System integration or prototypes development

Multi-stage design and experiments (LPI and LPA components driven by separate laser beams); full start-to-end analysis to optimise the choice of injection energy, and laser and plasma channel parameters in curved plasma channels.
## IV. RF Technology

**Keywords: RF drivers, RF injectors, low-level RF and synchronisation**

This cluster is related to the following three topics:

RF gun technology covers the development and optimisation of S-band guns, aiming to reach the best performances in terms of accelerating field gradient and breakdown rate, with particular attention to the new clamping technology that, with the help of special gaskets, allows to avoid the brazing procedure. The study and development of cathode materials like Mg and Yttrium is also covered, besides the more common Cu.

RF accelerating structures technology covers the development and optimisation of the S-band and X-band accelerating structures in terms of feed coupler optimisation for the S-band case, while work for the X-band deals with the design and optimisation of the single cell and of the whole structure in terms of maximum achievable gradient compliant with the optimum compact factor, feeding RF system optimisation and UH vacuum sustainability.

Synchronisation covers the precise synchronisation between components of the RF system as well as with the laser drivers. Feedback mechanisms will be designed and implemented in collaboration with the diagnostics cluster, to ensure the well-controlled and synchronised operation of the EuPRAXIA machines.

For both topics, the cluster will prepare a technical design of the RF technology for the two EuPRAXIA construction sites, carry out test and prototyping experiments, and coordinate the production and delivery of all necessary components for the construction phase.

## V. Magnets and Other Beamline Components

**Keywords: magnets, beam transport and manipulation, collimation, beam dumps**

This cluster covers magnets, beam transport, and manipulation as well as beam dumps.

Permanent magnet quadrupoles of variable strength, an original design developed at Synchrotron SOLEIL (QUAPEVA), will be investigated and optimised for the EuPRAXIA design, among others.

For the beam transport and manipulation, the different technologies investigated as part of the conceptual design study, from permanent quadrupoles and solenoids to plasma lenses (in collaboration with the cluster on plasma sources and components), will need to be optimised and downselected for a full technical design.

In the case of beam dumps, both conventional designs and plasma-based beam dumps as a possible future upgrade option are under investigation.

In all cases, test experiments and prototypes are planned to develop a robust technical design for the beamlines at the EuPRAXIA sites. Furthermore, the components required throughout the construction phase will be produced and delivered to the respective facility locations in a joint manner.

## VI. Diagnostics

**Keywords: electron diagnostics, positron diagnostics, photon diagnostics, plasma diagnostics**
This cluster is related to several topics:

**Electron and positron beam diagnostics:** Electron and positron beam diagnostics cover every acceleration scheme and all the positions along the machine, including the experimental areas. It includes the implementation of conventional diagnostics and R&D on new dedicated instrumentation for plasma acceleration. Non-intercepting or non-disturbing beam diagnostics are in the scope of this cluster. Particular emphasis will be put on innovative single-shot emittance measurement techniques, the development of CBPMs, and several longitudinal diagnostics. Among others, we will focus on the use of coherent radiation (Smith-Purcell-based measurements, implementation of high-throughput THz detector systems and readout electronics, near-field electro-optical sampling) and transverse deflecting structures (X-band TDS, gyrotron/gyroclystron-driven transverse deflecting structures). Betatron diagnostics, passive streakers (metallic or dielectric), and diagnostics based on plasma afterglow are also included in this task.

**Photon diagnostics:** Photon diagnostics – e.g. for measuring the spectra, divergence and emittance of radiation sources – cover all photons coming from applications, such as FEL, THz sources, X-rays, etc. that can be used to monitor the quality of the source. These diagnostics will be mainly located in the experimental areas. The photon diagnostics for positron experiments for material characterisation are also included.

**Plasma diagnostics:** Plasma diagnostics are devoted to the measurement of the plasma parameters, to optimise the plasma acceleration process. It includes the measurement of plasma density by the Stark broadening effect, interferometry-based techniques (including novel, robust, real-time second-harmonic interferometry methodology or wavefront measurement techniques for LPIs), and the measurement of the laser driver spectral modulation for the determination of average accelerating fields in long LPA plasmas. Particular emphasis will be placed on the multi-plane few-cycle shadowgraphy of wakefields to retrieve the electron density distribution by in situ interferometry and ex situ interference tomography of non-axisymmetric gas jets. There will also be a focus on diagnostics based on plasma afterglow and ultra-fast plasma diagnostics, which can be used to visualise plasma waves in a beam- or laser-driven electron wakefield accelerator. Here, electromagnetic probe pulses with wavelengths in the visible to mid-IR range will be used to diagnose plasmas covering a wide range of electron densities.

All of these areas of interest include theoretical simulations and mechanical design, device construction, as well as testing and characterisation at selected facilities with beams.

## VII. Applications

**Keywords:** positron generation, free-electron laser, undulators, inverse Compton source, betatron source, THz radiation, high-field physics, test beam generation, HEP detector testing, user end stations

This cluster focuses on the design, prototyping and delivery of all components related to EuPRAXIA's foreseen flagship applications. These include the following eight types of particle / radiation sources:

- free-electron laser beamlines for electron energies between 1 and 5 GeV

- a low-energy, high-charge positron source for material testing and industrial applications

- a high-energy, low-emittance positron source for experimental studies of positron acceleration and transport
- an X-ray betatron source for medical and life science imaging

- a hard X-ray to gamma-ray inverse Compton scattering source for fundamental science and industrial applications

- two electron test beamlines for high-energy physics detector testing, THz radiation, and other detector testing

- high-field physics (as a future development)

For each source, both the transport line of the electron beam to the source and the transfer beamline of the secondary beam after the source need to be designed. After testing and prototyping, the necessary components will be produced collaboratively with the cluster members and delivered to the respective construction sites. The cluster activities will be built on recent work devoted to the optimisation of several aspects of these sources, from their generation and characterisation, to their transport and manipulation, up to their use.

In the case of the free-electron laser, this includes, among others, the undulators and photon beamline. Superconducting undulators (SCU) tailored to EuPRAXIA will be conceptualised, prototyped, tested, and finally implemented according to field measurements. Technological options to be considered are Nb-Ti-based planar SCUs (available technology with the industrial partner Noell), superconducting transverse-gradient undulators, and SCUs with switchable period length (proof of feasibility in the TDR phase) as well as more advanced concepts like laser-scribed HTS tape-stacked undulators.

An additional focus for the cluster will be placed on preparing the user end stations, including general infrastructure (together with the cluster on layout & implementation), as well as the necessary instruments and diagnostics for user experiments. Strong interaction is also planned with the cluster on training, outreach & dissemination regarding the organisation of user workshops as well as with local groups at the host institutes of the construction sites on other topics related to user support.

VIII. Transformative Innovation Paths

**Keywords: hybrid accelerators, novel injection and radiation generation mechanisms, multi-pulse LWFA, plasma-based metrology and diagnostics**

This cluster concentrates on highly innovative technologies which emerged from EuPRAXIA phase 1 with steep TRL trajectories. These are R&D-intensive thrusts over the next phase and in turn have the potential to transform EuPRAXIA by boosting the quality and efficiency of electron and photon beam output partially by orders of magnitude. An overarching philosophy of this cluster is an "all-plasma" approach, i.e. where suitable using plasma not only for the accelerator, but also for the injector, for diagnostics and for radiation sources.

Here, we highlight six major thrusts or themes:

1. **Multi-pulse LWFA.** This thrust develops resonant excitation by multiple lower power laser pulses. This may allow using highly efficient high-repetition-rate laser drivers such as thin disk or fibre lasers, (optically) tailored plasma channels, and plasma photo-cathode injectors.

2. **Hybrid LWFA$\Rightarrow$PWFA.** This approach combines the best of LWFA and PWFA, centrally utilising high-current electron beams from compact LWFA stages as drivers for PWFA stages. This seeks to unlock key advantages of PWFA such as dephasing-free, practically diffraction-free, dark current-free operation and an abundance of ionisation-based injection mechanisms.

3. **Plasma photo-cathodes.** Fully or partially decoupled ionisation injection schemes such as the Trojan Horse plasma photo-cathode, plasma torch, 2PII, WII, and
multi-pulse injection schemes, etc., will be developed. A key prospect is the con-
trolled production of ultra-low-emittance, ultra-high-brightness electron beams as
the foundation for high-performance applications.

4. Plasma-based metrology and diagnostics. The ultra-low-emittance, ultra-high-brightness beams enabled by the above schemes exceed the metrology capabilities of many established diagnostics. Recent innovations show that ionisation-based and plasma-based detector and amplifier techniques allow minimally invasive, ultra-high sensitivity diagnostics, which will be developed.

5. Plasma undulators and advanced radiation sources. The above-listed technologies combined aim to realise high-efficiency, high-repetition-rate, and ultra-high-brightness electron beams in compact setups. These are ideally suited for driving light sources. Plasma wigglers/undulators (betatron/ICL), ICS, and XFEL based on ultra-high-brightness electron beams will be developed.

6. Plasma-based sources and targets. The above technologies partially require very specific, often multi-component plasma sources with selective ionisation capability and (plasma) density profiles. This theme will develop the corresponding sources in close collaboration with the cluster on plasma components and systems.

IX. Training, Outreach & Dissemination

**Keywords:** dissemination, outreach, education, public engagement, knowl-
edge transfer, training, user workshops

This cluster will ensure that EuPRAXIA maintains a high profile within the inter-
national landscape of scientific facilities. It will promote its activities with the aim of
attracting users and collaborators, and help to disseminate its research results to the
wider scientific community. A dedicated effort in outreach and public engagement
will aim at raising awareness of EuPRAXIA's research goals and its key technolo-
gies, maximising its impact on science and society. Moreover, a coordinated training
and education programme will ensure a continuous supply of highly skilled scientists
and engineers to develop, operate, and continuously optimise the diverse sites and
excellence centres of EuPRAXIA.

**Publications:** Regular contact with all participating partners provides a continuous stream of news articles that will be posted on the central website (www.EuPRAXIA-project.eu). All news will be further promoted through social media and targeted online and print articles. News topics include (but are not limited to) research results, journal publications, technological innovations, events, and position vacancies. Items of particular relevance will be selected for publication as feature articles in magazines. The newsletter *The EuPRAXIA Files* is a collection of abstracts from research articles that are relevant to the EuPRAXIA project and will continue to be published three times a year. It will be distributed to all the members of the EuPRAXIA collaboration and made available via the website to disseminate the latest advances in wakefield acceleration and its applications.

**Outreach events and educational resources:** EuPRAXIA partners will be encour-
aged to hold public events showcasing the achievements in plasma acceleration. Cen-
tral support in the form of education material and demo setups will be provided.
Based on experiences gained in past pan-European events, a combination of social
media campaigns and video live-streams, will be used to engage several institutions
at a time and maximise reach and impact. Events set up around a specific theme
– such as Physics of Star Wars or Marie Curie Day –, innovative educational mate-
rials – like the augmented reality app AcceleratAR, the computer game Surfatron
or poster resources that explain the science and technology of EuPRAXIA – will
be made available and used to communicate the science of EuPRAXIA to a range
of target audiences and ensure high visibility in the media. The cluster leader will continue to present EuPRAXIA findings at relevant international conferences via the project TEAM based at the Cockcroft Institute.

**Training:** The cluster members are very experienced in organising complementary skills schools for researchers at different career stages, topical workshops on selected R&D challenges, and international schools that train the next generation of researchers in the science and technology of EuPRAXIA. Online resources such as lectures for undergraduate and postgraduate students, as well as science short films targeting the general public will be part of the wider communication portfolio of EuPRAXIA. Its established YouTube channel will be used to disseminate these materials and coordinate the wider training efforts. Educational material from EuPRAXIA partners will continue to be made available via a dedicated section on the project website. Additionally, the cluster will work on the development of a training program foreseen as one of the user access modes at the future EuPRAXIA facility. In this context, collaborations and connections will be set up with universities and research institutes that can be developed into a full training scheme during the infrastructure's operational phase.

**User workshops:** Workshops and discussion sessions with potential future users will be organised throughout the different phases of EuPRAXIA. These will aim, first, at attracting future facility users and comparing the users' needs with EuPRAXIA's foreseen capabilities. During the operation phase, such regular user meetings will, in particular, also allow different user groups to exchange experiences, while providing the host sites with feedback on machine performance and possible improvements.

# X. Layout & Implementation

**Keywords:** control system, data acquisition, data analysis and storage, software and hardware development for operation, vacuum, cooling, power distribution, water, site construction, ventilation, support labs, laser safety, radiation safety, general safety, access system

The “Layout & Implementation” cluster covers several topics related to the control and operation, infrastructure, and safety of the future EuPRAXIA machine:

**Command control system:** The control system and control room for the two EuPRAXIA construction sites will be designed and implemented, with close collaboration with the technical groups at the EuPRAXIA host sites. Moreover, the integration of the laser and accelerator control into one consistent system will be a critical challenge particular to EuPRAXIA.

**Data acquisition, analysis, and storage:** A robust and reliable data acquisition, analysis, and storage infrastructure suitable for user operation needs to be set up for all EuPRAXIA sites. In this context, it will heavily rely on the extensive experience of some of the project partners in the design and building of DAQ systems, with particular efforts placed on developing fast diagnostic tools and beam analysis techniques based on GPU platforms to better adapt the data acquisition process to plasma-accelerator-based machines. Additionally, the needs and possibilities for data analysis and storage at the future EuPRAXIA facility need to be investigated and assessed, considering, among others, EU policy on open data access. Depending on the results of this investigation, a data analysis and storage system will be designed and implemented for EuPRAXIA. Strong interaction with the clusters on theory and simulation and diagnostics will be considered for this topic.

**Infrastructure:** The overall infrastructure of the two EuPRAXIA machine sites as well as the experimental support sites that require the setup of sizable, new test beamlines for EuPRAXIA will be targeted. Besides the building design, the utilities of
each new facility need to be considered, including ventilation, heating, air conditioning, electrical power distribution, cooling, and fresh water systems. The cooling and conditioning system needs to be designed and tested carefully, while new concepts for ultra-high-precision water cooling will be explored. For the electrical distribution system, feasible solutions assuring high stability will need to be developed according to best-practice rules. Additional responsibilities include the design and testing of the vacuum system as well as the setting up of a strategy for module and component testing during the construction and commissioning phase. Moreover, the cluster will also evaluate the necessary support laboratories and their infrastructure needs that must be foreseen both for the commissioning phase as well as for the user facility operation. This work will be carried out in close collaboration with the technical groups at the host sites as well as other experienced consortium facilities.

**Safety:** A safety infrastructure for each of the EuPRAXIA experimental sites will be designed and set up together with local technical groups responsible for safety aspects at the host institutes. This will include, among others, the implementation of interlock and access systems, but also safety procedures for users, safety training, etc. Particular emphasis will be placed on laser safety, coordinated closely with the work cluster on laser technology, and radiation safety. The latter will require detailed radiation protection simulations to be carried out and radiation monitoring to be designed and implemented into the EuPRAXIA machine sites. Because of the short timescale of plasma-generated electron beams and hence any generated radiation, the effects of how electromagnetic pulses in the EuPRAXIA machine can affect the surrounding experimental instrumentation will also be investigated as a potential safety risk.

### 3.3.2 EuPRAXIA Construction and Excellence Sites

The EuPRAXIA facility will be constructed in two central “construction sites”:

1. The EuPRAXIA site with a *beam-driven plasma accelerator, FEL, and other applications*. This site will feature a 1 GeV compact RF linear accelerator, based on the CERN-developed X-band technology. Initially, it has conventional undulators for 1 GeV FEL operation.

2. The EuPRAXIA site with a *laser-driven plasma accelerator, FEL, and other applications*. This site will feature an RF photo-injector, a laser-driven plasma injector, conventional undulators, and three major high-power lasers that drive the plasma accelerator. This facility is designed for producing 5 GeV electron beams and the according FEL radiation.

The two construction sites will be located at existing facilities to maximise reusing already available equipment. As explained before, this approach with two construction sites with complementary technological paths will establish a technical competition but avoids an institutional competition in Europe. There are a number of scientific and technical challenges that are particularly important and difficult. The pursuit of these problems is best performed at EuPRAXIA ‘excellence centres’. The already existing equipment in those locations can be upgraded to the EuPRAXIA needs, local teams can be best integrated into the project work, and the construction sites can focus in parallel on the optimal preparation of EuPRAXIA facility construction and later user operation. These excellence sites will be crucial during the R&D and TDR phases of the project but also during construction (e.g. component testing) and operation (testing and preparation of facility upgrades). They should therefore be operational for the TDR phase but are also a long-term investment into the capabilities of the European research infrastructure landscape, both for EuPRAXIA and other needs. The following excellence sites are envisaged at the moment:
<figure><img src="image_30.png" /><figcaption>**Fig. 3.3.** Overview of the distributed facility concept for EuPRAXIA proposing two flagship construction sites and five supporting excellence centres.</figcaption></figure>

- an excellence centre for application beamline R&D, prototyping, and testing,

- an excellence centre for laser-plasma acceleration and 1 GeV FEL R&D, prototyping, and testing,

- an excellence centre for plasma accelerator and high-repetition-rate developments,

- an excellence centre for theory and simulation, and

- an incubator for lasers and concepts at ELI Beamlines.

The scheme of the two construction sites, the excellence centres, and the connection to other major European research infrastructures and industry is visualised in Figure 3.3. In the following, we describe the two construction sites in more detail. Additional information on the excellence centres and possible candidates can be found in Chapter 3.6.

## EuPRAXIA Beam-Driven Plasma Accelerator Site

The EuPRAXIA concept of the beam-driven plasma accelerator site is shown in Figure 3.4. The schematic diagram lists the major components of the plasma accelerator, namely, the RF injector for producing electron beams, the RF linear accelerator for accelerating the electron bunch that drives the wakefield in the plasma, and two
plasma accelerators, one aimed at high charge (compromising quality) and one aimed at high beam quality (compromising charge). A line of conventional undulators is used to produce FEL radiation and to send X-rays to two user areas. Alternatively, the high-quality beam can be used to drive an inverse Compton scattering (ICS) gamma-ray source. The high-charge plasma accelerator is used for producing positrons and electron beams for high energy physics detector tests and preparational studies for a plasma linear collider. A full technical description of all components and system simulations can be found in Part 4.

## EuPRAXIA Laser-Driven Plasma Accelerator Site

The EuPRAXIA concept of the laser-driven plasma accelerator site is shown in Figure 3.7. The accelerator infrastructure includes high-power lasers for driving a 5 GeV plasma acceleration, RF and/or plasma injectors for producing electron beams at 150–500 MeV, and two high-energy plasma accelerators for high charge or high beam quality. A conventional undulator line produces X-ray FEL pulses from 1–5 GeV electrons and delivers them to two user areas. In addition, there is a user area for life science, material studies, and medical X-ray imaging. Two other user areas are designed for (1) an ultra-compact positron source and its imaging capabilities and (2) a table-top electron test beam area, e.g. for detector tests from high-energy physics.

The laser-driven plasma accelerator facility in EuPRAXIA will allow the testing of various concepts and approaches for producing high-quality beams in a laser-driven plasma accelerator. The variability of the infrastructure will provide important risk mitigation towards achieving the performance goals in EuPRAXIA. A full technical description of all concepts, components, and system simulations can be found in Part 4.

## 3.4 EuPRAXIA Construction Footprint (Towards Accelerator Miniaturisation)

EuPRAXIA is based on a large-scale infrastructure design but reduced in size over conventional technology by a factor of 3 at least. It pushes one of the biggest strengths of plasma-accelerator technology in a realistic step-wise approach.

It was one of the important goals of the EuPRAXIA design study to demonstrate the benefits of a facility that is based on plasma accelerators for total facility size and cost. Here, we discuss advantages in total facility footprint when including estimates for all relevant infrastructure, like laser laboratories, shielding walls, beam tunnels, conventional undulators, space estimates for X-ray optics, and user laboratories.

To address this task, a database of required components and their space requirements has been set up and was filled with input from collaboration members. The total footprints were calculated both in facility length but also in terms of required surface area. The summary of obtained facility lengths is shown in Table 3.3 where length estimates of RF-based facilities are also given in comparison. These latter values have been guided by existing facilities; for the SRF case we haven taken FLASH (DESY) as an example, whereas SwissFEL is a good example for C-band technology. EuPRAXIA aims at a realistic stepwise approach of miniaturisation. It is based on a large-scale infrastructure design but reduced in size over conventional technology by a factor of 3 at least, as listed in Table 3.3 and also visualised in Figure 3.6. EuPRAXIA therefore pushes one of the biggest strengths of plasma-accelerator technology. Once the expected size reduction by a factor of 3 has been demonstrated, a
<figure><img src="image_31.png" /><figcaption>**Fig. 3.4.** Schematic facility layout for the flagship construction site based on beam-driven plasma acceleration.</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>**Fig. 3.5.** Size comparison of the EuPRAXIA infrastructure in relation to a hospital in Copenhagen. The entire EuPRAXIA facility (up to 1 GeV beam energy) would fit onto the parking lot of the building. Note further that the facility footprint could be even smaller if a multi-storey building design is considered (image adapted from Google Maps).</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Fig. 3.6.** Comparison of the expected EuPRAXIA machine length with parameters for facilities of equivalent beam energies based on conventional RF-technologies [157,158]. The transverse size is not to scale. It is noted that such facilities tend to offer FEL performance parameters which are not achievable with the EuPRAXIA design. Gains in size must therefore be put into the context of performance limitations with the EuPRAXIA approach.</figcaption></figure>
**Table 3.3.** Comparison of the expected EuPRAXIA machine size with parameters for facilities of equivalent beam energies based on conventional RF-technologies [157,158]. It is noted that such facilities tend to offer FEL performance parameters which are not achievable with the EuPRAXIA design. Gains in size must therefore be put into the context of performance limitations with the EuPRAXIA approach. The presented EuPRAXIA cases include scenarios with an RF injector to create the electron beam (external injection) as well as an all-optical setup with a plasma injector generating the beam.

<table><thead><tr><th rowspan="2"></th><th colspan="2">Acc. length</th><th colspan="3">Application length</th><th rowspan="2">Total length</th></tr><tr><th>RF / laser</th><th>Plasma</th><th>Undulators</th><th>Photon beamline</th><th>User areas</th></tr></thead><tbody><tr><td>EuPRAXIA-PWFA ext. injection (1 GeV)</td><td>46 m</td><td>13 m</td><td>39 m</td><td>22 m</td><td>15 m</td><td>135 m</td></tr><tr><td>EuPRAXIA-LWFA ext. injection (1 GeV)</td><td>42 m</td><td>17 m</td><td>39 m</td><td>22 m</td><td>15 m</td><td>135 m</td></tr><tr><td>EuPRAXIA-LWFA ext. injection (5 GeV)</td><td>42 m</td><td>17 m</td><td>61 m</td><td>40 m</td><td>15 m</td><td>175 m</td></tr><tr><td>EuPRAXIA-LWFA all-optical (5 GeV)</td><td>20 m</td><td>17 m</td><td>61 m</td><td>40 m</td><td>15 m</td><td>153 m</td></tr><tr><td>Conventional (SRF, 1.25 GeV)</td><td colspan="2">144 m</td><td colspan="3">171 m</td><td>315 m</td></tr><tr><td>Conventional (C-band, 5.8 GeV)</td><td colspan="2">440 m</td><td colspan="3">300 m</td><td>740 m</td></tr></tbody></table>

continuation of the miniaturisation process towards an even more compact facility will be pursued. For example, external and RF-based injectors might be eliminated at some point, more compact undulator designs might become available, and also, laser technology could reduce in size and complexity. A decrease in user area sizes, on the other hand, will not be strived for, as no improvement in compactness is envisaged without having to reduce experimental possibilities significantly. Overall, a size reduction factor for the accelerator of 10 and even 20 thus seems ultimately feasible at high beam energy.

It is concluded that a significant factor of 3 in the reduction of the total facility size has been achieved with the EuPRAXIA design.

## 3.5 EuPRAXIA Digital Footprint (Data Sharing and Management)

EuPRAXIA proposes a data management strategy that will encourage the development of standards and common formats, improve data sharing and provide robust structures for data acquisition, analysis and storage within its facilities.

Accelerator science and, in particular, plasma-accelerator research is a data-intensive field. Experimentally, even compact plasma-accelerator beamlines tend to feature between five and fifteen different diagnostics, each taking images or other measurements with a repetition of a few hertz (with much higher rates expected for EuPRAXIA). This easily leads to the acquisition of multiple petabytes of data for a single experiment. On the theoretical side, plasma acceleration studies are heavily reliant on particle-in-cell (PIC) simulations. Typically run as massively parallelised calculations on CPU- or GPU-based systems at supercomputing facilities, these computational methods also produce several tens to hundreds of gigabytes of data per simulation.

In the past, most research groups have handled and evaluated their data, both experimental and computational, individually as challenges in accessibility (large data volumes, not all analysis / simulation software open source) and reusability (multiple possible techniques and setups to be studied, multiple different simulation codes)
<figure><img src="image_34.png" /><figcaption>**Fig. 3.7.** Schematic facility layout for the flagship construction site based on laser-driven plasma acceleration.</figcaption></figure>
have limited the possibility of data sharing among groups and institutes. With the European project EuPRAXIA, however, there is now a unique opportunity to change and optimise this way that data is handled in the field towards a more standardised and more “FAIR” approach [159]. Involving most plasma-accelerator groups in Europe (and worldwide) as project partners or more informal collaborators, EuPRAXIA could be a key factor in bringing together the research community and developing a strategy on how data is collected, evaluated, communicated, and shared in plasma-accelerator research. Such an effort to drive the community’s data management needs and contribute to standards directly from within the research field would also clearly be in line with the European e-IRG Roadmap 2016 recommendations aimed at setting up a common organisation of the electronic infrastructure across Europe in the near future [160].

Some specific points that should be included in EuPRAXIA’s digital strategy are the following:

**Benchmarking:** A multitude of different codes and computational tools exist in plasma-accelerator research. Following on from efforts started during the conceptual design study, the benchmarking of simulation codes with one another as well as with experimental data should be carried out. Moreover, more advanced data science techniques, such as machine learning, could be considered as additions to typical tool sets in the future. With large amounts of related and well-characterised (see next point) test data, EuPRAXIA would provide a great basis for such pursuits.

**FAIR principles:** Not only does the EuPRAXIA Consortium comprise a wide range of experimental and theoretical experts in accelerator, photon, plasma, and laser science, but also, a good number of simulation codes are developed by project partners. As such, the collaboration is in an excellent position to start the discussion on how plasma-accelerator data could be made more consistent with the FAIR (Findable – Accessible – Interoperable – Reusable) principles [159,161]. This should include, for example, the development of common computational formats and standards to improve data characterisation and identification, but also, a better alignment of experimental and computational data would be beneficial. Figure 3.8 highlights several further possible angles to this effort.

**Data acquisition, analysis, and storage:** An important aspect of the design and implementation of the future EuPRAXIA research infrastructure will be the setup of robust data acquisition, analysis, and storage structures for the EuPRAXIA sites, especially those active as user facilities. The realisation of such features will not be trivial in the case of EuPRAXIA requiring the combination of standards, procedures, and capabilities from both laser and accelerator-based infrastructure operation. It will thus set a precedent for future facilities of its kind.

**Data sharing:** An interesting goal for EuPRAXIA’s digital strategy could be the development of a system for sharing and managing data more efficiently within the consortium. Especially during the project’s technical design and construction phases, a large number of studies and measurements will need to be carried out within a limited set of resources. One strategy for tackling this challenge, which is described in more detail in Part 5, will be to design and plan experiments more effectively to consider several research questions in the same measurements. A similar approach, however, could also be employed directly to the data itself by considering data sharing and its possible use for multiple purposes across research groups. Taking a computational example, a single PIC simulation of a multi-GeV laser-driven plasma accelerator stage may cost a few hundred thousand CPU core hours and may be carried out with the aim of studying the dynamics of the injected electron beam. By also sharing this very rich data set with researchers interested in the drive laser evolution, the behaviour of the background plasma,
<figure><img src="image_35.png" /><figcaption>**Fig. 3.8.** Summary of some key strategies to make data management within the EuPRAXIA collaboration more consistent with the FAIR principles [159]. Many of the aspects listed could also have beneficial effects for the plasma-accelerator research community in general (image based on: Sungya Pundir, http://www.exeter.ac.uk/research/researchdatamanagement/open/fair/).</figcaption></figure>

or the effectiveness of the target design, resources can be used more efficiently and possibly better results achieved. A first proposed step to implement an appropriate infrastructure for this endeavour will be the setup of a specialist panel to assess specific challenges and possible implementation paths.

**European initiatives:** Finally, besides EuPRAXIA's own initiatives and activities, it will also be important to evaluate how the project can work within and towards the already existing European initiatives for data management. Examples of particular relevance include, among others, the European Open Science Cloud for data storage and sharing as well as the EU Data Infrastructure for high-performance computing [89,160].

Such a digital strategy could be pushed and developed in detail particularly by the technical cluster on theory and simulation (see Sect. 3.3.1) and the excellence centre for theory and simulation (see Sect. 3.3.2) that are proposed for EuPRAXIA.

## 3.6 Matching EuPRAXIA Needs and Existing Facilities / Capabilities in Europe

The European research area features unique infrastructures and capabilities. Those can match all the needs of the EuPRAXIA project. Attractive implementation options have been considered during the EuPRAXIA CDR.

The original goal of the EuPRAXIA design study was a site-independent design of a European plasma-accelerator facility. During the course of the design work, it became apparent that a number of outstanding facilities exist in Europe, that they would be suited to host parts of EuPRAXIA, and that those facilities would also be interested in considering their infrastructure as a EuPRAXIA site option. Therefore, some work has been performed to match the EuPRAXIA concept to existing capabilities and infrastructures in Europe.
<figure><img src="image_36.png" /><figcaption>**Fig. 3.9.** Map showing the proposed locations of the different construction sites and excellence centres at existing facilities in Europe. For the construction site for laser-driven plasma acceleration, several possible locations are under consideration; further details can be found in Section 3.6.7.</figcaption></figure>

It is noted that all the considerations here do not imply any commitment of the laboratories involved but are preliminary proposals. Those options can be followed up on if the relevant decision makers in the laboratories, the national funding agencies, and the EU commission would decide so.

In the following sections, a few of the implementation options are being described in more detail. A short overview of the proposed site and centre locations is shown in Figure 3.9.

### 3.6.1 INFN-LNF: Construction Site for Beam-Driven Plasma Acceleration

In this paragraph, we introduce the EuPRAXIA@SPARC_LAB project [162,163], intended to put forward the Laboratori Nazionali di Frascati (LNF) of the Italian Istituto Nazionale di Fisica Nucleare (INFN) as host of one of the EuPRAXIA European pilot user facilities. The new infrastructure is conceived to be able to accommodate in principle any machine configuration resulting from the EuPRAXIA design study that will find within the new LNF infrastructure the necessary technological background. Nevertheless, a particular emphasis has been addressed to the design of machine in the beam-driven configuration (PWFA) as a complementary facility to the laser wakefield acceleration (LWFA) configuration, developed elsewhere.
<table><caption>**Table 3.4.** Parameters achieved with the start-to-end simulations for the EuPRAXIA@SPARC_LAB facility.</caption><thead><tr><th></th><th>Units</th><th>1 GeV PWFA</th></tr></thead><tbody><tr><td>RMS Energy spread</td><td>%</td><td>1.1</td></tr><tr><td>Peak current</td><td>kA</td><td>2.0</td></tr><tr><td>Bunch charge</td><td>pC</td><td>30</td></tr><tr><td>RMS bunch length</td><td>µm(fs)</td><td>3.82(12.7)</td></tr><tr><td>RMS normalised emittance</td><td>mm mrad</td><td>1.1</td></tr><tr><td>Slice energy spread</td><td>%</td><td>0.034</td></tr><tr><td>Slice normalised emittance (x/y)</td><td>mm mrad</td><td>0.57/0.615</td></tr><tr><td>Undulator period</td><td>mm</td><td>15</td></tr><tr><td>Undulator strength K(a<sub>w</sub>)</td><td></td><td>1.13(0.8)</td></tr><tr><td>Undulator length</td><td>m</td><td>30</td></tr><tr><td>ρ (1D/3D)</td><td>×10<sup>-3</sup></td><td>2.5/1.8</td></tr><tr><td>Radiation wavelength</td><td>nm(keV)</td><td>2.98(0.42)</td></tr><tr><td>Photon energy</td><td>µJ</td><td>6.5</td></tr><tr><td>Photons per pulse</td><td>×10<sup>10</sup></td><td>10</td></tr><tr><td>Photon bandwidth</td><td>%</td><td>0.9</td></tr><tr><td>Repetition rate</td><td>Hz</td><td>10–100</td></tr></tbody></table>

In the PWFA scenario, in which a high-charge (200 pC) electron bunch driver excites the plasma wave (~300 µm) to accelerate a low-charge witness bunch (20 pC), the peak accelerating field in the non-linear regime is limited to twice the value of the peak decelerating field within the bunch (transformer ratio $R_T=2$). Therefore, the maximum possible energy gain for a trailing bunch is about twice the incoming driver energy. In this regime, a driver bunch energy of 500 MeV is enough to accelerate the witness bunch up to 1 GeV. This will be the main goal of the EuPRAXIA@SPARC_LAB project. In the second phase, it will be also possible to run the machine in a configuration with a transformer ratio larger than 4 so that the 5 GeV threshold for the witness beam will be also achievable with a 1 GeV driver and witness injection energy.

The core of the project will be the plasma accelerating module that will consist of a 0.5 m long, 0.6 mm diameter capillary tube in which the plasma is produced by a high-voltage discharge in hydrogen. The accelerating gradient is expected to be in excess of 1 GV/m.

Another fundamental component included in our design is the X-band accelerating technology adopted for the 1 GeV RF drive linac [164]. This choice keeps the overall linac length compact, taking advantage of the high gradient (up 80 MV/m) operation of the X-band accelerating structures.

EuPRAXIA@SPARC_LAB has been designed to fulfil the 1 GeV EuPRAXIA parameters [220]. To support our design, we have performed start-to-end simulations. In Table 3.4, the achieved parameters are listed (more details also in Chaps. 17 and 24.1). The reported performances show that EuPRAXIA@SPARC_LAB design is expected to meet the challenging requests for the SASE-FEL synchrotron radiation sources.

To meet the EuPRAXIA requirements, some preparatory actions are underway at LNF:

- provide LNF with a new infrastructure with the size of about 154 m × 35 m, as the one required to host the EuPRAXIA facility;
<figure><img src="image_37.png" /><figcaption>Fig. 3.10. Layout of the EuPRAXIA@SPARC_LAB infrastructure.</figcaption></figure>

- design and build a 0.5–1 GeV, X-band RF linac and an upgraded FLAME laser [165] up to the 0.5 PW range;

- design and build a compact FEL source, equipped with a user beamline at 2–4 nm wavelength, driven by a 1 GeV electron bunch.

The EuPRAXIA@SPARC_LAB project requires the construction of a new building to host the linac, the FEL, the experimental room, and the support laboratories. The executive project of the building is now in preparation; the construction is expected to start in 2021 and to be completed by mid-2023. The new facility will cover approximately an area of 5,000 m². The layout of the EuPRAXIA@SPARC_LAB infrastructure is schematically shown in Figure 3.10.

From left to right, one can see a 55 m long tunnel hosting a high-brightness 150 MeV S-band RF photo-injector equipped with a hybrid compressor scheme based on both velocity bunching and magnetic chicane. A chain of high-gradient X-band RF cavities will provide the energy boost from 150 MeV up to a maximum of 1 GeV. At the linac exit, a 5 m long plasma-accelerator section will be installed, which includes the plasma module and the required matching and diagnostics sections. In the downstream tunnel, a 40 m long undulator hall is shown, where the undulator chain will be installed. Further downstream, after a 12 m long photon diagnostic section, the user hall is shown with two end stations. The commissioning of the FEL beamline is expected to be completed by the end of 2026. An additional γ-ray Compton radiation source and a positrons source can be later installed in the other shown beamlines. The upper room in the figure will be dedicated to host klystrons and modulators to drive the X-band linac. In the lower light-blue room will be installed the existing 300 TW FLAME laser, eventually upgraded to 500 TW. The plasma-accelerator module can be driven in this layout either by an electron bunch driver (PWFA scheme) or by the FLAME laser itself (LWFA scheme). A staged configuration of both PWFA and LWFA schemes will be also possible to boost the final beam energy beyond 5 GeV. In addition, FLAME is supposed to drive, in the green room, electron and secondary particle sources that will be available to users in the downstream 30 m long user area.

The local collaboration is carried out with the support of groups from INFN (LNF, LNS, Milano, Roma, Napoli), the Universities of Rome “La Sapienza” and “Tor Vergata”, the University of Milan, ENEA Frascati, CNR-INO Pisa, and the CERN CLIC team. Associated partners are also the University of California at Los Angeles (UCLA) and the Hebrew University of Jerusalem (HUJ). A strong collaboration with the FLASH_Forward group at DESY will provide additional expertise in the field of PWFA acceleration techniques.
### 3.6.2 LAPLACE - France: Excellence Centre for Laser-Plasma Acceleration and 1 GeV FEL

Following discussions in the frame of the French GDR APPEL group (http://gdr-appel.fr/), a proposal to prepare a joint project emerged within the French laser-plasma-acceleration community. The present contribution to EuPRAXIA is based on these discussions and will structure future activities within the French laser-plasma community. The contribution is based on two projects: the Laser PLasma Acceleration CEntre (LAPLACE) bringing together mainly the Laboratoire d'Optique Appliquée (LOA), the Laboratoire de l'accélérateur linéaire (LAL), and SOLEIL, as well as the Prototype Accelerator Laser plASma (PALAS) project including the Laboratoire de l'accélérateur linéaire (LAL), the Laboratoire de Chimie Physique (LCP), the Laboratoire Le Prince Ringuet (LLR), and the Laboratoire d'Optique Appliquée (LOA). The two projects offer a complementary and joint research and development approach. LAPLACE proposes to implement high-repetition-rate laser-plasma-based accelerator developments and high-energy laser-plasma-acceleration studies with associated applications, such as a free-electron laser and pump-probe spectroscopy with X-rays or electrons. The PALAS project, on the other hand, aims to build an accelerator test facility to prototype a GeV multi-stage laser-plasma accelerator. Both the LAPLACE and PALAS projects are currently led by LOA and LAL in association with an open list of partners, including SOLEIL, LLR, the Laboratoire de Physique des Gaz et Plasmas (LPGP), LCP, and CEA. The LAPLACE project aims at developing these two facilities dedicated to laser-plasma accelerators and applications with the following specific aims:

**LAPLACE:** Development of high-repetition-rate LPAs producing tens-of-MeV, femtosecond-scale electron bunches at a minimum of 100 Hz repetition rate as well as high-energy LPAs driven at few-Hz repetition rates with dedicated beamlines for an FEL and FLASH radiobiology (NanotheRad [166]) applications.

**PALAS:** A laser-plasma-accelerator facility for GeV-class laser-plasma accelerator prototyping based on two laser-plasma accelerating sections driven by a 500 TW class laser system with a minimum repetition rate of a few Hz.

The two projects will rely on the upgrade of three main existing facilities, hosted in a renovated building with radiation shielding. In addition, the long focal area of the Apollon laser facility, designed by the CILEX Consortium (LLR, CEA, LULI, LOA, LPGP, Soleil, LAL, etc.), will provide a tool for testing multi-stage, PW-scale laser-driven electron acceleration in plasma.

The proposal is centred on solving several technological bottlenecks and will address optimisation studies with the longer term goals of demonstrating a multi-stage laser-plasma accelerator and short-wavelength FEL gain. It provides an answer to the necessity of rapid prototyping for the EuPRAXIA project, addressing issues such as (i) the reliability of a GeV-class multi-stage laser-plasma accelerator, (ii) the demonstration of FEL gain with a > 500 MeV laser-plasma driver and, finally, (iii) exploring the possibility of laser-plasma acceleration at high repetition rate. Increasing the repetition rate is presently the only way to reach average currents comparable to state-of-the-art room-temperature electron RF accelerators. In the meantime, several other technological challenges must also be addressed in this prototyping phase.

As mentioned above, the LAPLACE and PALAS projects will be developed as an upgrade of three main existing facilities. For the LAPLACE facility, the *Salle Noire* – a 1 TW, kHz-repetition-rate laser driving a few-MeV laser-plasma acceleration beamline – will be upgraded to a 10 TW laser system with a minimum repetition rate of 100 Hz. In addition, the *Salle Jaune* will be upgraded to a 500 TW-class driver. Depending on the funding, a larger building may be available to provide enough
<figure><img src="image_38.png" /><figcaption>Fig. 3.11. Overview of the different partners and activities involved in the LAPLACE and PALAS projects proposed to form the excellence centre for laser-plasma acceleration and a 1 GeV FEL.</figcaption></figure>

space for a short-wavelength FEL. The PALAS project will be based on the existing EXALT laser driver and upgraded pump laser with the last amplifier delivering 15 J to a PW-class optical compressor. The PALAS project could be hosted in the old GeV linac of the LAL injection hall at the DCI ring.

The French scientific and industrial communities have an important role to play in the EuPRAXIA project for the LPA-driven FEL and the development of optimised laser drivers for the optimisation of laser-driven plasma injectors and accelerator stages. The French contribution in the 2020-2025 period to the EuPRAXIA prototyping phase should rely on existing, but *upgraded* facilities with well-adapted technical support. For an efficient and significant contribution, the facilities should be dedicated to LPA development.

The LAPLACE and PALAS projects rely on the LOA and LAL sites for major developments in the EuPRAXIA framework. In addition to LOA and LAL expertise, the two projects and their contribution as an excellence centre to EuPRAXIA will benefit from an exceptional environment, with the accelerator expertise in IRFU-CEA, research and development on laser guiding and plasma components in LPGP, the “Maison de la Simulation” research laboratory for numerical simulations, and, in particular, the SMiLEI [167] team and the Centre for Data Science of the Université Paris Saclay. In addition, proximity and long-term scientific collaborations with the world-leading high-intensity laser industry is strengthening a French contribution to EuPRAXIA to push laser driver developments further. In this particular aspect, a common laboratory is being set up to develop advanced control and expertise on high-intensity laser systems for laser-plasma acceleration. Finally, the ambition to structure the French community is envisioned to bring higher visibility at the European and international level.
In the short term (2020–2023), the LAPLACE and PALAS projects’ research work will be organised in several work packages: WP1 to 5 are foreseen to work with existing facilities and are potentially part of the EuPRAXIA clusters. Completing this part of the project might require relatively modest additional funding that could be obtained via regional or national grants.

**Work Package 1: Laser-plasma injector** – relevant to the EuPRAXIA cluster on plasma components and systems;

**Work Package 2: Numerical simulation** – relevant to the EuPRAXIA cluster on theory and simulation;

**Work Package 3: Electrons beam diagnostics** – relevant to the EuPRAXIA cluster on diagnostics;

**Work Package 4: Advanced control command** – relevant to the EuPRAXIA cluster on layout and implementation;

**Work Package 5: Optics and lasers** – relevant to the EuPRAXIA cluster on laser technology.

In the long term (2023–2026), more ambitious goals relevant to EuPRAXIA will be addressed. Such a research plan will require a substantial national investment program that we believe should go to the LAPLACE and PALAS projects. This second phase implies crucial upgrades on the current facilities and important infrastructure works. In the context of the LAPLACE and PALAS projects, LAL and LOA have agreed to set up a joint research venture on the development of LPAs, while optimising the different funding sources through the various institutes. A balanced investment could be foreseen and optimised between, on the one hand, the laser upgrades for LAPLACE and PALAS and, on the other hand, the existing building renovation. We estimate that the investment (outside of the cost of the building) would be in the range of 10–13 M€ to acquire the necessary equipment for the LAPLACE and PALAS projects.

**Work Package 6: Short-wavelength FEL gain demonstration with 500 MeV electrons** – relevant to the EuPRAXIA cluster on applications;

**Work Package 7: Multi-stage GeV laser-plasma accelerator** – relevant to the EuPRAXIA clusters on plasma components & systems and magnets & other beamline components;

**Work Package 8: Prototype of a kHz laser-plasma accelerator operating in the 5–50 MeV range** – relevant to the EuPRAXIA cluster on transformative innovation paths

### 3.6.3 IST – Portugal: Excellence Centre for Plasma Simulations and Theory

Computer simulations are the third pillar of the scientific method. In plasma accelerators and in projects with the ambition of EuPRAXIA, the tight interplay and integration between experiments, simulations, and theory is even more critical, as demonstrated by the central role played by simulation in all stages of discovery science and disruptive technology development in plasma accelerators. The maturity of the numerical methods that support R&D in plasma accelerators – capable of taking advantage of enormous computational resources and infrastructures – and the emergence of exascale computing supported by the EU-wide flagship Joint Undertaking EuroHPC open a window of opportunity for the establishment of a world leading centre in plasma simulations and theory in Europe.

At IST, we envision to establish an excellence centre in plasma theory and simulations in the format of a “collaboratorium”, i.e. a platform for coordination, collaboration, and scientific & technical exchange that can i) lead the coordination efforts in
accelerator institutes (the Cockcroft Institute and the John Adams Institute). Since
2016, UK research in this field has been nationally coordinated by the Plasma Wake-
field Accelerator Steering Committee (PWASC, see http://pwasc.org.uk/), which
represents groups at 10 universities (Imperial College London, Lancaster, Liverpool,
Manchester, Oxford, Queen's University Belfast, St. Andrew's, Strathclyde, Univer-
sity College London, and York), as well as the Accelerator Science and Technology
Centre (ASTeC) and the Central Laser Facility (CLF).

Members of the UK groups have significant leadership roles in major international plasma-accelerator projects. Within EuPRAXIA, UK groups provide leaders and/or co-leaders in three of the eight Work Packages supported by the Horizon 2020 design study. Within the AWAKE programme, UK leadership positions include the deputy spokesperson, a member of the speakers and publications committee, and several task coordinators. UK researchers have held leadership positions or leading roles in many high-profile international experiments, including SLAC FACET I and II, DESY FLASHForward, and AWAKE. The UK has strong representation in several other international efforts, including the ICFA panel on Advanced and Novel Accelerators (ANA), which has formed the Advanced LinEar collider study GROup (ALEGRO) to coordinate the preparation of a proposal for an advanced linear collider in the multi-TeV energy range. Members of the UK novel accelerator community also provide leadership to large international projects through the co-authorship of technical design reports, e.g. for ELI-NP or for SLAC FACET-II.

The UK groups, in collaboration with their international colleagues, have a strong record of developing high-quality plasma-accelerated electron beams, including the first demonstrations of narrow-band electron beams, the first generation of GeV beams, the successful demonstration of electron acceleration by the AWAKE collaboration, and the first demonstration of the plasma photo-cathode scheme. They have also been strongly involved with developing the applications of these beams. These include applications to the following: betatron radiation for the tomographic imaging of medically relevant samples, security, electron-positron plasma generation, strong-field QED experiments, and tests of radiation reaction.

The application-directed UK funding environment and the strong track record and experience of international collaboration mean that the UK is ideally placed to host the EuPRAXIA Excellence Centre for Advanced Application Beamlines.

Existing and Future Facilities in the UK

Much of the experimental work by the UK groups on LWFAs has been performed at the CLF at the Rutherford Appleton Laboratory (RAL), which hosts the Astra laser (600 mJ, 40 fs laser pulses at a repetition rate of 5 Hz) and the Gemini laser (two synchronised laser beams of 15 J, 50 fs pulses). The experimental facilities at CLF are supported by laser systems based in the university labs, including systems at Strathclyde (SCAPA, three laser systems with a 350 TW / 5 Hz flagship, three bunkers, and up to seven beamlines), Queen's University Belfast (TARANIS-X), Imperial College London, and Oxford. The university-based systems play an important role in developing and testing new concepts, preparing for experiments at national and international facilities, and in training students.

An important project transforming the UK infrastructure for plasma-accelerator research will be the future £81.2 M Extreme Photonics Application Centre (EPAC) at CLF. This new centre will provide a 10 Hz petawatt laser pumped by CLF's world-leading DiPOLE technology, driving plasma accelerators for applications and science. Initially, EPAC will deliver two target areas for enabling experiments on the fundamental and technological aspects of plasma accelerators as well as the development of novel applications: a 40 m long target area dedicated to LWFA and hybrid
PWFA→LWFA as well as another multi-purpose target area with the capability for solid target interactions. The capacity for delivering additional laser sources (e.g. 100 Hz) and a third target area at a later point exist in EPAC. With additional funding opportunities, including the materialisation of EuPRAXIA, EPAC will have the capacity to expand its capabilities and its applications further, including via additional dedicated areas.

To date, the UK groups have also contributed to PWFA R&D via experiments at SLAC FACET and at CERN AWAKE. The CERN AWAKE programme has received funding from the UK institutes and from UK research councils (STFC) since 2012. There is scope to significantly enhance the facilities for PWFA and hybrid LWFA→PWFA research. CLARA, the Compact Linear Accelerator for Research and Applications, is a dedicated R&D facility for developing FEL R&D and to prepare and support UK X-FEL capabilities. CLARA currently produces 50 MeV electron beams and will eventually provide 250 MeV beams. There is also the plan to extract the 250 MeV beam into a beamline dedicated to science experiments requiring short pulses of relativistic beams; this capability could support UK PWFA research by providing a facility for electron-linac-driven PWFA.

## The Proposed UK Contribution

With our European partners, we will continue to take advantage of the positive feedback between pushing the performance of plasma accelerators and developing new, advanced applications. The UK offers world-leading expertise in optimising electron beam quality, aiming at controlled attosecond-scale beams with phase-space control and including the reduction of emittance and energy spread to levels required by FELs. It also has leading programmes on developing strategies for operation at high repetition rates, advanced plasma sources, and beamline diagnostics and beam transport.

The repetition rate requirement for the laser driver for EuPRAXIA is 10 Hz as a minimum, but ideally, it requires a 100 Hz repetition rate and beyond. Diode-pumped solid-state laser (DPSSL) technology is ideal for this task since it aims at delivering unprecedented high peak power (>1 kW) with high repetition rates (10 Hz–1 kHz) and high overall efficiencies (>20%). This is an area in which the UK is currently world leading; the DPSSL technology developed at the CLF (DiPOLE) can already produce 100 J pump lasers that can drive petawatt lasers at 10 Hz, with the final goal of achieving 100 TW, 100 Hz lasers.

For PWFA, linear accelerators such as LCLS, PAL XFEL, and SwissFEL are used to drive FELs at rates between 60 and 120 Hz. The technical challenges associated with operating these precision accelerators at such high rates compared to more typical 5 to 10 Hz accelerators are very significant. The CLARA accelerator is designed to operate at 400 Hz, making it ideally placed to further develop and test EuPRAXIA-related equipment on the quest towards up to 100 Hz plasma-driven systems in the future.

To guarantee the efficient use of beams at such a high repetition rate, significant and fundamental developments in the technology of secondary sources, their characterisation, and their applications have to be pursued hand-in-hand with the development of the accelerator itself. It is necessary to ensure that beam transport and handling lines, beam diagnostics, and detectors are developed to work at matching repetition rates, handling high-flux beams without deteriorating their unique properties, such as the femtosecond, if not attosecond, duration. This applies to the main electron beam as well as to the positron beam and X-ray / gamma ray end stations.
Such high repetition rates also require handling a large amount of data and fast active feedback on the machine. EuPRAXIA can take advantage of the experience available at UK facilities such as the Diamond Light Source and of the active interdis- ciplinary approach at national facilities for the development of high-repetition-rate technology. Likewise, the handling of big data sets is a UK STFC strength as it is crucial for existing programmes. Finally, there is a wealth of experience in applying machine-learning techniques for beam control and dynamic accelerator control. UK Centres for Doctoral Training had a focus on this technique for several years and are leading in this area.

The EuPRAXIA Consortium has identified the following pilot applications: free-electron laser (FEL), high-energy (GeV) and low-energy (sub-MeV) positron beamlines, a Compton source of MeV gamma-ray beams, and betatron X-ray sources. Several other applications can be envisaged, but these end stations are the ones that allow pursuing specific electron beam parameters that are thought to accommodate the widest range of future applications. The laser-driven construction site will have three laser-driven beamlines: FEL, X-ray medical imaging, and a low-energy positron beamline. Future developments will include accelerator R&D as well as high-field physics experiments. The beam-driven construction site – LNF is a candidate – will host three beam-driven beamlines: FEL, a high-energy positron beamline (including capability for testing novel accelerator technologies and high-energy physics detectors), and a Compton scattering photon source.

In comparison to the state of the art of plasma accelerators today, EuPRAXIA accelerators shall enable a paradigm shift: from R&D accelerators operating usually for a few weeks at a repetition rate often well below 1 Hz to accelerators with industrial beam quality operating 24/7 with a repetition rate of 20 Hz and growing over time to 100 Hz and higher. Given the UK's world-leading expertise in developing high-repetition-rate, high-quality plasma-accelerator technology and applications, we propose that the UK build prototypes, develop all beamlines, and play a major role in their delivery.

## Summary

The conjunction of nationally coordinated world-class expertise, matching funds to upgrade already competitive national and university-scale facilities, and existing programmes in similar areas makes the UK the ideal partner in EuPRAXIA to prototype and develop all beamlines and to play a major role in delivering them.

### 3.6.5 ELI Beamlines: Incubator for the Application of Novel Accelerator Technology to Laser Science Users

The Czech Republic has a long and remarkably impressive history of research and development in the field of high-power short-pulse lasers. As a reflection of this, the European Strategy Forum for Research Infrastructures (ESFRI) supported the location of one pillar of the pan-European Extreme Light Infrastructure (ELI) project, ELI Beamlines, in Dolni Brezany, south of Prague, under the auspices of the Institute of Physics of the Czech Academy of Science. The building for this new facility has been completed, and the envisaged high-repetition-rate lasers with unprecedented opportunities are under development at ELI Beamlines to drive advanced short-pulse optical, IR, X-ray, and particle beams (secondary sources) beyond the state of the art by controlling and extending the parameters of lasers and secondary sources, mainly their intensities, stability, synchronisation, quality, energy range, and repetition rates.
<figure><img src="image_39.png" /><figcaption>Fig. 3.12. ELI Beamlines facility in Dolni Brezany (image credit: ELI Beamlines).</figcaption></figure>

It allows the performance of new investigations spanning the range from fundamental to applied sciences and medicine, ultimately leading to a better understanding of nature and providing future societal benefits. The ELI Beamlines development, as a part of the pan-European ELI project, has two major aspects: enhancing the capabilities and versatility of the laser systems and subsequently using these improved lasers for new experimental possibilities, leading to the interconnectivity / interdependence of lasers and experimental applications. The ELI Beamlines project is a unique endeavour in the field of photonic-based research worldwide and the first large-scale facility in this domain. ELI Beamlines has to innovate in parallel to construction and commissioning to be an incubator for new coming projects, which will be initiated by the prospective user community. The specific nature of the ELI Beamlines user facility is its multi-disciplinary features, opening extremely wide opportunities for the worldwide user community to develop new secondary radiation and particle sources, thus, as a consequence, improving user capabilities at new end stations, creating new paths of applied and fundamental research, pushing the boundaries of science and technology. After the commissioning phase, ELI Beamlines will be able to produce ultra-short laser pulses of a few femtoseconds ($<30$ fs) duration at the peak power of up to 10 PW. The PW-class laser systems of ELI Beamlines aim to increase the repetition rate to up to 50 Hz in the nearest future. It will lead to the development of new techniques for time-resolved spectroscopy, scattering and diffraction techniques, medical imaging, medical diagnostics, and radiotherapy, including new tools for the design, development, and testing of new materials, as well as improvements in X-ray optics.

Apart from the facility construction and technology development, ELI Beamlines leads the following research activities (research programs):

**RP1 (Lasers):** development of the short-pulse laser systems for all research programs at ELI Beamlines, including the development, implementation, and optimization of the laser systems, their components and subsystems. Short-term activities: development and implementation of four main laser systems of the ELI Beamlines facility. Long-term activities: development of the laser systems to reach world-leading intensities and pulse parameters at high repetition rates.

**RP2 (X-ray sources, driven by ultra-short laser pulses):** development of a new generation of laser-driven secondary light sources from the “vacuum-UV” to the “gamma-ray” energy range. Short-term activities: provide the international user community access to ultrashort secondary light sources for applications in molecular, biomedical, and material science. Long-term activities: continuous development of the radiation sources, including the development of a laser-driven coherent X-ray.
**RP3 (Particle acceleration by lasers):** development of versatile and stable sources of high-energy electrons, protons, and ions, driven by various laser-acceleration mechanisms. Short-term activities: development of laser-driven ion and electron sources with world-leading beam parameters. Long-term activities: development of an ion source as a compact and low-cost laser-driven proton/ion source for cancer therapy. Laser-driven electron acceleration is aimed to develop a laser-driven X-ray FEL.

**RP4 (Applications in molecular, biomedical, and material sciences):** development of setups for time-resolved experiments using high-power lasers, secondary light sources, and a comprehensive set of pump beams for both fundamental and applied research. Short-term activities: implement capabilities in the VUV and soft X-ray range for time-resolved material science, coherent diffraction imaging for atomic molecular, and optical science; implement X-ray instruments for time-resolved scattering, diffraction, absorption spectroscopy, phase-contrast imaging, and pulse radiolysis; use high-intensity lasers directly for advanced optical spectroscopy applications, such as "fsec"-stimulated Raman scattering and 2D spectroscopy. Long-term activities: combine all of these methods with perfect synchronisation for complete investigations of complex phenomena.

**RP5 (Plasma and high-energy density physics):** explore both fundamental science and possible applications in the field of high-energy and high-intensity laser-plasma interaction. Research activities concentrate on ultra-high-intensity laboratory astrophysics, warm dense matter, and plasma optics.

**RP6 (Exotic physics and theory / simulation):** explore theoretical and experimental aspects of the exotic physics expected in the so-called ultra-relativistic regime (above $1 \times 10^{22} \text{ W cm}^{-2}$) of laser-matter interaction. Short-term activities: simulation support for experimental programs in high-energy and high-intensity laser-matter interaction. Long-term activities: explore new horizons in the physics of laser-matter interactions under extreme conditions.

ELI Beamlines has currently over 250 scientific-technical employees, and all of them participate in the development and implementation of the ELI Beamlines systems. All ELI Beamlines achievements will provide input and guidance on key components for the high-repetition-rate laser-driven FEL community, including the worldwide X-ray user community. ELI Beamlines is a platform for the development of new laser technology, the improvement of radiation sources, and the acceleration of particles (protons and electrons) as well as a platform for educating a new generation of scientists and engineers.

### 3.6.6 DESY – Germany: Excellence Centre for Plasma Accelerators and High-Repetition-Rate Developments

DESY has been building up capabilities for plasma-accelerator R&D in the past decade and is currently further expanding these activities. It is executing an ambitious development program in beam-driven and laser-driven plasma accelerators, targeted to high electron beam quality, high average power and first applications. In the context of the EuPRAXIA initiative, DESY is planning to bring in its unique capabilities and research interests as an 'excellence centre for plasma acceleration and high-repetition-rate developments'. The DESY capabilities are concisely summarised in the following paragraphs.

Particle beam drivers enable high-repetition-rate and high-average-power operation of plasma wakefield accelerators using the high-power accelerator technology
available today. DESY, as the leading laboratory in the development of superconducting radiofrequency resonators, which power many of the world's most advanced particle accelerators, utilises high-average-power technology to enable plasma wakefield accelerators with above 10 kW electron beams at MHz repetition rates in the FLASHForward facility. In particular, FLASHForward aims at realising a plasma booster module in the GeV range that operates at high efficiency, supporting FEL beam quality. To achieve this, research at FLASHForward seeks to demonstrate beam-quality preservation in emittance and energy spread, relying on fine control of the beam properties before plasma interaction and advanced diagnostic capabilities post-plasma. As part of the FLASH FEL user facility, FLASHForward benefits from advanced accelerator controls as well as feedback and feedforward systems that enable high stability and precise tunability for precision experiments. With these capabilities, FLASHForward will become the world's premier plasma wakefield accelerator for the development of high-average-power plasma technology, ultimately opening an avenue for future high-average-power applications.

These developments are synergistically complemented by KALDERA, the high-average-power, high-peak-power laser initiative at DESY. Starting in 2019, it will develop and commission by 2024 a laser system delivering 100 TW peak power at a 1 kHz repetition rate to drive a laser-plasma accelerator generating GeV-level electron beams. DESY has identified a kW average-power laser system as mission critical for the development of laser-plasma acceleration. The leap in repetition rate from today's few Hz to a kHz level will provide electron beams at a level of average power comparable to modern radiofrequency accelerator technology while enabling feedback loops and the active stabilisation of electron beam parameters. It represents a major step for advancing laser-plasma acceleration from a concept to a technology.

To host KALDERA, DESY is currently building a 400 m² clean room inside the SINBAD facility. It also prepares an 80 m long radiation-shielded area close by for the installation of a kHz plasma electron target. DESY strongly supports the KALDERA initiative with specialised and user-facility trained technical support personnel for the development of controls, data acquisition, machine protection, safety installations, and technical infrastructure.

The KALDERA system will provide the laser pulses required for the Helmholtz large investment project ATHENA. The ATHENA project builds up infrastructure for implementing a laser-driven electron plasma accelerator flagship at DESY. Inside the SINBAD facility, the capabilities of the high-repetition-rate KALDERA laser system are complemented by the S-band linear accelerator ARES. The ARES linac with beam energy up to 150 MeV is located in the direct vicinity of KALDERA and is presently under commissioning. ARES will provide access to ultra-short electron bunches with a 50 Hz repetition rate and advanced beam diagnostics. Coupling the capabilities of KALDERA and ARES will enable R&D on a compact laser-plasma booster with high-quality electron bunches at beam energies up to 1 GeV, repetition rates of initially up to 50 Hz, and advanced stabilisation systems as developed at DESY. The perspective for such a coupled KALDERA/ARES/ATHENA system aims at the development of a compact free-electron laser at 1 GeV with high beam quality and a repetition rate of 50 Hz initially and later towards the kHz regime.

In summary, the present and future DESY capabilities relevant for EuPRAXIA and partially supported in the Helmholtz ATHENA project include a MHz electron beam driver at FLASHForward, high-average-power laser pulses towards kHz at KALDERA, ultra-short electron bunches at up to 150 MeV from the S-Band linac ARES, advanced plasma and beam diagnostics, ultra-fast feedback systems, plasma accelerator components, as well as experimental plasma acceleration setups. In the context of these capabilities, investments, and research interests, DESY has
been defined as EuPRAXIA’s “excellence centre for plasma acceleration and high-repetition-rate developments”.

### 3.6.7 Possible Candidate Sites for the Laser-Driven Plasma Acceleration Construction Site

Given the strong competition of the laser-plasma accelerator field in Europe, several laboratories are examining the opportunity of hosting the laser-driven plasma acceleration construction site of EuPRAXIA. Developments in the European research and funding landscape are having a direct impact on possible options for EuPRAXIA implementation as boundary conditions are changing. Discussions are ongoing, and a final site concept will need to be worked out during the preparatory and technical design phases of EuPRAXIA. The following section describes some of the present ideas and concepts on specific site proposals. The final concept for EuPRAXIA must also involve additional detailed discussions with decision makers and funding agencies at the national and European level during later phases, as mentioned above.

#### General characteristics and requirements

The final construction site for the laser-driven part of EuPRAXIA shall fulfil a few basic requirements:

- The site shall be at a laboratory with existing infrastructures in one or several of the following areas: RF accelerators, laser installations, user access.

- The site shall have existing groups in place to address all safety requirements (laser, radio-protection, access control) and rules.

- The site shall provide a space of about 175 m in length and 35 m in width (details depending on the choices during the preparatory and technical design phases).

- The site shall provide the required services and facilities for the support of external users.

#### UK: EPAC

The possibility of hosting the laser-driven arm of EuPRAXIA in the UK is now stronger than ever before. The UK has played an internationally leading role in laser wakefield research for many years. The Central Laser Facility at the STFC Rutherford Appleton Laboratory (RAL) has been a major resource for research in this field and a pioneer of European collaboration, with currently 30% European users. It currently hosts Gemini, one of the pre-eminent centres for laser-driven accelerators. Research in CLF is augmented by recent centres, such as SCAPA, and university labs in Queen's University Belfast, Imperial College London, and Oxford as well as the UK accelerator institutes – the Cockcroft and John Adams Institutes – that develop both conventional and novel accelerator technology. This environment is complemented by STFC's Daresbury laboratory, which develops the underpinning accelerator and radiation source technology, with a strong track record in delivering key building blocks across the UK and EU, and contributing tailored solutions for plasma wakefield acceleration at CLARA.

The UK government has just invested £81.2 M in a new laser centre at CLF – the Extreme Photonics Applications Centre (EPAC) – in order to exploit the applications of laser-driven novel accelerators in industry and medicine. This facility will host a 10 Hz petawatt laser, pumped by CLF's world-leading, EU-grown DiPOLE
<figure><img src="image_40.png" /><figcaption>**Fig. 3.13.** Left: The EPAC facility under development in the UK. Right: Areal view of the EPAC site with space for a potential EuPRAXIA facility next to it (image credits: UKRI, right image adapted from Google Maps).</figcaption></figure>

technology, which will be used for driving plasma accelerators. EPAC will help to develop novel particle and radiation beams for applications in industry, medicine, and security, for example by providing new ways to inspect critical industrial components.

Linking EPAC with EuPRAXIA presents an ideal opportunity to exploit the UK government's investment in this area. EPAC will be a user facility that will be online in 2024, coinciding with the potential construction phase of EuPRAXIA. Initially, EPAC will deliver the 10 Hz petawatt laser to two heavily radiologically shielded target areas for enabling experiments on fundamental and technological aspects of plasma accelerators as well as the development of novel applications: a 40 m-long target area dedicated to LWFA and hybrid LWFA$\Rightarrow$PWFA as well as a multi-purpose target area with the capability for LWFA and solid target interactions. As currently planned, the EPAC building has a third, additional shielded area, which can host a dedicated beamline for programmes including EuPRAXIA. In addition, it would be possible to build an entirely new building for EuPRAXIA next to EPAC as there is a significant scope for expansion in the construction site with shared basic infrastructure.

In addition to this, as part of a Horizon 2020 programme, the CLF is currently developing the technology for a 100 Hz pump laser for a petawatt-class driver for high-repetition-rate laser wakefield accelerators. With EuPRAXIA funding, it would be possible to deliver a similar laser within the existing EPAC building or in any subsequent expansions to it.

It is clear that the missions of EPAC, CLF, and the UK plasma accelerator groups and university centres align very strongly with EuPRAXIA and its timescales. For example, the existing capabilities in EPAC (10 Hz petawatt laser driving wakefield accelerators for applications) will de-risk EuPRAXIA significantly and it is possible to have dedicated experimental R&D programmes for testing and prototyping EuPRAXIA concepts in EPAC and SCAPA. Further, EPAC will have the capacity for delivering additional laser sources (e.g. a 100 Hz PW-class laser) and a third target area at a later point, once the primary beamlines are commissioned. When additional funding opportunities, such as EuPRAXIA, materialise, EPAC will also have the capacity to expand its capabilities and its applications further, including via additional dedicated, radiologically shielded areas.

As mentioned above, the plasma wakefield acceleration community in the UK has formed a consortium – the Plasma Wakefield Acceleration Steering Committee (PWASC) – to coordinate major activities in this area in the UK, and to amalgamate organisational leadership of research centres and universities. The PWASC has just
published a roadmap that identifies the major research themes and milestones in this area for the next two decades. A clear theme is generating high-energy, high-6D-brightness electron beams that can drive an X-ray FEL (XFEL), ultimately based on compact and high-efficiency laser-plasma wakefield acceleration. While an ecosystem of linked conceptual recipes for this grand challenge exist, and remarkable experimental progress has been demonstrated over the past years through UK-EU-US collaborations, as reported to EuPRAXIA, further R&D, technology development, and implementation require high-energy, high-repetition-rate laser systems that would be available at EuPRAXIA@EPAC. A EuPRAXIA-led, collaborative technology development programme towards this in the UK with a focus on XFEL-ready beams will fully align with the EuPRAXIA goals and complement the efforts from other EuPRAXIA partners and centres of excellence.

This two-pronged proposal from the UK provides EuPRAXIA with a significant de-risking opportunity as well as a future-proofing pathway. The specifications of EPAC are very close to the EuPRAXIA baseline, and basing EuPRAXIA in EPAC will de-risk a key milestone of EuPRAXIA – having a user facility based on LWFA – and tick off its early goals. The ambitious technology development programme towards XFEL-ready beams will ensure that the technology for the upgrade paths of EuPRAXIA is simultaneously developed, making it a very attractive proposition to host EuPRAXIA in the UK.

## Czech Republic: ELI Beamlines

The construction of the ELI Beamlines (ELI-BL) building in Dolni Brezany (near Prague) was finished in 2016. The site area is 65 000 m², including the building area of 28 645 m². The footprint of the experimental building area is 60×100 m². The experimental building has three floors (from top to bottom): the service floor, the laser floor, and the experimental floor, which has six experimental halls. The L3 “HAPLS” laser beam transport from the laser hall to different experimental halls is under preparation now at ELI Beamlines. From November 2019, the L3 “HAPLS” laser beam (see Fig. 3.14), which is the highest repetition-rate PW system worldwide, will be available in the ELI Beamlines experimental areas, starting from the E3 and E4 halls. From the middle of 2020, the L3 “HAPLS” laser beam will be delivered into the E5 experimental hall, dedicated to electron acceleration using a laser wakefield. In addition, the L2 “Amos” laser system of ELI Beamlines, which is under development for the ELI-BL “laser-driven” FEL program (the LUIS project), will be available in the E5 experimental hall starting from the middle of 2021. It will allow to perform LFWA electron acceleration with a repetition rate of up to 25 Hz using an output pulse energy of 10 J with pulse compression down to 30 fs. The LUIS setup is under preparation now in the E5 experimental hall, which is a low-vibration area with a length of 60 m. To accommodate both electron beam and photon beam transport for the laser-driven FEL project, the E5 experimental area can be extended using the existing E6 experimental hall. In this case, the total length of the area, up to 100 m, will be available for the “laser-driven” FEL development at ELI Beamlines, with a possible upgrade in the future of up to 150 m, with a minimal width of 4 m and a height of 5 m (see Fig. 3.14). For this, high thermal stability (±0.5 °C) will be provided to avoid changing the undulator properties in the case of the “Swiss-FEL” type of undulator.

The E5 experimental hall, which has a clean room of ISO Class 7, has fully operating HVAC, lightning, and service distributions. The interlock system will be implemented during the first half of 2020. The floor in the E5 experimental area has superior vibration stability. The rails in the floor are implemented in the E5
<figure><img src="image_41.png" /><figcaption>**Fig. 3.14.** The L3 “HAPLS” laser system at the ELI Beamlines laser hall (image credit: ČTK).</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>**Fig. 3.15.** Possible allocation of the “laser-driven” FEL elements (both the electron beam transport and the photon beam transport) in the ELI Beamlines experimental areas E5 and E6, with the control room above the experimental area (image credit: ELI Beamlines).</figcaption></figure>

hall, allowing the flexible installation of beamline technologies. The estimated time to complete the E5–E6 merging is 6–12 months.

Taking into account the current status of the ELI Beamlines development (in particular, the readiness of the infrastructure, high-power lasers, and experimental halls), this centre can be considered as a possible option for the laser-driven plasma accelerator construction site and realisation of the EuPRAXIA project.

Italy: CNR-INO

CNR-INO is located inside the CNR Campus in Pisa (Fig. 3.16) and is currently hosting a sub-PW laser laboratory (the Intense Laser Irradiation Laboratory, ILIL [168]) with a fully equipped experimental area (Figs. 3.17 and 3.18) dedicated primarily to laser-plasma acceleration research. The ILIL Laboratory is a founding partner of
<figure><img src="image_43.png" /><figcaption>**Fig. 3.16.** Map (left) and aerial view (right) of the entire CNR campus in Pisa, with the red arrow indicating the Intense Laser Irradiation Laboratory of the National Institute of Optics (http://www.area.pi.cnr.it/).</figcaption></figure>

<figure><img src="image_44.png" /><figcaption>**Fig. 3.17.** A view of the actual layout of the Intense Laser Irradiation Laboratory at CNR-INO in Pisa (http://ilil.ino.cnr.it/).</figcaption></figure>

EuPRAXIA and is the first laboratory in Italy where laser-plasma acceleration was demonstrated experimentally in 2007 [169].

The laboratory is also a leading node of the Italian network on the Extreme Light Infrastructure with a focused research programme on medical applications of laser-driven plasma acceleration for advanced radiotherapy and X-ray diagnostic developments. Moreover, ILIL is the proposing institute of the Resonant Multi-Pulse Ionisation Injection scheme (ReMPI) [170] for the generation and acceleration of high-quality electron beams. The ReMPI scheme is one of the priority solutions within the EuPRAXIA model for the 150 MeV injector and represents an effective compact solution as a GeV-scale, fully laser-driven accelerator scheme. The implementation of such a scheme for the 150 MeV injector is already being pursued at the ILIL-PW lab based on the existing 250 m² infrastructure and national funding.

At the same time, CNR-INO is also pursuing a fairly large programme for the development of novel high-average-power laser technologies that will be carried out
<figure><img src="image_45.png" /><figcaption>**Fig. 3.18.** The interaction target chamber (left) and a section of the control room (right) at the Intense Laser Irradiation Laboratory (http://ilil.ino.cnr.it/).</figcaption></figure>

<figure><img src="image_46.png" /><figcaption>**Fig. 3.19.** Map of the CNR campus showing the INO laboratories, including the existing Intense Laser Irradiation Laboratory, compatible with the EuPRAXIA laser-driven accelerator construction site.</figcaption></figure>

in the 2020–2022 timeframe and, eventually, could be part of the final EuPRAXIA layout. In view of these activities, additional upgrades of the laboratory are in order, including a 50 m² extension of the shielded target area – an option in the current layout – and the construction of a new 100+100 m² clean room to host laser developments, all using already available building space.

From the point of view of local support infrastructures, CNR-INO is located inside the Pisa CNR campus (Area della Ricerca di Pisa), a large multi-disciplinary research site hosting 1,200 staff and unique research capabilities, with established high-quality support facilities and all necessary utilities (engineering, gas supplies, water cooling, UPS, electricity, etc.). The campus also hosts a major medical activity and is the site of the “.it” registration authority.

In view of the above, CNR-INO is an ideal candidate to host the construction of the laser + accelerator + beam transport to establish the EuPRAXIA electron beam
specifications required at the entrance of the undulator for FEL operation. This rep-
resents the most challenging milestone of EuPRAXIA and could justify a dedicated,
focused effort. Significant additional room would be needed to host the undulator
and user beamlines planned for the full EuPRAXIA laser-driven facility and could
be available subject to approved construction. Alternatively, the commissioned high-
quality accelerator could finally be integrated as an option to drive the undulator at
the EuPRAXIA@SPARC Lab in Frascati.

The convergence of the ILIL-PW research objectives and the EuPRAXIA main
goals is clearly an opportunity for both parties. Further discussion will be needed to
finalise the plan of EuPRAXIA at CNR-INO in Pisa and to secure funding for the
needed additional dedicated infrastructure and instrumentation developments.

Italy: INFN-LNF

The EuPRAXIA@SPARC_LAB project [162,163] of the Italian “Istituto Nazionale di Fisica Nucleare” (INFN) has been described in Section 3.6.1 as the candidate to host the beam-driven plasma acceleration (PWFA) construction site. Nevertheless, in the same infrastructure, the laser-driven plasma acceleration (LWFA) option can be also accommodated, in principle in any configuration resulting from the EuPRAXIA Design Study [171]. Among all the possible LWFA configurations, the EuPRAXIA@SPARC_LAB project has been investigated in detail by INFN to fulfil the 1–5 GeV EuPRAXIA parameters in the LWFA scheme with the external injection of an electron bunch [162]. This configuration is of particular interest for its promising capability to produce high-quality electron beams and would be very suitable for the INFN site, where a 0.5–1 GeV X-band RF linac is already foreseen to drive the PWFA scheme. To this end, a wide room (32×12 m², light blue in Fig. 3.10) will be devoted to the laser sources, including the 0.5–1 PW upgrade of the existing 300 TW FLAME laser system [165], which will operate at a 5–10 Hz repetition rate and will be synchronised to the RF linac at the fs level.

In the LWFA scenario with external injection, a low-charge (30 pC) electron bunch is accelerated to up to 0.5–1 GeV by the RF linac and then injected into the plasma module that will consist of a 0.5 m long, 0.6 mm diameter capillary tube, in which the plasma is produced by a high-voltage discharge in hydrogen. With a plasma density of $1 \times 10^{17} \text{ cm}^{-3}$ (quasi-linear regime) and on-target laser parameters of 25 J in a 112 fs (FWHM) long pulse with a 70 µm (RMS) spot size (corresponding to a $0 \sim 1.3$), the accelerating gradient is expected to be of the order of 10 GV/m. Start-to-end simulations show that the accelerated electron beam at 5 GeV keeps a high quality profile with an RMS normalised slice emittance of 0.35 µm, an RMS energy spread of $1 \times 10^{-3}$, and a peak current of 2 kA [172], thus being able to drive a short-wavelength SASE FEL as foreseen for the EuPRAXIA project.

## 3.7 Long-Term Scientific Programme

Considering the challenging nature of the EuPRAXIA project based on its use of
novel technologies, most of the design study and consequently this report focus on
the baseline design of the proposed EuPRAXIA research infrastructure. However,
looking at the long-term sustainability of such a facility, with research infrastructures
typically expected to run for several decades, it is also important to keep in view
potential future developments and directions of the science at EuPRAXIA.

This section thus presents the first version of a long-term scientific programme.
Defining such a strategy relatively early on will allow, on the one hand, the design
<figure><img src="image_47.png" /><figcaption>**Fig. 3.20.** The five main pillars of the EuPRAXIA long-term science programme and specific activities towards these topics. Key features that would be targeted in future upgrades are a further reduction in facility size as well as improvements in the machine performance and output beam properties.</figcaption></figure>

and building of the infrastructure with future upgrades in mind. On the other hand, it will also make it possible to already integrate parts of the necessary R&D for these developments very effectively into the technical design phase of the facility.

As Figure 3.20 shows, the proposed long-term programme is built on five main pillars, each of which is foreseen to (1) improve the impact of the EuPRAXIA research infrastructure, (2) ensure that it remains competitive as a facility in the overall research landscape, and (3) open up new possible experimental opportunities at EuPRAXIA:

**Reduced facility footprint:** An important factor limiting a reduction in facility size for the baseline design is the use of many well-tested but relatively large technologies for transport lines, diagnostics, and other parts of the machine. This is a risk-mitigation strategy to ensure that the main R&D efforts can flow into more critical sub-systems, such as the high-power laser and plasma stages. Once robust performance has been achieved for these essential parts, though, a further miniaturisation of the entire machine should be pursued in a stepwise approach, aiming for a size reduction of the accelerator by a factor of 10 or more in the long-term. A promising spin-off of this strategy could also be the development of ultra-compact, self-contained accelerator systems based on EuPRAXIA technologies and suitable for testing with collaborating institutes or companies.

**High power laser technology:** The EuPRAXIA laser system is developed to achieve the requirements set by the EuPRAXIA design aims but with technologies at a high technical readiness level to ensure robust and stable performance from the beginning of the facility operation. With this trade-off in mind, future improvements of the high-power laser system will be essential for the competitiveness of EuPRAXIA. Technical challenges of particular interest will, for example, be upgrades to kHz repetition rates and higher efficiency, strategies for which are being developed now (see Sect. 10.2.7).

**Accelerator technology:** A variety of different technological approaches has been investigated as part of the EuPRAXIA conceptual design study, some of which have shown great potential, but are still at a low technical readiness level. By developing these concepts, such as the hybrid plasma acceleration scheme (see
also Chap. 26), further in parallel to the baseline design, a future implementation
can be considered to improve the machine properties EuPRAXIA can offer to
users with time. Other aspects of accelerator technology, such as beam control
and multi-stage acceleration, may not have specific future innovation strategies
beyond the baseline yet; nonetheless, it will be important to upgrade these as
well, as the overall EuPRAXIA facility develops.

**Plasma-based FEL:** With plasma-based free-electron lasers (FELs) requiring significant research and development to become ready for user operation, the EuPRAXIA FEL design is aimed at achieving an acceptable but in particular also robust performance for this application. Once more experience has been gathered with this new type of radiation source, especially in user experiments, the next natural goal will thus be to improve the properties of the FEL both through optimisation of the beamline and through the integration of more advanced concepts.

**Method improvement for applications:** EuPRAXIA will be an ideal test bed for the secondary beam applications that plasma accelerators can provide based on their specific properties. The user experience gained over time will help to find new ways for improving these imaging and analysis tools at EuPRAXIA in terms of beam properties, reliability, and user-friendliness. Equivalently to the pursued accelerator footprint improvements, an interesting side effect of these developments could also be the prototyping of independent, more specialised, and highly compact accelerator application systems in collaboration with academic or industrial partners.

# 4 Preliminary Resource and Financial Plan

## 4.1 Cost Model and Definitions

EuPRAXIA is, at full scale, a 320 million euro investment into European compact accelerator innovation and scientific applications, including a 70 million euro investment into laser technology. Depending on budget availability minimal scenarios include options for a 60 million euro beam-driven and a 75 million euro laser-driven accelerator site (without R&D costs).

The budget estimate listed in this report is a preliminary estimate that is based on data collected from the consortium on the different required cost items. Here, we explain the categories of spending:

**Category Preparation Costs:** These are costs spent by the laboratories on preparing the sites and centres before the approval of the EuPRAXIA TDR and construction projects. These are costs that are pre-invested by the laboratories in ongoing projects and with the view of preparing the EuPRAXIA site. This category is not included in the EuPRAXIA cost estimate but might become relevant as in-kind contribution.

**Category Investment Costs (R&D and construction):** These are costs spent on hardware, contracts with industry, access to facilities, contractual labor, and other smaller items. These costs are given in 2019 terms in millions of euro.

**Category Personnel Costs (R&D and construction):** This category specifies the labor at collaborating institutes for performing the required tasks in the technical design and construction of EuPRAXIA. These costs are given in full-term equivalents (FTE). One FTE means that one person works full time for the EuPRAXIA project during one year.
**Category Operational Costs:** These are the costs related to hardware commissioning, beam commissioning, and the operation of the proposed facility after the completion of construction.

The major EuPRAXIA costs will incur during the following two phases of the project.

**Costs during Technical Design:** The technical design phase follows the conceptual design that we report on in this document. During the technical design phase, the detailed technical solutions are developed, prototypes will be constructed and tested, components will be industrialised, critical proof-of-principle experiments will be carried out and the construction will be prepared. At the end of this phase, a technical design report (TDR) will describe the detailed facility design.

**Costs during Construction:** This describes the costs that are required for constructing the facilities at the proposed sites as defined in this report. Of particular interest are the two flagship construction projects that realise the complementary technological paths of (1) laser-driven plasma acceleration (LWFA) and (2) beam-driven plasma acceleration (PWFA).

The sections below specify the total cost of the facility and the breakdown into the two main technologies (beam-driven and laser-driven plasma accelerators) as well as the breakdown into major technical systems. Both investments and person-power needs have been estimated from the project participants and are presented. At the end of this chapter, minimal systems are discussed that do not offer the benefits of a European research infrastructure but could be starting blocks for a EuPRAXIA facility. Also, these minimal systems give a first indication of a possible system cost after a successful EuPRAXIA project. For example, the final prize tag of a 5 GeV plasma accelerator after EuPRAXIA will be much lower than the cost of EuPRAXIA as R&D costs will not have to be paid again and central components will be available from the industrial partners of EuPRAXIA.

## 4.2 Summary of Total Costs

The estimated total resources required for the realisation of the EuPRAXIA facility are listed in Table 4.1. The table specifies the integrated investment costs, the personnel needs, and the project duration for the phases of TDR and construction. We note that the project durations for the beam-driven site (8 years) and the laser-driven sites (10 years) are unequal because of the technical readiness level associated with the respective technologies of X-band RF and high-peak-power lasers at 20–100 Hz. More details on the schedule can be found in Section 5.1.

The total costs in Table 4.1 are split into parts related to the site with beam-driven plasma acceleration and laser-driven plasma acceleration. It is seen that the laser-driven leg of EuPRAXIA is more expensive than the beam-driven leg. This is a result of the laser R&D required for realising the more powerful lasers in EuPRAXIA, the cost of laser procurement, and the undulator cost for the 5 GeV FEL user area. This will be explained and discussed in a subsequent section in more detail.

From the data in Table 4.1, a spending profile with yearly tranches of investment budget was estimated. This is shown in Figure 4.1. It is seen that the yearly investment budget would increase over eight years to 55 M€ and then drop in the last two years of the project. The average yearly spending would amount to 32 M€ over the full project duration.

The corresponding person-power effort during the project duration is shown in Figure 4.2. It is seen that this Europe-wide project would require the support of
<table><caption>**Table 4.1.** The estimated integrated costs of the TDR phase and construction are given (a) for the total project, (b) for the parts connected to the beam-driven plasma accelerator site (PWFA) and (c) for the parts connected to the laser-driven plasma accelerator sites.</caption><thead><tr><th></th><th>TDR phase</th><th>Construction phase</th><th>Sum</th></tr></thead><tbody><tr><td><strong>Invest</strong></td><td></td><td></td><td></td></tr><tr><td><em>Total</em></td><td>119 M€</td><td>204 M€</td><td>323 M€</td></tr><tr><td><em>Beam-driven</em></td><td>34 M€</td><td>85 M€</td><td>119 M€</td></tr><tr><td><em>Laser-driven</em></td><td>85 M€</td><td>119 M€</td><td>204 M€</td></tr><tr><td><strong>Personpower</strong></td><td></td><td></td><td></td></tr><tr><td><em>Total</em></td><td>981 FTE</td><td>854 FTE</td><td>1835 FTE</td></tr><tr><td><em>Beam-driven</em></td><td>294 FTE</td><td>283 FTE</td><td>577 FTE</td></tr><tr><td><em>Laser-driven</em></td><td>687 FTE</td><td>571 FTE</td><td>1258 FTE</td></tr><tr><td><strong>Duration</strong></td><td></td><td></td><td></td></tr><tr><td><em>Total</em></td><td>6 years</td><td>4 years</td><td>10 years</td></tr><tr><td><em>Beam-driven</em></td><td>4 years</td><td>4 years</td><td>8 years</td></tr><tr><td><em>Laser-driven</em></td><td>6 years</td><td>4 years</td><td>10 years</td></tr></tbody></table>

<figure><img src="image_48.png" /><figcaption>**Fig. 4.1.** The yearly tranches of the required investment costs are shown versus the duration of the project for the total and split by construction site.</figcaption></figure>

184 persons (average) and about 230 persons (peak) for its realisation. This personnel could, in some cases, be contributed by partners in kind and, in other cases, would require funding support from national or European funding agencies. During the EuPRAXIA conceptual design work, about 250 persons have been involved in the work, while an estimated 40 FTE have been invested per year at maximum. The FTE available to EuPRAXIA would therefore need to roughly grow by a factor of 6 in the project realisation phase. The high number of persons associated with this project shows that the required persons and resources in principle exist in the field and in the connected institutions.

It is noted that a final budget planning will be part of the project preparatory phase, which must be performed in parallel to the TDR phase. During the preparatory phase, all budget numbers will be updated, a detailed cost-benefit analysis will be performed, and funding models will be defined. During the CDR phase, we have performed a first study of a possible budget distribution over Europe, grouping the
<figure><img src="image_49.png" /><figcaption>Fig. 4.2. The yearly tranches of the required person power are shown versus the duration of the project for the total and split by construction site.</figcaption></figure>

funding by country of the institutes. The result is shown in Figure 4.3, integrating over investment and personnel costs. Here, it is assumed that one FTE costs 100 k€.

It is seen in Figure 4.3 that the project could be implemented with Italy, France, Germany, and the UK as major stakeholders at the 18% to 25% level of share in the total resources. Portugal could share at the 2% level given the mainly theoretical and numerical expertise that the connected institute would bring in. An additional 5% share, including general infrastructure and construction costs, would go to whichever country would host the construction site for laser-driven plasma acceleration. We conclude from this preliminary study that existing groups and facilities in Italy, France, Germany, the UK, Portugal, and other countries have the critical capabilities to support such a EuPRAXIA project if common resources and funding would be made available by the various funding agencies.

## 4.3 Breakdown of Investment Costs Related to the Beam-Driven Plasma Accelerator Site

The overall project costs listed above include a variety of cost items that enable a truly European effort. We first consider the site with the beam-driven plasma accelerator facility (PWFA). The total PWFA cost includes not only the resources required for the construction of the local accelerator facility and the FEL beamline but also prototyping and personnel costs around Europe. These additional resources will support the local construction by prototyping devices, testing them, and then delivering them to the PWFA site. While the cost of the final device should be included in the accelerator construction cost, the prototyping and testing at outside facilities can also be considered separately. The same applies to application beamlines, for example, for positron generation. The positron beamline will be developed, prototyped, and tested in excellence centres in another country with the final beamline delivered to the PWFA construction site.

Table 4.2 provides the breakdown of the PWFA costs into various categories, explained in the caption of the table. We can see, for example, that the PWFA accelerator plus FEL facility would cost ~68 M€, which is about half of the total cost. The other half of the total cost funds prototyping and testing activities around Europe, provides the additional application beamlines from non-local partners (thus
<figure><img src="image_50.png" /><figcaption>**Fig. 4.3.** A possible share of EuPRAXIA resources (investment plus personnel) over existing groups and facilities in connected European countries is given. General costs for the construction site for laser-driven plasma acceleration (LWFA) are listed separately as their allocation would depend on the location of the site.</figcaption></figure>

<table><caption>**Table 4.2.** The costs of the beam-driven plasma accelerator site are broken down into construction costs of the accelerator (costs of all infrastructure and installed equipment at the construction site), into FEL construction costs (undulators plus user area), into costs for the construction of lines and user areas for other applications (see description in the relevant section), into costs of testing and prototyping at all European partners, and into costs for other innovation parts (for example alternative radiation sources).</caption><thead><tr><td>Category</td><td>Invest</td><td>Person-power</td></tr></thead><tbody><tr><td>Accelerator construction</td><td>52 M€</td><td>162 FTE</td></tr><tr><td>FEL construction</td><td>16 M€</td><td>22 FTE</td></tr><tr><td>Other applications</td><td>17 M€</td><td>91 FTE</td></tr><tr><td>Prototyping EU wide</td><td>33 M€</td><td>285 FTE</td></tr><tr><td>Other innovation paths</td><td>1 M€</td><td>17 FTE</td></tr></tbody></table>

amplifying the scientific impact of the EuPRAXIA PWFA site), and prepares additional innovation paths.

As noted before, the presented numbers are preliminary estimates that must be detailed and adapted in a preparatory phase of EuPRAXIA. The final numbers will depend on available funding support, the final partners, and implementation details.

It is interesting to consider the breakdown of investment costs also by technical system. Such a breakdown is shown in Figure 4.4. As this concerns the beam-driven plasma accelerator facility, it is seen that, as expected, the main cost drivers are the cost of the linear RF accelerator, infrastructure, and applications.
<figure><img src="image_51.png" /><figcaption>Fig. 4.4. The total investment cost related to the beam-driven plasma accelerator facility is broken down into the various major technical systems. Diagnostics and controls costs are contained in the linac and plasma system costs. The listed investment costs include R&D, prototyping, and construction.</figcaption></figure>

<figure><img src="image_52.png" /><figcaption>Fig. 4.5. The total investment cost related to the laser-driven plasma accelerator facility is broken down into the various major technical systems. Diagnostics and controls costs are contained in the linac and plasma system costs. The listed investment costs include R&D, prototyping, and construction.</figcaption></figure>

## 4.4 Breakdown of Investment Costs Related to the Laser-Driven Plasma Accelerator Site

The breakdown of investment costs by technical systems is shown in Figure 4.5. It is seen that, as expected, the main cost drivers for the laser-driven leg of EuPRAXIA are the costs of the laser systems and of the science applications. The same breakdowns as explained above for the beam-driven plasma accelerator site have been done for the costs related to the laser-driven construction site. Table 4.3 gives the breakdown of the total costs per categories. We can see that the *investment cost of the 5 GeV laser-plasma accelerator facility plus the FEL user area amounts to a total of ~110 M€*, which, again, is about half of the total investment cost. The other half of the cost, similar to the case of the beam-driven plasma accelerator facility, is in part related to the development, prototyping, and testing in European research institutes and in close collaboration with the European laser industry. The laser investment will fertilise technological advances in laser technology and in the laser industry, required
**Table 4.3.** The costs of the laser-driven plasma accelerator site are broken down into construction costs of the accelerator (costs of all infrastructure and installed equipment at the construction site), into FEL construction costs (undulators plus user area), into costs for the construction of lines and user areas for other applications (see description in the relevant section), into costs of testing and prototyping at all European partners, and into costs for other innovation parts (for example alternative radiation sources).

<table><thead><tr><td>Category</td><td>Invest</td><td>Person-power</td></tr></thead><tbody><tr><td>Accelerator construction</td><td>86 M€</td><td>461 FTE</td></tr><tr><td>FEL construction</td><td>24 M€</td><td>27 FTE</td></tr><tr><td>Other applications</td><td>9 M€</td><td>75 FTE</td></tr><tr><td>Prototyping EU wide</td><td>84 M€</td><td>678 FTE</td></tr><tr><td>Other innovation paths</td><td>1 M€</td><td>17 FTE</td></tr></tbody></table>

for delivering the needed components but also allowing the European laser industry to maintain its world-leading capabilities. Other parts of the additional costs from collaborating institutes around Europe concern additional application beamlines that will again amplify and enhance the scientific potential of the constructed facility. Here, we list the positron-based imaging of materials as an example.

As noted before, the presented numbers are preliminary estimates that must be detailed and adapted in a preparatory phase of EuPRAXIA. The final numbers will depend on available funding support, the final partners, and implementation details.

## 4.5 In-Kind Contributions

There have been major investments in the infrastructures at the EuPRAXIA institutes in the past. As the EuPRAXIA sites will be located at existing sites, the installed facilities and equipment will be brought into the EuPRAXIA project, and its usage will be considered as in-kind contribution. In addition, new projects on the scale of 20 M€ to 90 M€ are ongoing that enhance the existing investments and possible in-kind contributions even further.

It is immediately clear that the selection of final sites will have an impact on the total costs. For example, if a required laser will be available locally as in kind, the construction cost will be reduced. We assume in this CDR that the required buildings are available and can be adapted to the needs of one of the EuPRAXIA sites. The costs for adaption are included, the building is considered in kind. The INFN/LNF laboratory in Frascati, Italy, is planning to provide the infrastructure (building, air-conditioning, etc.) as an in-kind contribution as a host of the beam-driven plasma accelerator site. A detailed list of in-kind commitments and final budget adjustments must again be performed during the preparatory phase after the CDR.

## 4.6 Operational Costs

Operational costs require a detailed analysis and study that could not be performed during this CDR. Based on past experience with accelerator-based facilities, a very rough operational cost (material plus operational personnel) can be estimated for the two construction sites:

**Operational cost of the beam-driven plasma accelerator facility:**

Estimate: ≈ 6 M€ per year

**Operational cost of the laser-driven plasma accelerator facility:** Estimate:

≈ 12 M€ per year
It is again stressed that the operational costs listed above are very rough estimates with large errors of uncertainty. The higher operating costs of the laser-driven facility can be assigned to a few arguments:

- The lifetime and mean time to failure of RF modulators and klystrons has been optimised over many decades, and maintenance costs are therefore optimised. On the other side, lasers are a revolutionary and rapidly developing technology that still must be optimised for the lifetime of pump diodes, for example.

- The laser-driven plasma accelerator site of EuPRAXIA includes laser power drivers and undulator length for a beam energy of 5 GeV. The beam-driven leg of EuPRAXIA includes undulators for a beam energy of 1 GeV. Both approaches and sites will profit strongly from new types of innovative and small-size undulators. Strong R&D is supported.

The distribution and financing of the operational costs would need to be defined with the funding agencies during the preparatory phase.

## 4.7 Alternative Minimal Systems

The EuPRAXIA project has aimed at a risk-minimised approach with a maximised scientific output. *Risk minimisation* is important as up to today, no plasma-accelerated electron beam has been brought into FEL lasing. Also, the available quality of electron beams from plasma accelerators is still quite limited, with up to an order of magnitude improvement required for certain applications, like highest-brilliance FELs. Another area of concern is the achieved repetition rate in plasma accelerators, with 0.5 Hz regularly maintained as average and peak rates of 5 Hz. Risk-mitigation measures are achieved in the EuPRAXIA concept by providing the infrastructure for various approaches of producing high-quality electron beams in a plasma accelerator. Examples include optimised plasma injectors in the bubble regime, plasma sources relying on the new resonance multi-pulse ionisation injection method, the injection of electrons from RF-based photo-cathodes, and novel two-stage acceleration schemes with intermediate phase space rotation.

*Maximised scientific output* is achieved by including various highly promising scientific applications that offer advantages compared to existing facilities. EuPRAXIA does not only include the FEL application but also includes, for example, positron-based imaging, positron beam acceleration, ICS and betatron sources for material and medical applications, etc.

Last but not least, EuPRAXIA brings together institutes and experts from around Europe, with close connections also to partners in Asia, Russia, and the United States. The *collaborational and knowledge-building* aspect of EuPRAXIA is not cost-neutral, but it provides the capabilities to advance faster with such broad support from around Europe. This strengthens the competitive position of Europe in this rapidly advancing technological field but also generates important socio-economic benefits for society. Examples can be the building of European connections, the training of young technicians and scientists in a technically challenging international environment, and last but not least, new jobs for well-trained young people in science and industry.

Given the above features and benefits of EuPRAXIA, it is clear that these come at a cost. We strongly believe that these investments into risk mitigation, science output, and more general socio-economic benefits in the European landscape of high-tech endeavours are well worth the costs. It is, however, interesting to consider minimal scenarios where a local facility targeted to one application only is considered. Cost estimates for a few reduced and minimal cases are summarised in Table 4.4.
**Table 4.4.** The investment costs for the full EuPRAXIA case are compared to estimates of minimal systems. For the beam-driven cases, the applications are reduced to only include the FEL. For the laser-driven cases, the applications in the first scenario are reduced to only include the FEL. In the second step, the mitigation measures on beam quality (multiple laser, RF injector, novel concepts) are reduced to a minimal one-laser, one-stage system. In both cases, the costs of European-wide R&D and prototyping are not included for the reduced scenarios.

<table><thead><tr><th>Scenario</th><th>Invest</th></tr></thead><tbody><tr><td><strong>Beam-driven plasma accelerator facility</strong></td><td></td></tr><tr><td>Full EuPRAXIA proposal</td><td>119 M€</td></tr><tr><td>Plasma accelerator facility with FEL</td><td>68 M€</td></tr><tr><td><strong>Laser-driven plasma accelerator facility</strong></td><td></td></tr><tr><td>Full EuPRAXIA proposal</td><td>204 M€</td></tr><tr><td>Plasma accelerator facility with FEL</td><td>110 M€</td></tr><tr><td>Minimal laser plasma accelerator with FEL</td><td>75 M€</td></tr></tbody></table>

# 5 Preliminary Project Organisation and Implementation

## 5.1 Project Schedule

A preliminary timeline for the project has been developed, as shown in Figure 5.2. It assumes that the project technical design, implementation and start of operations can be achieved within 10 years. This estimate as well as the proposed completion date depend strongly on the available financial resources throughout the project development, all of which are currently not clearly foreseeable yet. A certain amount of uncertainty thus needs to be expected from this schedule.

Conceptually, the development of EuPRAXIA as a multi-site infrastructure is divided into two separate yet related timelines. The design and implementation of the five excellence centres is scheduled within the “Technical Design Phase” over the next four to six years, whereas the two user machine sites will begin their technical design and construction at a later point and aim to reach completion at the end of the “Implementation & Construction Phase”. Between the construction sites, the facility based on beam-driven plasma acceleration is expected to require only a four-year technical design compared to six years of R&D and prototyping for the case of the site focused on laser-driven plasma acceleration. This means that the PWFA-based construction site could already be completed after an eight-year period, while the entire research infrastructure, including the LWFA construction site, would be ready by the ten-year mark. The reasons for this temporal separation in the development of the different sites are clear:

1. The excellence centres are smaller setups based strongly on existing infrastructures, thus requiring less design and construction time. Additionally, they are meant as advanced R&D facilities to mature the technologies to be implemented at the user sites, from the plasma-accelerator beamline and feedback systems to the FEL and other applications. They therefore need to be ready for use much earlier than the main machine sites, but not for performance as external user facilities. Figure 5.1 highlights some of the main technical milestones that will need to be achieved either as activities at the EuPRAXIA support facilities or even earlier as R&D at some of the EuPRAXIA Consortium facilities available for smaller testing and prototyping experiments (see Part 5 for more details).

2. The implementation of the laser-driven plasma accelerator construction site is largely dependent on the development of the required high-power,
Important Technical Milestones

<figure><img src="image_53.png" /><figcaption>**Fig. 5.1.** Main technical milestones to achieve during the EuPRAXIA technical design phase.</figcaption></figure>

high-repetition-rate laser sources as a significant advance from the current state of the art. This endeavour is expected to require considerable R&D and prototyping efforts. The X-band technology forming the basis of the second construction site for beam-driven plasma acceleration, on the other hand, is, while also a clear extension from the current state, already at a higher technical readiness and supported by long-lasting expertise developed in RF accelerator technology.

Furthermore, given the high degree of novelty in many of the technologies planned for EuPRAXIA as well as in view of funding uncertainties, a phased approach is also foreseen for the implementation and operation of each of the two user machine sites, as indicated briefly in Figure 5.2. With a detailed schedule to be defined during the technical design phase, first ideas for sensible phasing milestones include

- a staggered construction and operation of beamlines and user areas;

- an initial machine operation with 1 GeV electron energy with a later increase to 5 GeV; and

- an initial machine operation at a 20 Hz repetition rate, with a later increase to 100 Hz.

Particularly with regard to user operation, an approximately two-year-long pilot user phase will be extremely beneficial to develop a better understanding of the user and operation needs and challenges of EuPRAXIA as a novel type of plasma-accelerator-based facility. This will be followed by the main user operation phase, with a gradual ramp-up in operation hours, as the stability and performance of the machine is optimised and more beamlines / user areas become available.

Finally, for the long-term operation of the EuPRAXIA infrastructure, future machine upgrades will need to be developed and implemented over time, such as
<figure><img src="image_54.png" /><figcaption>Fig. 5.2. Preliminary timeline for the design, implementation, and operation of EuPRAXIA, including several milestones marked by crosses in the Gantt chart.</figcaption></figure>

high-repetition-rate operation and other topics of interest. These component improvements or machine upgrades will be prepared and optimised at the continuous testing areas of the four EuPRAXIA support facilities with a staggered transfer of technology to the user sites. In this way, the operation disruptions at all facilities are minimised, while a close technological and organisational connection between the many sites remains ensured.
<figure><img src="image_55.png" /><figcaption>**Fig. 5.3.** Implementation and re-evaluation process for the EuPRAXIA project structure. With the end of the current consortium agreement for the conceptual design study, a new agreement will be set up, which should be updated and revised as necessary throughout the different phases of EuPRAXIA.</figcaption></figure>

<figure><img src="image_56.png" /><figcaption>**Fig. 5.4.** Planned management structure for the EuPRAXIA Consortium for future project phases. The listed bodies will be integrated as the “project management” within the wider EuPRAXIA infrastructure concept shown in Figure 3.1. Note that the features shown in grey are planned to be implemented only during the construction and operation of the EuPRAXIA facility.</figcaption></figure>

## 5.2 Structure and Governing Model

The following legal and governance model described for EuPRAXIA represents the most suitable option, as foreseeable at this current project stage. Over the course of the design and implementation of the EuPRAXIA research infrastructure, it should be re-examined and possibly revised regularly, as varying external or internal conditions, but also, evolving perspectives within the collaboration may make changes in governance bodies as well as legal structure desirable over time. Figure 5.3 visualises how this continuous implementation and evaluation process for the collaboration structure could work.
The EuPRAXIA collaboration will, for the coming project phases, be structured through a consortium agreement. Following a survey of various international and national legal structures, the decision aligns well with the collaboration's priority to retain a high level of flexibility necessary to accommodate the current uncertainties in terms of funding, schedules, and the implementation of novel technologies still to be validated in the technical design phase. In addition, a consortium agreement will also easily retain the current collaboration constitution, including international partners and many smaller European institutes, without the need to organise the project at a higher, country-based level. Finally, with EuPRAXIA in a relatively early stage of development, the administrative effort associated with the collaboration governance can remain comparably small to develop and expand proportionately as the research infrastructure evolves.

The consortium's governance model is shown in Figure 5.4. It has been largely modelled after the collaboration structure during the conceptual design phase and will consist, among others, of the following bodies:

- The *Collaboration Board (CB)* is the highest decision-making body of the consortium. It is composed of one representative of each consortium member and can host observers. Its main function is to make proposals and decisions on aspects, such as changes to the consortium plan, finances, and intellectual property rights, changes to the consortium composition through withdrawal, default, or entry of a member, suspension or termination of the project, and appointment of positions in the other governance bodies.

- The *Steering Committee (SC)* is responsible for supervising the execution of the project. Similarly to the Collaboration Board, it consists of representatives of the consortium members as well as the Project Coordinator. Its tasks focus on executing and implementing decisions from the Collaboration Board as well as, more generally, overseeing the project progress.

- The bulk of the work carried out within the project is organised via ten *work clusters*, each focusing on a specific technical aspect of the project. Each cluster consists of multiple partner institutes and has a defined set of tasks to be achieved throughout the duration of the project.

- A range of advisory bodies will be set up to oversee the project, such as the *Scientific Advisory Committee* (advising on scientific matters), the *Open Innovation Forum* (connecting EuPRAXIA with industry), and the *Quality Insurance Panel* (overseeing quality control). The *Resource Review Panel* will represent the organisations and ministries funding EuPRAXIA to advise and oversee the project development. As such, it should be set up, once a funding path for implementation and operation is developed.

During the implementation and operation phases, further governance bodies should be installed, in particular the Audit Committee for monitoring consortium finances and the Ethical Advisory Board, overseeing the ethics of user experiments at EuPRAXIA. Moreover, the host institutions of the six EuPRAXIA sites will, of course, be integrated more strongly into the facility governance.

## 5.3 Operational Model

A detailed operational model for the future EuPRAXIA infrastructure will need to be developed, once the facility design, particularly the technical layout and the funding model, has been defined. As a first step, however, several more general arguments can be made regarding the facility operation:

- Both EuPRAXIA construction sites will be opened for user operation in the long term after commissioning and test operation periods are completed. The other
five excellence centres within the infrastructure, however, will be dedicated to
research and development instead.

- The entire infrastructure will be coordinated by a lean overarching project management structure, as described in Section 5.2, while much of the day-to-day organisation during operation will be carried out by the host institutions of the individual sites.

- The necessary support structures for user operation – including IT systems, user support, and machine maintenance – will be provided by the host institutions of the construction sites to integrate these services directly into the institutions’ existing operational infrastructure.

- It is assumed that the interdependencies between centres will be minimal as the targeted applications for both construction sites are complementary and most users will be interested in only one of the two machines.

- The interdependencies between individual beamlines, on the other hand, need to be examined more carefully during the technical design. For now, it is assumed that, at least during the beginning of the operational phase, each laser system (for the LPA-based site) and each RF accelerator (for the BPA-based site) will only feed one beamline and one user area at a time.

- To fully demonstrate the suitability of plasma acceleration for user experiments, the EuPRAXIA construction sites will aim to operate continuously for several days at a time (24/7), at least during a fraction of the year.

5.4 User Access

5.4.1 Types of User Access

The proposed EuPRAXIA research infrastructure is not a traditional user facility, but rather a demonstrator for future user infrastructures based on plasma wakefield acceleration. As such, a key goal of the project is the demonstration of high-quality machine performance required for different applications. Once this and other demonstration goals of EuPRAXIA have been achieved, the operation of pilot user experiments is envisaged.

An initial assessment of this potential user community for EuPRAXIA concluded
that three main types of user groups can be expected, as Figure 5.5 highlights. Based
on this evaluation, some considerations for a possible user access model are described
in the following; these will need to be revised and developed in further detail during
the preparatory phase of the project.

Beam users – especially in the beginning of the operation phase, a very small user
group – correspond to those interested only in the properties of the beams offered
at EuPRAXIA. The service that is most relevant for them is beam access, with the
technical support of the EuPRAXIA technical staff. What distinguishes this cate-
gory of users from others in particular is that they are not interested in the use of
plasma-accelerator technology. In contrast, the category of users termed co-developers
corresponds to those interested in developing and testing novel technologies related
to plasma accelerators. EuPRAXIA will be the first user facility where such tech-
nologies are in operational use. It is therefore expected to be a reference facility, if
not the only facility, where said technologies can be further developed and tested
in real conditions. The third category of users is composed of trainees of different
origins. As a demonstrator of more compact and economical accelerator technolo-
gies, EuPRAXIA is expected to attract (1) accelerator facility staff unexperienced
with the new technologies in use at EuPRAXIA, (2) accelerator facility as well as
company staff contemplating the adoption of new accelerator technologies in use at
<figure><img src="image_57.png" /><figcaption>**Fig. 5.5.** Main user categories expected at EuPRAXIA. The access model should be adapted to cater to the different needs of these types of users.</figcaption></figure>

EuPRAXIA, and (3) staff from research and academic institutions with no accelerator facility but interested in developing the theoretical and practical knowledge in view of carrying out experiments in different accelerator facilities. All three of these categories may contain academic as well as industrial users.

Because of their varying objectives for using the EuPRAXIA infrastructure, each user category will have their individual needs in terms of support, duration of beam time, user area setup, etc. To properly accommodate all, it is thus proposed to split access among these groups, offering each more individual services. While co-developers may be interested, for example, in longer experimental periods and more direct access to the accelerator beamline, beam users will be content with being placed in the end user areas, but will expect a reliable, continuously available beam. By assigning specific experimental periods throughout the year to each type of user activity, operation times could thus be optimised and longer maintenance periods in between utilised to adapt the machine setup accordingly.

For the initial period of pilot user operation, a stronger focus on co-developer experiments could be useful as a way to optimise the beamline performance without hindering experiments that are very reliant on strong beam performance or a high number of beam hours.

### 5.4.2 User Facility Modes

Based on the above categorisation of the expected EuPRAXIA user community, a simple access model can be envisaged encompassing five access modes, as summarised in Figure 5.6 and explained in the following. Note that wide access refers entirely to data and, as such, is discussed in Section 3.5.

It is considered that all access to EuPRAXIA could be organised in three steps: a feasibility review, an evaluation, and scheduling. The feasibility review would be common to all proposals and would be carried out by the EuPRAXIA support staff with the support of EuPRAXIA researchers, if needed. The evaluation step would depend on the access mode and be carried out by external panels. Finally, the scheduling would consist of accommodating all selected proposals within a timeframe compatible with the availability of beamlines, equipment, and other boundary conditions.
<figure><img src="image_58.png" /><figcaption>Fig. 5.6. Summary of the proposed access modes and procedures for users at EuPRAXIA.</figcaption></figure>

In the excellence-driven access mode, beam time is foreseen to be granted after an assessment of the quality of all experiment proposals by the Research Program Advisory Committee. The acceptance of proposals will be based largely on their scientific excellence, although other criteria featuring into the evaluation could include, for example, consistency with the mission of EuPRAXIA, societal impact, the applicants' track record, or the diversity of the applicant group. Quota-based access could be handled very similarly but with only proposals endorsed by one or several members of the EuPRAXIA Collaboration Board to be considered. This access mode thus ensures clear benefits for institutions and funding bodies to invest time and resources into EuPRAXIA while, at the same time, preserving the scientific excellence of experiments carried out.

Third, the concept of market-based access is designed specifically for industrial users in respect of the provisions of the General Block Exemption Rules 2014 applying to Research & Development & Innovation and the commission guidelines from the Framework for State Aid for Research and Development and Innovation. As the proposed activities at EuPRAXIA may vary significantly among users in this mode, access should be negotiated individually on a case-by-case basis from the initiation of a non-disclosure agreement and the feasibility review to assessing potential ethical concerns and negotiating a contract with the users. Finally, training at EuPRAXIA could be delivered to industrial users, students, accelerator facility operators, or any other type of users, so again, it makes sense to handle proposals in a customised way. Two main routes, however, could be foreseen: training access through a long-term agreement or through an ad hoc, one-off request. In both cases, a feasibility assessment will be necessary, considering, for example, available resources as well as the proposal's relevance for EuPRAXIA. Once successful, this can be followed by the definition of a training curriculum, contract negotiation, and training scheduling.

### 5.4.3 User Support

Beyond the actual machine time, further services and support measures will need to be considered and designed for user operation at EuPRAXIA before implementation. These will include, among others, the organisation of user meetings and workshops, a process that has been started already for potential users during the EuPRAXIA conceptual design phase, but should be continued and extended throughout the development of the EuPRAXIA infrastructure. For the operational phase in particular,
the setup of a team of local contacts and machine staff will be essential, who will
be responsible for user support before, during, and after experiments as well as for
feasibility reviews and scheduling. Furthermore, supporting laboratories and work-
shops at the construction sites will need to be set up or organised within existing
infrastructures, and adequate options for accommodation and catering on site must
be ensured.

While most of these measures should likely be integrated into the host institute's
existing infrastructure, especially for those institutions that host already other user
facilities, details still need to be devised.

5.5 Quality Assurance

For EuPRAXIA, an ambitious, innovative project bringing together more than forty
institutions across the world, quality assurance is extremely important and will be
tackled through a number of dimensions:

- **Governance structures:** EuPRAXIA's proposed governance model (see Sect. 5.2) foresees a number of bodies dedicated to oversight and quality control, most importantly the Quality Insurance Panel. For the implementation and operation phases, the Audit Committee and Ethical Advisory Committee will additionally come into play to ensure adherence to proper spending and ethical principles. On a scientific level, the planned network of technical clusters (see Sect. 3.3.1) will structure the collaboration's research activities and help to avoid inconsistencies or unintentional repetitions among partners.

- **Performance indicators:** As Sections 3.3.1 and 5.1 show, major milestones and deliverables have been defined for the next project phases. This will be extended with a detailed work breakdown structure, schedule, and list of responsibilities for each consortium partner to better measure project progress. Key Performance Indicators as suitable measures for continuous performance will be defined for EuPRAXIA's operational phase.

- **Standards and procedures:** While this design report acts as a comprehensive summary of the design decisions, layout considerations, and project planning to date, more common standards and procedures will need to be defined during the technical design phase in the form of a quality assurance plan. This will include, among others, technical design requirements, procedures for procurement and component testing, employee and user training decisions, and safety regulations (coordinated with the safety organisation described in Chap. 27.1). It will be essential to build on the expertise and existing structures at the project's partner institutions in this context.

- **Validation of scientific results:** A large fraction of the technical design phase will be dedicated to validating the results and concepts proposed in this conceptual design report. Through test experiments, prototyping and further computational studies, a robust comparison between different codes, simulations and experiment results as well as design parameters with fabricated components will become possible.

5.6 Outreach and Communication

The EuPRAXIA Outreach and Liaison Group based at the University of Liver-
pool/Cockcroft Institute will continue to work closely with the Management Support
Team and project partners to communicate project results and events internationally.
# Activities

## Existing External and Internal Communication Platforms

The project website www.eupraxia-project.eu will be maintained by the Outreach and Liaison Group as the main portal of information of EuPRAXIA for the external world. Relevant news about EuPRAXIA will be promoted via established social media channels. The University of Liverpool's Sharepoint system will continue to be used to facilitate the exchange of documents among the project partners. The group will also continue to showcase EuPRAXIA achievements at workshops and conferences around the world and continue to publish the EuPRAXIA files newsletter three times per year. Existing dissemination material (leaflets, brochures) will be updated on a regular basis and reflect project progress.

## Education and Engagement

The computer game Surfatron will be developed into an online version that can be accessed via the project website. Liverpool's pioneering augmented reality application, AcceleratAR (http://acceleratar.uk/), which allows building an accelerator by placing paper cubes on a tabletop and then bringing them to life using the app, will be expanded to also include plasma acceleration. This will allow for engaging with primary and secondary schools in a novel and interactive way. Furthermore educational material on the basis of the 2018 Symposium will be developed further and promoted through learning and teaching conferences, as well as publications in journals on education and teaching.

## Outreach

The group, in close collaboration with EuPRAXIA partners, will continue to communicate the specific research challenges and opportunities of plasma accelerators to a range of audiences with the aim to further increase the attractiveness of the field overall. This will include policy makers and funding councils.

The website (https://marie-curie-day-2017.org/) that was developed for Marie Curie Day 2017 will be extended to include information about female researchers in accelerator science and technology. There is a number of female researchers in EuPRAXIA at various career stages, and they will be amongst the first to be included. The aim is to establish the website as a careers portal for researchers whilst continuing to promote the posters and talks from the 2017 event.

Another Physics of Star Wars event will be held in November 2019 at the University of Liverpool. Using unique Star Wars concepts (hyperspace, the lightsaber, the Force, etc.), the physics and technology of EuPRAXIA will be explained and communicated via media internationally. Communication will benefit from the links that were established in the past and target an even larger audience. Educational material shall be made available for use by schools and university groups.

EuPRAXIA partners will be encouraged to hold public events showcasing the achievements in plasma acceleration, with supporting material and demo setups provided by the Outreach and Liaison Group. Based on experiences gained in past pan-European events using social media campaigns and video live-streams, emphasis will be placed on events involving more than one institution at a time.
# 5.7 Project Risk Assessment

EuPRAXIA needs to consider two different types of risks as part of the project: general project risks and technical risks for specific sub-systems of the machine. While a comprehensive risk assessment for the technical design, implementation, and operation phases of the project is ongoing, this chapter will highlight some of the most important examples.

## 5.7.1 General Project Risks

The general project risks that need to be considered for EuPRAXIA are largely financial, schedule related, or organisational in nature. Table 5.1 shows a few relevant example risks together with possible mitigation strategies. Also listed in each case is a rough assessment of each risk's probability and impact.

## 5.7.2 Technical Risks

With EuPRAXIA as a complex, high-performance machine, the technical risks contained in the project have been analysed carefully. Generally speaking, two particular issues could arise for components of the facility: (1) unforeseen technical problems could occur that limit the final performance of a sub-system, and (2) the expected performance of a component, currently based in many cases on simulations, could be overestimated compared to the experimental output because of computational limitations, new physics processes, or unavoidable imperfections. With these in mind, a set of risk-reduction actions has been defined as a roadmap for the next phases of the project:

1. The EuPRAXIA design is based, as far as possible, on proven concepts and solutions. The baseline for all machine sub-systems is thus either already at a high technical readiness level or an extrapolation of existing technology, particularly for critical components, such as the laser system and the free-electron laser.

2. Test experiments are planned for all machine components, and for the most critical sub-systems, full-scale prototypes will be developed and checked. Further details on the planned prototyping are described in Part 5.

3. The EuPRAXIA accelerators are designed to be very well-characterised machines with a high level of control. Extensive diagnostics, feedback, and correction systems are foreseen to be implemented.

4. Backup options are defined for the most critical sub-systems and will be included in the planning for R&D and prototyping to ensure that they could, if necessary, be implemented with as few changes in cost, schedule, and scope as possible.

5. A wide range of applications with varying levels of complexity and possible step-wise optimisation strategies were defined. This will allow one to fall back initially on less complex user applications, in case of problems with the technical machine performance.

A more detailed risk analysis for each sub-system of EuPRAXIA is provided in the following, with major risks summarised in Figure 5.7. It also shows an estimate of the current technical readiness levels of the components and techniques foreseen for each machine part.
<table><caption>**Table 5.1.** A list of the some of the most relevant general risks for the EuPRAXIA project during its technical design, implementation, and operation phases. Considering the proposed mitigation strategies, in each case, the risk probability and impact is shown on a qualitative scale.</caption><thead><tr><th>Risk</th><th>Description</th><th>Mitigation Strategy</th><th>Probability</th><th>Impact</th></tr></thead><tbody><tr><td>Failure, bankruptcy or withdrawal of one of the host partners</td><td>E.g. due to political circumstances</td><td>Development of a strong consortium agreement defining clearly the benefits for partners; early discussions / negotiations with institution higher managements and national ministries; distributed infrastructure concept with responsibilities spread between partners</td><td>Very low</td><td>High</td></tr><tr><td>Failure, bankruptcy or withdrawal of one of the non-host partners</td><td></td><td></td><td>Low</td><td>Medium</td></tr><tr><td>Delays, difficulties in finalising the research infrastructure (RI) technical design</td><td>E.g. due to limited facility access for simulations / test experiments or failure to coordinate individual partners' work</td><td>Setup of a strong management structure for the technical design phase; efforts to secure common consortium funding for R&amp;D activities; continuation of current work on prototyping and test experiment coordination through a dedicated panel</td><td>Low</td><td>Medium</td></tr><tr><td>Failure to obtain funding for infrastructure implementation</td><td>Assuming the completion of a convincing technical design</td><td>Application to ESFRI roadmap; planned discussion with EU representatives and institute management of EuPRAXIA partners on possible opportunities</td><td>Very low</td><td>Very high</td></tr><tr><td>Failure to obtain funding or sufficient guarantees for funding of operational phase</td><td>Assuming completion of a convincing technical design and success in obtaining implementation funding</td><td>Early negotiation with host institutions / funding bodies</td><td>Very low</td><td>Very high</td></tr><tr><td>Failure to acquire sufficient qualified human resources</td><td>Workforce too small to carry out all tasks and deal with all issues for technical design leading to possible project delays</td><td>Timely start of recruitment; inclusion of all current EuPRAXIA partners to carry out design work; wide advertisement of positions using also close connections of EuPRAXIA partners and associated labs to university PhD programs</td><td>Low</td><td>Medium</td></tr><tr><td>Failure to timely complete construction</td><td>Possibly due to inaccurate schedule estimation or communication / coordination issues between partners</td><td>Inclusion of contingencies in cost and schedule planning; building on expertise of collaboration for realistically assessing construction schedule and progress; definition of clear responsibilities during construction</td><td>Low</td><td>High</td></tr><tr><td>Failure to timely procure equipment</td><td>Possibly due to delays from external suppliers, underestimation of time requirements for procurement (e.g. through publicly funded procurement processes), failure of supply chain because of economic conditions / limited suppliers or flaws in procured equipment and need for re-procurement</td><td>Building on expertise of the many experts in the collaboration in handling procurement</td><td>Low</td><td>Medium</td></tr><tr><td>Increase in equipment cost (or underestimation of costs)</td><td>More likely for newly designed equipment</td><td>Establishment of an expert committee on accelerator and laser costing to provide best knowledge estimates based on extensive project experience; inquiry of price estimates from industry</td><td>Low</td><td>Medium</td></tr><tr><td>Need for changes / rework of research infrastructure design</td><td>Possibly due to mistakes in the final technical design or due to required changes in the project scope (based on changes in user needs, scientific program, etc.)</td><td>Thorough validation of components, design and layout during technical design; contingencies in both cost and schedule</td><td>Low</td><td>Medium</td></tr><tr><td>Shift in an industrial partner's priorities</td><td>Leading to changing interest / investments in essential technologies</td><td>Discussion and collaboration with companies in relevant industries early on since conceptual design study</td><td>Low</td><td>Medium</td></tr><tr><td>Insufficient user base or failure to attract users (academic &amp; industrial)</td><td></td><td>Early start to user demand analysis through discussions with potential users to align their needs with the design and inform about EuPRAXIA; plans for user / industry workshops during technical design and beyond; development of multiple complementary applications within the RI concept</td><td>Very low</td><td>High</td></tr></tbody></table>
## Laser System

For the laser system, as one of the most challenging components of the EuPRAXIA machine, a whole range of testing and prototyping exercises is foreseen, as described in more detail in Chapter 10. Three high-level risks, in particular, are being targeted by these activities and need to be considered for backup solutions in case the technical design R&D discovers fundamental performance limitations. First, there is a possibility, although highly unlikely, that the laser stability requirements for a reliable plasma-accelerator operation cannot be fulfilled. In this scenario, a reduction in the plasma density in the LPI and LPAS components would be possible to relax the operational requirements yet at the cost of a lower accelerating gradient and hence larger machine size. A second risk related to the laser stability lies in the possibility that the final focus laser beam handling is insufficient for the staging of multiple plasma targets. While experimental staging tests should provide a clear confirmation of the performance requirements, a final mitigation strategy in case of fundamental issues could foresee the design of an alternative single-stage plasma accelerator. Third, the thermal management of the laser components is a significant challenge and risk to high-repetition-rate operation. As a consequence, the laser design is set up in a stepwise approach with a baseline at 20 Hz, ideal performance at 100 Hz, and a future development option to the kHz level. If the planned R&D activities reveal any physical constraints in this respect, operation at lower repetition rate will thus still be possible, or a redesign of the laser system based on alternative technologies could be considered.

## Plasma Components

Although in general, plasma targets can be produced by now in quite a standardised manner, EuPRAXIA will add some additional challenges to the design and fabrication of components by requiring, on the one hand, particularly long plasma cells with laser guiding capabilities and, on the other hand, components that can withstand significantly higher repetition rates than is usually the case in plasma acceleration experiments. To overcome both of these issues for the target design, several possible technological options have been explored conceptually, as described in Chapters 13 and 16, and will be tested extensively in the coming technical design phase. This strategy will also help to reduce the risk of simulation errors that the current computational approach for estimating the machine performance entails. Additional to experimental testing, the next project phase will focus on cross-checking simulation results with multiple codes as well as carrying out thorough tolerance studies.

## RF Components

The radiofrequency-based sections of the machine layout are some of its more low-risk components and in themselves are integrated in the design as a risk-mitigation strategy. Nonetheless, combining RF injectors with plasma acceleration is a relatively new concept and, as such, brings certain risks with it. It is in particular possible that the conditions on the electron beam properties and synchronisation required for a stable plasma acceleration process, as Chapters 14 and 19 highlight, cannot be met. In this case, again, operation at lower plasma density and hence lower accelerating gradient would be a backup option if consultations with experts and operational accelerator facilities outside the project do not result in alternative conventional accelerator solutions.
<figure><img src="image_59.png" /><figcaption>**Fig. 5.7.** List of the main technical risks associated with each major sub-system of EuPRAXIA. General and more specific mitigation strategies for each are described in the main text. Additionally, an estimate of the technical readiness levels (TRL) of the components of each sub-system is listed based on the TRL scale from 1 to 9 defined for Horizon 2020 [173].</figcaption></figure>
## Diagnostics

The diagnostics layout for the EuPRAXIA machine should be customised to the specific properties of the plasma-accelerator-based setup and, as such, involves a large number of novel diagnostic techniques with a compact size and / or the capability to take single-shot measurements (e.g. single-shot emittance measurements, compact cavity BPMs, single-shot coherent radiation-based bunch length measurements). While the testing and prototyping of these devices is planned during the technical design phase, there is a risk that these diagnostics will not perform as well as required for machine operation. As a mitigation strategy, the conceptual design foresees more conventional backup options for the diagnostics sections, which have been validated through operation in other machines but would result in a less compact and less efficient EuPRAXIA accelerator.

## FEL

Lasing with an FEL based on plasma acceleration has not been achieved to date, thus proving one of the most high-risk aspects of EuPRAXIA. As such, multiple mitigation strategies are being pursued in parallel. First, several test experiments are currently ongoing (see Sect. 32.7), with further large-scale prototyping activities planned during the technical design phase. As a second point, conventional undulator technology is chosen as a baseline for the beamline design to minimise risks for this application as much as possible. Third, specific beamline components designed to compensate large energy spread, one of the potential issues with plasma acceleration, are being investigated, such as transverse gradient undulators and decompression chicanes (see Chap. 18). Both could potentially ease the strict electron beam requirements necessary for FEL operation. Finally, if fundamental show-stoppers should emerge based on the studies above, the machine design can be altered during the technical design phase, avoiding, as much as possible, additional costs or schedule delays. For the beam-driven construction site, for example, a backup option is already integrated as the X-band accelerator could be extended to 1 GeV, allowing FEL operation without the need for a plasma acceleration stage.

## Other Applications

Similarly to the free-electron laser application, there is a risk that the electron beam requirements for the other applications foreseen for EuPRAXIA cannot be fulfilled. Beyond the necessary prototyping and testing to reduce this risk, this possible problem is also mitigated by selecting the set of highlight applications chosen for EuPRAXIA based not only on their scientific relevance but also their electron beam requirements. Hence, technically advanced applications with lower requirements, such as the betatron source, can be used, if necessary, as a basis for testing and improving accelerator performance, until it becomes optimised enough for more challenging use cases.

# 6 Impact Assessment

## 6.1 Strategic Significance for European Science and Innovation

### Direct Scientific Impact

The proposed EuPRAXIA research infrastructure will make available several types of particle and radiation beams with capabilities in a number of different scientific fields.
<figure><img src="image_60.png" /><figcaption>**Fig. 6.1.** Map of large-scale user facilities providing access to similar types of particle and radiation sources as EuPRAXIA will offer (X-rays from free-electron lasers, high-energy electron test beams, gamma rays, and high-energy positrons). The list of facilities shown here is not exclusive, but it does provide an overview of the main research infrastructures of each type.</figcaption></figure>

For many of these, EuPRAXIA will be one of only a few facilities worldwide with such characteristics. As Figure 6.1 shows, for example, there is only a handful of free-electron laser infrastructures in the X-ray regime operational today. Similarly, for the case of electron accelerator R&D, few dedicated user facilities are available in Europe and worldwide; for plasma-accelerator research or studies with ultra-short electron beams, there are even less and none with the capabilities expected at EuPRAXIA. Operational gamma-ray sources for user experiments are almost entirely based on Compton scattering using the GeV-scale electron beams at synchrotron facilities. Hence, both for these as well as high-brightness, non-coherent X-ray sources, largely covered by synchrotrons, EuPRAXIA will be one of the first few compact machines. Finally, to our knowledge, FACET-II will provide the only other high-energy positron source (multi-GeV) in the next years, thus making EuPRAXIA an essential addition to offer similar capabilities within the European research area. It is also important to consider that EuPRAXIA will not only provide access to all of these types of sources and beams but also do so in a single infrastructure (at two locations), thus enabling new types of multi-species experiments which are difficult to coordinate in the current landscape.

A direct scientific impact is therefore expected from the implementation and operation of the EuPRAXIA facilities; the overall capacity for user experiments in photon and accelerator science in Europe will be increased, especially in key scientific areas of strong competition for facility access, such as X-ray free-electron lasers. In addition to a rise in scientific output (through papers and patents) from European research communities, however, these new and increased capabilities of EuPRAXIA could also be beneficial in broadening the community of users of accelerator-based technologies. Through providing beam characteristics not available elsewhere, for example, the entry barrier for accelerator-assisted research can be lowered, allowing access also to research groups typically not making use of large-scale accelerators.
## Impact on Technological Development Capacity of the European Research Area

Plasma accelerators are a highly demanding technology with requirements close to technical feasibility limits in several scientific areas. With significant synergy created from the proposed combination of state-of-the-art plasma acceleration, modern lasers, latest accelerator technology, top-of-the-line environmental control, and promising applications, EuPRAXIA will contribute to strengthening the technological development capacity and effectiveness of the European Research Area at the frontiers of these fields. Technological impact can be expected, among others, on the following topics:

**Ultra-fast synchronisation, electronics, and correction loops:** The timing and accuracy requirements of plasma accelerators are in the femtosecond regime, requiring state-of-the-art synchronisation on the order of single femtoseconds. EuPRAXIA will foster further progress on this technological frontier. Lasers that are timed precisely to an external reference and ultra-fast electronics can be implemented in many technological devices, like laser heaters, laser vibration monitoring, laser wires, ultra-precise timing reference systems, femtosecond stabilisation loops, etc. We note that mechanical miniaturisation (e.g. in nanotechnology or plasma structures) leads to much-reduced stabilisation tolerances and goes hand-in-hand with ultra-fast technologies that must react before tolerances are violated. Ultra-fast technology will thus contribute to the successful development of miniaturised technology.

**Compact accelerator magnets with high field quality:** The need for matching electron beams in and out of plasma channels requires magnetic focusing to a small beam size in the single micrometre regime. While this has been achieved in RF accelerators, the conventional schemes have to be adopted for plasma-accelerator needs. Required are, for example, sufficient flexibility for adapting to changes in plasma focusing fields and small dimensions. Additionally, challenges for the chromatic correction of magnetic beam transport will need to be considered, even with the improved low-energy spread electron beams foreseen from EuPRAXIA. The project will therefore foster developments towards compact accelerator magnets with high field quality, chromatic correction, and a certain tuning range. Both permanent magnets as well as electromagnetic magnets are options that will be considered together with industrial suppliers and accelerator labs. We expect that the magnets and lattice solutions from EuPRAXIA can have many other applications in the 30,000 accelerators around the world.

**Stabilised and high-repetition-rate PW-scale laser technology:** It has recently been demonstrated that the 5 GeV target energy of EuPRAXIA can be achieved with a PW-scale laser [9]. While the peak power in such state-of-the-art systems is sufficient, there are limitations in efficiency, stability, and, in the long term, average power. EuPRAXIA will not consider efficiency problems as these are being addressed in other European efforts. It has, however, investigated laser stability limits and possible design solutions. For example, it is estimated that the laser spot transverse jitter at the location of the beam acceleration can be reduced by optimising the laser transport (e.g. mirror stabilisation) as well as by optimising the laser itself (see Chap. 10). In a similar context, an increase in laser repetition rate up to the 100 Hz to kHz level is expected to significantly improve the feedback capabilities of both the laser system and accelerator-related processes, thus further enhancing machine shot-to-shot reliability. These developments will be carried out in close collaboration with manufacturers and laser institutes, thus opening the path to a broader application of such next-generation laser systems and placing Europe as a leader in laser technology and stability.
**Plasma cell technology:** EuPRAXIA proposes the design of a plasma cell that can be cascaded and fulfils the required tolerances for user applications. The end goal is an "industrial" plasma cell design that provides a path to higher energy by cascading. The in- and out-coupling of the laser is included. EuPRAXIA will thus provide Europe with a technological solution for plasma cell technology to be used with lasers from the European and worldwide industry.

**Compact FELs:** The reduction of length in the acceleration will significantly reduce the length of FELs. However, the undulator sections require significant space and in many cases will determine the length required. EuPRAXIA relies on existing undulator designs for its baseline. However, compact innovative developments and solutions will be followed in parallel, comparing benefits and disadvantages. EuPRAXIA will thus provide Europe with a detailed assessment of compact FELs and, if adequate, with solutions for length-reduced undulators.

**Fast photon science detection technology:** The length of electron bunches in plasma accelerators will necessarily be very short, at around 1 femtosecond, with similarly short photon pulses produced from these beams. This will open a unique science reach into ultra-fast processes that can be analysed at the electronic level without perturbation. The detector technology for ultra-fast photon science has to be adapted to the plasma beam features. EuPRAXIA will foster progress in ultra-fast photon science detection technology, allowing Europe to stay at the leading edge of photon science detectors.

**High-energy physics detector technology:** EuPRAXIA will provide a dedicated application area for developing and commissioning the latest HEP detector technology. Detector components can profit from the additional beam time and the (controllable) high density beam characteristics, either directly in the (collected) electron beam or after a conversion target. EuPRAXIA will contribute to the development of state-of-the-art HEP detectors for maximum event rate and multiplicity.

**Medical accelerator technology:** Medical applications are included in EuPRAXIA in the form of medical imaging. Compared to other accelerator uses in medicine, such as radiation therapy, this application will allow for the development and demonstration of the required stability of plasma accelerators. This knowledge and these achievements in EuPRAXIA towards stable plasma accelerators can then be directly applied as a path to wider medical applications with a long-term perspective to using plasma acceleration as a technology in hospitals and medical research.

The technical work carried out in EuPRAXIA is thus foreseen to significantly strengthen the technological development capacity and effectiveness as well as the scientific performance, efficiency, and attractiveness of the European Research Area.

## Impact on International Cooperation

EuPRAXIA will develop a model of international cooperation and a strategic vision for bringing plasma accelerators to the users and to the market. The R&D on electron plasma acceleration is carried out in multiple projects in Europe, Asia, and the United States. The relatively small size and cost of a plasma accelerator has attracted a large number of excellent university groups, in addition to major accelerator labs. Scientific results are published at the highest level of journals, including *Nature*, *Science*, and *Physical Review Letters*. Scientific highlight results have often the clear priority over working on already achieved parameters for improving beam quality and demonstrating user readiness. With EuPRAXIA, a new kind of international cooperation has been established in the field of plasma acceleration. The 41
consortium partners in Europe and abroad will continue to team up with the clear
goal to bring plasma accelerators to user readiness and to the market. It will estab-
lish a common goal for the international field and will give a vision for the funding
and implementation of common projects across different universities, labs, and coun-
tries. We believe that a common European project on electron plasma accelerators
can also contribute to the advancement of the field in other parts of the world. The
EuPRAXIA strategy, results, and design choices will be disseminated and discussed
widely in the community, especially outside of Europe. The activity can contribute
to a global consensus on the best way for bringing plasma accelerators to the users
in photon science, high-energy physics, industry, and medicine. It can be used as a
basis for improving methodologies and developing standards within this relatively
new field within accelerator science. EuPRAXIA will thus foster a new kind of trans-
parent and use-case-oriented international cooperation in the diverse field of plasma
accelerators. It will develop a model for the construction of plasma accelerators in
a fair and distributed open innovation approach beneficial to consortium partners,
industry, as well as researchers beyond the collaboration.

## 6.2 Socio-Economic Opportunities

The opportunities produced by a project such as EuPRAXIA go beyond its scientific impact reaching, as Figure 6.2 shows, into economic, societal, and environmental areas. This is also reflected in the range of societal project goals introduced in the executive summary (see Part 1).

The role of the project for the European high-tech industry as a source for knowl-
edge transfer and technological spillover, for example, should be mentioned. With
the high-power laser industry as a most direct beneficiary and partner, EuPRAXIA
will help keep Europe's leading position in this field and will contribute to further
strengthening the European technological capacity in laser- and accelerator-related
industries. European companies will be able to connect to the EuPRAXIA research
infrastructure not only as suppliers but also as users or co-developers. It can be envis-
aged, for example, that industry tests and develops new laser stability measures and
other technologies under the very challenging EuPRAXIA conditions and tolerances.
The high demands will inspire and foster technological progress, keeping European
laser industry at a leading edge. Similar trends can be envisaged also in the area of
accelerator systems and components as well as IT and software development, with
the latter benefiting particularly from efforts and challenges in plasma simulation.

Additionally, EuPRAXIA will contribute to opening new market opportunities for industry with its concept as an open-innovation platform. The quality improvements, compact size, and cost-effectiveness of plasma accelerators that EuPRAXIA strives to push forward will, in the long term, make accelerators feasible space-wise and/or affordable to additional users. Our future vision based directly on the developments at a future EuPRAXIA research infrastructure includes, for example, compact plasma-based FELs in the basement of every large university, plasma accelerators for fast medical imaging in hospitals, and very compact plasma colliders at high-energy physics laboratories. In many cases, conventional facilities would not be replaced but complemented, and industry could be involved very closely in these developments.

Another important role of a future EuPRAXIA infrastructure will also lie in
training and education. On the one hand, the EuPRAXIA collaboration provides
an excellent environment for such efforts, with strong cooperation among research
centres, universities, and industrial partners across Europe. On the other hand, the
EuPRAXIA research infrastructure will also represent one of the first test facilities
worldwide with plasma accelerators at its core, thus offering unique expertise with