# EuPRAXIA Conceptual Design Report

R W Assmann, M K Weikum, T Akhter, D. Alesini, I. Andriyash, A S

Alexandrova, M P Anania, N E Andreev, M Artioli, A Aschikhin, et al.

► To cite this version:

R W Assmann, M K Weikum, T Akhter, D. Alesini, I. Andriyash, et al.. EuPRAXIA Conceptual Design Report. The European Physical Journal. Special Topics, 2020, 229, pp.3675 - 4284. 10.1140/epjst/e2020-000127-8 . hal-03097230

HAL Id: hal-03097230

https://hal.science/hal-03097230v1

Submitted on 16 May 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Review

EuPRAXIA Conceptual Design Report

R.W. Assmann¹,ª (editor), M.K. Weikum¹ (editor), T. Akhter², D. Alesini³,
A.S. Alexandrova⁴,⁵, M.P. Anania³, N.E. Andreev⁶,⁷, I. Andriyash⁸, M. Artioli⁹,
A. Aschikhin¹, T. Audet¹⁰, A. Bacci¹¹, I.F. Barna¹², S. Bartocci¹³, A. Bayramian¹⁴,
A. Beaton⁴,¹⁵, A. Beck¹⁶, M. Bellaveglia³, A. Beluze¹⁷, A. Bernhard¹⁸, A. Biagioni³,
S. Bielawski¹⁹, F.G. Bisesto³, A. Bonatto⁴,²⁰, L. Boulton⁴,¹⁵, F. Brandi²¹,
R. Brinkmann¹, F. Briquez²², F. Brottier²³, E. Bründermann¹⁸, M. Büscher²⁴,
B. Buonomo³, M.H. Bussmann²⁵,²⁶, G. Bussolino²¹, P. Campana³, S. Cantarella³,
K. Cassou²⁷, A. Chancé²⁸, M. Chen²⁹, E. Chiadroni³, A. Cianchi³⁰,³¹, F. Cioeta³,
J.A. Clarke⁴,³², J.M. Cole³³, G. Costa³, M.-E. Couprie²², J. Cowley³⁴, M. Croia³,
B. Cros¹⁰, P.A. Crump³⁵, R. D'Arcy¹, G. Dattoli³⁶, A. Del Dotto³, N. Delerue²⁷,
M. Del Franco³, P. Delinikolas⁴,¹⁵, S. De Nicola²,³⁷, J.M. Dias³⁸, D. Di Giovenale³,
M. Diomede³, E. Di Pasquale³, G. Di Pirro³, G. Di Raddo³, U. Dorda¹,
A.C. Erlandson¹⁴, K. Ertel³⁹, A. Esposito³, F. Falcoz⁴⁰, A. Falone³, R. Fedele²,⁴¹,
A. Ferran Pousa¹,⁴², M. Ferrario³, F. Filippi³,⁴³, J. Fils⁴⁴, G. Fiore²,⁴¹,
R. Fiorito⁴,⁵, R.A. Fonseca³⁸, G. Franzini³, M. Galimberti³⁹, A. Gallo³,
T.C. Galvin¹⁴, A. Ghaith²², A. Ghigo³, D. Giove¹¹, A. Giribono³, L.A. Gizzi²¹,⁴⁵,
F.J. Grüner⁴²,⁴⁶, A.F. Habib⁴,¹⁵, C. Haefner¹⁴, T. Heinemann¹,⁴,¹⁵,⁴²,
A. Helm³⁸, B. Hidding⁴,¹⁵, B.J. Holzer⁴⁷, S.M. Hooker³⁴, T. Hosokai⁴⁸,
M. Hübner³⁵, M. Ibison⁴,⁵, S. Incremona³, A. Irman²⁵, F. Iungo³, F.J. Jafarinia¹,
O. Jakobsson²⁰, D.A. Jaroszynski¹⁵, S. Jaster-Merz¹, C. Joshi⁴⁹, M. Kaluza⁵⁰,⁵¹,
M. Kando⁵², O.S. Karger⁴², S. Karsch⁵³, E. Khazanov⁵⁴, D. Khikhlukha⁵⁵,
M. Kirchen¹,⁴⁶, G. Kirwan⁴,¹⁵, C. Kitégi²², A. Knetsch¹, D. Kocon⁵⁵, P. Koester²¹,
O.S. Kononenko⁵⁶, G. Korn⁵⁵, I. Kostyukov⁵⁴, K.O. Kruchinin⁵⁵, L. Labate²¹,⁴⁵,
C. Le Blanc¹⁷, C. Lechner¹, P. Lee¹⁰, W. Leemans¹, A. Lehrach²⁴, X. Li⁵⁷,
Y. Li²⁰, V. Libov⁴², A. Lifschitz⁵⁶, C.A. Lindström¹, V. Litvinenko⁵⁸,⁵⁹, W. Lu⁶⁰,
O. Lundh⁶¹, A.R. Maier⁴²,⁴⁶, V. Malka⁸, G.G. Manahan¹⁵, S.P.D. Mangles³³,
A. Marcelli³, B. Marchetti¹, O. Marcouillé²², A. Marocchino³, F. Marteau²²,
A. Martinez de la Ossa¹, J.L. Martins³⁸, P.D. Mason³⁹, F. Massimo¹⁶, F. Mathieu¹⁷,
G. Maynard¹⁰, Z. Mazzotta⁶², S. Mironov⁵⁴, A.Y. Molodozhentsev⁵⁵, S. Morante³⁰,
A. Mosnier²⁸, A. Mostacci⁶³,⁶⁴, A.-S. Müller¹⁸, C.D. Murphy⁶⁵, Z. Najmudin³³,
P.A.P. Nghiem²⁸, F. Nguyen³⁶, P. Niknejadi¹, A. Nutter⁴,¹⁵, J. Osterhoff¹,
D. Oumbarek Espinos²², J.-L. Paillard¹⁷, D.N. Papadopoulos¹⁷, B. Patrizi⁶⁶,
R. Pattathil³², L. Pellegrino³, A. Petralia³⁶, V. Petrillo¹¹,⁶⁷, L. Piersanti³,
M.A. Pocsai¹²,⁶⁸, K. Poder¹,³³, R. Pompili³, L. Pribyl⁵⁵, D. Pugacheva⁶,⁷,
B.A. Reagan¹⁴, J. Resta-Lopez⁴,⁵, R. Ricci³, S. Romeo³, M. Rossetti Conti¹¹,
A.R. Rossi¹¹, R. Rossmanith¹, U. Rotundo³, E. Roussel¹⁹, L. Sabbatini³,
P. Santangelo³, G. Sarri⁶⁹, L. Schaper¹, P. Scherkl⁴,¹⁵, U. Schramm²⁵,
C.B. Schroeder⁷⁰, J. Scifo³, L. Serafini¹¹, G. Sharma⁷¹, Z.M. Sheng¹⁵,²⁹,
V. Shpakov³, C.W. Siders¹⁴, L.O. Silva³⁸, T. Silva³⁸, C. Simon²⁸, C. Simon-
Boisson⁷², U. Sinha³⁸, E. Sistrunk¹⁴, A. Specka¹⁶, T.M. Spinka¹⁴, A. Stecchi³,
A. Stella³, F. Stellato³⁰,³¹, M.J.V. Streeter³³, A. Sutherland⁴,¹⁵, E.N. Svystun¹,
D. Symes³⁹, C. Szwaj¹⁹, G.E. Tauscher¹, D. Terzani²,⁴¹, G. Toci⁶⁶, P. Tomassini²¹,
R. Torres⁴,⁵, D. Ullmann⁴,¹⁵, C. Vaccarezza³, M. Valléau²², M. Vannini⁶⁶,
A. Vannozzi³, S. Vescovi³, J.M. Vieira³⁸, F. Villa³, C.-G. Wahlström⁶¹,
R. Walczak³⁴, P.A. Walker¹, K. Wang²⁷, A. Welsch⁴,⁵, C.P. Welsch⁴,⁵,
S.M. Weng<sup>29</sup>, S.M. Wiggins<sup>4,15</sup>, J. Wolfenden<sup>4,5</sup>, G. Xia<sup>4,20</sup>, M. Yabashi<sup>73</sup>, H. Zhang<sup>4,5</sup>, Y. Zhao<sup>4,20</sup>, J. Zhu<sup>1</sup>, and A. Zigler<sup>74</sup>

1. Deutsches Elektronensynchrotron – Hamburg, 22607 Hamburg, Germany

2. INFN, Sezione di Napoli, 80126 Napoli, Italy

3. INFN, Laboratori Nazionali di Frascati, 00044 Frascati, Rome, Italy

4. Cockcroft Institute, Warrington WA4 4AD, UK

5. University of Liverpool, Liverpool L69 7ZE, UK

6. JIHT of RAS, Moscow 125412, Russia

7. Moscow Institute of Physics and Technology, Dolgoprudny 141701, Russia

8. Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel

9. ENEA – Centro Ricerche Bologna, 40129 Bologna, Italy

10. LPGP, CNRS, Univ. Paris-Sud, Université Paris-Saclay, 91405 Orsay, France

11. INFN, Sezione di Milano, via Celoria, 16, 20133 Milano, Italy

12. Wigner Research Centre for Physics, H-1121 Budapest, Hungary

13. Università degli Studi di Sassari, Dip. di Architettura, Design e Urbanistica ad Alghero, 07041 Alghero, Italy

14. Advanced Photon Technologies, NIF & Photon Science Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

15. SUPA, Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK

16. LLR, CNRS, École Polytechnique, Palaiseau and Université Paris Saclay, Palaiseau Cedex, France

17. LULI, École Polytechnique, CNRS, CEA, Sorbonne Université, 91128 Palaiseau, France

18. Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany

19. Université de Lille, CNRS, UMR 8523 – PhLAM, Lille, France

20. University of Manchester, Manchester M13 9PL, UK

21. CNR Istituto Nazionale di Ottica, 56124 Pisa, Italy

22. Synchrotron SOLEIL, Gif-sur-Yvette 91192, France

23. Europ opportunities OÜ, Sopruse pst 9, 10615 Tallinn, Estonia

24. Forschungszentrum Jülich, 52428 Jülich, Germany

25. Helmholtz-Zentrum Dresden-Rossendorf e.V., 01328 Dresden, Germany

26. Center for Advanced Systems Understanding (CASUS), Görlitz, Germany

27. LAL, CNRS/IN2P3 Univ. Paris Sud, Orsay, and Université Paris Saclay, Orsay, France

28. CEA, IRFU, DACM, Université Paris Saclay, F-91191 Gif-sur-Yvette, France

29. Shanghai Jiao Tong University, Shanghai 200240, P.R. China

30. University of Rome Tor Vergata, 00133 Rome, Italy

31. INFN Sezione di Roma Tor Vergata, 00133 Rome, Italy

32. STFC Daresbury Laboratory, Sci-Tech Daresbury, Warrington, UK

33. John Adams Institute, Blackett Laboratory, Imperial College London, London, UK

34. Department of Physics & John Adams Institute, University of Oxford, Oxford OX1 2JD, UK

35. Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, 12489 Berlin, Germany

36. ENEA, Centro Ricerche Frascati, 00044 Frascati, Rome, Italy

37. SPIN-CNR, Complesso Universitario di M.S. Angelo, 80126 Napoli, Italy

38. GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal

39. Central Laser Facility, RAL, Didcot, Oxfordshire OX11 0QX, UK

40. Amplitude Technologies, 91029 Evry, France

41. Università di Napoli “Federico II”, 80126 Napoli, Italy

42. Universität Hamburg, 22761 Hamburg, Germany

43. ENEA, Centro Ricerche Casaccia, 00124 Santa Maria di Galeria, Rome, Italy

44. GSI Helmholtzzentrum für Schwerionenforschung GmbH,

45. INFN, Sezione di Pisa, Pisa, Italy
⁴⁶ Center for Free Electron Laser Science, 22607 Hamburg, Germany

⁴⁷ CERN, 1211 Geneva 23, Switzerland

⁴⁸ Osaka University, Osaka Prefecture, 565-0871, Japan

⁴⁹ University of California Los Angeles, Los Angeles, CA 90095, USA

⁵⁰ Helmholtz Institute Jena, 07743 Jena, Germany

⁵¹ Institut für Optik und Quantenelektronik, 07743 Jena, Germany

⁵² KPSI-QST, Kyoto 619-0215, Japan

⁵³ Ludwig-Maximilians-Universität München, 80802 Munich, Germany

⁵⁴ IAP RAS, Nizhnij Novgorod 603950, Russia

⁵⁵ ELI-Beamlines, Dolni Brezany, Czech Republic

⁵⁶ LOA, ENSTA-CNRS-École Polytechnique UMR 7639, Palaiseau F-91761, France

⁵⁷ Deutsches Elektronensynchrotron - Zeuthen, 15738 Zeuthen, Germany

⁵⁸ Brookhaven National Laboratory, Upton, NY 11973, USA

⁵⁹ Stony Brook University, Stony Brook, NY 11794, USA

⁶⁰ Tsinghua University, Beijing 100084, P.R. China

⁶¹ Lund University, 221 00 Lund, Sweden

⁶² ARCNL, University of Amsterdam, 1098, XG Amsterdam, Netherlands

⁶³ Sapienza, University of Rome, 00161 Rome, Italy

⁶⁴ INFN Sezione di Roma 1, Rome, Italy

⁶⁵ Department of Physics, University of York, Heslington YO10 5DD, UK

⁶⁶ CNR Istituto Nazionale di Ottica, I-50019 Sesto Fiorentino, Italy

⁶⁷ University of Milan, 20133 Milan, Italy

⁶⁸ University of Pécs, Institute of Physics, H-7624 Pécs, Hungary

⁶⁹ School of Mathematics and Physics, The Queen's University of Belfast, BT71NN Belfast, UK

⁷⁰ Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

⁷¹ Diamond Light Source, OX11 0DE Didcot, UK

⁷² Thales Laser S.A., 91400 Orsay, France

⁷³ RIKEN SPring-8 Center, Hyogo 679-5148, Japan

⁷⁴ Hebrew University of Jerusalem, Jerusalem, Israel

Received 17 December 2019 / Accepted 21 July 2020

Published online 23 December 2020
**Abstract.** This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science – through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The EuPRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full EuPRAXIA facility within 8–10 years.

<footnote>a e-mail: ralph.assmann@desy.de</footnote>
# Contents

<table><tr><td>Part 1 Executive Summary</td><td>3686</td></tr><tr><td>Part 2 Short Description of EuPRAXIA</td><td>3690</td></tr><tr><td>Part 3 The Research Infrastructure Concept</td><td>3701</td></tr><tr><td>1 Introduction</td><td>3701</td></tr><tr><td>2 Opportunities for Science and Innovation</td><td>3713</td></tr><tr><td>2.1 Flagship Science and Innovation Goals</td><td>3713</td></tr><tr><td>2.1.1 Flagship Innovation Goals</td><td>3714</td></tr><tr><td>2.1.2 Flagship Science Goals</td><td>3715</td></tr><tr><td>2.1.3 Unique Selling Points of EuPRAXIA</td><td>3716</td></tr><tr><td>2.2 The Needs of External Users from the Academic Environment</td><td>3720</td></tr><tr><td>2.2.1 Summary of User Needs and Possibilities</td><td>3720</td></tr><tr><td>2.2.2 Accelerator Science</td><td>3722</td></tr><tr><td>2.2.3 Laser Science</td><td>3723</td></tr><tr><td>2.2.4 Photon Science</td><td>3725</td></tr><tr><td>2.2.5 High-Energy Physics</td><td>3727</td></tr><tr><td>2.2.6 Medical Physics</td><td>3729</td></tr><tr><td>2.2.7 Inspection and Material Studies</td><td>3731</td></tr><tr><td>2.3 The Role of Industry</td><td>3733</td></tr><tr><td>2.4 Added Value for the European Research and Technology Landscape</td><td>3734</td></tr><tr><td>2.5 The Role of EuPRAXIA for Innovation and Science in the Global Environment</td><td>3736</td></tr><tr><td>3 Description of the EuPRAXIA Infrastructure</td><td>3737</td></tr><tr><td>3.1 Introduction</td><td>3737</td></tr><tr><td>3.2 Overview on Design and Performance Parameters</td><td>3738</td></tr><tr><td>3.3 The Concept of Distributed EuPRAXIA Construction</td><td>3738</td></tr><tr><td>3.3.1 EuPRAXIA Clusters for Organising Europe-Wide Work</td><td>3741</td></tr><tr><td>3.3.2 EuPRAXIA Construction and Excellence Sites</td><td>3751</td></tr><tr><td>3.4 EuPRAXIA Construction Footprint (Towards Accelerator Miniaturisation)</td><td>3753</td></tr><tr><td>3.5 EuPRAXIA Digital Footprint (Data Sharing and Management)</td><td>3756</td></tr><tr><td>3.6 Matching EuPRAXIA Needs and Existing Facilities / Capabilities in Europe</td><td>3759</td></tr><tr><td>3.6.1 INFN-LNF: Construction Site for Beam-Driven Plasma Acceleration</td><td>3760</td></tr><tr><td>3.6.2 LAPLACE - France: Excellence Centre for Laser-Plasma Acceleration and 1 GeV FEL</td><td>3763</td></tr><tr><td>3.6.3 IST - Portugal: Excellence Centre for Plasma Simulations and Theory</td><td>3765</td></tr><tr><td>3.6.4 PWASC - UK: Excellence Centre for Advanced Application Beamlines</td><td>3766</td></tr><tr><td>3.6.5 ELI Beamlines: Incubator for the Application of Novel Accelerator Technology to Laser Science Users</td><td>3769</td></tr><tr><td>3.6.6 DESY - Germany: Excellence Centre for Plasma Accelerators and High-Repetition-Rate Developments</td><td>3771</td></tr><tr><td>3.6.7 Possible Candidate Sites for the Laser-Driven Plasma Acceleration Construction Site</td><td>3773</td></tr><tr><td>3.7 Long-Term Scientific Programme</td><td>3779</td></tr><tr><td>4 Preliminary Resource and Financial Plan</td><td>3781</td></tr><tr><td>4.1 Cost Model and Definitions</td><td>3781</td></tr><tr><td>4.2 Summary of Total Costs</td><td>3782</td></tr><tr><td>4.3 Breakdown of Investment Costs Related to the Beam-Driven Plasma Accelerator Site</td><td>3784</td></tr></table>
<table><tbody><tr><td>4.4</td><td>Breakdown of Investment Costs Related to the Laser-Driven Plasma Accelerator Site</td><td>3786</td></tr><tr><td>4.5</td><td>In-Kind Contributions</td><td>3787</td></tr><tr><td>4.6</td><td>Operational Costs</td><td>3787</td></tr><tr><td>4.7</td><td>Alternative Minimal Systems</td><td>3788</td></tr><tr><td>5</td><td>Preliminary Project Organisation and Implementation</td><td>3789</td></tr><tr><td>5.1</td><td>Project Schedule</td><td>3789</td></tr><tr><td>5.2</td><td>Structure and Governing Model</td><td>3792</td></tr><tr><td>5.3</td><td>Operational Model</td><td>3793</td></tr><tr><td>5.4</td><td>User Access</td><td>3794</td></tr><tr><td>5.4.1</td><td>Types of User Access</td><td>3794</td></tr><tr><td>5.4.2</td><td>User Facility Modes</td><td>3795</td></tr><tr><td>5.4.3</td><td>User Support</td><td>3796</td></tr><tr><td>5.5</td><td>Quality Assurance</td><td>3797</td></tr><tr><td>5.6</td><td>Outreach and Communication</td><td>3797</td></tr><tr><td>5.7</td><td>Project Risk Assessment</td><td>3799</td></tr><tr><td>5.7.1</td><td>General Project Risks</td><td>3799</td></tr><tr><td>5.7.2</td><td>Technical Risks</td><td>3799</td></tr><tr><td>6</td><td>Impact Assessment</td><td>3803</td></tr><tr><td>6.1</td><td>Strategic Significance for European Science and Innovation</td><td>3803</td></tr><tr><td>6.2</td><td>Socio-Economic Opportunities</td><td>3807</td></tr><tr><td>6.3</td><td>Consistency with the Scientific Strategy of Research Areas</td><td>3808</td></tr><tr><td>6.4</td><td>Competition and Worldwide Context</td><td>3809</td></tr><tr><td>6.5</td><td>Long-Term Future Accelerator Roadmap and the Role of EuPRAXIA</td><td>3811</td></tr><tr><td>Part 4</td><td>Systems Design and Performance</td><td>3813</td></tr><tr><td>7</td><td>Technology Background</td><td>3813</td></tr><tr><td>7.1</td><td>Plasma-Accelerator Physics</td><td>3813</td></tr><tr><td>7.1.1</td><td>Laser-Driven Wakefield Acceleration (LWFA)</td><td>3814</td></tr><tr><td>7.1.2</td><td>Beam-Driven Wakefield Acceleration (PWFA)</td><td>3816</td></tr><tr><td>7.1.3</td><td>Challenges in Plasma Acceleration</td><td>3816</td></tr><tr><td>7.1.4</td><td>Plasma Sources</td><td>3818</td></tr><tr><td>7.2</td><td>Laser Science</td><td>3819</td></tr><tr><td>7.3</td><td>Free-Electron Lasers</td><td>3823</td></tr><tr><td>8</td><td>Description of the Overall Facility Layout and Its Building Blocks</td><td>3826</td></tr><tr><td>8.1</td><td>EuPRAXIA Site for Beam-Driven Plasma Acceleration</td><td>3826</td></tr><tr><td>8.2</td><td>EuPRAXIA Site for Laser-Driven Plasma Acceleration</td><td>3829</td></tr><tr><td>8.3</td><td>Beam Distribution Concept</td><td>3830</td></tr><tr><td>9</td><td>Summary Tables of Parameters and Technical Data</td><td>3832</td></tr><tr><td>9.1</td><td>Laser System</td><td>3833</td></tr><tr><td>9.2</td><td>RF Injector and Accelerator</td><td>3833</td></tr><tr><td>9.3</td><td>Plasma Injector</td><td>3834</td></tr><tr><td>9.4</td><td>Plasma Accelerator</td><td>3834</td></tr><tr><td>9.5</td><td>Beam Transport Line</td><td>3835</td></tr><tr><td>9.6</td><td>Free-Electron Laser</td><td>3836</td></tr><tr><td>9.7</td><td>Other Radiation Sources</td><td>3837</td></tr><tr><td>9.8</td><td>Secondary Particle Sources</td><td>3837</td></tr><tr><td>9.9</td><td>Accelerator Start-to-End Simulations</td><td>3838</td></tr><tr><td>10</td><td>Laser Systems as Innovative Power Drivers</td><td>3839</td></tr><tr><td>10.1</td><td>Introduction</td><td>3839</td></tr><tr><td>10.2</td><td>Baseline Design</td><td>3839</td></tr><tr><td>10.2.1</td><td>Layout and Amplifiers</td><td>3839</td></tr><tr><td>10.2.2</td><td>Front End</td><td>3841</td></tr><tr><td>10.2.3</td><td>Power Amplifiers</td><td>3842</td></tr><tr><td>10.2.4</td><td>Ti:Sapphire Amplifier Structure and Geometry</td><td>3842</td></tr><tr><td>10.2.5</td><td>Pump Lasers</td><td>3850</td></tr></tbody></table>
<table>
  <tbody>
    <tr>
      <td>10.2.6 Transport to Plasma</td>
      <td>3851</td>
    </tr>
    <tr>
      <td>10.2.7 Development Paths</td>
      <td>3854</td>
    </tr>
    <tr>
      <td>10.2.8 Risk-Mitigating Solutions</td>
      <td>3856</td>
    </tr>
    <tr>
      <td>10.3 Conclusion</td>
      <td>3859</td>
    </tr>
    <tr>
      <td>11 Laser Control-Command System</td>
      <td>3859</td>
    </tr>
    <tr>
      <td>11.1 Introduction</td>
      <td>3859</td>
    </tr>
    <tr>
      <td>11.2 Overview</td>
      <td>3860</td>
    </tr>
    <tr>
      <td>11.3 Needs and Requirements</td>
      <td>3862</td>
    </tr>
    <tr>
      <td>11.3.1 Operational Requirements</td>
      <td>3862</td>
    </tr>
    <tr>
      <td>11.3.2 Personnel Safety System</td>
      <td>3867</td>
    </tr>
    <tr>
      <td>11.3.3 Electrical Timing and Synchronisation</td>
      <td>3868</td>
    </tr>
    <tr>
      <td>12 RF Accelerator to 500 MeV as Beam Driver</td>
      <td>3870</td>
    </tr>
    <tr>
      <td>12.1 Introduction</td>
      <td>3870</td>
    </tr>
    <tr>
      <td>12.2 S-band RF Photo-Injector</td>
      <td>3870</td>
    </tr>
    <tr>
      <td>12.2.1 RF Gun</td>
      <td>3871</td>
    </tr>
    <tr>
      <td>12.2.2 RF Gun Solenoid</td>
      <td>3873</td>
    </tr>
    <tr>
      <td>12.2.3 Photo-Cathode Drive Laser System</td>
      <td>3873</td>
    </tr>
    <tr>
      <td>12.2.4 S-band Linac</td>
      <td>3874</td>
    </tr>
    <tr>
      <td>12.2.5 High-Power System Components</td>
      <td>3877</td>
    </tr>
    <tr>
      <td>12.3 X-band RF Linac</td>
      <td>3878</td>
    </tr>
    <tr>
      <td>12.3.1 Introduction</td>
      <td>3878</td>
    </tr>
    <tr>
      <td>12.3.2 Linac Technology and Gradient Choice</td>
      <td>3879</td>
    </tr>
    <tr>
      <td>12.4 Accelerating Section Design</td>
      <td>3880</td>
    </tr>
    <tr>
      <td>12.4.1 X-band Linac Layout</td>
      <td>3883</td>
    </tr>
    <tr>
      <td>13 Plasma Injector</td>
      <td>3884</td>
    </tr>
    <tr>
      <td>13.1 Introduction</td>
      <td>3884</td>
    </tr>
    <tr>
      <td>13.2 Injection Mechanisms</td>
      <td>3885</td>
    </tr>
    <tr>
      <td>13.3 Low-Energy Plasma Injector</td>
      <td>3886</td>
    </tr>
    <tr>
      <td>13.3.1 Simulation Results</td>
      <td>3887</td>
    </tr>
    <tr>
      <td>13.3.2 Plasma Structure</td>
      <td>3892</td>
    </tr>
    <tr>
      <td>13.4 GeV Plasma Injector</td>
      <td>3893</td>
    </tr>
    <tr>
      <td>13.5 Implementation in the EuPRAXIA Beamline</td>
      <td>3894</td>
    </tr>
    <tr>
      <td>13.5.1 Staging Requirements</td>
      <td>3894</td>
    </tr>
    <tr>
      <td>13.5.2 Laser Removal</td>
      <td>3895</td>
    </tr>
    <tr>
      <td>13.5.3 Stability</td>
      <td>3895</td>
    </tr>
    <tr>
      <td>13.5.4 Repetition Rate</td>
      <td>3895</td>
    </tr>
    <tr>
      <td>13.6 Conclusion</td>
      <td>3896</td>
    </tr>
    <tr>
      <td>14 RF Photo-Injector</td>
      <td>3897</td>
    </tr>
    <tr>
      <td>14.1 Introduction</td>
      <td>3897</td>
    </tr>
    <tr>
      <td>14.2 Laser-Driven External Injection Scheme</td>
      <td>3898</td>
    </tr>
    <tr>
      <td>14.3 Full RF Compression Scheme</td>
      <td>3898</td>
    </tr>
    <tr>
      <td>14.4 Hybrid Compression Scheme</td>
      <td>3904</td>
    </tr>
    <tr>
      <td>14.5 Particle-Driven External Injection Scheme</td>
      <td>3906</td>
    </tr>
    <tr>
      <td>14.6 Future Plans and Conclusions</td>
      <td>3912</td>
    </tr>
    <tr>
      <td>15 Sub-Femtosecond Synchronisation of Electrons to the Laser</td>
      <td>3913</td>
    </tr>
    <tr>
      <td>15.1 A Scheme for Timing Jitter Compensation</td>
      <td>3914</td>
    </tr>
    <tr>
      <td>15.2 Linear Model</td>
      <td>3915</td>
    </tr>
    <tr>
      <td>15.3 Numerical Simulations</td>
      <td>3916</td>
    </tr>
    <tr>
      <td>15.4 Conclusion</td>
      <td>3918</td>
    </tr>
    <tr>
      <td>16 Laser-Driven Plasma Accelerator</td>
      <td>3918</td>
    </tr>
    <tr>
      <td>16.1 Introduction</td>
      <td>3918</td>
    </tr>
    <tr>
      <td>16.2 Parameters of Plasma Accelerator Sections</td>
      <td>3920</td>
    </tr>
    <tr>
      <td>16.2.1 Density</td>
      <td>3920</td>
    </tr>
    <tr>
      <td>16.2.2 Interaction Length</td>
      <td>3920</td>
    </tr>
    <tr>
      <td>16.3 Intense Laser Beam Confinement</td>
      <td>3921</td>
    </tr>
  </tbody>
</table>
<table>
  <tbody>
    <tr>
      <td>16.3.1 Requirements</td>
      <td>3921</td>
    </tr>
    <tr>
      <td>16.3.2 Possible Waveguides</td>
      <td>3921</td>
    </tr>
    <tr>
      <td>16.3.3 Example of Waveguide Implementation</td>
      <td>3922</td>
    </tr>
    <tr>
      <td>16.4 Staged Accelerator Implementation</td>
      <td>3923</td>
    </tr>
    <tr>
      <td>16.5 Conclusion</td>
      <td>3925</td>
    </tr>
    <tr>
      <td>17 Beam-Driven Plasma Accelerator</td>
      <td>3926</td>
    </tr>
    <tr>
      <td>17.1 Start-to-End Simulation for the 1 GeV Case</td>
      <td>3927</td>
    </tr>
    <tr>
      <td>17.1.1 Plasma Simulation Parameters and Code</td>
      <td>3927</td>
    </tr>
    <tr>
      <td>17.1.2 Optimum Bunch Acceleration in the Plasma Section</td>
      <td>3927</td>
    </tr>
    <tr>
      <td>17.1.3 Summary of the Simulation Results</td>
      <td>3928</td>
    </tr>
    <tr>
      <td>17.2 Numerical Design for the 5 GeV Case</td>
      <td>3930</td>
    </tr>
    <tr>
      <td>17.2.1 Plasma Parameters</td>
      <td>3930</td>
    </tr>
    <tr>
      <td>17.2.2 Driver Train Structure and Transformer Ratio</td>
      <td>3931</td>
    </tr>
    <tr>
      <td>17.2.3 Bunch Acceleration</td>
      <td>3932</td>
    </tr>
    <tr>
      <td>17.3 Outlook and Further Research and Development</td>
      <td>3933</td>
    </tr>
    <tr>
      <td>18 Magnets, Undulators, and Plasma Lenses</td>
      <td>3934</td>
    </tr>
    <tr>
      <td>18.1 Introduction</td>
      <td>3934</td>
    </tr>
    <tr>
      <td>18.2 Specific Magnetic Elements</td>
      <td>3936</td>
    </tr>
    <tr>
      <td>18.3 Review of State-of-the-Art Short-Period Undulators</td>
      <td>3940</td>
    </tr>
    <tr>
      <td>19 Beam Transport and Handling Systems</td>
      <td>3950</td>
    </tr>
    <tr>
      <td>19.1 Beam Transport for the Low-Energy Beam</td>
      <td>3950</td>
    </tr>
    <tr>
      <td>19.1.1 Introduction</td>
      <td>3950</td>
    </tr>
    <tr>
      <td>19.1.2 Compact Beamline with Six Permanent Quadrupoles</td>
      <td>3951</td>
    </tr>
    <tr>
      <td>19.1.3 Beamline with Eight Permanent Quadrupoles</td>
      <td>3953</td>
    </tr>
    <tr>
      <td>19.1.4 Beamline with Two Active Plasma Lenses</td>
      <td>3954</td>
    </tr>
    <tr>
      <td>19.1.5 Beamline with Two Solenoids</td>
      <td>3956</td>
    </tr>
    <tr>
      <td>19.1.6 Conclusion</td>
      <td>3958</td>
    </tr>
    <tr>
      <td>19.2 Beam Transport for the High-Energy Beam</td>
      <td>3959</td>
    </tr>
    <tr>
      <td>19.2.1 Introduction</td>
      <td>3959</td>
    </tr>
    <tr>
      <td>19.2.2 Scheme 1: LPI-5 GeV</td>
      <td>3960</td>
    </tr>
    <tr>
      <td>19.2.3 Scheme 2: LPI-150 MeV + LPAS-5 GeV</td>
      <td>3961</td>
    </tr>
    <tr>
      <td>19.2.4 Scheme 3: RFI-500 MeV + LPAS-5 GeV</td>
      <td>3962</td>
    </tr>
    <tr>
      <td>19.2.5 Scheme 4: RFI-240 MeV + LPAS-2.5 GeV + Chicane + LPAS-5 GeV</td>
      <td>3962</td>
    </tr>
    <tr>
      <td>19.2.6 Scheme 5: RFI-500 MeV + PPAS-1 GeV</td>
      <td>3967</td>
    </tr>
    <tr>
      <td>19.2.7 Conclusion</td>
      <td>3969</td>
    </tr>
    <tr>
      <td>20 Plasma Diagnostics</td>
      <td>3972</td>
    </tr>
    <tr>
      <td>20.1 Introduction</td>
      <td>3972</td>
    </tr>
    <tr>
      <td>20.2 Plasma Diagnostics</td>
      <td>3973</td>
    </tr>
    <tr>
      <td>20.2.1 Interferometric Density Measurement</td>
      <td>3973</td>
    </tr>
    <tr>
      <td>20.2.2 Spectroscopic Methods</td>
      <td>3978</td>
    </tr>
    <tr>
      <td>20.2.3 Group Velocity Measurements</td>
      <td>3978</td>
    </tr>
    <tr>
      <td>20.2.4 Summary</td>
      <td>3978</td>
    </tr>
    <tr>
      <td>20.3 Plasma Wave Diagnostics</td>
      <td>3979</td>
    </tr>
    <tr>
      <td>20.3.1 Longitudinal Probing</td>
      <td>3979</td>
    </tr>
    <tr>
      <td>20.3.2 Transverse Probing</td>
      <td>3981</td>
    </tr>
    <tr>
      <td>20.3.3 Summary</td>
      <td>3981</td>
    </tr>
    <tr>
      <td>20.4 Conclusion</td>
      <td>3981</td>
    </tr>
    <tr>
      <td>21 Electron Beam Diagnostics</td>
      <td>3982</td>
    </tr>
    <tr>
      <td>21.1 Introduction</td>
      <td>3982</td>
    </tr>
    <tr>
      <td>21.2 Charge and Trajectory Measurements</td>
      <td>3982</td>
    </tr>
    <tr>
      <td>21.3 Transverse Size Measurement</td>
      <td>3983</td>
    </tr>
    <tr>
      <td>21.4 Longitudinal Diagnostics</td>
      <td>3983</td>
    </tr>
    <tr>
      <td>21.5 Time-of-Arrival Monitors</td>
      <td>3985</td>
    </tr>
    <tr>
      <td>21.6 Beam Loss Monitors</td>
      <td>3985</td>
    </tr>
  </tbody>
</table>
<table>
  <tbody>
    <tr>
      <td>21.7</td>
      <td>Transverse Emittance Monitors</td>
      <td>3986</td>
    </tr>
    <tr>
      <td>21.8</td>
      <td>Accelerator Diagnostics Layout Before the Plasma-Accelerator Stage</td>
      <td>3988</td>
    </tr>
    <tr>
      <td>21.9</td>
      <td>Accelerator Diagnostics Layout After the Plasma-Accelerator Stage (LPAS / PPAS, relevant for Schemes 1 to 6)</td>
      <td>3992</td>
    </tr>
    <tr>
      <td>21.10</td>
      <td>Conclusion</td>
      <td>3995</td>
    </tr>
    <tr>
      <td>22</td>
      <td>Electron Beam Dump</td>
      <td>3996</td>
    </tr>
    <tr>
      <td>22.1</td>
      <td>Introduction</td>
      <td>3996</td>
    </tr>
    <tr>
      <td>22.2</td>
      <td>The SwissFEL Beam Dumps, Aramis and Athos</td>
      <td>3996</td>
    </tr>
    <tr>
      <td>22.3</td>
      <td>The Plasma Beam Dump</td>
      <td>3997</td>
    </tr>
    <tr>
      <td>22.3.1</td>
      <td>Simulation Study of a Plasma Beam Dump</td>
      <td>3998</td>
    </tr>
    <tr>
      <td>22.3.2</td>
      <td>Passive Beam Dump for EuPRAXIA</td>
      <td>3998</td>
    </tr>
    <tr>
      <td>22.4</td>
      <td>Conclusions</td>
      <td>3999</td>
    </tr>
    <tr>
      <td>23</td>
      <td>Start-to-End Simulations of the Accelerator</td>
      <td>4001</td>
    </tr>
    <tr>
      <td>23.1</td>
      <td>General Aspects</td>
      <td>4001</td>
    </tr>
    <tr>
      <td>23.2</td>
      <td>Study of Injection Techniques</td>
      <td>4003</td>
    </tr>
    <tr>
      <td>23.3</td>
      <td>Study of Acceleration Stages</td>
      <td>4005</td>
    </tr>
    <tr>
      <td>23.4</td>
      <td>Selection of the Configurations to Be Submitted to Start-to-End Simulations</td>
      <td>4006</td>
    </tr>
    <tr>
      <td>23.5</td>
      <td>Preserving Beam Emittance When Matching In and Out of Plasma Stages</td>
      <td>4008</td>
    </tr>
    <tr>
      <td>23.6</td>
      <td>Optimising and Designing Beam Transfer Lines</td>
      <td>4010</td>
    </tr>
    <tr>
      <td>23.7</td>
      <td>Start-to-End Simulations for Scheme 1: LPI-5 GeV</td>
      <td>4012</td>
    </tr>
    <tr>
      <td>23.8</td>
      <td>Start-to-End Simulations for Scheme 2: LPI-150 MeV + LPAS-5 GeV</td>
      <td>4021</td>
    </tr>
    <tr>
      <td>23.9</td>
      <td>Start-to-End Simulations for Scheme 3: RFI-500 MeV + LPAS-5 GeV</td>
      <td>4028</td>
    </tr>
    <tr>
      <td>23.10</td>
      <td>Start-to-End Simulations for Scheme 4: RFI-240 MeV + LPAS-2.5 GeV + Chicane + LPAS-5 GeV</td>
      <td>4035</td>
    </tr>
    <tr>
      <td>23.11</td>
      <td>Start-to-End Simulations for Scheme 5: RFI-500 MeV + PPAS-1 GeV</td>
      <td>4041</td>
    </tr>
    <tr>
      <td>23.12</td>
      <td>Summary of Start-to-End Simulations</td>
      <td>4046</td>
    </tr>
    <tr>
      <td>24</td>
      <td>Photon User Applications</td>
      <td>4047</td>
    </tr>
    <tr>
      <td>24.1</td>
      <td>Free-Electron Laser</td>
      <td>4047</td>
    </tr>
    <tr>
      <td>24.1.1</td>
      <td>Introduction</td>
      <td>4047</td>
    </tr>
    <tr>
      <td>24.1.2</td>
      <td>Features of the Electron Beams at the Plasma Exit and the Transfer Line</td>
      <td>4048</td>
    </tr>
    <tr>
      <td>24.1.3</td>
      <td>Undulator Line Characteristics</td>
      <td>4049</td>
    </tr>
    <tr>
      <td>24.1.4</td>
      <td>FEL Results</td>
      <td>4050</td>
    </tr>
    <tr>
      <td>24.1.5</td>
      <td>Conclusion</td>
      <td>4057</td>
    </tr>
    <tr>
      <td>24.2</td>
      <td>Betatron X-Ray Source</td>
      <td>4057</td>
    </tr>
    <tr>
      <td>24.3</td>
      <td>Compton Source</td>
      <td>4063</td>
    </tr>
    <tr>
      <td>25</td>
      <td>Particle User Applications</td>
      <td>4066</td>
    </tr>
    <tr>
      <td>25.1</td>
      <td>High-Energy Physics Test Beams</td>
      <td>4066</td>
    </tr>
    <tr>
      <td>25.2</td>
      <td>Secondary Particle Sources</td>
      <td>4069</td>
    </tr>
    <tr>
      <td>25.2.1</td>
      <td>Low-Energy Positrons</td>
      <td>4069</td>
    </tr>
    <tr>
      <td>25.2.2</td>
      <td>High-Energy Positrons</td>
      <td>4070</td>
    </tr>
    <tr>
      <td>26</td>
      <td>Additional Innovation Paths</td>
      <td>4073</td>
    </tr>
    <tr>
      <td>26.1</td>
      <td>Hybrid Laser-Beam-Driven Plasma Wakefield Acceleration</td>
      <td>4073</td>
    </tr>
    <tr>
      <td>26.1.1</td>
      <td>Introduction</td>
      <td>4073</td>
    </tr>
    <tr>
      <td>26.1.2</td>
      <td>LWFA Beams as PWFA Drivers</td>
      <td>4074</td>
    </tr>
    <tr>
      <td>26.1.3</td>
      <td>LWFA→PWFA Transition</td>
      <td>4075</td>
    </tr>
    <tr>
      <td>26.1.4</td>
      <td>Experimental Status</td>
      <td>4076</td>
    </tr>
    <tr>
      <td>26.2</td>
      <td>LWFA→PWFA Staging as a Beam Energy and Brightness Transformer</td>
      <td>4077</td>
    </tr>
    <tr>
      <td>26.2.1</td>
      <td>Injector for a Hybrid Plasma Accelerator</td>
      <td>4078</td>
    </tr>
    <tr>
      <td>26.3</td>
      <td>Plasma Injection Mechanisms</td>
      <td>4086</td>
    </tr>
    <tr>
      <td>26.3.1</td>
      <td>Optimisation of Ionisation Injection</td>
      <td>4086</td>
    </tr>
    <tr>
      <td>26.3.2</td>
      <td>Generation of Attosecond Electron Bunches by Density-Gradient-Controlled Injection</td>
      <td>4087</td>
    </tr>
  </tbody>
</table>
<table><tbody><tr><td>26.4</td><td>Compact Radiation Generation</td><td>4090</td></tr><tr><td>26.4.1</td><td>Betatron Radiation Control in a Plasma Channel</td><td>4090</td></tr><tr><td>26.4.2</td><td>Schemes for Enhanced Betatron Radiation</td><td>4091</td></tr><tr><td>26.4.3</td><td>Coherent Bright XUV and X-Ray Radiation from LWFA</td><td>4091</td></tr><tr><td>26.5</td><td>Dielectric Technology for Diagnostics &amp; Beam Control</td><td>4092</td></tr><tr><td>26.6</td><td>Additional Application Beamlines</td><td>4093</td></tr><tr><td>27</td><td>Safety and Environmental Impact</td><td>4094</td></tr><tr><td>27.1</td><td>Safety Aspects</td><td>4094</td></tr><tr><td>27.2</td><td>Environmental Impact</td><td>4099</td></tr><tr><td>Part 5</td><td>EuPRAXIA Pre-Construction R&amp;D and Technical Design Phase</td><td>4100</td></tr><tr><td>28</td><td>Necessary Experiments and Prototypes for the EuPRAXIA Design</td><td>4100</td></tr><tr><td>28.1</td><td>Laser System</td><td>4100</td></tr><tr><td>28.2</td><td>Plasma Components</td><td>4102</td></tr><tr><td>28.3</td><td>RF and Accelerator Technology</td><td>4103</td></tr><tr><td>28.4</td><td>Diagnostics</td><td>4104</td></tr><tr><td>29</td><td>EuPRAXIA Consortium Facilities</td><td>4107</td></tr><tr><td>29.1</td><td>APOLLON (France)</td><td>4107</td></tr><tr><td>29.2</td><td>CLARA (United Kingdom)</td><td>4108</td></tr><tr><td>29.3</td><td>COXINEL (France)</td><td>4109</td></tr><tr><td>29.4</td><td>ELBE Centre for High Power Radiation Sources (Germany)</td><td>4110</td></tr><tr><td>29.5</td><td>ELI - Extreme Light Infrastructure</td><td>4111</td></tr><tr><td>29.6</td><td>FLASHForward (Germany)</td><td>4113</td></tr><tr><td>29.7</td><td>HI Jena (Germany)</td><td>4114</td></tr><tr><td>29.8</td><td>ILIL (Italy)</td><td>4115</td></tr><tr><td>29.9</td><td>Imperial College London (United Kingdom)</td><td>4116</td></tr><tr><td>29.10</td><td>KIT (Germany)</td><td>4118</td></tr><tr><td>29.11</td><td>LMU-CALA (Germany)</td><td>4119</td></tr><tr><td>29.12</td><td>LPA-UHI100 (France)</td><td>4119</td></tr><tr><td>29.13</td><td>Lund Laser Centre (Sweden)</td><td>4120</td></tr><tr><td>29.14</td><td>LUX (Germany)</td><td>4120</td></tr><tr><td>29.15</td><td>Queen's University of Belfast (United Kingdom)</td><td>4121</td></tr><tr><td>29.16</td><td>Rutherford Appleton Laboratory (United Kingdom)</td><td>4122</td></tr><tr><td>29.17</td><td>SCAPA (United Kingdom)</td><td>4122</td></tr><tr><td>29.18</td><td>SINBAD (Germany)</td><td>4123</td></tr><tr><td>29.19</td><td>SPARC_LAB (Italy)</td><td>4124</td></tr><tr><td>29.20</td><td>University of Oxford (United Kingdom)</td><td>4126</td></tr><tr><td>29.21</td><td>Wigner Institute (Hungary)</td><td>4127</td></tr><tr><td>30</td><td>International Consortium Facilities</td><td>4128</td></tr><tr><td>30.1</td><td>Berkeley Lab Laser Accelerator Center (United States)</td><td>4128</td></tr><tr><td>30.2</td><td>Institute of Applied Physics at the Russian Academy of Sciences (Russia)</td><td>4129</td></tr><tr><td>30.3</td><td>Osaka University / KPSI-QST / RIKEN (Japan)</td><td>4130</td></tr><tr><td>30.4</td><td>Shanghai Jiaotong University (China)</td><td>4131</td></tr><tr><td>30.5</td><td>Tsinghua University Beijing (China)</td><td>4131</td></tr><tr><td>31</td><td>Computing Facilities</td><td>4132</td></tr><tr><td>Part 6</td><td>Appendix</td><td>4135</td></tr><tr><td>32</td><td>Further Working Package Results</td><td>4135</td></tr><tr><td>32.1</td><td>Assumptions and Numerical PIC Codes Used</td><td>4135</td></tr><tr><td>32.2</td><td>Further Considerations for High-Gradient Plasma-Accelerator Structures</td><td>4144</td></tr><tr><td>32.2.1</td><td>Introduction</td><td>4144</td></tr><tr><td>32.2.2</td><td>Plasma Stage Parameters</td><td>4144</td></tr><tr><td>32.2.3</td><td>Plasma Creation</td><td>4146</td></tr><tr><td>32.2.4</td><td>Laser Confinement</td><td>4151</td></tr><tr><td>32.2.5</td><td>Injection Techniques for the LPI</td><td>4163</td></tr><tr><td>32.2.6</td><td>Summary: Preliminary Design Elements</td><td>4175</td></tr><tr><td>32.3</td><td>Enabling Laser Design and Optimisation</td><td>4176</td></tr></tbody></table>
<table><tr><td>32.3.1 Introduction</td><td>4176</td></tr><tr><td>32.3.2 Pump Laser Technology</td><td>4178</td></tr><tr><td>32.3.3 Pump Array Layouts</td><td>4179</td></tr><tr><td>32.3.4 Conclusive Remarks</td><td>4180</td></tr><tr><td>32.4 Outreach and Liaison</td><td>4181</td></tr><tr><td>32.4.1 Resources</td><td>4181</td></tr><tr><td>32.4.2 Publications</td><td>4184</td></tr><tr><td>32.4.3 Activities</td><td>4185</td></tr><tr><td>32.5 Experience from PWFA Test Experiments</td><td>4187</td></tr><tr><td>32.5.1 Active Plasma Lenses</td><td>4187</td></tr><tr><td>32.5.2 Plasma Dechirper</td><td>4190</td></tr><tr><td>32.5.3 Plasma Sources</td><td>4193</td></tr><tr><td>32.5.4 Beam-Driven Plasma Wakefields</td><td>4195</td></tr><tr><td>32.6 Use of Other Novel Technologies</td><td>4196</td></tr><tr><td>32.6.1 Possible Alternative Injector Concepts</td><td>4196</td></tr><tr><td>32.6.2 Ultra-Cold Electron Source</td><td>4196</td></tr><tr><td>32.6.3 Dielectric Laser Acceleration (DLA)</td><td>4197</td></tr><tr><td>32.6.4 State of the Art in Fibre Optics Lasers</td><td>4199</td></tr><tr><td>32.7 Experience from FEL Test Experiments</td><td>4200</td></tr><tr><td>32.8 Accelerator Prototyping - Report on Test Experiments</td><td>4202</td></tr><tr><td>32.8.1 Introduction</td><td>4202</td></tr><tr><td>32.8.2 Testing Laser Wakefield Accelerator Sources for EuPRAXIA</td><td>4203</td></tr><tr><td>32.8.3 Non-Destructive Bunch Length Measurement Based on Smith-Purcell Radiation</td><td>4204</td></tr><tr><td>32.8.4 3D-Printed Capillary for a Hydrogen-Filled Discharge for Plasma Acceleration</td><td>4204</td></tr><tr><td>32.9 CAD Model of the Conceptual EuPRAXIA Facility Layout</td><td>4207</td></tr><tr><td>33 EuPRAXIA Publications and Conference Contributions</td><td>4209</td></tr><tr><td>33.1 List of EuPRAXIA Peer-Reviewed Publications</td><td>4209</td></tr><tr><td>33.2 List of EuPRAXIA Conference Proceedings</td><td>4217</td></tr><tr><td>References</td><td>4221</td></tr><tr><td>List of EuPRAXIA Partner and Associated Partner Institutes</td><td>4268</td></tr><tr><td>Commonly Used Acronyms</td><td>4270</td></tr><tr><td>List of Figures</td><td>4272</td></tr><tr><td>List of Tables</td><td>4281</td></tr></table>
# Part 1
## Executive Summary

The conceptual design of a new European research infrastructure, EuPRAXIA, is presented in this report. The concept has been established over the last 4 years in a unique collaboration of 30 institutes in Europe and 11 laboratories in Asia, Russia, and the United States. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma concepts and laser technology. It focuses on the development of particle accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in France, Germany, Italy, and the UK. The conceptual design of EuPRAXIA is timely; recent breakthroughs in laser technology realised through the significant investments by the EU and its member states in the Extreme Light Infrastructure (ELI) and parallel advancements in plasma accelerators worldwide underpin the realisation of a plasma-accelerator-based user facility for science and research. EuPRAXIA has involved the international laser community to build links and bridges with accelerator science as well as to realise synergies, identify disruptive ideas, innovate, and foster knowledge exchange.

The EuPRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency (RF)-based accelerators. EuPRAXIA envisions a beam energy of 1 to 5 gigaelectronvolts (GeV) and a beam quality (single pulse) equivalent to present RF-based linacs.

The worldwide interest in plasma-based electron accelerators is driven by the need of a new technology to overcome the multi-TeV barrier for particle accelerators and the desire to make electron-beam and associated radiation facilities (FELs, synchrotrons, etc.) available to a larger user base. Advancing into the next generation of light sources, a size and cost reduction for the facility of at least one order of magnitude must be realised to harvest the scientific and commercial prospects. EuPRAXIA aims at realising the first laser plasma user facility worldwide, demonstrating feasibility and gaining operational and user experience.

EuPRAXIA will demonstrate high-quality beam generation from a plasma-accelerator module and is therefore the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier, that would combine many such modules. The EuPRAXIA energy range and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, tabletop test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing.

Consistent with a high-confidence approach, the EuPRAXIA project includes measures to retire risk by establishing scaled technology demonstrators. The EuPRAXIA facility will integrate lessons learned and establish user readiness by 2030. A risk reduction roadmap has been defined and will ensure that risks are minimised and challenges will be addressed by appropriate mitigation measures.

Initial system designs have been developed using start-to-end simulations to achieve the single- and multi-stage acceleration of electron beams, with a quality much superior to present state-of-the-art plasma acceleration experiments. In some parameters, a factor 10 improvement is predicted. At the same time, the design initially realises at least a factor 6 gain in required floor space for the accelerator if compared to existing accelerator facilities. This includes the accelerator system itself and its support infrastructure. A final gain of a factor 10 and more for the accelerator-only footprint seems achievable in a phased approach.
The development and construction of new generations of pulsed lasers with petawatt peak power as drivers for plasma wakefield accelerators will be performed jointly with industry and national laboratories specialised in energetic laser development. These lasers will operate with high stability at 20 to 100 Hz, a modest advancement of a factor 2 to 10 over the current state of the art. The high repetition rate, however, enables laser-based and beam-based fast feedback loops as well as the application of advanced data science techniques, such as machine learning. In parallel, focused R&D activities will be pursued on the rapid development of laser systems that can operate at kHz repetition rates and deliver peak-power levels at 100 TW or more. Such drivers would allow a factor 10 increase in average brilliance and pave the path to even more compact and efficient accelerator systems. The parallel development is envisioned to facilitate innovation in high-repetition-rate plasma-accelerator technology, enabling additional applications and science reach while maintaining the 100 Hz laser systems as a project baseline for a risk-balanced approach. The improved feedback efficiency with such increased repetition rates compared to the current state of the art will allow even better pulse-to-pulse stability control and beam quality.

Finally, EuPRAXIA also includes the development and construction of a compact X-band RF accelerator based on technology from CERN to realise a beam-driven plasma accelerator. Such an X-band linear accelerator will exploit the capabilities with the most compact RF technology available today and provide an excellent science test facility with a complementary technological approach that decouples the plasma accelerator from laser science.

Distributed and versatile user areas will be set up to exploit the inherent advantageous features of the plasma accelerator, for example multiple parallel user lines for laser-driven accelerators, the generation of ultra-fast electron and photon pulses with naturally short pulse lengths, the quasi-point-like emission of X-rays inside plasmas with the potential for ultra-sharp imaging, and unique pump-probe configurations with the synchronised EuPRAXIA particle and laser beams.

The EuPRAXIA project will serve society in a number of ways. The broad and interdisciplinary EuPRAXIA collaboration in Europe and with international partners will create a critical mass of expertise and capabilities in Europe. It will support and further position Europe as a world-leading competitor in accelerator innovation. The EuPRAXIA project will also challenge and support the European and world-wide laser industry to further develop and improve their products on high-power pulsed lasers. New generations of scientists and technicians in the EU will be exposed to innovative and highly challenging technical and intellectual problems in centrally located and well-integrated R&D facilities. The proximity to major universities and laboratories in Europe will amplify the capability of EuPRAXIA to fascinate young generations for science and technology, to foster innovative out-of-the-box thinking, to serve as a high-tech training base, and to strengthen the job base for technical work. Finally, a compact particle accelerator product as a result of the EuPRAXIA project could make accelerators available as versatile tools to new users and in new locations, e.g. laboratory spaces at university, hospitals, mobile platforms, and beyond.

The EuPRAXIA infrastructure is implemented through several components: clusters, excellence centres, and construction site(s).

Clusters of European institutes collaborate jointly to address well-defined challenges and guide the overall R&D and design of the project. This is complemented by EuPRAXIA centres of excellence at existing large infrastructures in Hamburg (DESY – Germany), on the Plateau de Saclay (France), in Lisbon (IST – Portugal), in the UK (CLF and SCAPA), and at ELI Beamlines (Czech Republic). The centres of excellence perform mission-critical R&D, prototyping, testing, and construction tasks bringing in and upgrading their existing infrastructures and delivering fully tested components to the EuPRAXIA construction site(s). As a central component to the
infrastructure concept, one or two construction sites will host new large research
facilities, exploiting beam-driven (BPA) and laser-driven plasma-accelerator (LPA)
technology. At these construction site(s), the consortium will set up several plasma-
accelerator beamlines realising complementary technologies and applications, and
providing pilot access to academic and industrial users, once target parameters have
been reached. INFN – Frascati (Italy) has been agreed as the construction site for
a beam-driven plasma accelerator facility, including several applications, such as a
free-electron laser and positron sources. It is ready to proceed and will be realised in
the first phase of the project. For the laser-driven construction site, a strong concept
has been developed at this stage, and multiple candidate sites have been identified
based on the varied research landscape for laser-driven plasma acceleration already
existing across Europe. A site decision for the construction of the laser-driven plasma
accelerator will be taken during the preparatory project phase of EuPRAXIA.

The EuPRAXIA clusters, excellence centres, and construction sites are coordi-
nated by a lean project management structure. It organises the overall technical
design (TD) and prototyping, it oversees construction work, and it links to industrial
partners through work at the collaborating institutes.

Funding options for EuPRAXIA start at 68 million euros for a beam-driven and 75
million euros for a laser-driven EuPRAXIA implementation (minimal systems). It is
estimated that the full-scale EuPRAXIA facility with two construction sites requires
an investment of 320 million euros and about 1,800 FTE person power integrated over
an 8-10 year project duration (technical design, prototyping, and construction). The
320 million euro cost includes about 83 million euros that is invested in laser tech-
nology with industry. Funding will be leveraged by co-funding from the EuPRAXIA
partners. The full-scale EuPRAXIA implementation is a truly European project that
realises a major competitive advantage, strong scientific impact, and important soci-
etal benefits. Proceeding at full speed, we estimate that the EuPRAXIA research
infrastructure would start full operation in 8-10 years. Parts of EuPRAXIA could go
into operation at significantly earlier times.

Role and Structure of the CDR

This report describes the conceptual design of a European plasma accelerator facil-
ity EuPRAXIA. It includes technical results and organisational concepts that are
required for assessing the feasibility of such a research infrastructure based on plasma
acceleration and other technologies. The report does not provide a final technical
design of the facility to be implemented, but instead proposes solutions for critical
issues, while also identifying additional challenges and developing strategies on how
to possibly approach them in the future. Based largely on theoretical and simulation
studies, together with in-kind funded experimental activities, the CDR summarises
the activities and developments of a large collaboration of research organisations.
Most of the concepts developed in the context of EuPRAXIA and presented here
have been published as EuPRAXIA articles in scientific journals and are referenced
accordingly herein. As an added value, this report brings these findings together and
applies them in the context of constructing a compact, large-scale research facility.

On the basis of the outcomes of this report, it is acknowledged that, while some
technical issues remain to be solved, several novel solutions have been developed
and confirmed in the standard peer review process before publication in journals
like Physical Review Letters. No fundamental show-stoppers have been identified
for the concept of the EuPRAXIA facility. The continuation of the project through
a technical design phase will validate the proposed design in more detail. This next
phase will also be focused on developing further the detailed technical specifications of
the proposed facility including required prototypes and experimental demonstrations, upon which the final design of the EuPRAXIA implementation will be based.

The remainder of this report consists of five main sections. While the first two present an introductory description of the project suited for non-expert readers, Parts 4 to 6 focus on the technical challenges, proposed solutions and planned experimental demonstrations that determine the strategy for realising the proposed infrastructure.

In this context, Part 2 provides a short description of the EuPRAXIA project, including an introduction to plasma acceleration and the EuPRAXIA Consortium, a definition of the project goals, a summary of the implementation concept of EuPRAXIA and its benefits to participants, as well as an outline of the project's cost, schedule and risk estimates.

Subsequently, the non-technical aspects of EuPRAXIA are presented in Part 3. After an introduction of the project and the field of plasma accelerators in Chapter 1, the case of EuPRAXIA for science and innovation is put forward in Chapter 2 and the concept of the proposed infrastructure is discussed in Chapter 3. Chapter 4 outlines the preliminary financial project planning, whereas Chapter 5 summarises various organisational aspects from schedule and governing model to risk mitigation. Finally, Chapter 6 provides a brief overview of the predicted impact of EuPRAXIA in a scientific and societal context.

Part 4 focuses entirely on the conceptual design of the beamlines foreseen at the EuPRAXIA facility. Chapters 7 to 9 present the technical background, an overview of the facility sub-systems, and a summary of technical parameter tables, respectively, before different machine components are examined in detail in Chapters 10 to 26. Chapter 27.2 summarises aspects relating to the safety and environmental impact of the project.

Consequently, an outlook on the planned technical design phase of EuPRAXIA – dedicated with a large part to prototyping and test experiments – is given in Part 5. After a description of key future activities for different machine sub-systems in Chapter 28, a variety of relevant and suitable test facilities related to the EuPRAXIA Consortium is introduced in Chapters 29 to 31.

Finally, Part 6 presents the Appendix of the CDR, where further details are provided on many of the technical topics discussed in the conceptual design, and a list of EuPRAXIA journal publications can be found.
Part 2
Short Description of EuPRAXIA

Plasma Wakefield Acceleration: A Transformative Technology for
Future Particle Accelerators

Plasma wakefield acceleration opens the path to new particle accelerators that can be much more compact and cost-effective. A strong competition is ongoing throughout the world involving laser and accelerator industry.

Europe has played a leading role in the development of particle accelerators since the first radiofrequency (RF) accelerator was built in Aachen in 1927 [1]. Today Europe operates most of the leading particle accelerators in the world. Tens of thousands of users from science, industry, and medicine are provided with beam time and rely on the availability of cutting-edge, high-quality particle and photon beams for performing their work.

Providing particle beams that are competitive on a worldwide scale requires con-
tinuous optimisation and the improvement of operating facilities, including strategic
research and development. Adopting new ideas and novel technologies has constantly
advanced the field of particle accelerators. Prime examples are the 1968 invention of
stochastic cooling by Simon van der Meer at CERN [2], opening the route to higher
density beams and recognised by the 1984 Nobel Prize in Physics [3]. In 1979, Tajima
and Dawson proposed laser-driven plasma accelerators that can produce accelerating
fields 1,000 times stronger than in RF accelerators [4], thus opening the possibil-
ity for accelerator facilities with a much-reduced size and cost. Such machines have
since been realised experimentally and reproducibly demonstrated multi-GeV elec-
tron beams with promising beam quality. In parallel, high-power lasers have been
developed over the past decades since the invention of chirped pulse amplification
and are today produced with European laser industry in a leading position. This
innovation in laser technology was awarded with the 2018 Nobel Prize for Physics,
going to Donna Strickland and Gérard Mourou [5].

A strong competition is ongoing throughout the world (with the United States, Europe, Japan, and China being the key players) involving laser and accelerator industry. The common breakthrough goal of these activities is an accelerator facility that will support users with particle and photon beams, while demonstrating benefits in size and cost. Such a technology leading to compact accelerators will have unforeseen applications in hospitals, universities, emerging countries, etc., while also being a required stepping stone to future high-energy accelerators for discovery science. EuPRAXIA is the design of such a new facility that offers unprecedented features, enables (if constructed) advances otherwise not possible, and prepares a future with compact accelerators.

The EuPRAXIA Consortium

EuPRAXIA enables interdisciplinary collaboration in Europe and beyond as
a key to success.

The EuPRAXIA Consortium has formed around an EU-funded Horizon 2020 con-
ceptual design study to develop the concept of a “European Plasma Research Accel-
erator with eXcellence In Applications”. It serves as an open innovation platform
bringing together 16 participants and 25 associated partners from Europe, Asia, and
the United States. Among the 41 members of the consortium are international organisations, major national laboratories, and universities combining interdisciplinary expertise from particle accelerators, laser science, plasma physics, theory, simulations, and accelerator-based user facilities.

EuPRAXIA is the first European project that develops a dedicated accelerator research infrastructure based on novel plasma concepts and laser technology. It keeps a strict focus on the development of particle accelerators, their user communities, and the exploitation of existing accelerator infrastructures in France, Germany, Italy, and the UK. The laser community, in particular the Extreme Light Infrastructure (ELI) and its new laser infrastructures, is connected as associated partners, ensuring complementarity and full knowledge exchange.

The EuPRAXIA work is organised in 15 work packages (WP), each led by two senior scientists from European institutes. The consortium also connects to representatives from leading European laser companies through its scientific advisory board and its laser work package, that are, in this way, directly contributing to the project.

## Technical and Scientific Goals

EuPRAXIA will demonstrate compact and highly innovative plasma-accelerator technology for higher energy applications in the European research landscape.

The EuPRAXIA project aims at the construction of an innovative electron accelerator based on plasma wakefield acceleration, with a beam energy of 1 to 5 gigaelectronvolts (GeV), a beam quality (single pulse) equivalent to present RF linacs, and demonstrated benefits in size and cost when compared to RF-based machines.

The EuPRAXIA pilot facilities with improved electron beam quality will establish user-readiness and will deliver electron beams and photons as a first demonstrator to users of this new technology. The EuPRAXIA energy range and its performance goals will enable versatile applications in various domains, e.g. a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, tabletop test beams for particle detectors, and highly mobile but deeply penetrating X-ray and gamma-ray sources for material testing.

EuPRAXIA will demonstrate high-quality beam generation from a plasma-accelerator module and is therefore the required stepping stone to possible plasma-based linear colliders at the high-energy physics (HEP) energy frontier, which would combine many such components.

In more detail, the EuPRAXIA scientific and technical goals include the following:

- Achievement of the single- and multi-stage acceleration of electron beams to final energies between 1 and 5 GeV, with a pulse duration of a few femtoseconds, with transverse emittance of about 1 mm mrad, and with relative energy spread reaching from a few percent down to a few $10^{-3}$ total and a few $10^{-4}$ in a 1-micrometre-slice of the beam. The EuPRAXIA specifications approach the regime of modern FELs, open various application paths, and also fulfil basic requirements for a 5 GeV plasma-accelerator stage of a linear collider.

- Implementation of a highly compact machine layout, initially realising at least a factor 3 gain in required floor space for the facility with accelerator and user areas. This includes all necessary components and infrastructure. EuPRAXIA additionally implements a realistic stepwise strategy for the miniaturisation of the particle accelerator itself by a factor 10 and beyond in its most compact version.
- Development and construction of new generations of petawatt-scale pulsed lasers as drivers for plasma wakefields, together with industry and laser institutes. These lasers will operate with high stability at 20 to 100 Hz, therefore enabling laser-based and beam-based fast feedbacks. In parallel, major R&D activities will be pursued on the rapid development of laser systems with higher efficiency, which can operate at kHz repetition rates. Here, we mention the KALDERA system that will be developed at DESY. The new kHz laser system would replace the more conventional 100 Hz laser system during an upgrade of the EuPRAXIA infrastructure and would enable additional applications and science reach. The improved feedback efficiency with such increased repetition rates compared to the current state of the art will allow much better pulse-to-pulse stability control and beam quality.

- Development and construction of a compact beam driver based on X-band RF technology from CERN. The X-band linac foreseen for EuPRAXIA's beam-driven plasma accelerator infrastructure will be the RF linear accelerator with the highest acceleration per metre worldwide.

- Development and construction of distributed and versatile user areas that exploit the inherent advantageous features of the plasma accelerator, namely:

* the possibility of multiple parallel user lines for laser-driven accelerators,

* the generation of ultra-fast electron and photon pulses with naturally short pulse lengths,

* the quasi-point-like emission of X-rays inside plasmas with the potential for ultra-sharp imaging,

* unique pump-probe configurations with the synchronised EuPRAXIA particle and laser beams.

The EuPRAXIA Consortium has, since 2015, developed and published a number of concepts that describe novel solutions for improved beam quality and will allow reaching these goals (see Chap. 33 for a full list of publications). More controlled electron injection techniques, phase-space manipulation concepts as well as enhanced stability and synchronisation for laser and electron beams, for example, will be combined with conventional, well-established components. Those solutions will be tested in a phased approach during the technical design phase to confirm the validity of the facility concept.

## Societal Goals

EuPRAXIA will involve the European industry and younger generations in inspiring technological challenges close to major universities and research institutions in Europe.

The EuPRAXIA project will serve society in a number of ways:

- The broad and interdisciplinary EuPRAXIA collaboration in Europe and with international partners will create a critical mass of expertise and capabilities in Europe. It will support and further position Europe as a world-leading competitor in accelerator innovation.

- The EuPRAXIA project will challenge and support the European and world-wide laser industry to further develop and improve their products on high-power pulsed lasers. This will strengthen the laser industry overall but in particular also enable European laser companies to stay world leading in a fair and competitive effort.

- New generations of scientists and technicians in the EU will be exposed to innovative and highly challenging technical and intellectual problems in centrally located
and well-integrated R&D facilities. The proximity to major universities and laboratories in Europe will amplify the capability of EuPRAXIA to fascinate young generations for science and technology, to foster innovative out-of-the-box thinking, to serve as a high-tech training base, and to strengthen the job base for technical work.

- A compact particle accelerator product as a result of the EuPRAXIA project could make accelerators available as versatile tools to new users and in new locations, e.g. laboratory spaces at university, hospitals, mobile platforms, and beyond. This would multiply access to accelerators and could create major advances in knowledge and capabilities, some of them yet unimaginable. We can, for example, foresee that ultra-fast X-ray pulses from compact accelerators could serve tens of thousands of PhD students and post-docs to scan features of many million types of viruses and bacteria, materials and medical compounds. The most promising cases would then be studied further and with more accuracy in big science facilities.

## Implementation Concept for EuPRAXIA

EuPRAXIA builds a new kind of European research infrastructure on the foundations of existing facilities and past national investments.

The implementation concept for the EuPRAXIA project has been inspired by the design and construction of large detectors for particle physics, for example ATLAS and CMS for the Large Hadron Collider at CERN. Dozens of institutes form such a detector collaboration; they develop, prototype, and produce the components of the detector and finally assemble the full device at the site of the experiment. During this process, groups of institutes ("clusters") collaborate on certain well-defined parts of the detector, combining their expertise and involving the local infrastructures at their laboratories. This distributed model of R&D and production has proven very efficient and highly successful for building together some of the world's most complex scientific installations.

EuPRAXIA aims at implementing such a concept for the common construction of a European plasma-accelerator facility, in the form described below. We also note that such a concept, almost unique in the current scenario of collaborative efforts in the field, is an open innovation approach. As such, following the EU definition, it aims to "open up the innovation process to all active players so that knowledge can circulate more freely and be transformed into products and services that create new markets" [6].

The proposed EuPRAXIA implementation model relies on five pillars:

1. The common goals of the EuPRAXIA Consortium, as described above.

2. Several clusters of institutes that address particular and well-defined challenges in the EuPRAXIA project. International partners are included through bilateral or EU programs, within the rules of funding agencies.

3. A set of EuPRAXIA centres of excellence in plasma-based acceleration, in plasma theory and simulation, and in laser technology. They rely heavily on already-existing large infrastructures active in the field which provide the scientific and technical capabilities required for solving critical R&D challenges for the project in due time. At these facilities, mission-critical R&D, protoyping, testing, and construction tasks are performed, bringing in and upgrading their existing infrastructures.

4. One or two EuPRAXIA site(s) will host the construction of the beam-driven and laser-driven plasma accelerator facility(ies). At these construction site(s)
and possibly also at excellence centres, the consortium will set up several plasma-accelerator beamlines realising complementary technologies and applications and providing pilot access to academic and industrial users once target parameters have been reached.

5. An overarching and lean EuPRAXIA project management structure. It coordinates the overall technical design and prototyping, it oversees construction work, and it links to industrial partners through work at the collaborating institutes (funding passes through existing institutes and administrative structures).

The EuPRAXIA proposal foresees that scientific concepts, technical components, and assemblies are developed and prototyped in existing national facilities in France, Germany, Italy, Portugal, the UK and at ELI Beamlines, close to major universities and research institutions in Europe. This solution will ensure that the existing R&D infrastructure in Europe is fully exploited, it avoids expensive duplication of technical and administrative structures, it optimises the need for investment, and it provides access to a large pool of a technically educated and young work force, which is required for constructing such a facility.

EuPRAXIA proposes clusters of collaborating institutes that solve critical challenges in certain technical areas, including high peak-power laser technology.

The EuPRAXIA Consortium proposes clusters of institutes for addressing the critical challenges in our project. These clusters will bring together the partner institutes to address specific developments of interest for EuPRAXIA; the results will then be shared within the consortium and implemented at the excellence centres and eventually at construction site(s). This model provides a way to extend resources for a specific topic, and benefit the whole consortium. Important clusters proposed include the following:

1. cluster on theory and simulation: start-to-end simulations, performance predictions, evaluation of measured hardware performance, code benchmarking, development of new theoretical concepts, high-performance computing, new codes, and new approaches;

2. cluster on laser technology: R&D, testing, prototyping and construction of final hardware for laser drivers, laser diagnostics, optical components, laser transport lines, and kHz laser technology, all with a link to leading European laser expertise and industry;

3. cluster on plasma components and systems: R&D, testing, prototyping and construction of final hardware for plasma sources, plasma injectors, plasma-accelerator stages, plasma mirrors, and plasma lenses;

4. cluster on RF technology: R&D, testing, prototyping and construction of final hardware for RF drivers, RF injectors, low-level RF, and synchronisation;

5. cluster on magnets and other beamline components: R&D, testing, prototyping and construction of final hardware for magnets, beam transport and manipulation, collimation, and beam dumps;

6. cluster on diagnostics: R&D, testing, prototyping and construction of final hardware for electron diagnostics, positron diagnostics, photon diagnostics, and plasma diagnostics;

7. cluster on applications: R&D, testing, prototyping and construction of final hardware for positron sources, the free-electron laser, undulators, an inverse Compton source, a betatron source, test beam generation, HEP detector testing, and user end stations;

8. cluster on transformative innovation paths: R&D on novel concepts and techniques possibly suitable to implement at EuPRAXIA, including hybrid
accelerators, novel injection and radiation generation mechanisms, multi-pulse LWFA, and plasma-based metrology and diagnostics;

9. cluster on training, outreach, and dissemination: dissemination of results, out-reach, education, public engagement, knowledge transfer, training, and user workshops;

10. cluster on layout and implementation: testing, prototyping and, partially, con-struction of final hardware for the infrastructure of the construction site(s), including the control system, data acquisition, analysis and storage, safety sys-tems, cooling, and other basic support systems.

A more detailed description of these clusters is given in Section 3.3.1.

EuPRAXIA proposes five centres of excellence at existing facilities inside the EU, to be implemented and operated in close collaboration with the technical clusters.

Five EuPRAXIA centres of excellence are being proposed:

1. A EuPRAXIA laser and prototyping excellence centre, located on the Plateau de Saclay, close to Paris in France. A EuPRAXIA prototype laser will be installed by French EuPRAXIA institutes together with the laser industry to generate and accelerate electron beams to 1 GeV. High-quality feedback systems will be devised, acting on the parameters of the laser and the plasma structure developed in the French institutes. The Saclay prototype will be optimised to achieve the required stability goals for EuPRAXIA. In continuation of the COXINEL project in France, FEL quality beams will be demonstrated by generating FEL light. Both the plasma-accelerator technology and the FEL process will then be optimised for EuPRAXIA. In parallel, high-repetition-rate laser applications will be developed.

2. A EuPRAXIA excellence centre for plasma acceleration and high-repetition-rate developments located at DESY. For the past decade, DESY has been building up a program in plasma acceleration and is currently further expanding its capabilities to advance the science and technology of plasma accelerators. It is executing an ambitious development program in both electron 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 capabilities and research interests as an excellence centre for plasma acceleration and high-repetition-rate developments. The present and future DESY capabilities relevant for EuPRAXIA include a GeV electron beam driver at a MHz repetition rate at FLASHForward, high peak-power laser pulses (>100 TW) delivered at a kHz for an average power at the multi-kW level 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. Several of these capabilities are supported through the Helmholtz-wide ATHENA project on the laser-plasma accelerator R&D infrastructure in Germany.

3. A EuPRAXIA theory and simulation centre, located at the Instituto Superior Técnico in Lisbon, Portugal. The centre will coordinate EuPRAXIA work on theory and simulations that will be performed at the various partner institutes. Common standards and comparability of results will be ensured. An increase in simulation capabilities through smarter algorithms and access to supercomputers will be essential to establish a full operational model of EuPRAXIA performance, including imperfections, feedback systems, and user needs.

4. A EuPRAXIA advanced application beamline centre, located in the UK. The Central Laser Facility CLF (STFC Rutherford Appleton Laboratory), the CLARA
facility (STFC Daresbury Laboratory), and the SCAPA facility (University of Strathclyde), together with several UK universities, combine unique expertise in plasma accelerators, X-ray-based medical imaging, industrial applications, and schemes for positron production. The UK-based EuPRAXIA consortium will design and prototype EuPRAXIA application beamlines.

5. A EuPRAXIA incubator at a laser centre, located at ELI Beamlines in Prague, Czech Republic, for user aspects and for the transfer of plasma-accelerator technology into ELI. The EuPRAXIA incubator at ELI will provide input and guidance on key components for high-repetition-rate laser-driven FELs from the laser community to EuPRAXIA. In addition, it will provide ELI with a direct open innovation access to the accelerator technology developed inside the EuPRAXIA project, such that it can provide this to its users. Industrial technology transfer from and to the laser industry can pass through ELI but will also directly involve the clusters, excellence centres, and construction site(s) of EuPRAXIA.

It is at the core of EuPRAXIA that the excellence centres will be implemented and operated in close collaboration with the clusters. For example, the laser and prototyping facility will be defined, set up, and operated in collaboration with the cluster on laser technologies, thus ensuring a wide European share of knowledge and work. The concepts, technical designs, technical drawings of prototype devices, and results achieved at the excellence centres listed above and at the construction site(s) (see below) will be made available in the form of open innovation to the whole consortium. The facilities described above will serve as continued development and testing areas during the operation of EuPRAXIA, developing necessary improvements for various components or preparing facility upgrades.

EuPRAXIA proposes a staged approach for the construction of sites for two complementary technical approaches in plasma acceleration. This will ensure competition in technology instead of competition among institutes.

In parallel to and after the technical design and prototyping work, we foresee that the consortium will construct the joint EuPRAXIA plasma accelerator and user facilities. Considering constraints and interests in the collaborating institutes and countries, the conceptual design has converged on the construction of a site devoted to beam-driven acceleration. For the laser-driven construction site, a strong concept has been developed at this stage, and multiple candidate sites have been identified, based on the varied research landscape for laser-driven plasma acceleration already existing across Europe.

1. The EuPRAXIA facility for beam-driven plasma acceleration is proposed to be constructed in Frascati, Italy, and is ready to proceed. The host lab is INFN-LNF, and the electron beam driver will rely on the most compact RF technology available, namely, X-band structures developed at CERN. The Frascati site of EuPRAXIA will build on the investments in beam-driven plasma acceleration at SPARC_LAB. This proposal also reflects on the Italian interest in an FEL user facility that combines a 1 GeV RF-based FEL option with a plasma-based advanced FEL setup at possibly higher energy. EuPRAXIA@SPARC_LAB would be the first FEL on the Frascati site. The dual approach will ensure that a new FEL user community at Frascati can be served with maximum availability and particle flux. User applications for EuPRAXIA@SPARC_LAB will focus on a 1 GeV free-electron laser with an upgrade to 2–5 GeV, an inverse Compton scattering photon source, high-energy positron beams, and test beams.

2. The EuPRAXIA facility for the laser-driven plasma accelerator could be constructed at an excellence centre or at one of the several options described in Section 3.6.7. Laser pulses will be provided from lasers developed in collaboration
with industry and scientific institutes for the EuPRAXIA needs. The laser-driven EuPRAXIA site aims at the most compact accelerator solution for future free-electron lasers, complementing the existing big science FELs at various large research centres in Europe. User applications for EuPRAXIA will focus on a compact 1–5 GeV free-electron laser, plasma-based medical imaging, a compact positron source for material science applications, and highly compact test beams.

The complementary approaches followed in the two construction sites induce some level of com- petition in technology that will help speed up progress. At the same time, the two approaches offer different benefits and application reach. The two legs of EuPRAXIA will therefore have an excellent and long-term sustainability in tech- nology development and user markets.

The combination of clusters, excellence centres, and construction site(s) will enable a truly European project that integrates the European R&D landscape in this research field and binds together existing facilities and investments. This approach allows the fastest progress in a situation of strong competition throughout the world. It is therefore proposed in the EuPRAXIA CDR and will be refined in the next project phases.

It is a crucial aspect of the EuPRAXIA concept that the implementation model described above is coordinated and governed in the common EuPRAXIA project that we propose. The overall EuPRAXIA management will match tasks across all partner laboratories with available expertise, interests, and capacity. It will ensure that common components will be used in the construction site(s) and the connected facilities, wherever possible. The EuPRAXIA project will thus prevent the duplica- tion of work and loss of resources from internal competition. The proposed EU-wide coordinated project will produce critical mass, synergy, and added value, positioning the EuPRAXIA Consortium as a highly competitive and success-enabled player in the worldwide competition for a new accelerator technology.

## Added Value for EuPRAXIA Participants

EuPRAXIA partners participate in the knowledge creation process, gain in institutional capabilities, share EuPRAXIA intellectual property, and transfer innovations back into their nations.

The participants in the EuPRAXIA project will profit from a number of benefits for their work and their countries:

- Participation in the knowledge creation process that will be fostered by EuPRAXIA with its interdisciplinary and international project team ("attracting and educating the brains for our future").

- Gain in capabilities in the participant's laboratory from project tasks performed and from additional resources procured via the EuPRAXIA project.

- Access to EuPRAXIA infrastructures and expertise across the whole project, profiting from larger critical mass and enabling faster progress in R&D activities.

- Institutional share in intellectual property that is generated in the open innovation format of EuPRAXIA.

- Possibility to transfer compact applications to the participant's country, users, and industry.

- Bringing the competitive advantage of the EuPRAXIA collaboration to the participant's institute and country in a world-wide race for high technology leadership as well as modern industries and applications.
- Supporting accelerator development using laser plasma technology, bringing together the accelerator community interested in beam quality and the laser plasma physicists interested in high-level advances in laser science.

## Preliminary Cost Estimate

EuPRAXIA is, at full scale, a 320 million euro investment into European compact accelerator innovation and scientific applications, including an 83 million euro investment into laser technology. Depending on budget availability, minimal scenarios include options for a 68 million euro beam-driven and a 75 million euro laser-driven accelerator site.

The EuPRAXIA concept relies heavily on existing R&D facilities at major European laboratories. The institutes operating there today will ensure the availability of the required administrative structures and basic infrastructures. EuPRAXIA can therefore be highly cost-effective and focus on innovation and leadership in technology and science. Using existing large laboratories as hosts guarantees the sustainability of the EuPRAXIA centres and site(s) and opens the possibility of co-funding.

Detailed discussions on funding provisions for the full EuPRAXIA project will only take place after the completion of the Conceptual Design Report (CDR) in October 2019. In Section 4, the CDR provides a preliminary cost estimate for the EuPRAXIA research infrastructure, the required technical design and prototyping. Options start at a cost of 68 million euro for a beam-driven and 75 million euro for a laser-driven EuPRAXIA implementation (minimal systems). It is estimated that the full-scale EuPRAXIA facility with two construction sites requires an investment of 320 million euro and about 1,800 FTE person power integrated over a 8–10 year project duration (technical design, prototyping, and construction). The 320 million euro cost includes about 83 million euro that is invested in laser technology with industry and will be leveraged by co-funding from the EuPRAXIA partners. The full-scale EuPRAXIA implementation is a truly European project that realises the competitive advantage, scientific impact, and societal benefits described above.

The full-scale EuPRAXIA project is a strategic investment into accelerator innovation, laser industry, the technical education of young generations, and European collaboration.

It is explicitly acknowledged that the development of a EuPRAXIA funding scenario will require detailed discussions with decision makers at the national and EU levels. Only the support and injection of significant funding from the European Commission can enable an open innovation infrastructure project like EuPRAXIA, a project that is focused on capital-intensive high technology and operates across Europe.

The interim funding from the completion of the conceptual design report in October 2019 to the possible start of (not yet defined) Horizon Europe funding for the technical design at the end of 2022 is an issue of critical importance. The shift of the next roadmap exercise of the European Strategy Forum for Research Infrastructures (ESFRI) by one year creates a likely gap in funding that should be urgently addressed. A funding of 4 million euro would be sufficient to keep the EuPRAXIA collaboration active and advance preparatory work on concepts and implementation details. Additional funding at the level of a former “Joint Research Activity” (JRA, 15 million euro) would allow the start of urgent R&D work and the performance of critical feasibility demonstrations.
## Schedule

EuPRAXIA is a project that requires a technical design phase of four to six years and a construction phase of four years.

The definition of the EuPRAXIA project phases and the correspondence with European decision processes (such as the ESFRI roadmap) are discussed in Section 5.1. We note that the international race towards compact particle accelerators with applications and progress in back-bone technologies (e.g. high-power lasers from industry) requires a fast decision process. Fast decisions will make it possible to retain the competitive edge. Once funding is available, the EuPRAXIA R&D can start swiftly, based on the work done in the EuPRAXIA conceptual design study and the reliance on existing research infrastructures. It is acknowledged that the EuPRAXIA project is a highly innovative project with a number of risks in technical challenges and project setup. Therefore, a decision process will need to move in a stepwise fashion, from the approval of the technical design phase to a project readiness review and finally the approval for construction.

Proceeding at full speed, we estimate that the EuPRAXIA research infrastructure would start full operation in 8–10 years. Parts of EuPRAXIA could go into operation at significantly earlier times.

## Risk Reduction Roadmap

EuPRAXIA is a highly innovative project and, as such, has areas of technical and managerial risk. A risk reduction roadmap will ensure project success.

The EuPRAXIA project team has identified major risks in various important areas:

1. Laser technology, hardware, and transport might face unforeseen technical problems.

2. The readiness of a user application could be delayed, or user demand might be lower than anticipated initially.

3. One of the novel EuPRAXIA schemes might not work as predicted.

4. Certain EuPRAXIA simulation results might be too optimistic given unavoidable imperfections, new physics processes, and computational limitations.

5. A partner with a relevant commitment for EuPRAXIA might withdraw from the project given funding constraints.

6. A strategic industrial partner might change priorities.

These are just a few major examples. More can be found in Chapter 5.7. Based on these challenges, the EuPRAXIA Consortium has defined a risk reduction roadmap including the following measures:

1. In its CDR, EuPRAXIA relies as much as possible on proven concepts and solutions, requiring challenging but realistic extensions in technology. For example, the baseline EuPRAXIA laser technology is based on an extrapolation of the presently available industrial solutions. Also, the envisaged beam energy in plasma acceleration has been demonstrated. Higher risk solutions have been defined as backup and development paths.

2. Full-scale prototypes of critical components will be tested at EuPRAXIA excellence site(s) before the final construction decision. Examples include the high-power laser, the beamlines, the undulator and FEL performance at 1 GeV, and the plasma targets.
3. The EuPRAXIA concept foresees full and very accurate diagnostics of the laser, the plasma, and the electron beams. Optical control elements and electron beam correctors have been integrated or foreseen to counter performance limitations.

4. Backup options have been defined for critical schemes and can be tested in a timely matter, if needed. Examples include various types of electron sources, various plasma schemes, options for diagnostics, and different transfer line designs.

5. The approach of a distributed construction model guarantees that the consortium can continue operation even if a partner would withdraw from the collaboration.

6. EuPRAXIA has already, in its CDR phase, connected to several companies and will continue relying on multiple industrial partners. This will minimise risks in case of changing priorities in a connected company.

7. EuPRAXIA offers opportunities for outstanding science, innovation, and multiple user applications at excellence centres and site(s). This offers risk mitigation in the case that one science or user application is delayed because of problems in technical parameters.

8. EuPRAXIA has reached out to the user communities in particle physics, photon science, health, and industry to better integrate their insights and needs into the facility design. This effort will be continued.

We are convinced that the combination of a highly innovative EuPRAXIA concept, advanced technologies, new ideas and risk-mitigation strategies will ensure a fully successful project.
# Part 3
## The Research Infrastructure Concept

<figure><img src="image_2.png" /><figcaption>Fig. 1.1. Partners and associated partners of the EuPRAXIA Consortium.</figcaption></figure>

# 1 Introduction

## The EuPRAXIA Consortium

The EuPRAXIA Consortium is a collaboration of 41 institutions across 14 countries (as of November 2018) in Europe and beyond, as shown in Figure 1.1. It consists of 16 full members as well as 25 associated partners, which joined the collaboration after the beginning of the design study and hence have added to the project largely through in-kind contributions.

The EuPRAXIA collaboration came together in a unique attempt to combine interdisciplinary expertise from across all areas relevant for the construction of a novel plasma-based accelerator facility. As such, EuPRAXIA comprises not only international organisations and national laboratories but also research institutes and universities. Its partners have demonstrated excellence in accelerator science and plasma physics as well as laser technology, photon science, and high-energy physics. Finally, through its Scientific Advisory Committee (SAC) as well as more informal partnerships, the EuPRAXIA Consortium is also connected to industry, such as the world-leading laser companies Amplitude, Thales, and Trumpf Scientific, as well as other excellent research hubs like the European XFEL, Fermi, and ALBA light sources, CERN, and the Lawrence Livermore National Laboratory.

The contributions of the consortium partners to the EuPRAXIA conceptual design study have been coordinated through a set of fifteen technical work packages (WPs) with specific foci on the main sub-systems of the EuPRAXIA machine design. In this context, each work package is directed by a WP leader and co-leader, as Figure 1.2 shows, and includes representatives from around two to ten different partner institutes. Work Packages 9 to 15 are not part of the Horizon 2020 project funding; hence, all work carried out by these groups (and, to a large extent, presented in this report) has been entirely based on in-kind contributions.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.2.** Project management structure of the EuPRAXIA design study. The leaders and co-leaders of each work package are shown as well as the project coordinator and the head of the EuPRAXIA Collaboration Board. The Scientific Advisory Committee consists of (top row, from left) C. Biscari (ALBA), F. Falcoz (Amplitude Laser), L. Giannessi (Elettra/ENEA), E. Gschwendtner (CERN), (bottom row, from left) C. Haefner (LLNL), K. Michel (Trumpf Scientific), C. Simon-Boisson (Thales), and T. Tschentscher (European XFEL). The Management Support Team is comprised of (top row, from left) P.A. Walker, M. Weikum, (bottom row, from left) T. Minniberger, R. Rudolph, R. Torres, and A. Welsch.</figcaption></figure>
## State of the Field and Promise for the Future

Particle accelerators are widely used tools for industry, medicine, and science. Today there are around 30,000 particle accelerators in use worldwide, all relying on long-proven and highly developed methods for particle acceleration based on metallic radiofrequency (RF) devices [8]. The possible energy of particles, however, is often limited by practical boundaries on size and cost, for example the available space in hospitals, the available university funding for accelerator-based light sources, or the cost society as a whole can afford for science projects at the energy frontier. In past years, novel acceleration methods with plasma-based structures have demonstrated the acceleration of charged particles in a length reduced by a factor 100–1000. A recent success at Berkeley in the United States, for example, showed an electron beam being accelerated over a distance of 20 cm to 7.8 GeV with a 1 PW laser pulse [9]; in comparison, an RF-based machine requires several tens to hundreds of metres to reach this energy. Many accelerator applications exist at the energy range from 1 GeV to 8 GeV, which could benefit from more compact acceleration technologies. Moreover, the electron pulses from plasma accelerators are short (few femtoseconds), thus opening a groundbreaking route into novel applications for ultra-fast science.

The EuPRAXIA conceptual design study is relying on the impressive progress achieved at LBNL and other laboratories. EuPRAXIA is not aiming at advancing plasma acceleration towards higher beam energies. Instead, it investigates ways for achieving high-quality electron beams and for establishing attractive use cases. To accomplish these goals in a realistic, risk-mitigated manner, the project proposes to combine a mixture of novel concepts, proven phenomena, established technologies, and well-known, yet to be demonstrated solutions.

## Developments in Plasma-Accelerator Research

Plasma accelerators have developed in recent years as one of the most promising novel accelerator technologies capable of overcoming some of the limitations that conventional machines based on radiofrequency acceleration experience, such as limited accelerating field strengths from material breakdown effects and consequently large size and cost at high particle energies. By employing plasmas, i.e. ionised gas targets, instead of vacuum-filled metallic cavities as an accelerating medium, field gradients on the order of tens of gigavolts per metre – up to three orders of magnitude larger compared to RF-based technology – can be achieved. Consequently, plasma wakefield acceleration has the potential to significantly reduce acceleration distances and hence the dimensions and costs of future accelerator-based machines. Figure 1.4 highlights with examples from both RF accelerators and plasma acceleration experiments the order of magnitude of this reduction in acceleration length for various particle beam energies. At the same time, Figures 1.5 and 1.6 visualise the macroscopic and microscopic scales of a plasma-accelerator stage.

Although theoretically proposed already in the late 1970s by Tajima and Dawson [10], it was not until the early 21st century that plasma accelerators demonstrated the first promising experimental results as controllable high-energy electron sources [11–13]. Since then, a rapid development has been observed for both laser-driven (LWFA) and beam-driven (PWFA) plasma accelerators reaching, on the one hand, new energy records, while, on the other hand, significant milestones related to beam quality and applications were achieved. A recent version of the Livingston curve, for example, as shown in Figure 1.7, highlights well how this new technology approaches conventional accelerators in terms of energy still growing at an exponential rate.

Laser-driven plasma acceleration (LWFA) has been linked very closely with the development of laser technology over the last few decades, in particular the invention
<figure><img src="image_4.png" /><figcaption>**Fig. 1.3.** Participants in the EuroNNAc and EuPRAXIA Workshop on a European Plasma Accelerator in Pisa, June 2016. This EuPRAXIA collaboration meeting was organised as a combined event with the European Network for Novel Accelerators EuroNNAc [7].</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 1.4. Comparison of size and achievable electron beam energy for different existing radiofrequency and plasma accelerators. It becomes clear that a trade-off between machine size and beam quality at equivalent beam energies distinguished the two technologies to date (image credits: R. Assmann).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 1.5. Basic principle of a plasma accelerator. While the left image shows the macroscopic technical components, the schematic on the right highlights the physical process of plasma acceleration on a microscopic scale (image credits: DESY, H. Mueller-Elsner, R. Assmann).</figcaption></figure>

of chirped pulse amplification in 1985 [14], a technique that Donna Strickland and Gérard Mourou received the Nobel Prize in Physics for in 2018 [5]. In LWFA, a short, high-power laser pulse drives the accelerating plasma wakefield, while electron beams can be brought to high energies in this field after injection into the accelerator either from an external source or through generation inside the plasma (self-injection). With the latter, the more prominent method used in experiments in the past, a multitude of different self-injection mechanisms has been developed and tested over the last decade ([15–20] and others), with new ideas and concepts created even today (e.g. [21–23]). Beyond the generally very compact setup, self-injection thus promises flexible options for creating electron beams with varying properties. For external
<figure><img src="image_7.png" /><figcaption>**Fig. 1.6.** Small, novel plasma accelerator compared to the FLASH accelerator at DESY (Germany) (image credits: DESY, H. Mueller-Elsner).</figcaption></figure>

injection, on the other hand – a technique that is pursued for its improved control over the electron beam quality – theoretical and computational results also show great promise. Most experimental studies are still in preparation [24,25], with some first successful experiments studying beam capture efficiency and other major challenges [26].

Overall, research in LWFA has focused on a broad range of topics, as many small and larger institutes have worked relatively independently on their experimental setups or employed computational methods to study plasma acceleration. Major milestones thus include not only electron acceleration to a few GeV [9,27], but also successful experiments for high-charge beam acceleration [28] and the generation of short electron pulses in LWFA [29–31]. Over the last few years, research has also moved towards improving electron beam quality, one of the main challenges in plasma acceleration. In this context, for example, energy dechirping concepts to remove the typically large correlated energy spread have been developed [32–34], and techniques for preserving electron beam emittance throughout the plasma accelerator are being studied through plasma target shaping [35–37] and high-gradient focusing [38]. Improved diagnostics and plasma characterisation are further topics of interest with successful studies on measuring wakefields [39] and controlling plasma properties [40]. With a view towards reaching even higher beam energies, different concepts for staging multiple plasma accelerators [41,42] are also under development, including, for example, the study of new types of plasma targets that allow acceleration over long distances by keeping the drive laser focused [43,44].

In beam-driven plasma acceleration (PWFA), the community is generally smaller, with experimental infrastructures limited to a few major facilities, such as SLAC, SPARC_LAB, FLASHForward, and CERN. PWFA uses a relativistic particle beam as a plasma wakefield driver, typically an electron beam from an RF accelerator. This technique comes with some advantages, such as longer possible accelerating distances and hence higher energy gains, but also disadvantages, including the generally very large and expensive experimental setups required. As energy records were achieved
<figure><img src="image_8.png" /></figure>

**Fig. 1.7.** Livingston curve for accelerators, showing the maximum reach in beam energy versus time. The grey bands visualise accelerator applications. The left fork shows the progress in conventional accelerators from the first ideas in the 1920s. This main fork splits into two lines for electron/positron machines and for proton accelerators. A new fork of laser-driven plasma accelerators emerged in 1980, reaching multi-GeV energies by now. Beam-driven plasma acceleration results are indicated by the square point. Data beyond 2014 (vertical dashed line) indicate goals for the various technologies (image credits: R. Assmann).

quite early on [45], efforts in recent years have focused experimentally on improv-
ing beam quality and controlling beam properties [46,47], in some cases with similar
concepts as in LWFA, such as energy dechirping mechanisms [48]. Additionally, with
particular promise for high energy physics, efforts have been placed on improving
energy gains through improvements in transformer ratio [49]. With a similar goal in
mind, other types of particles have been investigated as wakefield drivers, such as pro-
ton beams (AWAKE) [50] and even positron beams (both as drivers [51] and witness
beams [52]). Finally, more unconventional solutions are also pursued, especially with
regard to solving the issue of size and cost of PWFAs. One particular path has been
hybrid acceleration, which uses an LWFA to generate a driver for a second PWFA.
In this context and more generally, recent years have looked at different possible
internal injection mechanisms in PWFA stages, such as density downramp injection,
wakefield-induced ionisation injection [53,54], and Trojan horse injection [55].

As will be shown in the later parts of this report, the EuPRAXIA design brings
together many of these developed techniques and combines them with new concepts
targeted towards specific remaining issues. The wide range of institutes involved in
the collaboration thus plays an essential role in providing the necessary expertise to
apply these various developments in a comprehensive and application-driven context,
moving from plasma accelerators as experiments towards plasma-based tools and user
machines.

Status of Flagship Free-Electron Laser Experiments

After the first demonstrations of undulator radiation generated by electron beams
from a laser-plasma accelerator [56–59], demonstrating free-electron laser (FEL)
amplification has become the next goal [60,61]. Theoretical studies [62–64] point out
that while the high brightness of an LWFA beam comprises a significant advantage
for FEL gain, the comparably high initial values of divergence and energy spread
that are typical for laser-plasma beams generated in the lab today, represent a major obstacle.

To address these issues, the community developed a twofold approach: in addition to the continuous efforts to improve the intrinsic quality of LWFA beams, new concepts have been developed to compensate the inferior characteristics of plasma-generated electron beams by re-distributing the electron phase space in the transport lines. These strategies include the stabilisation of the beam emittance and the spatial sorting of electrons by their energies and form the basis of the experimental projects of LWFA-based FELs currently under development.

Presently, several projects of LWFA-based FELs are either in the commissioning phase or have already started operation. In the framework of EuPRAXIA, the following collaborations are represented:

1. Collaboration of Synchrotron SOLEIL, Laboratoire d'Optique Appliquée (LOA), PhLAM of the University of Lille (all France), and the Weizmann Institute of Science (Israel).

2. Collaboration of the Center for Free-Electron Laser Science and University of Hamburg, Deutsches Elektronen-Synchrotron (both Germany), and ELI Beamlines (Czech Republic).

3. Collaboration of the Karlsruhe Institute of Technology, Helmholtz Institute Jena, and Friedrich-Schiller University in Jena (all Germany).

The experiment of Collaboration 1 is based at the Salle Jaune hall of LOA, where a 10-metre electron transport beamline, COXINEL, was designed and installed by SOLEIL. The laser-plasma accelerator uses the 60 TW, 800 nm, 30 fs Ti:Sapphire laser of LOA, and an additional laser beam of several mJ is provided to seed the FEL. The electron beam is handled by a series of permanent-magnet variable gradient quadrupoles designed for this experiment, followed by a decompressing magnetic chicane, electro-quadrupoles, and the 18 mm-period undulator U18 of SOLEIL. The chicane-based beam transport follows the strategy of the longitudinal sorting of the electrons by energy to enable FEL amplification for high beam energy spreads.

The UHH-DESY Collaboration 1 operates a dedicated laser-plasma accelerator beamline, *Lux*, driven by the *Angus* Ti:Sapphire laser system with a peak power of up to 200 TW. The beamline and the drive laser are fully integrated into the accelerator controls system available on the DESY campus. The laser beamline has been designed following the DESY machine vacuum standards. It is connectable to the conventional S-band linac machine *Regae*, and, for this, the interaction chamber is equipped with a dedicated differential pumping system for gas extraction, *Evoc*. The beam transport strategy is similar to the one of COXINEL, and it comprises several electro-quadrupoles, a decompressing magnetic chicane, and the 5 mm-period short cryogenic undulator *Frosty*.

The third project is developed by the KIT-FSU Collaboration 1 and is based at the 40 TW Jena laser facility *JETI-40*. The beamline is designed targeting an alternative strategy of beam energy-spread compensation. Here, a transverse gradient undulator (TGU) is used, where electron energies are sorted transversely via an achromat-like dogleg chicane. The TGU employed in the experiment is a 40-period superconducting iron-free undulator, fabricated with the coils of wounded Nb-Ti multifilament wires, and installed at the *JETI-40* facility inside a dedicated cryostat filled with liquid helium.

Both the COXINEL and *Lux* projects are now fully commissioned, and proper beam transport with first incoherent light was successfully demonstrated at these installations in 2017 [65,66]. At the present time, both projects are actively following scientific programs towards FEL amplification. The KIT-FSU project has, at the
<figure><img src="image_9.png" /><figcaption>**Fig. 1.8.** Schematic of the COXINEL experiment, including, from left to right, the laser hutch (grey), gas jet (cyan), removable permanent magnet quadrupoles (grey), magnetic chicane (dipole magnet in red) with a slit (brown) inserted in the middle of the chicane, electromagnetic quadrupoles (blue), undulator (purple), cavity beam position monitors (yellow), dipole dump (red), beam dump (grey), and CCD camera (black). Also shown are the measured (top) and simulated (bottom) electron beam profiles at different positions along the line (image reproduced from [65]).</figcaption></figure>

time of writing, successfully demonstrated LWFA beam transport, while the SCTGU device was thoroughly measured and tested at external facilities [67].

## Progress Towards Applications

Besides radiation generation using free-electron lasers, there exists a wide range of other applications typically implemented with the use of particle accelerators. First steps have been undertaken to apply the rapid progress in plasma acceleration to many of these, yet at the same time, new use cases have been identified for such a compact accelerator technology over the last few years.

One particular field of interest is the generation of a compact radiation source based on plasma acceleration, and different technologies have been studied in this context. Besides the use of undulators to generate synchrotron radiation from plasma-accelerated electron beams [56–59], a more compact method is based on betatron radiation. In the latter case, brilliant photon pulses are produced from the intrinsic transverse oscillations that electrons experience inside a plasma accelerator based on their strong radial focusing fields. The radiation properties are thus similar to synchrotron sources with photon energies in the keV range and a broad radiation bandwidth. Successful experiments have also shown that femtosecond-scale pulse duration and source sizes of a few micrometres are typical characteristics of these sources [60–73]. Betatron radiation has been demonstrated to be useful for tomographic and phase-contrast imaging [71, 74, 75]. Figure 1.9 depicts an example of such
<figure><img src="image_10.png" /><figcaption>**Fig. 1.9.** Tomographic imaging of a mouse embryo carried out at the Gemini laser at the Science and Technology Facilities Council's (STFC) Rutherford Appleton Laboratory. Single X-ray projections (A and B) and sagittal slices from 3D reconstruction (C and D) are shown, with A and C acquired with a betatron source, and B and D measured with a commercial microfocus scanner (image reproduced from [68]).</figcaption></figure>

an experiment, where a plasma-accelerator-based betatron source was shown to provide beam quality at least equivalent to commercial X-ray sources but with higher photon flux [68]. If such a source could be generated with a higher repetition rate than current plasma-accelerator experiments allow for, it would be extremely useful for fast and detailed medical imaging.

An additional kind of radiation generation that has been studied both theoretically and experimentally with plasma accelerators is inverse Compton or Thomson scattering. By scattering a laser pulse off an accelerated electron beam, this technique produces, depending on setup and laser-electron properties, photons with MeV energies and a quasi-monochromatic distribution [76–78]. With possible applications also in imaging, another interesting use case that has been investigated for this type of
source is the active interrogation of materials and nuclear characterisation [79]. In this context, plasma accelerators could bring possible future improvements to existing techniques thanks to their tunability and compact size.

In medical physics, plasma accelerators have been investigated for cancer treatment. While research in this field is still in the very early stages, using plasma-accelerated electron beams directly as both particle sources [80,81] and for the generation of X-rays is considered [82].

Finally, one of the potentially most promising application fields for compact accelerators is high-energy physics. With RF-based future collider designs reaching ever larger and more expensive scales, a plasma-based particle collider could mitigate these practical size and cost limitations. While a technical design of such a machine will still require considerable and long-term research efforts, first theoretical concepts have already been developed [83,84] and are complemented by many relevant accelerator-based R&D activities, such as plasma staging and multi-GeV acceleration experiments [41–43].

## Open vs. Closed Innovation

Although the term ‘open innovation’ has been originally developed in a business context, it has, in recent years, also found first applications in science [85,86]. A more general definition hence describes it as “open[ing] up the innovation process to all active players so that knowledge can circulate more freely and be transformed into products and services that create new markets” [85]. This stands in contrast to a closed innovation strategy, as traditionally followed by most companies, where the entire innovation and product development process is kept within the institution until a product’s release. All research & development (R&D), intellectual property, and technology is created only within the organisation and remains internal in such a system, at least partially as a historic consequence of the relatively little interaction between research institutions and companies in the past. Many of the arguments speaking for an open-innovation environment in a commercial setting are thus consequences of modern society; the increased mobility of highly educated people makes it hard to retain knowledge exclusively. Equally, the added availability of venture capital and similar concepts today allows to develop ideas and technologies outside of institutions through, for example, spin-offs or licensing agreements. Third, a stronger interconnectivity among institutions in a product or innovation process is required given the increasingly complicated systems and more global markets tackled by most companies [87].

All of these are also relevant topics in academic environments, however, where it is common for research staff to remain mobile throughout their career, for research to be carried out via collaborations of many institutions, and even schemes and programs designed to encourage knowledge exchange and the commercialisation of technologies are becoming more frequent (see, for example, [88] for initiatives in England in 2019/20). At the same time, knowledge-sharing practices are already being pushed strongly in research under the keyword of “open science”, with programs such as the European Open Science Cloud and European Data Infrastructure [89].

Driven by the ambition to act as a stepping stone for future compact accelerator technologies and applications available to new users and new locations, EuPRAXIA could thus provide an ideal platform to combine the more commercial and academic concepts of open innovation and science. As Figure 1.10 shows, there are two main levels of knowledge circulation foreseen within the EuPRAXIA infrastructure. In the first instance, a strong exchange of expertise within the community will be necessary based on the distributed infrastructure concept that EuPRAXIA foresees (more
<figure><img src="image_11.png" /><figcaption>Fig. 1.10. Schematic demonstrating the possible integration of the concept of open innovation in the EuPRAXIA research infrastructure [90].</figcaption></figure>

information in Chap. 3). Besides ensuring an optimised facility operation, this will be particularly beneficial for providing a means to reapply the technologies developed for the large-scale machines at EuPRAXIA directly back to the laboratory scale, thus advancing them in the direction of miniaturisation as a long-term goal. Second, the innovation process is opened up at EuPRAXIA further through the facility’s user access, as described in Section 5.4; by defining an access policy and providing an environment suitable for users from across the range of application development, it can provide a unique platform for these different groups to come together and exchange their knowledge, needs, and interests. Including trainees as future researchers, end users, and those working in the technology development stage between – here called co-developers – EuPRAXIA is able to act as a mediator not only providing a testing infrastructure for all but also indirectly helping to streamline the innovation process for plasma-accelerator technologies across all development stages. The long-term goals of the entire process lie directly in line with EuPRAXIA’s societal aims: to enable new technologies and applications, to support European industry in the development of new products, and to foster a new generation of experts.

## EuPRAXIA's Role in Europe

Europe is a strong player in accelerator science and especially novel accelerators, but much work in this topic is currently carried out by smaller institutes and universities working in their own laboratories and with their own individual resources. To make progress more efficiently and stay competitive worldwide, it is thus essential to bring the community together more strongly in a collaborative manner. The EuPRAXIA design study has been the first attempt in this direction, with much focus placed on comparing, combining, and optimising the various methods and technologies developed across the different project partners. A EuPRAXIA infrastructure would be the next step by providing a common platform for research with capabilities that are currently not available but are strongly needed for the development of the field.

While the next decade is expected to bring into operation several facilities with similar types of beamlines as EuPRAXIA – such as ELI Beamlines or the Extreme Photonics Applications Centre, recently funded in the UK – these infrastructures, foremost laser facilities in nature, will not be able to provide by themselves the same opportunities as a dedicated plasma-accelerator infrastructure, such as EuPRAXIA,
could. Focusing only partially on plasma acceleration and largely on laser-driven wakefield acceleration, there are significant differences in the scientific aims of these planned or developing facilities compared to EuPRAXIA to an extent that attempting to integrate the latter's goals into these other scientific programs would exceed their capabilities. At the same time, the areas of existing thematic overlap, such as plasma-based free-electron lasers and compact X-ray sources, are fields of significant user demand. For hard X-ray FELs in particular, operational facilities have shown a demand high enough to warrant more than one facility to push this concept, especially as there are expected to be variations in parameters and designs.

A possibly more important argument, however, is the unique synergy and potential for technological innovation that a dedicated common facility for plasma-accelerator technologies would provide. Through a design focused on various applications as well as varying techniques and methods, including both laser- and beam-driven plasma acceleration, a multitude of research avenues can be pursued in a very complementary manner. Setting up this design as a single yet geographically distributed infrastructure additionally provides a strong optimisation between cost-efficiency, which is much higher than for separate national facilities, and the use of existing infrastructures, which is much better than with a new single-site green-field structure. This last point is particularly essential in the design of EuPRAXIA as existing facilities with plasma-accelerator capabilities, such as ELI Beamlines and EPAC, but also smaller centres, such as CALA, SCAPA, and others, are all integrated into this infrastructure and thus allow a very effective exchange of knowledge and competencies beneficial to the development of the field as well as all partners.

Another aspect to consider is the potential future outlook of the project. EuPRAXIA is strongly focused on developing and providing tools for optimising applications, such as techniques for analytical sciences, which it shares with other infrastructures. However, simultaneously, it sets itself up to play a strong role as an intermediary step for future high-energy physics and plasma colliders by implementing important techniques such as external injection and staging, the integration of RF, laser and plasma technologies, and the possibilities for positron beam transport and acceleration experiments. While such milestones have or could be also achieved in smaller laboratories, their implementation in a user test facility opens entirely new perspectives with regard to stability, maintainability, and other operational challenges essential to high-energy accelerators.

Finally, concerning industry, EuPRAXIA will play an important role by guiding demand, thus pushing the European industry but also providing a platform to involve industry more strongly in novel accelerators.

# 2 Opportunities for Science and Innovation

## 2.1 Flagship Science and Innovation Goals

The EuPRAXIA Consortium has produced a conceptual design report of a new kind of European research infrastructure. During the proposed EuPRAXIA phases of technical design (TD), construction, and operation, the facility would achieve a number of important innovation and science goals. While overall ambitious, these aims and planned achievements are supported by the technical solutions presented in detail in the later chapters of this report. To reach the required beam quality for the proposed machine performance and application range, for example, new plasma injection and acceleration schemes have been developed (such as the Resonant Multi-Pulse Ionisation Injection scheme, described in Chap. 23), existing schemes have been improved (e.g. ionisation-induced injection in tailored density
profiles, described in Chap. 13) and experimentally validated (e.g. hybrid acceleration schemes, described in Chap. 26), new chirping and synchronisation concepts have been defined (described in Chaps. 15 and 23) and a laser system has been designed that not only advances main output parameters, but also incorporates high precision and stability (as described in Chap. 10). This is complemented by design approaches for the electron diagnostics (see Chap. 21) and beam transport systems (see Chap. 19) as other examples, which balance stable performance from well-known technologies with novel, more compact-sized solutions.

Building on these results in a step-wise approach with predicted intermediate milestones, it is foreseen that the innovation goals will be mainly reached during the technical design and construction phases, realising the improved technical components and building up the innovative new concepts and layouts that have been developed for this project. The innovation goals of EuPRAXIA have been defined with the consortium and interested European industry. The completed facility will then, during the operational phase, allow the achievement of science goals that will be pursued with both external and internal users from the European research community.

While the final realisation and exact process of implementation of the proposed EuPRAXIA facility will eventually depend on the available funding and political landscape, the following sections list the most important “flagship” goals foreseen for the project.

### 2.1.1 Flagship Innovation Goals

**Flagship Innovation Goal 1:** EuPRAXIA will develop the first beam-driven and laser-driven plasma-based accelerator facility. Accelerator know-how and methods are used for the design and construction of plasma accelerators. Sufficient investment, especially in laser technology, is provided such that the required new technical developments are performed and the plasma acceleration concept is developed into a usable accelerator technology.

**Flagship Innovation Goal 2:** EuPRAXIA will develop, together with the laser industry, a new generation of high peak-power lasers, advancing the presently leading technology into the regime of 20–100 Hz repetition rates for 100-Joule-class lasers and pulse durations of 50 femtoseconds. The extended repetition rates advance the state of the art by an order of magnitude and will be used for improved feedbacks for higher pulse-to-pulse stability.

**Flagship Innovation Goal 3:** EuPRAXIA will further develop, together with the European Organisation for Nuclear Research (CERN), RF technology at the X-band frequency (12 GHz) and will construct the world’s first 1 GeV X band linear accelerator, demonstrating the most compact electron linac to date, with an average accelerating gradient of 80 MV/m. This linac would be 30% shorter than available today for the same energy.

**Flagship Innovation Goal 4:** EuPRAXIA will construct the required technical schemes for achieving a much-improved quality of the electron beam from a plasma accelerator, leading to a 10-fold increase in the 6D phase space density at best. Predicted performances include 30 pC of charge at 5 GeV beam energy, sub-micron transverse emittances, and an energy spread of below 1% to 0.2% (total) and below 0.1% to 0.03% (for a short slice of the beam).

**Flagship Innovation Goal 5:** EuPRAXIA will demonstrate a factor 7 reduction in the total size for a 5 GeV electron beam facility without a subsequent user beamline. If the FEL undulator and user beamlines are included, the EuPRAXIA facility aims at demonstrating a factor 3 reduction in total size.
**Flagship Innovation Goal 6:** EuPRAXIA will establish a distribution scheme that optimises science return versus investment, minimising the number of expensive items (e.g. lasers, RF electron beams) while distributing less expensive items (e.g. plasma beam/radiation sources). The facility therefore combines the capabilities of relatively high-cost lasers (final pulses easily distributed to various close-by sites), of relatively low-cost plasma sources (up to GeV beams produced locally at end of laser beamlines) and of high-energy electron beams (drivers for plasmas, FEL generation of hard X-rays).

**Flagship Innovation Goal 7:** EuPRAXIA will establish a continuous mode of beam operation for the internal and external users over at least one week (24/7 operating mode) before a maintenance day may occur.

**Flagship Innovation Goal 8:** During its operational phase, EuPRAXIA will perform continuous innovation work to further reduce drastically the total facility footprint required for plasma-based user sources. The aim is to advance the reduction from a factor 6 to a factor 10 and beyond for the accelerator. This will involve, for example, innovative undulator technology, novel beam transfer and diagnostics schemes, and reduced-size optical elements.

**Flagship Innovation Goal 9:** During its operational phase EuPRAXIA will perform continuous innovation work to research and adopt novel laser technology that delivers high peak-power laser pulses at improved efficiency and a higher repetition rate, reaching into the kHz regime.

### 2.1.2 Flagship Science Goals

**Flagship Science Goal 1:** EuPRAXIA will deliver free-electron laser (FEL) X-rays with $10^9 - 10^{13}$ photons per pulse to user areas, covering wavelengths of 0.2 nm to 36 nm. The EuPRAXIA FEL pulses are naturally short (down to 0.4 fs) and will therefore provide users with tools for investigating processes and structures in ultra-fast photon science at a reduced facility footprint.

**Flagship Science Goal 2:** EuPRAXIA will deliver betatron X-rays with about $10^{10}$ photons per pulse, up to 100 Hz repetition rate, and an energy of 5-18 keV to users from the medical area. The much-reduced longitudinal length of the X-ray emission area (point-like emission) leads to an important improvement in image resolution compared to other techniques.

**Flagship Science Goal 3:** EuPRAXIA will deliver positron beams at energies from 0.5 MeV to 10 MeV and a repetition rate of 100 Hz for material science studies. Per pulse, about $10^6$ positrons will be produced in a duration of 20-90 picoseconds on the sample, allowing time-resolved studies. Here, EuPRAXIA will advance the capabilities of existing positron sources in flux and time resolution.

**Flagship Science Goal 4:** EuPRAXIA will deliver electron and positron beams at energies from a few 100 MeV up to 5 GeV for high-energy-physics-related R&D (detectors, linear collider topics). R&D goals include the demonstration of a linear collider stage, a "tabletop" HEP test beam, and studies on positron transport and acceleration towards a linear collider.

**Flagship Science Goal 5:** EuPRAXIA will deliver photons from an inverse Compton scattering (ICS) source. The photons of up to 600 MeV and with a narrow-band spectrum will enable precision nuclear physics and highly penetrative radiography for users.

**Flagship Science Goal 6:** EuPRAXIA will provide access to a multi-stage high-repetition-rate plasma accelerator in the GeV range to users from accelerator science. This R&D platform will allow the testing of novel ideas and concepts, the
<figure><img src="image_12.png" /><figcaption>**Fig. 2.1.** The length of a typical RF-based accelerator facility with 5 GeV electron beams (C-band technology including injectors) is compared to the length of the EuPRAXIA 5 GeV design (including injectors). It is noted that undulator sections, target areas, and user areas are not included. Therefore, the miniaturisation factor of EuPRAXIA for the full facility is significantly lower than the factor 6–7 shown here. In EuPRAXIA, we aim at a factor 3 improvement in total facility length.</figcaption></figure>

full optimisation of a plasma collider stage, certain fixed target experiments (also in combination with lasers), and performance studies of conventional versus novel accelerator technology.

**Flagship Science Goal 7:** EuPRAXIA will provide access to cutting-edge laser technology with short pulse length in combination with high-energy photon pulses and short electron/positron bunches. Novel schemes of pump probe configurations and ultra-precise timing will be researched, feeding back into laser science.

### 2.1.3 Unique Selling Points of EuPRAXIA

EuPRAXIA will, as a facility, be the first of its kind. It offers a number of unique selling points.

The technologies of plasma accelerators, electron beams, and high-power lasers enable some unique features and advantages that the EuPRAXIA infrastructure aims to realise:

**Demonstrating a compact facility design in a realistic stepwise approach:** EuPRAXIA exploits plasma, electron beam, and laser technologies to increase accelerating fields by two to three orders of magnitude. While accelerating lengths are reduced by this factor, the total footprint of an accelerator facility includes all the required infrastructure (laser room, shielding, etc.) and user facilities. A “tabletop accelerator” then quickly becomes a large setup with reduced benefits in size, if any. EuPRAXIA is based on a large-scale infrastructure design but reduced in size over conventional technology by a factor of 3 at least. It therefore pushes one of the biggest strengths of plasma-accelerator technology in a realistic stepwise approach, as Figure 2.1 highlights schematically. Once a factor-3 reduction in facility size has
<figure><img src="image_13.png" /><figcaption>**Fig. 2.2.** Size, frequency, and field strength comparison of different types of RF cavities. It can be seen that both the X-band and plasma technology proposed for EuPRAXIA offer not only smaller structure size but also a higher accelerating gradient, leading to an overall small accelerator for similar energy gain (image credits: the University of Strathclyde and [91]).</figcaption></figure>

been demonstrated, a miniaturisation process towards even more compact designs will be pursued inside EuPRAXIA. A reduction factor of 10 and even 20 for the accelerator itself seems feasible at high beam energy.

**Advancing compact, high-average-power, high-repetition-rate technologies for driving plasma wakefields:** EuPRAXIA includes both high-power lasers and RF technology in the X-band range as drivers (power sources) for exciting the plasma wakefields required for particle acceleration. For high-power lasers, a baseline with development paths has been defined, advancing repetition rates for petawatt-class lasers from 5 Hz today over 20 Hz to 100 Hz and ultimately even into the kHz regime (see Fig. 2.3). In parallel, critical performance parameters will be advanced, for example in terms of shot-to-shot stability and laser pulse quality. EuPRAXIA will pursue work with the laser industry in bringing this key technology to maturity and to the required level for next-generation plasma-accelerator systems. X-band RF technology will be developed with CERN as it is presently the most compact RF technology available for creating high-energy and high-power electron beams, as visualised in Figure 2.2.

**Developing compact user application capabilities of plasma-accelerator systems:** EuPRAXIA will develop user application capabilities of plasma-accelerator systems, including through the parallelisation of user lines, the generation of secondary particle and photon sources, and synchronised pump-probe capabilities. Laser-driven plasma accelerators offer unique possibilities for the parallelisation of user applications: the large laser infrastructure can be centrally located and laser pulses be distributed over 360 degrees into the small-size plasma accelerators and downstream user areas. The scheme of a circular synchrotron light source with dozens of user beamlines could effectively be miniaturised. This is illustrated in Figure 2.4.

**Advancing plasma-accelerator performance:** EuPRAXIA will advance plasma accelerators towards high beam quality, approaching that of conventional accelerators, so that they could then become suitable for existing accelerator applications and open up new application paths. Several novel ideas and concepts are described
<figure><img src="image_14.png" /><figcaption>**Fig. 2.3.** Comparison of different lasers and applications in terms of stored energy in a single laser pulse and repetition rate of the laser. The orange dots and areas indicate existing lasers, the blue areas target for various applications. The red/green dots and the white circle indicate the design goals for EuPRAXIA (reprinted from [92] with the permission of Elsevier).</figcaption></figure>

in the technical part of this CDR. The simulated progress in energy spread with EuPRAXIA is shown in Figure 2.5. It is seen that the EuPRAXIA conceptual design report describes a novel recipe on how to gain a factor 10 in performance and how to approach the beam quality in RF-based free-electron lasers. EuPRAXIA thus presents a design complementary with current FEL facilities – enabling additional sites with reduced peak brightness but compact size, the goal of improved cost efficiency, and promising development opportunities. The approach is also complementary with future collider projects for particle physics – facilitating research into novel, compact collider concepts with adequate beam quality.

**Increasing science performance reach with unique plasma-accelerator features:** Plasma acceleration and the related processes, like betatron radiation, happen necessarily in a small plasma volume given by basic physics processes. The length of the plasma accelerator ranges from a few tens of micrometres (the diameter of a human hair) to a few hundreds of micrometres. Electron beams from plasma accelerators are therefore necessarily short and have pulse durations in the few femtoseconds regime. As such, they are unique tools for investigating processes in ultra-fast science, with additional benefits from pump-probe capabilities enabled by the synchronised EuPRAXIA lasers. Considering betatron radiation in the plasma volume, important gains in resolution can be obtained because of the much more localised emission of X-rays.

**Developing a staging solution and positron sources for high-energy physics:** The EuPRAXIA concept relies in many aspects on plasma-accelerator stages. EuPRAXIA will therefore demonstrate the critical features of coupling in and out of a plasma in the presence of a drive pulse. Of particular importance in this context is the beam-driven construction site of EuPRAXIA, where a compact electron beam driver will power plasma-accelerator stages. At this site also, positron beam generation and acceleration methods will be pioneered and developed. EuPRAXIA will thus construct an essential stepping stone towards high-energy plasma-accelerator applications.
<figure><img src="image_15.png" /><figcaption>**Fig. 2.4.** Comparison of user concepts in (left) light sources relying on electron storage rings (without injectors) and in (right) the EuPRAXIA laser-driven plasma-accelerator concept. While on the left, the electron beam distribution requires a storage ring of large size, the EuPRAXIA laser distribution can be much more compact, with electrons, photons, and secondary particles being generated with highly compact plasma accelerators locally. The EuPRAXIA concept foresees conventional undulators but could work much more beautifully also with compact undulators that are being developed in the field. The parallel scheme will profit from high-repetition-rate lasers as many lines could be supported by the same laser. EuPRAXIA implements the first basic demonstration of this scheme with up to three beamlines in parallel.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 2.5.** Summary of the electron beam energy and relative energy spread for the different EuPRAXIA schemes (from start-to-end simulations). For comparison, the values from typical plasma-accelerator experiments, operational RF-based FEL facilities, as well as proposed future particle collider projects (CLIC, ILC) are also shown.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 2.6.** Scientific fields for which the EuPRAXIA research infrastructure would be interesting based on an initial survey of potential future EuPRAXIA users. The answers listed under "Others" are shown in Figure 2.7 in further detail.</figcaption></figure>

## 2.2 The Needs of External Users from the Academic Environment

An important aspect in the design of a new research infrastructure is the study of potential user communities to be able to match future user needs and interests with what the planned facility will offer in terms of experimental capabilities and capacity. This section of the CDR presents a qualitative analysis of potential application fields and their current status for EuPRAXIA. It will be complemented by a full quantitative user demand analysis in the future.

### 2.2.1 Summary of User Needs and Possibilities

The EuPRAXIA facility design, as proposed in this report, offers opportunities for a variety of different applications. Scientific fields where EuPRAXIA would be relevant include, in particular, accelerator and laser science, high-energy physics, material processing and analysis, photon science, as well as medicine and life sciences. In most of these areas, accelerators already play an important role; nonetheless, as the following sub-sections will demonstrate, EuPRAXIA aims not only to improve existing capabilities for users but also to open opportunities for new accelerator uses as part of these research directions.

Figure 2.6 demonstrates the significance of the topics above very well based on an initial survey of research groups interested in a future EuPRAXIA infrastructure. All of the fields listed are supported by at least 9% of survey participants. In Figure 2.7, the breadth of relevant subjects is further highlighted by the set of the most common keywords from the survey's responses given for "other" scientific fields; these range from "Laser Technology" and "Plasma Physics" to "Nanomaterials" and "Molecular Science", among others. Finally, Figure 2.8 shows that such a new facility as EuPRAXIA could be useful for a diverse set of activities with potential users interested in fundamental research as well as more applied studies and simple test experiments. Thirty groups with a total of more than 1,000 researchers were involved in the survey, demonstrating a clear interest in the European research community for a new facility, such as EuPRAXIA proposes. A larger scale inquiry and discussion with potential users is foreseen during the project's technical design phase to back up this initial analysis and ensure an optimal agreement between user needs and EuPRAXIA's facility design.
<figure><img src="image_18.png" /><figcaption>**Fig. 2.7.** Word cloud describing scientific fields and topics of interest relevant to potential future users of the EuPRAXIA research infrastructure. These keywords are directly extracted from an initial survey of organisations interested in EuPRAXIA.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Fig. 2.8.** Types of activities for which the EuPRAXIA research infrastructure would be interesting based on an initial survey of potential future EuPRAXIA users.</figcaption></figure>

Based on this first assessment, a number of flagship applications have been defined for which EuPRAXIA can offer particular benefits to users. While the opportunities for these different user communities are described in more detail in the following six sections, a general summary of the advantages of EuPRAXIA in each case can be given by three possible arguments:

1. **Machine performance:** The proposed EuPRAXIA research infrastructure may provide better machine performance than other existing facilities or solutions. This would be the case for medical X-ray imaging, for example, where EuPRAXIA's compact betatron source would be a technology prototype with improved resolution and photon yield compared to standard X-ray sources.
2. **Machine availability:** The proposed EuPRAXIA research infrastructure may improve the overall availability of certain types of beams and user services. An example for this lies in hard X-ray free-electron laser science, where user beam time is extremely competitive. The future EuPRAXIA facility would not only increase the capacity for FEL experiments but also build a roadmap towards future compact and cost-efficient FEL beamlines.

3. **New features:** The proposed EuPRAXIA research infrastructure may provide machine features and beam properties different from existing facilities. The use of plasma-accelerator technology at EuPRAXIA leads to a series of unique characteristics, such as ultra-short particle and photon beam durations, small beam spot sizes, a small machine footprint, as well as multi-species sources. These could be used, for example, for improved performance in imaging experiments but would also be relevant for the testing of advanced detector systems, both in accelerator science and high-energy physics.

Beyond these well-defined flagship application fields, EuPRAXIA aims to remain open to other types of user experiments. With plasma acceleration as a relatively novel and tunable technology, it can be envisioned easily that entirely new application areas may develop over time, once the facility has come into operation and gained experience with first users.

EuPRAXIA users could come from a range of application fields. They would benefit from improved machine performance, better machine availability, or unique beam properties compared to existing facilities.

### 2.2.2 Accelerator Science

Particle accelerators have been developed and optimised over multiple decades, creating a variety of machine types, components, and technologies, from RF accelerators to novel acceleration schemes, such as plasma and dielectric acceleration (e.g. [93,94]). While, as such, it is a highly developed field, there are still many groups worldwide that investigate and develop new concepts and methods within accelerator science.

Some of these studies, especially on a smaller scale, can be performed at local facilities. For other, especially more complex experiments, however, dedicated accelerator test facilities are required, of which there are a handful in operation currently, as Table 2.1 shows. The parameter overview in the table shows well the variety of available test beams, as is necessary considering the diversity of accelerator-based systems and machines. However, at the same time, it demonstrates clearly that until now, there is a lack of machines producing ultra-short electron beams, a feature that is becoming more and more useful for light-source-related and novel acceleration technologies, for example. This shortage is partially covered by some of the facilities due to come online in the next years, such as the HELL beamline at ELI Beamlines [95], FACET-II [96,97], and SINBAD (with the ARES linac) [98]. EuPRAXIA will thus be another addition to this group of test machines with ultra-short beams, however, bringing further beam properties of interest with it, such as, among others, an increased repetition rate on the scale of 100 Hz (with a possible upgrade to kHz in the long run). For plasma accelerators in particular, this will represent a new regime, thus opening up multiple new research opportunities.

More generally, the EuPRAXIA infrastructure is designed to implement many different techniques and components into one machine, including plasma injectors with different beam energies, multiple staged plasma targets, RF injectors for external injection into a plasma, as well as advanced beam transport lines and diagnostics sections. This makes it uniquely suited for accelerator science experiments, especially
<table><caption>**Table 2.1.** Summary of the electron beam properties of several accelerator test facilities currently in operation or under development. To provide a reasonable comparison with the performance proposed for EuPRAXIA, only infrastructures with beam energies of at least 100 MeV and allowing external users for experiments are considered.</caption><thead><tr><th></th><th>E [GeV]</th><th>ΔE/E [%]</th><th>Q [pC]</th><th>σ<sub>τ</sub> [fs]</th><th>ε<sub>n</sub> [mm mrad]</th><th>f [Hz]</th></tr></thead><tbody><tr><td colspan="7"><strong>Operational</strong></td></tr><tr><td>FLASH Forward [99]</td><td>0.4-1.25</td><td></td><td>50-800</td><td>50-6,000</td><td>1-3</td><td>4 × 10<sup>4</sup>-3 × 10<sup>6</sup></td></tr><tr><td>SPARC LAB [100]</td><td>0.03-0.15</td><td>0.1-0.2</td><td>20-1,000</td><td>1 × 10<sup>4</sup>-2 × 10<sup>4</sup></td><td>1-5</td><td>10</td></tr><tr><td>CLEAR (CERN) [101]</td><td>0.06-0.22</td><td>&lt;0.2</td><td>10-500</td><td>1.67 × 10<sup>3</sup>-8 × 10<sup>3</sup></td><td>3-20</td><td>1-25</td></tr><tr><td colspan="7"><strong>Planned / under development / commissioning</strong></td></tr><tr><td>FACET II [96,97]</td><td>10</td><td>0.4-1.8</td><td>500-3,000</td><td>3.3-333</td><td></td><td>1-30</td></tr><tr><td>SINBAD-ARES [98]</td><td>0.1</td><td></td><td>0.5-200</td><td>0.8-10</td><td>&lt;0.5</td><td>50</td></tr><tr><td>ELI-Beamlines (HELL) [95]</td><td>0.1-5</td><td>0.1-10</td><td>10-1 × 10<sup>5</sup></td><td>1-10</td><td></td><td>10</td></tr><tr><td>EuPRAXIA</td><td>0.1-5.9</td><td>0.1-4</td><td>20-100</td><td>0.8-12</td><td>0.1-1.5</td><td>20-100</td></tr></tbody></table>

for topics related to novel accelerator technologies. Plasma acceleration would likely be the largest user community with, for example, possible proof-of-principle experiments on new plasma injection / acceleration concepts, the prototyping or stability testing of plasma targets and components, as well as R&D into specific accelerator setups. While other sites, especially high-power laser infrastructures, exist in Europe and worldwide where plasma-accelerator studies can be carried out, EuPRAXIA will be, at least in its beginning, the only dedicated plasma-accelerator user facility. This means that, on the one hand, the machine and its properties are all designed specifically for the user operation of plasma accelerators, with very well-characterised beams and components as well as unprecedented system stability. On the other hand, the machine also already includes multiple dedicated and stable plasma-accelerator stages; thus, the acceleration process and generation of electrons, unlike in other facilities, does not have to be part of the experiment, and instead, the experimental focus can be placed on more specific challenges.

There are, however, also other topics in accelerator science that could benefit from a facility such as EuPRAXIA. The development and testing of novel diagnostics, for example, is one such case. The tunability in beam parameters, the availability of various photon and particle beam types, and the overall single-femtosecond duration of these pulses are unique and, as such, will allow for a multitude of varied experiments. Similarly, other types of beamline elements and beam transport concepts could be investigated. Compact accelerator applications in high-energy physics, for example, are discussed as a particular topic of interest [83,84], which will require considerable R&D on the transport and manipulation of beams from such machines. EuPRAXIA would be well suited for these activities.

### 2.2.3 Laser Science

The request for sufficient average particle or photon flux for the envisioned applications of EuPRAXIA will require a rather high repetition rate for the EuPRAXIA laser systems on the order of 20 to 100 Hz. This means that, as a matter of fact, the EuPRAXIA lasers will feature, besides very high peak power, an unprecedented average power. These features pose interesting challenges and, at the same time, will make EuPRAXIA a unique platform for testing novel concepts in the field of ultra-short and ultra-intense laser science.

Research aimed at increasing the average power of ultra-intense lasers, in particular, would greatly benefit from the design and construction of the facility. As it is
<figure><img src="image_20.png" /><figcaption>**Fig. 2.9.** 3D visualisation of a plasma wave (blue) excited by a high-intensity laser pulse (red) and accelerating an electron beam generated with the VisualPIC code [102]. There is much potential for exploring new concepts and technologies in both plasma acceleration and RF accelerators as well as the combination of these two (image credits: A. Ferran Pousa, DESY / University of Hamburg).</figcaption></figure>

well known, this is a very active research field since increasing the repetition rate of ultra-short laser systems is seen as a crucial issue for translating laser-driven accelerator technologies to fields as diverse as medicine, material science, and so on, even on a smaller scale than the EuPRAXIA facility, thus allowing a widespread diffusion of advanced, possibly all-optical particle and secondary sources. In this respect, a major research topic is the thermal management in pump and main lasers amplifiers. Substantial improvements beyond the current state-of-the-art architectures are needed to increase the overall laser system efficiency and thus allow their repetition rate and average power to be increased (up to the multi-kW or more range). A lot of efforts are made already toward this goal – an example being the HAPLS laser system, as seen in Figure 2.10 – with several approaches, each at a different stage of maturity. The project would thus certainly give a strong boost to this research field.

Additionally, research related to the transition to diode pumping would benefit from the project. More in perspective, all novel approaches towards an ever-increasing efficiency – such as, for instance, direct pumping, multi-pulse extraction, and so on – would come into play. With this in mind, the possible transition from TiSa-based architectures to new laser materials is of strong interest for the laser community and could benefit significantly from developments at EuPRAXIA.

Besides the energy efficiency issue, other aspects related to thermal management in the laser chain are expected to become relevant for high average and peak-power lasers such as those foreseen for EuPRAXIA. These will require substantial efforts in laser science and technology. As an example, the research in the field of novel grating materials/coatings able to withstand the expected average power without introducing strong wavefront aberrations can be cited.

A rather peculiar aspect of the EuPRAXIA laser systems will be the constraints on their long-term operation; indeed, the combined requirements on the repetition rate and the system availability (uptime) will need new solutions for optical coatings with longer lifetime. Incidentally, the high degree of complexity of the optical architecture of the laser systems coupled with their required reliability in the long term will speed up the availability of advanced technological solutions for the automatic control, alignment, and maintenance procedures of ultra-short laser chains, which would benefit the entire laser community.

One of the critical issues for the EuPRAXIA laser systems given the usage of special target designs and the need for a stable and reliable operation of the beamlines
<figure><img src="image_21.png" /><figcaption>**Fig. 2.10.** HAPLS laser at Lawrence Livermore National Laboratory. Now installed at the ELI Beamlines facility, this laser system is among the current state of the art for petawatt-scale laser systems (image credits: Lawrence Livermore National Laboratory).</figcaption></figure>

for the users will be the pointing stability. Besides already consolidated measures, the search for novel, possibly active techniques will be of interest for laser science.

Another very critical issue for the successful operation of the EuPRAXIA lasers will be the active correction of wavefront distortions, so as to allow a high Strehl ratio to be achieved. Research on both wavefront characterisation techniques and devices and feedback loop algorithms suitable for high-repetition-rate operations would be stimulated.

Finally, we mention here the role that the original design, construction, and further development of the EuPRAXIA lasers may play in the research field of longitudinal diagnostics for ultra-short and ultra-intense pulses. The development of both diagnostics for measuring spectral amplitude and phase as well as diagnostics for the characterisation of pulse contrast could benefit. This is true, in particular, in view of the need for single-shot diagnostics, as well as fast reconstruction algorithms to possibly provide a full shot-by-shot laser pulse characterisation for high-repetition-rate lasers in the future.

### 2.2.4 Photon Science

The investigation of matter using short-wavelength light sources has benefited from recent developments with the advent of various VUV-X-ray sources. Among them, the X-ray laser [103] as well as high-order harmonic generation in gas (HHG) [104,105] reaching attosecond duration [106] or on solid targets [107] take advantage of the light emission properties of matter. Accelerator-based light sources [108–110] are also valuable tools in this respect.

Third-generation synchrotron light sources rely on synchrotron radiation generated from charged particles in bending magnets or undulators, creating a periodic permanent magnetic field. They are delivering high-brilliance photon beams with partial transverse coherence. These light sources, delivering beams simultaneously to various beamlines, are workhorses for matter investigation. They presently evolve towards low-emittance (picometer-scale) diffraction-limited light sources (DLSR) thanks to
<figure><img src="image_22.png" /><figcaption>**Fig. 2.11.** Diffraction image of a complex biomolecule under high-intensity X-ray light (image credits: E. Reimann, DESY).</figcaption></figure>

multibend achromat optics requiring high-gradient quadrupoles. The DLSR high bril-
liance and degree of transverse coherence set a new paradigm and give access to serial
crystallography for drug discoveries (pharmaceutical industry) and to new techniques
(imaging, spot scanning, small focal spot spectroscopies) for material development,
addressing the 21st century societal challenges of health, environment, energy, infor-
mation technology, and cultural heritage.

Longitudinal coherence, on the other hand, is achieved in free-electron lasers
[111] by setting in phase the electrons thanks to an energy exchange between the
electrons and a light wave (the spontaneous emission or an external seed), resulting
in bunching, coherent emission, and light amplification. Several decades after the
laser invention [112,113] and the first free-electron laser (FEL) [114], tuneable high-
power X-ray lasers became a reality all over the world. FEL-based fourth-generation
light sources presently offer femtosecond tuneable radiation in the X-ray [115–119]
and in the VUV to soft X-ray [120–122] domains. These tuneable devices open the
path for deciphering unexplored areas of matter and cells (such as protein structure
and function) in a time-resolved way. An example of this is shown schematically in
Figure 2.11. The League of European Accelerator-based Photon Sources (LEAPS),
consisting of 19 research infrastructures in 10 states with 220 beamlines alone, has
served more than 24,000 direct users during the last 5 years.

The recent advent of tuneable coherent X-ray FELs (XFELs) [123,124] opened
a new era for the investigation of matter [125]. They enable us (1) to decrypt the
structure of biomolecules and cells [126–128], (2) to provide novel insight into the elec-
tronic structure of atoms and molecules [129–132], and (3) to observe non-equilibrium
nuclear motion, disordered media, and distorted crystal lattices thanks to progress
in femtosecond spectroscopy [133] and pump-probe techniques [134]. Detailed struc-
tural dynamics can be inferred from spectroscopic signatures [135]. XFELs can also
reveal movies of chemical reactions. With new imaging techniques [136,137], they
are exceptional tools for the investigation of the ultra-fast evolution of the electronic
structure in atoms and provide a deeper insight into the extreme states of matter
[138].

Another revolution in the field could now arrive with the emergence of laser-
wakefield-accelerator-based free-electron lasers [60,61]. These could provide, on the
one hand, compact “laboratory-scale” short-wavelength undulator radiation sources
or, on the other hand, new FELs available for the user community. After the first
observations of LWFA-based undulator radiation [56–59,63,65], ultra-short VUV to
X-ray compact undulator sources can be seen as attractive for the user community
[66,139]. Achieving a plasma-accelerator-based FEL is more demanding in terms of
electron beam quality, especially in terms of divergence – requiring strong focus-
ing with, for instance, variable permanent magnet quadrupoles [140] – and energy
spread – with possible electron sorting with a magnetic chicane [62,141] or a trans-
verse gradient undulator [64]. With the expected improvements of plasma-based elec-
tron beam quality, however, the realisation of plasma-accelerator-based FELs could
become possible and open up exciting, new opportunities for the ultra-short photon
science community.

2.2.5 High-Energy Physics

Currently, the largest operational particle collider worldwide is the Large Hadron Collider, a 27 km ring of superconducting magnets at CERN. At its best performance, it provides proton–proton collisions with a centre-of-mass energy of 13 TeV using detectors, such as the one seen in Figure 2.12 to measure the particle interactions occurring in the process. Before that, the Large Electron–Positron Collider (LEP) created electron–positron collisions with a maximum energy of 209 GeV. The Large Hadron Collider’s last most iconic result was the detection of the Higgs boson and the determination of its mass. This represented a brilliant confirmation of the Standard Model (SM) that, however, is known to be incomplete. It does not account for important aspects of fundamental physics observed in cosmology, including dark matter, dark energy, and the excess of matter over antimatter. Additionally, important features of the model, including the spectrum of quark and lepton masses and the presence of a phase transition that breaks its gauge symmetry, are put in by hand rather than being explained from physics principles [142]. To date, the extensive search for additional particles that would generalise the SM has been in vain, and there is a growing need in the community for the next generation of electron–positron colliders that would reach and eventually surpass the TeV barrier.

Conventional RF technology has intrinsic limitations on the maximum acceler-
ating gradients that it can sustain, of the order of tens of MV/m. As an exam-
ple, the proposed Compact Linear Collider (CLIC) is planning to reach 100 MV/m,
implying an overall accelerating length to reach 1 TeV of 100 km. The sheer scale
and, subsequently, cost of these machines have motivated the quest for alternative
accelerating technologies. Arguably, plasma-based wakefield acceleration is one of
the most promising, with landmark results already experimentally obtained for elec-
trons. Accelerating fields of up to 100 GV/m have been demonstrated in a plasma
[143]. Recent promising results in this area include, for instance, the demonstration
of energy doubling of a 42 GeV electron beam in less than one metre of plasma [45], a
2 GeV energy gain of a positron beam in one metre of plasma [52], and the laser-driven
acceleration of electrons up to 8 GeV in only 20 cm of plasma [9]. These promising
results have motivated international large-scale projects to study the feasibility of
building a plasma-based electron–positron collider. For instance, plasma-based par-
ticle acceleration for the next generation of colliders is included as a major area of
investment in the Advanced Accelerator Development Strategy Report in the United
<figure><img src="image_23.png" /><figcaption>Fig. 2.12. CMS detector at the Large Hadron Collider (image credits: Luigi Selmi, CC-BY 2.0, https://www.flickr.com/photos/susterru/14972545150).</figcaption></figure>

States [144], it is the main driver for the European consortium ALEGRO (Advanced Linear Collider Study Group) [142], and it is one of the main areas of development identified by the Plasma Wakefield Acceleration Steering Committee (PWASC) in the UK [145].

While the plasma-based acceleration of electrons is rapidly progressing, positron acceleration is far more difficult, mostly because of the intrinsic and strong asymmetry of the wakefields in the plasma. There are four main regimes that are currently being investigated: the quasi-linear regime, the non-linear regime, the hollow channel regime, and the wake-inversion regime. Whilst each regime has its unique advantages and attractive characteristics, any one of them presents significant challenges that must be overcome before reaching maturity. In general, it is challenging to provide a positron beam with sufficient quality to be synchronised with the positron-accelerating region of a plasma wakefield, and this makes experimental progress in this area slow. In particular, one would need low-emittance and short (less than tens of fs) beams with a non-negligible charge. To date, no positron facility suitable for advanced plasma-wakefield studies is available in Europe, and the only facility existing worldwide is FACET [146] in the United States, together with its proposed upgrade, FACET-II [147]. FACET-II is designed to provide a 10 GeV beam containing approximately 1 nC of charge with a transverse normalised emittance of 20×20 mm mrad and a duration of 60–70 fs. Currently, the construction of a positron beamline in FACET-II is on hold, with the next stage of funding expected to start after 2021.

The capabilities proposed in the EuPRAXIA facility design, however, could help to close this gap in the international research landscape. The main positron parameters of FACET and FACET-II are shown in Table 2.2 together with beam properties conservatively achievable within EuPRAXIA [148]. Whilst, FACET-II is designed to provide a higher energy beam with a lower normalised emittance, EuPRAXIA will have the capability of providing fs-scale beams at a high repetition rate. The advantage of the design in EuPRAXIA is the absence of a damping ring that, even though it allows for particle storage and therefore a higher bunch charge, introduces a temporal broadening in the beam. The short duration of the beam, however, is of
<table><caption>**Table 2.2.** Main positron beam parameters obtainable in FACET and FACET-II compared with those achievable with EuPRAXIA. EuPRAXIA positron parameters are based on the wakefield accelerated electron beam proposed for the beam-driven site of EuPRAXIA (see details in Chap. 25.2.2).</caption><thead><tr><th>Quantity</th><th>Units</th><th>FACET-I</th><th>FACET-II</th><th>EuPRAXIA</th></tr></thead><tbody><tr><td>Energy</td><td>GeV</td><td>21</td><td>10</td><td>1</td></tr><tr><td>Rel. energy spread</td><td>%</td><td>1.5</td><td>0.7</td><td>5</td></tr><tr><td>Repetition rate</td><td>Hz</td><td>1</td><td>1</td><td>100</td></tr><tr><td>Average power</td><td>W</td><td>7.4</td><td>10</td><td>3</td></tr><tr><td>Beam charge</td><td>pC</td><td>350</td><td>1000</td><td>1</td></tr><tr><td>Transverse spot size in x and y</td><td>μm</td><td>30 x 30</td><td>16 x 16</td><td>20 x 20</td></tr><tr><td>Bunch duration</td><td>fs</td><td>50</td><td>20</td><td>>8</td></tr><tr><td>Transverse norm. emittance in x and y</td><td>mm mrad</td><td>200 x 50</td><td>20 x 20</td><td>500 x 500</td></tr></tbody></table>

critical importance for precision studies of wakefield acceleration for positrons and to ensure maximum energy extraction from the wakefield itself. Moreover, the proposed positron beamline will be the only one available in Europe and thus of fundamental importance for the progress towards the design of a novel TeV-scale particle accelerator, the primary goal of several high-level research projects in Europe, including the European consortium ALEGRO [142].

### 2.2.6 Medical Physics

Particle accelerators have widespread use in medical physics, from the low-energy (tens of MeV) linacs that are used to generate hard X-rays for radiotherapy, to the high-uptime cyclotrons used in the production of short-lived radioisotopes employed in both diagnosis (CT) and treatment.

There is a need for a role for high-precision high-energy electron accelerators for advances in medical physics too. Primarily, EuPRAXIA will be a relatively small-scale FEL that can provide X-ray radiation with unprecedented quality, which can be vital in determining biological pathways and in drug discovery. However, the overall EuPRAXIA facility has a number of other avenues in which it can be of interest to medical physics. Of these, two applications are considered of great interest at the moment: Very High Energy Electron Therapy (VHEET) and phase-contrast imaging (PCI). We expand briefly on some of these applications below.

#### FEL Applications for Medical Physics

One of the major uses of X-ray sources lies in determining the structure of biological material at the atomic scale. However, the scattering efficiency of X-rays is low, and to be able to attain enough signal before causing excessive damage to the material, large regular crystals of the material need to be grown, hence the reason this field is usually referred to as *crystallography*. However, this limits the range of materials that can be investigated: some do not form large regular crystals, whilst others change significantly when crystallised as compared to their form when in use, usually in solution. X-ray diffraction with FELs overcomes the restriction on crystal size by allowing strong diffraction from small samples given the very large photon flux they produce. Furthermore, the short temporal duration of the source means that the diffraction takes place before the material under investigation is destroyed by the large radiation flux. These measurements can even be extended to complex structures,
<figure><img src="image_24.png" /><figcaption>Fig. 2.13. Phase-contrast X-ray image of a spider (image credits: Excillum AB, Sweden).</figcaption></figure>

such as viruses, allowing unprecedented knowledge of the working of pathogens and other complex proteins. The ultra-fast nature of FELs also allows movies of these chemicals performing reactions, so in addition to providing structures, they are also elucidating biological pathways. FELs are already revolutionising the knowledge of biochemical processes with its resultant effect on medicine and treatment.

Very High Energy Electron Therapy

Cancer will affect an ever-increasing proportion of the population as people grow to older age and medicine tackles other diseases and illnesses. More than half of all cancer cases are treated by some form of radiation therapy, where an intense beam of penetrating radiation is directed at the tumour to kill cancerous cells. Again, in the vast majority of cases, this radiation is provided as high-energy photons produced by bremsstrahlung of a low-energy (~10 MeV) electron beam with a solid target. However, photons have a relatively small interaction cross-section as compared to the electrons themselves, and the only reason the electron beam itself is not used is that they do not have sufficient energy to reach deep-seated tumours. For these, electrons of energy in excess of 100 MeV are required, and the cost of a linac producing these energies has been considered to be prohibitive. Wakefield accelerators with their reduced footprint, on the other hand, are considered an interesting source of electrons for VHEET, and a number of studies are underway to demonstrate the use of these devices for this application. The laser-plasma injector (LPI) being developed for the EuPRAXIA facility would be able to produce an electron beam that would be ideal to do experimental work in this area, and a prime goal of EuPRAXIA will be ensuring the reliability and robustness of the electron source, which is vital for this kind of application.
## Imaging Applications

Of course, as important as treatment is the development of techniques to diagnose illness. Many of the advanced techniques now commonly used in hospitals are based on advanced physical techniques, such as PET, CT, or MRI. However, in most cases, the first and most trusted workhorse of medical imaging is the medical X-rays which has mostly remained unchanged in basic operation since the discovery of the X-ray. Yet, with typical X-ray sources, the low resolution and low photon number lead to grainy images which can cause inconclusive and sometimes even incorrect diagnosis. This is especially true when imaging soft tissue, where the contrast between different types of tissue (such as healthy or cancerous tissue) is limited.

High-quality synchrotron and FEL radiation sources have been demonstrated to provide diagnostic capability comparable to invasive and time-consuming biopsies. If the source has additional spatial coherence, then it can be used for phase-contrast imaging, as shown in the example in Figure 2.13. This technique provides extra contrast in soft tissues, such as the ability to detect tumours with high confidence. The radiation sources envisioned for EuPRAXIA, both the FEL and the broadband betatron source, would be able to provide compact and versatile setups that can be applicable to medical imaging. These sources could then be a forerunner of a more widespread adoption of these techniques in hospitals and medical centres.

## 2.2.7 Inspection and Material Studies

Quality assurance and innovation in manufacturing is underpinned by metrological methods and techniques that improve and drive forward the measurement infrastructure available to industry. Metrology is especially important in advanced manufacturing areas producing high-performance and high-value components that are made in and are required to perform under hostile environments. Heat and pressure treatments, new welding methods, radiation exposure, and impact damage are all examples of scenarios that can leave sub-micron defects in materials during advanced manufacturing or extreme performance use.

It is thus highly desirable for industry to be able to assess the uniformity and quality of materials over a wide range of thicknesses, sizes, and composition, ideally while being under significant stress. Moreover, it is vital that any inspection be carried out in a non-destructive manner. Generally speaking, non-destructive inspection can be easily performed at the surface of materials, but several difficulties are encountered when performing sub-surface, volumetric inspections. Several techniques have been developed, which can be classified depending on the particles used to probe the materials: neutron driven, X-ray driven, or positron driven.

Of them all, positron annihilation lifetime spectroscopy (PALS) is arguably the only one that can provide nanometre-scale resolution over a significant range of material thicknesses and detect defects and vacancies in materials down to a few parts per million (Fig. 2.15, left). When in a material, a positron enters a Bloch state, but it is rapidly localised in regions of missing matter; in other words, positrons get easily trapped at vacancy defects. In a timescale of the order of 100 ps, the trapped positron annihilates with an electron in the material, producing annihilation radiation, i.e. two X-ray photons each with an energy close to 511 keV. A sketch of the physical principles behind PALS is given in Figure 2.15 (right). By studying different characteristics of the emitted radiation, such as their spectrum and temporal footprint, one can extract a detailed map of vacancies in the material under study.

Traditionally, positron inspection of materials is carried out using $\beta^+$ radioactive sources, such as $^{22}$Na. The broadband positron populations that are continuously
<figure><img src="image_25.png" /><figcaption>Fig. 2.14. Accelerators used for inspection and material testing (image credits: FCC / Code of the Universe / CERN / Lois Lammerhuber).</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Fig. 2.15. Left: Typical range of operation of different methods of material inspection. Positron annihilation techniques allow for the smallest defects to be detected, and the EuPRAXIA facility will allow for micron-to-mm thicknesses to be volumetrically probed. Right: Cartoon of the physical principle behind PALS (image credits: M. Butterling, HZDR).</figcaption></figure>

generated from these atoms are then captured, energy-filtered, and time-gated in bursts with a duration of the order of a nanosecond.

As an example, the ELBE Centre at Dresden-Russendorf provides positron bunches with a duration (FWHM) of 250 ps and an energy tuneable from 0.5 to 15 keV [150]. The SPONSOR area can instead reach up to 36 keV [151]. The maximum intensity achievable by the machine is $10^6$ positrons per second. The NEPOMUC machine at the Technical University of Munich exploits neutron-induced positron production in $^{113}$Cd and produces approximately $10^9$ positrons per second with an energy of 1 keV [152]. The PLEPS line in Munich [153] instead provides a DC beam with approximately $5 \times 10^4$ particles per second. Four minutes are needed to get a full
<table><caption>**Table 2.3.** Calculated positron lifetimes (ps) for perfect lattice (bulk) and monovacancy defects in selected ABO₃ and related materials (adapted from [149]).</caption><thead><tr><th>Material</th><th>Bulk</th><th>Defect structure</th><th>V<sub>A</sub></th><th>V<sub>B</sub></th><th>V<sub>O</sub></th></tr></thead><tbody><tr><td rowspan="2">PbTiO<sub>3</sub></td><td rowspan="2">161</td><td>Unrelaxed</td><td>292</td><td>204</td><td>165</td></tr><tr><td>Relaxed</td><td>290</td><td>185</td><td></td></tr><tr><td rowspan="2">SrTiO<sub>3</sub></td><td rowspan="2">152</td><td>Unrelaxed</td><td>280</td><td>195</td><td>161</td></tr><tr><td>Relaxed</td><td>281</td><td>189</td><td></td></tr><tr><td>SrRuO<sub>3</sub></td><td>150</td><td>Unrelaxed</td><td>288</td><td>200</td><td>161</td></tr><tr><td>Sr<sub>3</sub>Ru<sub>2</sub>O<sub>7</sub></td><td>180</td><td>Unrelaxed</td><td>301</td><td>207</td><td>187</td></tr><tr><td>LaMnO<sub>3</sub></td><td>145</td><td>Unrelaxed</td><td>282</td><td>196</td><td>158</td></tr><tr><td>TbMnO<sub>3</sub></td><td>152</td><td>Unrelaxed</td><td>259</td><td>199</td><td>161</td></tr><tr><td>BiFeO<sub>3</sub></td><td>154</td><td>Unrelaxed</td><td>290</td><td>198</td><td>161</td></tr></tbody></table>

lifetime spectrum, whereas up to two hours are necessary to get a complete depth profiling, accumulating 25 spectra.

Despite the high performance of these machines and their wide use for industrial applications, it is difficult to produce high-quality positron beams with higher energy (up to a few MeV), able to penetrate deeper into the material under study. Also, the relatively long duration of the beams, comparable to the typical timescales of annihilation in materials (see Tab. 2.3, adapted from [149]), limits the resolution of the systems and makes data extraction rather complicated and prone to uncertainties. This is because it is necessary to deconvolute the signal with the positron duration and the detector response, both on timescales of the order of, if not longer than, 100 ps.

An alternative method to produce high-flux and short beams of positrons has been recently proposed. In a nutshell, the positron production exploits the quantum electro-dynamic cascade initiated by an ultra-relativistic electron beam propagating in a high-Z solid target [154–156]. The positrons escaping the rear surface of the solid target present a duration comparable to that of the primary electron beam, a broad spectrum and divergence, and an overall number of positrons that is a significant fraction of the number of electrons in the primary beam [148].

To generate high-flux, short, and mildly relativistic positron beams, one would then require a primary electron beam with the highest possible charge, modest energy (of the order of 10s to 100 MeV), and short duration. However, higher energy electron beams, as achievable within EuPRAXIA using a multi-staged approach, are also usable for this purpose. On the other hand, the unavoidable spectral broadening introduced by the cascade inside the solid significantly relaxes any requirement on spectral quality of the primary electron beam, which is virtually irrelevant for the positron production. These characteristics are guaranteed by the laser-plasma-based electron beams which are foreseen to be created at the EuPRAXIA infrastructure and which are discussed in more detail in Section 25.2.1.

## 2.3 The Role of Industry

As a relatively young field of science, plasma acceleration has not yet established its own industries, while at the same time relevant existing industry sectors connected to high-power laser and RF accelerator systems have only limited experience with this technology. To push plasma accelerators towards user application and perhaps even eventual commercialisation, closer interactions with industry will thus be essential. In this context, EuPRAXIA can provide an ideal platform for such a development
through involving industry in several different roles. First, with their vision as suppli-
ers of finished and usable equipment, industry partners can help laboratories in the
design of robust and reliable products. Thus, they will be able to participate actively
in risk-analysis studies and thus identify which activities must be carried out in
the prototyping phase to move from a low-technical-readiness to a high-technical-
readiness technology. Industry partners may also be able to directly participate in
prototyping projects during the EuPRAXIA technical design phase, through collab-
orative research projects or via students, whether for thermal studies for the laser
system (pump laser, mirrors, deformable mirror, gratings), studies on the materials
needed for optics or for electron transport and electron diagnostics, just to name
a few examples. In addition to these, other topics for collaboration could include
operational safety aspects or studies on the integrated logistical support necessary
depending on the mission profile and operation mode of the facility to guarantee a
controlled operating cost over the lifetime period of the infrastructure. In this way,
the manufacturers will be able to commit to the delivery of a turnkey facility for the
laser and the functional and reliable installation of particle beam parts in the manu-
facturing phase thanks to an extensive collaboration in the design. They will also be
able to commit more easily to maintenance procedures under operational conditions
of the facility over the expected lifetime, as they have already been involved in their
development. Additionally, however, industry could also take on a second role as users
of the future EuPRAXIA research infrastructure. As the previous sections showed,
many of the potential features of the machines designed for EuPRAXIA also have
great potential for industrial applications. As such, industrial users could take advan-
tage of this infrastructure, on the one hand, as an analysis or processing tool. On the
other hand, they could also see it as a testing ground for applying plasma-accelerator
technology to their own facilities and processes, a possibility unique to a facility such
as EuPRAXIA thanks to the potential for compactness and cost-efficiency of plasma
acceleration. This last point in particular directly links to EuPRAXIA's ambition
for open innovation (see Chap. 1) as collaborations and joint projects could foster a
unique exchange of knowledge and technology. While providing new research insights
and potentially improved performance for EuPRAXIA and its academic users, indus-
try partners could equally benefit from novel R&D results with the potential for future
product development opportunities.

2.4 Added Value for the European Research and Technology Landscape

The European research and technology landscape features thousands of particle acce-
erators that provide particle and photon beams to applications in health, industry,
and science. State-of-the-art particle beams and the produced photons are used, for
example, for cancer therapy in hospitals, for electron beam welding in factories, for
security X-ray scans at borders, for investigations of bacteria and viruses in research
centres, and, last but not least, for discovering new particles and fundamental forces.
Accelerators range in size from a few meters to the 27 km circumference of the Large
Hadron Collider at CERN. As undisputed masterpieces of modern technology, they
provide indispensable tools to the modern society in Europe, supporting innovation
and economical growth.

Today particle accelerators are a mature technology that has started encountering
some practical limitations because of size and cost. The application reach of particle
accelerators could be dramatically increased if their size and cost could be reduced.
For example, the availability of brilliant, time-resolved X-ray pulses in every univer-
sity could multiply the number of bacteria or virus structures that can be resolved.
Young scientists could have early access to powerful research tools, new ideas could
<figure><img src="image_27.png" /><figcaption>**Fig. 2.16.** Variety of possible topics and application fields available to users to investigate at the EuPRAXIA research infrastructure.</figcaption></figure>

be tested quickly, and unconventional research directions followed. Experience shows that such wider access to powerful research tools will strongly accelerate knowledge gain and innovation. Broader access will also enable more inventions and tests of new solutions, for example in medicine. Once the structure and dynamical behaviour of a bacterium has been understood, new approaches for medication can be developed and tested more quickly.

The EuPRAXIA CDR describes the design of a new European research infrastructure built around the concept of plasma accelerators. Those plasma accelerators aim at enabling a drastic reduction in size and cost for accelerator facilities. EuPRAXIA realises a facility that would, for the first time, demonstrate a gain in facility size while enabling various applications and providing users with additional access to particle and photon beams. As such a demonstration facility, EuPRAXIA targets different application fields in a complementary way with the existing and planned research landscape in Europe. The application fields and particular subjects that are addressed in the EuPRAXIA facility are presented in Figure 2.16.

From Figure 2.16, it becomes clear that EuPRAXIA aims at a broad spectrum of users and therefore also a broad added value for the European research and technology landscape. The added value from EuPRAXIA starts with developing the plasma, accelerator and laser technologies required, thus providing Europe with access to this innovative field and a clear leadership role. Added value continues through collaboration with related industry, which will ensure a swift industrialisation of the required high-tech components. Users from different domains will experience added value, when the highly innovative and novel accelerator technology will be made accessible to them. The users will, on one side, have the benefit of access to the particle and photon beams with some unique features (e.g. access to ultra-fast science). However, the users will also provide feedback on the particles and photon beams, often contributing requests and new ideas.

EuPRAXIA will, last but not least, also generate added value through a coordinating role with its many partner laboratories. EuPRAXIA has been proposed by members of the European Network for Novel Accelerators (EuroNNAc) as a European project that ensures the competitiveness of Europe in novel plasma accelerators. It builds on the expertise from several national projects in plasma acceleration, like the
ATHENA project in Germany, the SPARCLab project in Italy, the CILEX project in France, the EPAC project in the UK, and the ELI laser pillars. EuPRAXIA is connected to the LEAPS initiative through the LEAPS task on “Compact Sources”. It is fully complementary to the AWAKE plasma acceleration project, which can only be performed at CERN, with its powerful proton accelerator infrastructure. During its technical design and construction phases, EuPRAXIA will create significant added value by bundling European expertise in a major project with clearly defined common goals and interfaces to alternative approaches.

## 2.5 The Role of EuPRAXIA for Innovation and Science in the Global Environment

Plasma wakefield acceleration is an important research topic in various institutes across Asia and America, many of whom are already involved in EuPRAXIA as associated partners. Continuing and strengthening scientific exchange between EuPRAXIA and the rest of the world could thus strongly advance this field of science.

Several possible avenues for scientific collaboration could be foreseen for EuPRAXIA. A most direct route is, of course, the reciprocal use of experimental facilities. The project's technical design phase will provide excellent opportunities for common R&D and prototyping projects at different international facilities, such as the PEARL facility in Russia or the Laboratory for Laser Plasmas in China. A more complete list of European and international consortium facilities is given in Sections 29 and 30 in this context. Once the EuPRAXIA infrastructure is set up, it will provide a unique test facility and collaboration partner for users worldwide, especially also those from regions where plasma accelerator or, generally, accelerator facilities do not yet exist in a widespread manner.

In the long term, it could be envisaged that EuPRAXIA will be able to exchange knowledge and technologies with other facilities worldwide. Applying some of the novel techniques from the EuPRAXIA design at other research infrastructures, for example, would demonstrate very well their applicability and user readiness in different setups. At the same time, EuPRAXIA will be in a unique position to share experience in setting up and operating a large-scale test facility based on plasma acceleration. This expertise could be shared through training and knowledge exchange programs as an international effort. A particularly interesting perspective could be offered to developing countries in this way for whom compact accelerator technologies could be ideal entry points to accelerator science considering their reduced cost and size.

Finally, EuPRAXIA brings together and represents a large fraction of the European plasma-accelerator community. As such, it is expected to become an important contact point for international collaborations with European institutes in the field, fostering knowledge exchange and pushing progress for some of the hardest future challenges for novel accelerators, such as diagnostics development, applications, etc.

In the following, some further aspects for EuPRAXIA's possible role in the global research environment are exemplified with the cases of two collaboration partners in Russia and China.

### Statement from the Institute of Applied Physics in Russia (I. Kostyukov)

Plasma-based acceleration is one of the important directions of research at the Institute of Applied Physics of the Russian Academy of Sciences (IAP-RAS). The scientific exchange programs between EuPRAXIA and the non-European partners could
strongly advance this field of science. Furthermore, the EuPRAXIA infrastructure is excellently suited as a platform for the training of young scientists (not only from European countries but also from non-EU organisations like IAP-RAS), where they will learn advanced techniques for plasma-based acceleration, innovative methods of diagnostics, experiment planning, and organisation.

There are several experimental setups devoted to laser-plasma acceleration in IAP-RAS. Given its flexible structure, the EuPRAXIA facility can be used to repeat and verify experimental results obtained in IAP-RAS facilities (and vice versa) to extend them into new parameter spaces which allows to explore new regimes of acceleration and radiation generation.

The diagnostic methods for experimental investigation of the plasma-based accel-
eration are still a challenge. The modern methods cannot yet provide the necessary
spatial-temporal resolution of laser-plasma structures and phase parameters of the
electron bunches during acceleration. The versatile EuPRAXIA infrastructure could
make a significant contribution to the development of diagnostic techniques. More-
over, many elements of laser-plasma accelerators like laser targets, laser pulse con-
ditioning and manipulation, electron bunch transportation, etc. can be developed
and tested at the EuPRAXIA facility. The developed technologies can then be re-
examined and applied in other facilities, like the PEARL facility in IAP-RAS.

Statement from Shanghai Jiao Tong University (M. Chen)

EuPRAXIA will be the most important partner of the key Laboratory for Laser
Plasmas (LLP) at Shanghai Jiao Tong University. The two sides will conduct in-
depth collaborative research on high-quality electron acceleration (such as two-colour
ionisation injection, plasma dechirper devices, etc.), high-efficiency staged wakefield
acceleration (such as curved-plasma-channel-based staged laser wakefield accelera-
tion), and the wide applications of electron beams and radiation sources. LLP wel-
comes EuPRAXIA partners to carry out experimental studies on its 200 TW and
the future upgraded 200 + 300 TW laser and target systems. LLP would also like
to make theoretical and simulation contributions to EuPRAXIA. LLP looks forward
to performing high-quality electron acceleration, X-ray, and electron beam imaging
research on EuPRAXIA facilities and sharing the advanced experience of EuPRAXIA
in the construction of an acceleration facility and high-average-power laser systems.

3 Description of the EuPRAXIA Infrastructure

## 3.1 Introduction

In this chapter of the CDR, we describe the envisaged overall EuPRAXIA infras-
tructure, focusing on the concept of the facility and preliminary ideas on the possible
implementation. The technical studies that have been performed for the EuPRAXIA
CDR will be described in detail in Part 4. The research field of plasma accelerators
is distributed over Europe, and considerable effort has been invested in discussing
a model that includes the existing expertise and facilities while delivering a well-
defined European research infrastructure. The EuPRAXIA infrastructure and asso-
ciated project, therefore, will exhibit particular features that are not present in other
projects. This is explained and discussed in this chapter.
## 3.2 Overview on Design and Performance Parameters

The EuPRAXIA infrastructure will deliver laser, electron, positron, and photon beams to various users at two central sites. The site concept will be explained later in this chapter. The flagship innovation and science goals have been defined in Chapter 2.1. The detailed performance simulations are discussed later, and multi-page performance tables can be found in Chapter 9. For the convenience of the reader, we show a summary of design and performance parameters in Table 3.1.

Here, we remind the reader of a few main objectives of the EuPRAXIA design study:

1. To develop the disruptive technology of plasma accelerators, modern lasers, and compact beam drivers into a viable option for a new generation of compact and cost-effective accelerators.

2. To solve the beam quality problem of plasma accelerators such that the produced electron beams can support demanding applications like FELs.

3. To demonstrate benefits in terms of the performance, size, and cost of plasma acceleration with respect to RF technology.

While the latter point is discussed in Section 3.4, the expected machine performance of EuPRAXIA is summarised in Table 3.1. It was also visualised in Figure 2.5 for the critical parameter of electron beam energy spread. In the technical part of this design report, various solutions will be presented that allow extending the performance reach of plasma accelerators significantly.

The overview table reflects the two complementary technical approaches of beam-driven and laser-driven plasma accelerators, respectively. Both technologies offer, for example, different benefits on compactness (the laser-driven option is potentially more compact) and stored energy (the beam-driven option stores more energy). The two approaches share a number of common challenges and will be developed together in a synergistic approach. The foreseen user applications have been selected to optimally reflect the benefits of each approach.

## 3.3 The Concept of Distributed EuPRAXIA Construction

As written before, the plasma accelerator community in Europe is dispersed, with some 20 groups and about 15 facilities of 10–30 M€ scale per facility. Major groups and activities exist in France, Italy, Portugal, Sweden, the UK, the Czech Republic, Switzerland, and Germany. The situation can be compared to the field of particle physics detectors that have groups and facilities in various institutes, universities, and countries. The EuPRAXIA concept of distributed construction was therefore inspired by the construction model of big particle physics detectors in Europe:

### Model of particle physics detectors

- Many groups work together in experimental collaborations; the collaborations are organised in working groups. Working group leaders organise the distributed work and prepare reports.

- The collaboration elects spokespersons that manage agreement on the deliverables of the various institutes and track the overall progress.

- Local institutes in the various countries carry out the R&D work, perform local prototyping and tests, and construct the final components.

- The collaboration assembles the delivered components in the location of the particle physics detector.