67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

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JACoW Publishingdoi:10.18429/JACoW-FLS2023-MO3A5

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FLASH: STATUS AND UPGRADE
M. Vogt∗ , S. Schreiber, J. Zemella
Deutsches Elektronen-Synchrotron DESY, Germany
Abstract
FLASH, the Soft X-Ray and Extreme-UV Free Electron
Laser at DESY, is undergoing a substantial upgrade and refurbishment project, called FLASH2020+. The project will
finally enable external seeded and SASE FEL operation fora wavelength range down to 4 nm with the EEHG method.
This is achieved in two long shutdowns from November
2021 to August 2022 and from June 2024 to August 2025.
Key ingredient of the upgrade were installation of a laser
heater, replacing two early TTF-type L-band SRF accelerating modules by modern, high-gradient XFEL-type modules, redesign of the 2nd bunch compressor, and complete
redesign of the FLASH1 beam line for HGHG/EEHG seeding.
This talk will report on the project and the status of
FLASH after the first shutdown with emphasis on beam
dynamics aspects.

FLASH
This contribution is a modified version of a contribution [1] to the 21st International Conference on RadioFrequency Superconductivity, held this June in Grand
Rapids, MI, USA. There the emphasis was on the energy
upgrade, while this contribution emphasizes certain beam
dynamics aspects of the FLASH2020+ Upgrade.
FLASH [2–8] is a superconducting high-gain vacuumultraviolet (VUV) / soft X-ray free-electron laser (FEL), operated mainly as a photon user facility with up to two beamlines operated simultaneously. FLASH is segmented into
three functional beamlines: the common injector and linac
FLASH0, preparing and accelerating bunch trains suitable
for the FEL process, and the two FEL beamlines FLASH1and FLASH2 which finalize and diagnose the bunch preparation, accommodate the FEL process each in their own
internal undulator beamline, and finally dispose of the spent
beam in a beam dump. The FLASH3 beamline is used by
the plasma wake field acceleration experiment FLASHForward [9] which can be activated alternatively to FLASH2 bypowering a DC dipole. The FLASH accelerating RF consistsof 7 L-band (1.3 GHz) superconducting modules of 8 9-cell
Tesla-type cavities.
At the moment FLASH is undergoing a substantial upgrade and refurbishment project, called FLASH2020+ (seenext section). The first of two shutdowns started November
2021 and ended August 2022 successfully and the secondscheduled to start June 2024 and to end August 2025, isbeing prepared now.
The original FLASH injector (before the upgrade) consisted of a normal-conducting photo-cathode 1.6-cell RF∗ vogtm@mail.desy.degun (1.3 GHz), an accelerating L-band module (ACC1), a
third harmonic linearizer (ACC39) operated at 3.9 GHz, a
BACCA longitudinal feedback cavity [10], the first bunch
compression chicane, two more L-band accelerating modules (ACC2,ACC3), and the second bunch compression chicane — at that time an S-type 6-dipole chicane.
The bunches are produced in up to three trains mapped tothe three injector lasers.
In its standard setting, the RF-gun is operated with approximately 5 MW and produces a 600 µs flat top. The bunchat the exit of the gun then has a momentum of 5.6 MeV/“c”
FLASH0 can therefore provide up to 6000 bunches per second (in 10 trains) at 1 MHz bunch repetition frequency and
10 Hz pulse repetition frequency. If the RF pulse is split
between two sub-trains (for FLASH1 and FLASH2), a minimum of 70 µs has to be subtracted for transient effect of
the extraction kicker and for interpolating between the RFparameters of the two flat tops which are otherwise withincertain ranges independent.
The third harmonic linearizer ACC39 is typically operated
in decelerating mode with a nominal set point is 19.5 MeV.
The first bunch compression chicane is a 4-dipole Cchicane designed for bending angles from 15° (longitudinal
dispersion 𝑀56 =120 mm) to 15° (𝑀56 =255 mm). Prior
to the upgrade it used to be operated at a typical deflection
angle of 18° (𝑀56 =181 mm). Downstream of the chicane
used to be a comfortably equipped transverse diagnostics
and matching section.
Before the upgrade, the second (ACC2) and third (ACC3)
L-band Module were among the weakest in FLASH. Well
tuned they were capable of (together) providing an 𝐸-gain of
304 MeV at an off-crest angle of up to 30°, i.e. an effective
total amplitude of at most 350 MeV
In addition prior to the first shutdown, the second bunch
compression chicane was a 6-dipole (S-type) chicane blocking the space for proper a proper second beam re-match
upstream of the “main-linac”.
The FLASH0 “main-linac” consists of four L-band modules (ACC4/5/6/7). The maximum attainable energy gainof the main linac is about 800 MeV so that the maximum
𝑒− -beam energy of FLASH was ∼1250 MeV
Downstream of the last module the combined collimation and switch-yard section starts. Switching between
FLASH1 and FLASH2 is achieved via a kicker-septum
scheme with two vertically deflecting flat top kickers, deflecting the FLASH2 sub-train into the horizontally deflecting
channel of a DC Lambertson septum. In order to achieve therequired stability for the kicked bunches, the gap between thebunch trains needs to be >70 µs to cover the kicker rise-timeand the damping of the initial ringing.

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The FLASH1 beamline was originally designed and optimized for SASE FEL operation.
The FLASH2 beamline also provides SASE FEL radiation. It consists of an extra bunch compression chicane(called FL2BC1), a transverse diagnostic section capable ofperforming a quad-scan, the 12 variable gap FLASH2 undulators, the PolariX TDS [11–13], the horizontal photon/electron separator dipole which also generates the dispersion
for the longitudinal phase space mapping with PolariX, thevertical dump dipole and finally the dump beamline.
Downstream of the separators of FEL and electron beam,
both tunnels (FLASH1/2) are equipped with photon diagnostics, i.e., screens, pulse energy detectors, and spectrometers
for tuning the FEL. Then the FEL beams are delivered to thetwo experimental halls: Albert Einstein Hall (FLASH1), andKai Siegbahn Hall (FLASH2). Both halls contain further
photon diagnostics, hardware for photon beam manipulation [14], and of course the experimental end-stations.
The history of FLASH is nicely covered in Ref. [5, 8].

THE FLASH2020+ UPGRADE
To keep FLASH at the forefront of science, anambitious upgrade and refurbishment project, called
FLASH2020+ [15–17], was started in 2020. The goals are
to replace outdated hardware, and at the same time upgrade
the FLASH facility to stay attractive and competitive for atleast the next 15 years.

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JACoW Publishingdoi:10.18429/JACoW-FLS2023-MO3A5

the longer wavelengths and Echo Enabled Harmonic Generation mode (EEHG) [20, 21] for the shorter wavelengths.
Many pump-probe experiments at FLASH1 make use of
the THz undulator that produces radiation in the THz regime
using the spent electron bunches from the FLASH1 FEL
undulators. This was quite successful with bunches tweaked
to be a little more spiky than necessary for standard SASE
operation, but still compatible with producing useful SASE
pulse energies. A beam optimized for high stability, efficient
HGHG/EEHG operation however, is not likely to generate
decent THz pulse energies. Therefore a post compressor chicane will almost surely be required at some stage to optimize
the THz radiation.
In order to reliably provide beams with high quality andstability for seeding in FLASH1 and SASE in FLASH2,
the operability of FLASH needs an upgrade too. The design includes more and improved sections with transverse
diagnostics to match the incoming beam to a downstream
optics which is optimized for best performance, more appropriate locations for the transverse deflecting structure
LOLA [22, 23], an intra-train orbit feed forward to remove the systematic part of intra train orbit correlations,quad/skew-quad corrector packs in the new designed second
bunch compression chicane for removing systematic longitudinal to transverse correlations within the bunches, a laser
heater [24] to ameliorate the unwanted micro-bunching effects [25], and of course a upgraded diagnostics at all levels.
All these measures will make the operation of FLASH more
systematic and predictable.

Motivation

The Injector Upgrade

FLASH is currently the only superconducting FEL in the
wavelength regime from VUV to soft X-ray. In its current
injector configuration it can provide ∼ 5000 bunches per
second producing SASE FEL radiation from 60 nm down to∼4 nm with pulse durations from some tens to several hundreds of fs and with pulse energies from some µJ to ∼1 mJ.
Part of the motivation for the upgrade was to extend the wavelength range towards shorter wavelengths. In fact ∼4 nm is
just the upper end of a wavelength range, called “the water
window”, which is of special interest to many of the users.
The energy upgrade will allow to reach higher beam energies and thus create photons of shorter wavelengths <4 nm In
addition, so called afterburner undulators can significantly
enhance the 3rd harmonic content of the FEL radiation,
thereby extending the usability of the third harmonic down
to ∼1.3 nm.
SASE is a powerful production mechanism of FEL radiation however, it is a stochastically seeded process, withnotable shot-to-shot fluctuations of the photon spectrum andpotentially several uncorrelated modes (spikes) in a single
bunch. Thus the longitudinal coherence of SASE FELs israther low. The FEL process can however also be externallyseeded with a highly coherent external laser pulse. It has
been decided that FLASH1 should ultimately cover the wavelength range from ∼4 nm to ∼60 nm with external seeding inHigh Gain Harmonic Generation mode (HGHG) [18, 19] for

The first shutdown 2021/22 was dedicated to upgrading
and refurbishing the injector section of FLASH0. Figure 1
shows a schematic layout of FLASH after the first upgrade
shutdown 2021/22. Installation of two new injector lasers
was started. All components of the laser beamlines that
require access to the accelerator tunnel have been installed
during the shutdown. The new lasers are scheduled to becommissioned by the beginning of next year.
Kickers have been installed to reduce the systematic (shortterm reproducible) orbit slopes over the bunch train, between
gun ad ACC1, and around the two chicanes. Downstream of
BACCA, space has been generated to install an incouplingchicane for the laser heater laser, an undulator for the actualenergy modulation process, and the necessary surrounding
lattice elements (quadrupoles, steerers, screens). The laser
heater installation has been completed in the shutdown and it
is already commissioned to a large extent. We have achieved
transverse and temporal overlap, optimized the undulator gap
for the actual beam energy. We have observed the heatingprocess, i.e., increased slice energy spread in the heated partof the bunch, directly by both LOLA and PolariX, and byobserving the expected reduction of the micro-bunching gain
and the FEL gain.
In order to make space for the laser heater the first Burchcompression chicane (now called FL0BC1) had to be moved
about 2 m downstream. Thereby the space for the transverse
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67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

RF−stations

laser heater

BACCA

3.9GHz SCRF

newinjectorlasersystemRF−station

D−chicane

FLASH0movable D−chicane

FLS2023, Luzern, Switzerland

RF−stations

RF gun (1.3GHz/warm)

FLASH−INJECTOR

143 MeV

550 MeVbeam dump
THz undulator

XSeed experimentmatching

MAIN−LINAC

old injector lasers (1&2&3)

5.6 MeV

FLASH1
LOLA−TDS

old seed laser

upgraded 1.3GHz SCRF
1.3GHz SCRF
matching (w/ upgr. rf−distribution)
matching

1.3GHz SCRF

JACoW Publishingdoi:10.18429/JACoW-FLS2023-MO3A5550 MeV to 1350 MeV

photon diag
FLASH2APPLE−III undulator
beam dump‘‘Albert Einstein’’FEL user halls‘‘Kai Siegbahn’’pre−ionization laser (25 TW)
FLASHForward Exp.
PolariX−TDSFLASH3

PETRA

Figure 1: Schematic layout of FLASH (not to scale) after the first shutdown 2021/22.
diagnostics and matching section (between FL0BC1 andthe fixed position of ACC2) was reduced. This required an
updated concept for measuring emittance and mismatch amplitude as well as for re-matching into the design optics [26].
During the first shutdown 2021/22 two of the oldest, andweakest, TTF-type L-band modules were replaced by newly
refurbished, XFEL-type modules PXM2.1 and PXM3.1 [27].
We now routinely achieve an 𝐸-gain of around 417 MeV at
an off-crest phase of at most 25° which suggests an effective total amplitude of ∼460 MeV. This allows to operate
FL0BC2 at 560 MeV — 110 MeV more than the before theenergy upgrade.

Figure 2: 3D CAD model of FLASH’s 2nd bunch compression chicane FL0BC2
The section around the second bunch compressor chicane(now called FL0BC2) was completely redesigned in order togenerate space for a proper diagnostics and re-matching section, capable of performing symmetric quads-cans and multiquad-scans. The old (horizontal) 6-dipole S-shaped chicane
with flat vacuum chamber was replaced by a new, shorter,
C-shaped design with round vacuum chambers and skewquad/BPM/quad packs in the two chicane legs [28]. Figure 2
shows at 3D CAD model of the new chicane FL0BC2. The
inner dipoles are movable on rails so that the chicane is tunable from 0° to 6° (nominal 5°) with maximum a 𝑀56 of∼100 mm (nominal ∼70 mm). The magnet packs in the chicane legs consist of a quadrupole, followed by a beam position monitor, followed by a skew-quadrupole, in one leg andthe mirror image of the pack in the other leg. The goal is toemploy the horizontal chicane dispersion in the quadrupole
to reduce linear longitudinal-to-horizontal correlations inside the bunch, the combined action of horizontal dispersion in skew-quad and quad to reduce linear longitudinal-tovertical correlations inside the bunch [28–30].

The new chicane could so far only be commissioned in
part: Transmission with various magnet currents from 0.5°
to 5° and the corresponding nominal slider positions is perfect. With the nominal deflection angle of 5° the compression factor for given bunch chirp is in the right ball park.
Since mid-August the new FL0BC2 is routinely operated at
560 MeV.
In Early October 2022 commissioning with beam started
for about 3.5 weeks prior to the first user run with the new
injector [26]. Since then we take more and more features
into operation during the regular FEL studies.1 In the shorttime we managed to establish beam transport through bothbeamlines, commission a large part of the new updated diagnostics, match the beam from the gun to the design optics
upstream of FL0BC1, correct the dispersion in injector andlinac, and get the intra train orbit feed forward into operation.

The FLASH1 Upgrade Towards External(HGHG/EEHG) Seeding
The second shutdown 2024/25 was originally scheduled
to convert FLASH1 into an externally seeded beamline for
HGHG and EEHG operation, preserve the capability of providing also saturated SASE, and preserve the capability of
producing THz radiation of high pulse energy simultaneous to seeding and/or SASE for pump-probe experimentsin the Albert Einstein Hall. Figure 3 shows FLASH after
this stage of the project has completely been finalized. The
FLASH1 beamline is described in some detail in Ref. [31]with emphasis on the seed radiator section and in Ref. [32]with emphasis on the remaining parts. We refer to this original design as stage-FULL. However it turned out that in the2024/25 shutdown, not all goals can be achieved. Instead a
reduced design, called stage-0 will be implemented at first.
It enables HGHG and EEHG seeding with slightly reduced
performance but in particular will not provides SASE in saturation at shortest wavelengths or high THz pulse energies.
The first new section downstream of the septum will be atransverse diagnostic and matching section. The first modulator section contains a laser incoupling chicane FL1CH1
for the first seed laser, an undulator (1st modulator) in which
1 studies explicitly aiming at improving the FEL operation of FLASH.

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67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

RF−stations

BACCA

3.9GHz SCRF

newinjectorlasersystemRF−station

D−chicane

FLASH0movable D−chicane

matching &collimation

RF−stations

upgraded 1.3GHz SCRF
1.3GHz SCRF
matching (w/ upgr. rf−distribution)
matching

1.3GHz SCRF
RF gun (1.3GHz/warm)

FLASH−INJECTOR

FLASH1143 MeVbeam dump
THz undulator
LOLA−TDS

quasi−DBA
seed lasers
post−compr.
photon diag

matching

MAIN−LINAC

FLASH2APPLE−III undulator

old injector lasers (1&2&3)

5.6 MeV

JACoW Publishingdoi:10.18429/JACoW-FLS2023-MO3A5550 MeV to 1350 MeV

550 MeVbeam dump‘‘Albert Einstein’’FEL user halls‘‘Kai Siegbahn’’pre−ionization laser (25 TW)
FLASHForward Exp.
PolariX−TDSFLASH3

PETRA

Figure 3: Schematic layout of FLASH (not to scale) after the shutdown 2024/25 (Stage-Full).

amplitude / DC

10-1

σp
3 keV
5 keV
7 keV

a) WP1

10-2
10-3
10-4
10-5

2

5

10
wavelength / μm

20amplitude / DC

10-1

σp
3 keV
5 keV
7 keV

b) WP2

10-2

30

10-3
10-4
10-5

2

5

10
wavelength / μm

2030

Figure 4: Microbunching simulations using a realistic shotnoise model for two compression working points with thesemi-Lagrangian code SelaV1D
the electron bunch becomes energy-modulated by laser 1,
and the so-called over-folding chicane FL1CH2 to strongly
over-fold the energy modulation from the first laser in EEHG
mode. The over-folding chicane also serves for coupling seedlaser 1 out and for coupling seed laser 2 in. The second modulator section contains a second undulator to modulate theincoming phase space, and the so-called bunching chicane
FL1CH3 that is used to shear the doubly modulated structure
in order to create sharp spikes in the charge distribution —with potentially very high harmonic content. Downstream
follows the radiator section, with its (originally) 11 helical
APPLE-III undulators.
Seeded FEL operation is much more severely perturbed
by microbunching than SASE. We have performed startto-end simulations to minimize the impact of microbunching in FLASH1. In particular we have developed a code,
SelaV1D [33], that describes the evolution of the longitudinal
phase space density by low-noise backward tracking (semiLagrangian method) of a tree-based grid. The tree-based
grid allows extreme fine resolution for the 𝜖-support of theexotic FEL densities without wasting memory on unpopulated areas. Figure 4 compares too compression working
points (WPs), a heuristically designed WP and a carefully
optimized WP, by means of microbunching spectra provided
by SelaV1D simulations, starting the microbunching fromrealistic shot-noise without the artifacts of macro particle
tracking. Each WP shows three different slice energy spreads(intrinsic, moderate laser heater, cranked up laser heater).
It becomes clear that the microbunching amplitude can be
kept small (∼ 10−4 ) in the relevant wavelength regime withmoderate energy spread if a suitable wP is chosen.
Downstream of the radiators follows the new location
for LOLA downstream of the radiators. One key aspectof the new FLASH1 beamline is to disentangle the photon
beamline from the spent electron beamline, and eliminate geometrical coupling. A quasi double bend achromat structure(qDBA) will divert the beam by 5° to the port side2 . In orderto post-compress the “soft” seed beam for enhanced THz
output, stage-FULL includes a 4th chicane FL1CH4 [32]between the qDBA and the actual THz undulator. Finallythe beam is transported into the dump via a coupling-freebeamline much alike the FLASH2 dump line.
The CDR [15] foresees for FLASH2, the installation of
the PolariX TDS, the installation of a helical third harmonic
afterburner undulator (APPLE-III) and not further specified
modifications for advance lasing schemes. PolariX is installed and operational although not yet at full RF power(conditioning was slower than expected), the afterburner will
be installed this fall.

Stage-Full vs. Stage-0
As mentioned before not all features of the FLASH1 upgrade (stage-FULL) can already be achieved in the 2024/25
shutdown. However, the so called stage-0, to be implemented
2024/25, was designed by delaying features in way that allows implementation at a later stage with minimal effort.
Features that do not immediately affect the primary goal of
external seeding were delayed. This includes the ability tosaturate SASE at short wavelengths and high pulse-energy
THz radiation. Only 6 of the 11 APPLE-III radiators will be
installed at first, the first three radiators will be replaced byrecycled planar Xseed [34] radiators as buncher, 2 more willbe left empty at first. The post-compressor chicane FL1CH4
and some THz related electron beam diagnostic will be delayed. The intersections of the radiators will not equipped
2 in beam direction left

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laser heater

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Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035
with wire-scanners in the first implementation. The space
will however be reserved.

CONCLUSION

FLS2023, Luzern, Switzerland

JACoW Publishingdoi:10.18429/JACoW-FLS2023-MO3A5[9] R. D’Arcy et al., “FLASHForward: plasma wakefield accelerator science for high-average-power applicationsd”, Phil.
Trans. R. Soc. A, vol. 377, p. 20180392, 2019.
doi:10.1098/rsta.2018.0392

FLASH was and is a competitive FEL user facility. The
FLASH2020+ upgrade will extend the wavelength range andmake FLASH the only seeded VUV / soft X-ray FEL capableof supplying several thousand FEL pulses per second. The
first upgrade shutdown was successfully finished and user
operation was established after a very short commissioning
phase. The second shutdown planning is within schedule.
The originally planned second stage of the upgrade will
not be fully achievable after the next shutdown. However,delayed features can without exception be implemented later.
We are looking forward to the next step of the FLASH2020+
upgrade.

[10] B. Lautenschlager et al., “Arrival Time Stabilization
at FLASH Using the Bunch Arrival Corrector Cavity(BACCA)”, Proc. 12th Int. Particle Acc. Conf. (IPAC’21),
Campinas, SP, Brazil, 2021, paper TUPAB302.
doi:10.18429/JACoW-IPAC2021-TUPAB302

ACKNOWLEDGMENTS[13] P. Gonzalez Caminal, “Beam-Based Commissioning of a
Novel X-Band Transverse Deflection Structure with Variable
Polarization”, submitted to Phys. Rev. Accel. Beams.

We thank all the people from the FLASH team and the
FLA2020+ project who made the FLASH2020+ upgrade
possible. In particular we thank the people and groups thatworked on preparing, performing the 1st shutdown, and on
re-commissioning FLASH after the 1st shutdown and to
those preparing the 2nd shutdown.

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ISSN: 2673-7035

FLS2023, Luzern, Switzerland[24] C. Gerth et al., “Layout of the Laser Heater for
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Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI[25] Ph. Amstutz and M. Vogt, “Microbunching Studies for the FLASH2020+ Upgrade Using a SemiLagrangian Vlasov Solver”, in Proc. IPAC’22,
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