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

FLS2023, Luzern, Switzerland

JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P12

C. Davut∗ ,1,3 , O. Apsimon1,3 , S. S. Percival2,3 , and B. L. Militsyn2,3
1
The University of Manchester, Manchester, UK
2
STFC UKRI, Daresbury, UK
3
Cockcroft Institute of Accelerator Science and Technology, Daresbury, UK
Abstract
In this contribution, we review the low-energy electron
injectors for the existing X-ray Free-Electron Laser (XFEL)
facilities, focusing on the buncher and booster sections. The
technology choices are parallel to the increasing demand forstricter six-dimensional phase space quality. The current
capabilities for beam parameters and future requirements arelaid out, alongside a discussion on challenges and technological bottlenecks. In light of this review, preliminary results
for a high-capability injector providing a high repetition rateand continuous wave emission are presented as an option
for the UK XFEL.

INTRODUCTIONXFEL facilities are an unprecedented tool for probing
matter at the atomic and molecular scales. The concept of
the Free-Electron Laser (FEL) was introduced in the early
1970s [1], and since then developments in FEL technology
led to the construction of XFEL facilities. The first lasingin the vacuum ultra-violet range was achieved in 2005 at
the Deutsches Elektronen-Synchrotron (DESY) [2]. The
FLASH user facility commenced operation in the same year
and extended its capabilities into the soft X-ray range. A
milestone was reached in 2009 when the Linac Coherent
Light Source (LCLS) at the SLAC National Accelerator Laboratory achieved X-ray lasing, starting a new era of X-ray
science [3]. Since then, ongoing efforts have focused on advancing XFEL capabilities with the exploration of different
concepts such as multi- colour, two-pulse X-rays, variable
polarisation states, high spectral purity X-rays, higher brightness X-rays, sub-femtosecond and attosecond X-ray pulses
by manipulating the electron bunch before self-amplified
spontaneous emission (SASE) takes place [4]. However,increasing demand in beam quality through expanding thescope of XFEL applications leads to upgrades of existing
facilities [5, 6] and motivates the construction of new ones.
One such machine, the UK XFEL, is currently undergoing
the Conceptual Design and Options Analysis phase basedon the published science case [7]. An exhaustive sciencecase for UK XFEL demonstrated a community preference
for high repetition rate, photon energy, energy per pulse,and some additional features such as high repetition ratelaser seeding, high spectral purity X-rays, attosecond pulses
across all photon energies, combining X-rays with otheradvanced capabilities for EUV/gammas to have a unique
X-ray light source [8].
∗ can.davut@manchester.ac.uk

UK XFEL aims to operate across a large portion of therepetition rate-photon energy parameter space as shown in
Fig. 1. In light of the science case requirements, UK XFEL
has been proposed to operate at a 1 MHz repetition rate thatwill be likely driven by an 8 GeV beam generated using superconducting technologies. A more detailed discussion on thetechnologies proposed to be used based on the preliminary
focus of the UK XFEL can be found in [7].
High brightness plays a central role in determining theability of a light source to access new domains of ultra-fast
X-ray science [9]. Therefore, the main design objective for
the UK XFEL injector is to explore the minimum possible
transverse slice emittance at the end of the injector.
The preliminary design of the UK XFEL proposes the useof a normal conducting very high frequency (VHF) gun operating either at the 6th or 7th subharmonic of the main linac
RF frequency. The VHF gun will be followed by a single
or 2-cell buncher operating at either harmonic or subharmonic of the main linac RF frequency to compress bunches
to short lengths. Following that, a booster section will bring
the beam to the emittance-dominated regime, where the energy of the beam reaches ∼220 MeV after acceleration. A
magnetic chicane will then perform the final bunch compression before the main linac. The design goal of the UK
XFEL injector is preserving the transverse core slice emittance while compressing the bunch as much as possible anddelivering it to the main linac at a high repetition rate.

Figure 1: Research areas potentially covered by the UK
XFEL, based on the preliminary repetition rate and photon
energy capabilities. Figure courtesy of [7].
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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 DOIINJECTION INTO XFELs, A REVIEW OF TRENDS AND CHALLENGES

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 DOIOVERVIEW OF INJECTORS FOR XFELs
The performance and the components of the injectors
employed in XFELs rely on the type of electron gun used.
The electron bunch is typically generated using either photoemission or thermionic emission. In RF photo guns, thedistribution of the bunch is controlled by the laser pulsethat drives the photoemission process when illuminating thephotocathode, while the resonant RF structures of the gun
generate the accelerating field to extract the emitted electrons. RF photo-guns for XFELs are generally classed in twoemission types as either high-field or medium to low-field,
based on their cathode accelerating field, where the transverse brightness directly depends on the beam intensity [10].
A high-field RF photo-gun enables photoemission of highpeak current however delivering a very low transverse emittance can be challenging due to the space charge defocusing.
In this emission type, the bunch length, 𝜎𝑧 , is smaller than
the transverse beam size, 𝜎𝑥,𝑦 . The transverse emittanceand brightness can be estimated using ‘pancake’ approximation [11]. Such an emission type can be obtained using
high-frequency pulsed guns (>1 GHz) to reach high cathodefields up to ∼100 MV/m [12]. The beam is then matched
into the main linac using solenoidal focusing as shown in
Fig. 2(a). This ensures the compensation of the space charge
effect and minimises the projected emittance.

(a)

(b)

Figure 2: A simplified layout of (a) high (b) medium to low
accelerating field RF photo-gun based electron injector.
Most operational XFEL facilities are based on pulsed RF
photo-guns. The repetition rate is an engineering challenge
and is limited by RF power dissipation on the walls of thecavity [10]. Therefore, the operation of an XFEL in Continuous Wave (CW) mode with high RF frequency and high RF
field is not ideal. CW operation of normal conducting (NC)gun-based injectors can be performed either by reducing thefield or the frequency, which is typically ∼200 MHz [10].
An alternative technology is the superconducting RF(SRF) gun. In addition to their power efficiency, SRF guns
have the advantage of reducing the migration of dislocations on the photocathode which can convert to dark current
sources [13]. However, SRF gun technology is still in the

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development stage and faces some challenges. SRF cavities
have poor compatibility with insertable photo cathodes dueto the risk of contamination as a result of frequent photo
cathode changes. In addition, the use of either cryogenic orroom temperature solenoids far away from the gun cavity
also poses challenges, due to Meissner field exclusion which
can reduce the effect on the emittance compensation [12].
Since SRF guns can operate up to 50 MV/m (as demonstrated at KEK [14], with R&D studies pursuing higher
gradients) with CW operation that is lower than the limitsof high-frequency NC RF guns. Different groups at HZDRELBE, HZB-bERLinPro, DESY (for EU-XFEL), and KEK
are working on the development of L-band SRF guns toachieve CW operation under high accelerating fields [10, 12,
14, 15].
The second type of emission involves lasing the photocathode to produce electrons in an elongated, ‘cigar’ likepattern to reach low emittance by reducing space charge. In
this case, a longer laser pulse is used to emit the electrons,
therefore the bunch length is much larger than the radiusof the bunch in the transverse direction, where 𝜎𝑧 >> 𝜎𝑥,𝑦 .
The transverse emittance and brightness can be estimatedusing ‘cigar’ approximation [16]. Such an electron gun istypically operated at a subharmonic of the main linac (6thor 7th) that is sufficiently low frequency that allows to host along bunch. The advantage of a low-frequency gun is to havenegligibly small phase slippage, thus the emission field isalmost the same as the cathode peak field. The long bunch isthen matched (using a solenoid) into the buncher for ballistic
compression before its acceleration in the booster as shownin Fig. 2(b). The UK XFEL injector is based on a VHF-band(185.7 MHz) NC CW gun to operate at 1 MHz repetition
rate. Similar technology was demonstrated at LBNL for
Advanced Photoinjector EXperiment (APEX) designed for
the LCLS-II injector [17]. One should note that the APEX
gun was then improved to the APEX2 design by increasing
both the cathode launching field and the output energy to
reduce RF heating and increase the cathode field [18].
There are currently two CW XFELs under construction,
one is LCLS-II at SLAC, which improves the repetition ratefrom 120 Hz to 1 MHz compared to LCLS, and SHINE in
China besides EU XFEL CW upgrade [5, 19, 20].

UK XFEL LOW ENERGY BEAM LINE
The proposed UK XFEL low energy beamline will generate a beam from a VHF gun bunched by a one or 2-cellbuncher having harmonic or subharmonic of the main linac
RF frequency followed by a booster linac to reach the emittance dominated regime at 220-250 MeV. The preliminary
design presented here is based on a long electron bunch
via ‘cigar’ emission, reaching slightly higher than 1 MeV
energy level at the end of the gun and then compressing thebunch using a 2-cell harmonic buncher to ensure a linear
longitudinal phase space before acceleration in the booster.
A diagram of the injector beamline used in OPAL [21] simulations is shown in Fig. 3. This preliminary simulation study

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JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P12

uses field maps constructed by cylindrical single-cell field
distributions for VHF gun, buncher, and booster cavities
with the parameters given in Table 1 to demonstrate a ballistic compression scheme while preserving the normalised
core emittance below the target 1000 nm rad.

Figure 3: Diagram of the UK XFEL injector line showing thecomponents used in the simulations. A half-cell VHF gun(operating at 185.7 MHz, 7th subharmonic of the linac RF),
2-cell 1.3 GHz buncher, and 2-cell 1.3 GHz linac (booster)
were used along with the solenoids.

Figure 4: OPAL simulation results showing the RMS bunch
length as well as the relative energy spread of the electron
bunch along the beamline.

Table 1: OPAL simulation parameters used for the simulation study. The provided bunch parameters correspond to
the initial flattop distribution.
Parameter

Value

Unit

Bunch

ChargeSize
Length

300
0.75
150

pC
mm
ps

Gun

Phase
Frequency
Field

on crest
185.7
30.0

MHz
MV/m

Buncher

Phase
Frequency
Field

-90
1300.010degreeMHz
MV/mBooster

Phase
Frequency
Field

on crest
1300.0
100.0

MHz
MV/m

Figure 4 shows the RMS bunch length and energy spreadof the electron bunch along the injector line. The ballistic
bunching implemented aims to compress the bunch down to10∘ of the RF frequency corresponding ∼20 ps at the twocell 1.3 GHz buncher. However, the energy spread increases
in the buncher cavity due to a 90∘ phase shift relative tothe gun phase where the energy of the tail of the bunchincreases while it decreases at the head of the bunch. Thisis compensated in the booster during acceleration.
Normalised core emittance of the bunch (which refers to
the emittance calculated using only the three slices at thecenter of the bunch out of 9 slices in total) was calculated
along the beamline as shown in Fig. 5. The core emittanceon the x- and y-axis at the end of the simulation window
were found 588.5 and 576.57 nm rad, respectively. The
studies of the low-energy injector will continue to investigate
optimising the longitudinal phase space during the bunch
compression as a function of the number of the cells, RF

Figure 5: The variation of the normalized core emittance on
the x- and y-axis along the beamline.
frequency, as well as the field in the buncher cavity; andpreservation of the core emittance. The next steps include
the demonstration of acceleration of the bunch using a fullbooster linac.

CONCLUSIONThe UK XFEL conceptual design was initiated following
a comprehensive science case study [7]. A possible lowenergy injector design was studied to demonstrate the ballistic compression scheme using a 2-cell harmonic buncher
and preserve the normalised core emittance. A single-cellbuncher operating at the second subharmonic of the main
linac RF frequency, along with various booster accelerating
fields, is also under consideration for further simulations
based on multi-objective optimisation.

ACKNOWLEDGEMENTS
The Authors would like to thank Alexander Malyzhenkov
from CERN and Ben Hounsell from ASTeC for helpful
discussions.

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

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

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