Electron RF Injectors for Next
Generation FELs
Boris Militsyn on behalf of UK XFEL team
STFC Daresbury Laboratory, UKFuture Light Sources Workshop, 28 August – 1 September 2023
Lucerne, Switzerland

Outline• Motivation• Main constraints of the RF gun
development• RF guns of the operational FELs
• Modern trends on the RF gun
development• Normal conducting RF guns
Image © STFC John Dawson• CW RF guns
• UK XFEL RF injector• Conclusion

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Concept of UK XFEL

xfel.ac.uk
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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UK XFEL. General concept Transform-limited operation across the entire X-ray range(initial focus on 0.1 – 20 keV and 100 as – 100 fs).
 High efficiency facility, with a step change in thesimultaneous operation of multiple end stations.
 Evenly spaced, high rep. rate pulses to match samples anddetectors.
 Improved synchronisation/timing data with external lasers to< 1 fs.
 Widely separated multiple colour X-rays to at least one endstation.
 Full array of synchronised sources: XUV-THz, e-beams, highpower & high energy lasers at high rep. rate.

See also TU4P13 “An Introduction to the UK XFEL
Conceptual Design and Options Analysis”, D. Dunning etal.

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Typical requirements to the beams
for next generation FELs
Wavelength of a photon with given energy is defined as:ℎ𝑐𝜆𝑒𝑈That gives 62 pm at U=20 keV (HXR) and 13.4 nm for U=100 eV (SXR)
For generation of photons with these energies by standard SASE schemes
geometrical slice emittance of the electron beam at the entrance to the undulator
may be estimated as:𝜀or 𝜀𝜀

1.9 𝑚𝑚 · 𝑚𝑟𝑎𝑑 for generation of 100 eV photons with 1 GeV beam energy and𝜀0.078 𝑚𝑚 · 𝑚𝑟𝑎𝑑 for generation of 20 keV photons with 8 GeV beam

Bunch charge

100-300 pC

Peak current

3 kA

Bunch repetition rate

1+ MHzRMS energy spread ≤1.0·10-4

Average current

0.3+ mA

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Generation of the high brightness beams
 Before injection into the FEL undulator high brightness electron beam:• Emitted from photocathode with as low emittance as possible• Accelerated to MeV scale energy in the gun to preserve low emittance• Longitudinally compressed to the length, necessary for injection into the booster linac
and accelerated to the energy of emittance dominated regime• Accelerated and compressed until required energy and length necessary for FEL
generation

 We will consider first two stages of this process

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Electron emission in the RF gunEmittance of the beam emitted from the photocathode in photoinjector is defined by laser beamtransverse size and structure and Mean Transverse Energy (MTE) which is defined by property of thephotocathode and laser pulse energy:𝜀 ,at ℎ𝜈

𝜑𝑟 𝑀𝑇𝐸𝑘𝑇2 𝑚𝑐2𝑚𝑐ℎ𝜈 𝜑𝑀𝑇𝐸3∝~1.0 𝑒𝑉, normalise RMS emittance at room temperature 𝜀 ,~0.41𝑟

2𝑟

FEL injector typically operate with “cigar emission” when
Photocathode𝜏∝For “cigar” emission𝑟⁄

𝑇

D. Filippetto et al., Phys. Rev. ST
Accel. Beams 17, 024201 (2014)36∝B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

G. Shamuilov et al., Appl. Phys.
Lett. 113 204103 (2018)7

Photocathodes for RF photoinjectors
Material

Wavelength

Quantum
Efficiency

Robustness

World Expertise

Application

High Work Function Metals(Cu, Mo, Nb)

UV

10-5

HighHigh

NCRF highfield guns

UV

0.05 – 0.1

Medium-HighHigh

NCRF highfield guns

Low Work Function
Metals (Mg)UV

10-4 – 10-3??

MediumLow Field
SRF guns

Reduced Work Function
Metal Oxides (MgO)

Blue??

Medium-Low

Low

N/A

Sb-based (Cs2KSb)

Green

0.05 – 0.1

MediumHigh

Low/Medium field
VHF/SRF guns

GaAs

IR-Green

0.001 – 0.4

LowHigh

DC/Polarised guns

Te-based (Cs2Te)

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Photoemission from Cs2Te – potential
photocathode for CW XFELs
𝑟 2𝐸
2 3𝑚 𝑐𝜀 ,where𝐸𝐸𝐸

𝐸

0.55 𝑒𝑉that gives
𝜀 ,

0.85σ

Klaus Floettmann, “Notes on the thermal emittance of electrons emitted by
Cesium-Telluride photo cathodes”, TESLA FEL-Report 1997-01

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Reduced Work Function Photocathodes:
Ultra-thin Metal Oxide Films
Photocathode:MgO on Ag(100)• MgO film of around two monolayers
reduced the WF by ~1 eV

WF [eV]QE @ 266 nm

Ag(100)4.36 ± 0.05

1.1 x 10-4

MgO/Ag(100)3.40 ± 0.10

9.2 x 10-4

• Spectral response shown up to 360 nm• 3rd harmonic of Nd:YVO4 laser• QE increase at 266 nm by a factor of eighthttps://doi.org/10.1063/5.0124528

Courtesy L.B. Jones & Co

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Beam acceleration in the gun Maximum cathode electric field achievable in RF injectors directly linkedwith cavity frequency
 Breakdown field limited by Kilpatrick criterion:
𝑓 𝑀𝐻𝑧
1.64 𝐸0 𝑀𝑉 𝑚 · 𝑒 RF average and peak power dissipation. / Possible technologies for pulsed operation
 High frequency (S-,C-,X- bands) NCRF guns

 Possible technologies for CW operation

 DC photoinjectors
 Normal Conductive RF photoinjectors with a frequency of about 200 MHz (VHF)
 Superconducting RF photoinjectors
 Thermionic injectors

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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RF guns of operational FELs
Technology

Numberof cells

Operationmode

Bunch
repetition
rate, HzRF
frequency,
GHz

Cathodefield,
MV/m

LCLS

NCRF

1.6

Pulsed

120

2.856

115

SwissFEL

NCRF

2.6

Pulsed100 (400)

2.856

100

PAL-FEL

NCRF

1.6

Pulsed

60

2.856

120

EuXFEL

NCRF

1.6

Train-Pulsed

10x2700

1.3

50-60

ELBE

SRF

3.5

CW

50000250000

1.3

22

Facility

LCLS gun scheme

EUXFEL L-band gun

PAL-FEL gun
ELBE SRF gun
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

SwissFEL 2.6 cel gun
12Modern trends in the development of electronguns for XFELs
 Extra high cathode field high brightness pulsed photocathode guns

 Increasing photocathode field as high as possible, typically higher than 120 MV/m
 Development of the guns in S-, C- and X- bands
 Cryogenic NCRF guns
 Typically metal photocathodes

 High cathode field SRF CW photocathode guns

 L-band TESLA type cavities
 Operation at a field of 40-50 MV/m
 Operation with low work function metal and/or Cs2Te photocathodes

 Low cathode field CW SRF and NCRF photocathode gun Operation at a cathode filed of 20-30 MV/m
 Operation with low work function metal or alkali (Te and Sb) photocathodeB.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Potential normalized slice emittance of the RFguns
Photocathode guntechnology

DC*Q-wave SRF
Q-wavae NCRF
Q-wave NCRF
Q-wavae SRF
L-band SRF
L-band NCRF
S-band NCRF
C-band NCRF

Operation
mode

CW
CW
CW
CW
CW
CW
Train-pulsed
PulsedPulsedPhotoinjector
operationalfrequency, MHz

Cathode field, Emission pulse Normalised beam emittance, nm·rad at bunchcharge, pC**MV/mlength, psDC112186186217
1300
1300
3000
6000

1020
20
30
30
40
60
100
200

149.34248.02149.34149.34128.01
21.37
21.37
9.26
4.63

20

50

100

300

28.1
10.0
14.1
9.4
10.4
25.7
17.1
18.0
14.3

51.8
18.5
25.9
17.3
19.1
47.4
31.6
33.1
26.3

82.3
29.3
41.1
27.4
30.4
75.2
50.1
52.5
41.7

171.1
61.0
85.6
57.0
63.2
156.4
104.3
109.3
86.7

* - injector with DC gun and 186 MHz buncher** - injection at maximum field

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

14Daresbury 400 Hz High Repetition
Rate Gun (HRRG)
H shape coupler

RF probe
Bucking coil

Main solenoid

Waveguide split/pumping

Cathode plug
Gun Cavity

Cathode plug
Cooling channels
Phase adjustment screw
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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HRRG commissioning status•

HRRG has been recently commissioned at a fieldof 70 MV/m and demonstrated 100 pC bunches
following with a repetition rate of 100 Hz.

•The beam was delivered by hybrid Cu-Mo
photocathode with QE=2.2·10-4

VCYAG1YAG4YAG4B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

16NCRF VHF photocathode guns
 Under development for LCLS-II and SHINE FELs.
 Most recent results have been presented in the presentations
 MO3A1 – Dong Wang, “Progress on SHINE machine”
 MO3A3 – Daniel Gonnella, “Status LCLS-II Superconducting linac”
where first LCLS-II lasing was reported

Our congratulations to the LCLS-II team!!!

Rev. Sci. Instrum. 90, 033304 (2019)
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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SRF L-band photocathode gun, HZB

doi:10.18429/JACoW-IPAC2018-TUPML053

Cavities for operation with Cs2KSb photocathodes

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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SRF Quarter Wave photocathode gun, BNL
Operates with Cs2KSb photocathodes

UnitsParameters

ValueGun Voltage, MV

1.25

Charge per Bunch, pC

DOI: 10.1103/PhysRevLett.124.244801

100-20,000

Average Beam Current, mA @ 1500 pC

0.15

Normalized transverse RMS slice emittance @ 100 pC, mm-mrad

0.15

Normalized transverse RMS projected emittance @ 100 pC, mm-mrad

0.3

Longitudinal RMS slice emittance @ 100 pC, keV-ps

0.7

Quantum Efficiency, %1-4

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Wisconsin SRF gun. Design

R. Legg et al. IPAC2012, MOPPP045

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Quarter wave cavity SRF gun

J.W. Lewellen, NAPAC2022, WEPA03
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Concept of UK XFEL Low Energy Beamline
Subharmonicbuncher

Main frequency linac(Subharmonic) booster

Magnetic compressor

Subharmonic
VHF/SRF gun
Some RF linearization andlaser heater may be required

For 𝐼To mainlinac

15 𝐴, 𝜀Beam diagnostic line
2𝐼 1·𝛾𝐼 𝛾′𝜀⁄0.078 𝑚𝑚 · 𝑚𝑟𝑎𝑑 𝑎𝑛𝑑 𝛾
39 ⁄ 20𝛾 580, 𝐸 296 𝑀𝑒𝑉See also TU4P12 “Injection into XFELS, a review of trendsand challenges”, Can Davut et al.
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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GPT simulation of 300 pC beam in the 30 MV/m
186 MHz VHF gun

Courtesy Suzanna Percival
B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Conclusions
 New XFELs projects are dedicated for generation of photons with an energy of 20keV and higher that requires
extra high brightness electron beams to limit energy of the drive linacs
 Some of the project including UKXFEL also require repetition rate of the photon pulses of 1 MHz and higher Existing injector technologies do not allow to meet these two requirements simultaneously. So photoinjector
R&D should be concentrated on two directions: Development of extra high brightness high field photocathode guns for operation in pulsed mode
 Development of high brightness high repetition rate injector for operation in CW mode. That may be reached bytwo ways
 Development of high frequency medium field SRF photocathode guns. That requires
 Design of RF cavities and photocathode infrastructure Development of low dark current photocathodes which are able to operate in SRF gun environment and deliver mA
scale average current

 Development of NCRF and/or SRF VHF low field photocathode guns
 In case of use SRF cavities resolve the same photocathode problem as for high frequency SRF guns
 Development of the emittance preserving compression technologies which would allow for delivering high brightness
beam into the main drive linac

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Acknowledgement
 Axel Neumann, HZB Can Davut, The University of Manchester Feng Zhou, SLAC Dave Dunning, ASTeC

 Houjun Qian, DESY Lee Jones, ASTeC

 Irina Petrushina, BNL Oznur Apsimon, The University of Manchester Luca Cultrera, BNL Suzanna Percival, ASTeC

 Masahiro Yamamoto, KEK Olga Tanaka, KEK Rong Xiang, HZDR Thorsten Kamps, HZB

B.L. Militsyn, FLS2023, Lucerne, Switzerland, 28.08-01.09

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Thank you

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