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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-TU4P05

DESIGN OF THE TEST PLATFORM FOR HIGH AVERAGE CURRENT
VHF ELECTRON GUN
Zipeng Liu1,2 , Houjun Qian1,2 , Guan Shu1,2 , Xudong Li1,2 , Zenggong Jiang1,2 , Haixiao Deng1,2,∗
1
Shanghai Synchrotron Radiation Facility, Shanghai, China
2Shanghai Advanced Research Institute, Shanghai, China
Abstract
Recently a high-average-current CW VHF electron gun is
under construction at Shanghai Advanced Research Institute,which is aimed to develop the high average current and highbeam quality technologies. The high average current electron source is the key component of a kW-power-order freeelectron laser facility. The average current and the frequencyof this electron gun is 1-10 mA and 216.7 MHz, respectively.
The energy of electron is over 500 kV, and repetition rate isabout 1-9 MHz. To validate the performance of this instrument, a test platform has been designed. The R&D of its
vacuum and diagnostics design are presented in this work.

INTRODUCTION
In recent years, several projects involving free electron
lasers (FEL) and energy recovery linacs (ERL) facilities
have been proposed and commissioned. These facilities require high repetition rates, low emittance, and high power.
Some facilities, such as LCLS-II, SHINE, and APEX, have
employed normal-conducting (NC) RF electron guns [1–4].
The electron gun (VHF) test platform in this work is underconstruction based on a NC continuous-wave (CW) RF photogun at the Shanghai Advanced Research Institute (SARI).
The core of our photogun is a NC copper RF cavity operating at the VHF band, specifically at 216.7 MHz. Thiscavity generates a 22.5 MV/m accelerating field when supplied with nearly 90 kW of power. The accelerating gap
measures 4 cm. To operate high quantum efficiency semiconductor photocathodes (Cs2Te), an expected vacuum levelof 10−10 − 10−9 mbar is necessary.Beam quality, including
energy, normalized emittance, beam size, beam current, andthe thermal emittance of the photocathode, significantly influences the electron source. The layout of the test platform
is outlined in this paper, focusing mainly on diagnostics andvacuum design. The theoretical vacuum levels for the electron gun and test line are 3 × 10−8 mbar and 1 × 10−7 mbar,respectively. The main tube diameter is 50 mm, and theentrance diameter of the dump is 80 mm.

MANUSCRIPTS
The design objectives are presented in Table 1, and thelayout of the test platform is illustrated in Fig. 1. The heightof the beamline is 1.3 m. The beamline comprises a load
lock, electron gun, solenoids, dipole, profile monitors, ICT(Integrated Current Transformer), and BPMs (Beam Position∗ denghx@sari.ac.cn

Monitors). The positions of the main elements are indicatedin Table 2.
Table 1: Design Objectives
Parameter

Value

Unit

EnergyAverage current
Frequency
Repetition rate
Charge

>500
>1
216.7
1-9200keVmAMHzMHzpC

Table 2: Test Platform Layout
ElementElectron gun
Solenoid 1,2Laser injector
Profile 1,2,3DumpPosition

Unit

00.27,1.8
0.92
1.5,0.4(to the line),2.20.65(to the line)mm
mmm

Diagnostics Design
The essential characteristics of the electron gun’s photocathode are crucial for the electron source. Therefore, thiswork includes simulations to measure beam energy, darkcurrent, normalized emittance, and transverse momentum.
Dark current imaging Dark current is a primary focusof the electron source, particularly because superconducting
linacs such as LCLS-II and SHINE require low dark currents
for optimal acceleration quality and extended lifespan. Dark
current imaging plays a role in enhancing the photo-cathodemanufacturing process. As depicted in Equation 1, dark current emitted from the same position with varying transverse
momentum should reach the same position on the profile
by matching the strength of the solenoid (𝑀12 = 0). The
relationship between imaging magnification and the distance
between the photo-cathode and the profile is demonstrated
in Fig. 2b, where the primary emission of dark current isfrom the 5 mm center of the photo-cathode. As depicted
in Fig. 2a, the dark current is imaged, resulting in a loss oftransmission of the profile.
[𝑀11𝑀21

𝑀12 𝑟0𝑟] [ ] = [ ′]𝑀22 𝑟0′𝑟(1)

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JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P05

Figure 1: Layout of VHF electron gun test platform. The test line include photo-cathode system, electron gun system,
diagnostics system, vacuum system, magnet system and radiapretection system. The elements include solenoids, dipole
magnet, profile, ion pump, correctors, SBPMs, BBPM, ICT and dump.
(a)

(b)

Figure 2: The simulation of dark current. (a) The relationship between position of profile and imaging magnification.
(b) The dark current is imaging in profile of 1.5 m distance,
and the beam transmission loss is also shown.
Transverse momentum imaging Transverse momentum is the key characteristic of photo-cathode. As shownin Equation 1, the dark current emitted from the different
position with same transverse momentum should arrive thesame position of profile through matching the strength ofsolenoid (𝑀11 = 0). The relationship between the distance
between the profile and the photo-cathode and the imaging
magnification was simulated using ASTRA. As shown inFig. 3(b), the distance of the profile is 2.2 meters, in addition
to the resolution of the CCD. The influence of space charge
is also presented in Fig. 3(a), the space charge can be ignored when the beam charge is below 0.05 pC. Additionally,
GAGG with high quantum efficiency will be chosen.
Normalized emittance measurement The normalized
emittance is significant for FEL lasing. In this work, thesingle-slit method was used to measure the normalized emittance. However, the space charge will increase the measured
normalized emittance. As shown in Fig. 4, the space charge
influence was simulated with different slit gaps. The redand blue lines represent the intensity distribution with andwithout space charge, respectively. Different slit gaps of60, 100, 140, and 200 µm were simulated using ASTRA.

（a）

（b）

Figure 3: The theoretical simulation of transverse momentum imaging. (a) The influence of space charge for imaging
the transverse momentum of 2.2 m profile. (b) The relationship between the imaging magnification and position ofprofile.
Additionally, the gap of slit should be below 100 µm to avoid
the influence of space charge.

Vacuum Design
An excellent vacuum level is necessary for an RF cavity
system, and a low vacuum level will result in radiation passing through activated air. In our test platform, the vacuum
level of the RF cavity should be maintained at the orderof 10−8 . Simultaneously, the vacuum level of the test line
should achieve an order of 10−7 . The RF cavity maintains
a high vacuum level using nine Z400 NEG pumps and a
200 L/s ion pump. To attain such a high vacuum level, four
dump ports will also be utilized. Three of these consist ofone 50 L/s ion pump and one Z400 NEG pump each. Thepump port in front of the copper dump contains a 400 L/s
ion pump. This pump will prevent the main line from experiencing a poor vacuum level resulting from the high average
current. As shown in Fig. 5, the top figure depicts the 3D
grid of the entire test platform in Molflow+, while the bottom figure illustrates the vacuum level distribution from thephotocathode to profile 3. The vacuum level of the electron
gun is nearly 3 × 10−10 mbar, and the test line measuresaround 2.7 × 10−9 mbar.
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81

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

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-TU4P05

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

SUMMARY

In this work, the electron gun test platform at SARI is described. Additionally, the diagnostic and vacuum design arepresented. The test platform can be used to validate the quality of electrons by measuring dark current, average current,normalized emittance, transverse momentum, charge, andmore. The theoretical vacuum level is also discussed in thiswork, with the vacuum level of the test line and electron gun
being approximately 3 × 10−10 mbar and 2.7 × 10−9 mbar,respectively.

ACKNOWLEDGEMENTS
Figure 4: The space charge influence for measuring normalized emittance. The read line and blue line is the rms
beam size without and with space charge, respectively. The
slit gaps of 60, 100, 140, and 200 µm are presented. The
influence of space charge can be ignored when the slit gap
less than 100 µm.
(a)This work was supported by the CAS Project for Young
Scientists in Basic Research (YSBR-042), the National Natural Science Foundation of China (12125508, 11935020),Program of Shanghai Academic/Technology Research Leader(21XD1404100),and Shanghai Pilot Program for Basic Research – Chinese Academy of Sciences, Shanghai Branch(JCYJ-SHFY-2021-010).

REFERENCES[1] F. Zhou et al., “First commissioning of LCLS-II CW injector
source” in Proc. 10th Int. Particle Accelerator Conf. (IPAC’10),
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doi:10.18429/JACoW-IPAC2019-TUPTS106[2] H. Chen et al. “Analysis of slice transverse emittance evolution
in a very-high-frequency gun photoinjector”, Phys. Rev. Accel.
Beams, vol. 24, p. 124402, 2021.
doi:10.1103/PhysRevAccelBeams.24.124402[3] Z. Y. Zhu et al., “SCLF: An 8-GEV CW SCRF linac-based
x-ray FEL facility”, Proc. 38th Int. Free Electron Laser Conf.
(FEL’17), Santa Fe, NM, USA, Aug. 2017, pp. 182–-184.
doi:10.18429/JACoW-FEL2017-MOP055[4] P. Emma et al., “First lasing and operation of an ångstromwavelength free-electron laser”, Nat. Photonics, vol. 4,pp. 641–647, 2010. doi:10.1038/nphoton.2010.176(b)Figure 5: The simulation of vacuum. (a) The 3D grid of test
platform in Molflow+. (b) The vacuum level distribution
from photo-cathode to the profile 3.

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