Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THOA02

SIMULATION OF A NEW PHOTOCATHODE RF GUN IN KU-FEL
Yuhao Zhao†, Hideaki Ohgaki, Heishun Zen
Graduate School of Energy Science, Kyoto University
Abstract
Upgrade project of Mid-infrared Free Electron Laser in
Kyoto University (KU-FEL) has been started to achieve
high peak power and ultra-short pulse operation by installing a new photocathode RF gun under the High-Repetition-Rate attosecond pulse source based on the High Harmonic Generation (HHG) project, Q-LEAP. A start-to end
simulation was performed to optimize the magnet component of the accelerator and to evaluate an expected FEL
performance. The 1.6-cell photocathode RF gun manufactured by KEK is expected to generate a 1-nC bunch charge
beam. A beam compression study has been conducted andthe RMS bunch length has been success-fully compressed
to 0.42 ps which can generate a 1 kA of peak current. The
FEL simulation using the optimized beam parametershows that the FEL intra-cavity power can saturate at 109
Watts by only 5 roundtrips.

INTRODUCTION
The “Q-LEAP” project is the research project to develop
the fundamental technology for high-repetition attosecond
light sources driven by next-generation lasers and Free
Electron Lasers. We aim to make the development and realization of a high repetition rate ultrashort pulse lightsource (10 MHz or higher) by generation of few-cycle
pulses in the mid-infrared wavelength region using the existing FEL facility and performing high-order harmonic
generation (HHG).

A beam simulation study on the electron beam and FEL
with the 1.6-cell photocathode has been performed to evaluate the electron beam characteristics by the beam simulation code GPT [3] including the optimization of the accelerator component. FEL simulation is performed by using
GENESIS [4] with an oscillator calculation.

ELECTRON BEAM SIMULATIONThe beam simulation study from the RF gun to the endof accelerator tube has been conducted with a 1-nC bunchcharge beam in 2021 [5]. The laser injection phase and theacceleration phase of the accelerator tube was investigated
and the1-nC bunch charge beam can be successfully accelerated to be 28.5 MeV. In addition, beam parameter was
also optimized at the exit of accelerator tube with the RMS
emittance of 4.12 mm-mrad and RMS energy spread of
0.6% but the bunch length was very large, 2.14 ps, which
can only obtain a peak current of 200A [5]. To achieve
higher peak current with a high bunch charge beam, abunch compression study is indispensable. In the KU-FEL
facility, the electron beam can be compressed by a 180° arc
section.
In the bunch compression simulation with the spacecharge effect caused by 1-nC bunch beam, the relationshipbetween RMS bunch length and several R56 parameter
have been calculated and the result is shown in Fig. 2. We
can perceive that when the R56 was set to be 0.1 m, theRMS bunch length was minimum at 0.42 ps and a 1011 Aof peak current, which is 8.4 times higher than the previous
condition with 4.5-cell thermionic cathode RF gun (photocathode operation) was achieved.

0.42psFigure1: Schematic view of KU-FEL.
At Institute of Advanced Energy, Kyoto University, an
oscillator-type mid-infrared Free Electron Laser named as
KU-FEL, which has been developed and covers the wavelength range from 3.5-23µm [1], joins the Q-LEAP project.
To achieve a higher efficiency for an ultra-short pulse oscillation, a new photocathode RF electron gun has been introduced in KU-FEL [2]. Figure 1 shows the schematic
view of KU-FEL including the new electron gun and solenoid magnet.
____________________________________________† zhao.yuhao.83s@st.kyoto-u.ac.jp

Figure 2: Relation of R56-RMS bunch length.
To optimize the electron beam for FEL generation, theRMS emittance, beam size were calculated by changing theparameter of triple quadruple magnets before and after thearc section. According to the simulation result, magnetic
field gradient of the three quadrupole magnets before arc
section QAC21,22,23 was set to be 1.375, -2.75 and 1.375- 115 -

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THOA02

T/m and the magnets after the arc section QUND1,2,3 wereset to be 3.5, -7, 3.5 T/m which can get the minimum average radius of 0.96 mm at the entrance of the undulator.
Finally, the magnetic field strength of the solenoid which
located at the exit of the photocathode RF gun has been
tuned. The solenoid magnetic field strength was originally
set to be 2006 Gauss [5] and to get the smaller average
beam size at the undulator, the magnetic field strength wasincreased to 2069 Gauss, 2131 Gauss and 2194 Gauss.

Gauss, but once it rises to 2194 Gauss, both the emittanceand radius were increased significantly. Considering thethree parameters of RMS emittance, average radius and theRMS energy spread, the magnetic field strength with 2069
Gauss was chosen (Fig. 3). Tables 1 and 2 show the beamparameter with 4.5-cell thermionic cathode RF gun (photocathode operation) [6] and the new 1.6-cell photocathodeelectron gun at the entrance of the undulator, respectively.
Table 1: Beam Parameter of 4.5-Cell Thermionic CathodeRF Gun (Photocathode Operation) at the Entrance of theUndulator
4.5-cell thermionic cathode RF gun(Photocathode operation)
Bunch charge

120pC

RMS bunch length

0.43psPeak current

120A

RMS energy spread

0.4%

RMS emittance

4.68mm-mrad

Table 2: Beam Parameter of New 1.6-Cell PhotocathodeRF Gun at the Entrance of the Undulator
1.6-cell photocathode RF gun
Bunch charge

1nC

RMS bunch length

0.42psPeak current

1011A

RMS energy spread

1.08%

RMS emittance

5.08 mm-mrad

FEL SIMULATIONKU-FEL consists of a 1.8-m Hybrid-type undulator
whose parameter is shown in Table 3. The FEL simulation
with the new photocathode RF gun (Table 2) was performed using GENESIS1.3.
Table 3: Parameters of Hybrid-Type 1.8-m Undulator
KU-FEL Undulator

Figure 3: RMS Emittance(up), Average Radius(mid), RMS
Energy Spread(down) with magnetic field strength at 2069
GAUSS.

Total length

1.8m

Period number52

Undulator period

33mm

Minimum Gap

15mm

Maximum magnetic field

0.559T

Maximum K value
1.72
First, to find the maximum FEL amplification wavelength, the one-pass gain calculation was conducted. The
result is shown as the black curve in Fig. 4. The highest
gain of FEL (307%) is obtained at the wavelength of
12.285 µm.

The result shows that the smaller average radius is obtained when the magnetic field is 2069 Gauss and 2131

- 116 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THOA02

The evolution of FEL peak power with the round-trip isdemonstrated in Fig. 6. The result shows that the FEL peak
power is saturated by 5 round-trips with 109 Watts.

CONCLUSION

Figure 4: FEL wavelength investigation.
Second, for the trajectory correction in the x-axis, thecollector strength parameter CX was investigated as shownin Fig. 5. As the result, the minimum summation of (Δx)2
occurs when CX= -40.7.

Figure 5: Electron beam trajectories for different CX values.
Finally, the multi-pass time-independent simulation using Genesis 1.3 is performed to calculate the FEL gain.

The beam simulation from the exit of acceleration tube
to the entrance of the undulator and the FEL simulation wasconducted with the new 1.6 cell photocathode RF gun with
1-nC bunch charge beam whose maximum acceleration energy is 28.5 MeV. The RMS bunch length was successfullycompressed to 0.42 ps which can achieve a 1011 A peakcurrent by optimization of the arc section component. The
FEL simulation result proves that the FEL intra-cavity
power can saturate at 109 Watts by only 5 roundtrips that ishigher FEL gain than that of the 4.5-cell thermionic cathode RF gun with the photocathode operation. The installation of the new 1.6 cell photocathode RF gun will be finished at the end of 2022 and the beam commissioning isscheduled in early 2023.

REFERENCES[1] H. Zen et al., “Present Status and Perspectives of Long Wavelength Free Electron Lasers at Kyoto University”, 84, 2016,
pp. 47-53.
[2] Hajima, R.; Nagai, R.; Kawase, K.; Ohgaki, H.; Zen, H.;
Hayakawa, Y.; Sakai, T.; Sumitomo, Y.; Shimada, M.;
Miyajima, T., “Application of Infrared FEL Oscillators for
Producing Isolated Attosecond X-Ray Pulses via High-Harmonic Generation in Rare Gases”, Proceedings of the 39th
International Free-Electron Laser Conference (FEL-19),
Hamburg, Germany, 26–30 August 2019; pp. 272–275.
[3] S.B. van der Geer, M.J. de Loos, General Particle Tracer User
Manual Version 3.10.
[4] S. Reiche, “GENESIS 1.3: a fully 3D time-dependent FEL
simulation code”[5] 梶田駿汰 2021 年度修士学位論文, “KU-FEL 加速器の光陰極高周波電子銃導入に向けたシミュレーション”[6] Hideaki. Ohgaki, Toshiteru Kii, Kai Masuda, Satoshi Sasaki,Takumi Shiiyama, Heishun Zen Masao Kuriki, Nobuhiro
Terunuma, Junji Urakawa, Yoshio Kamiya, Masakazu
Washio, “NUMERICAL EVALUATION OF OSCILLATORFEL WITH MULTI-BUNCH PHOTO-CATHODE RF-GUN
IN KYOTO UNIVERSITY”, Novosibirsk, Russia, Proceedings of FEL 2007, 390-393.

Figure 6: Peak power evolution of multi-pass time-independent simulation (Undulator Gap:16mm).

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