Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP002

Electron Beam Generation by a Photocathode RF-gun at Kyoto UniversitySuphakul Sikharin 1,*, Kantaphong Damminsek2, Heishun Zen 1, Toshiteru Kii 1 and Hideaki Ohgaki 1
1Institute of Advanced Energy Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

2Department of Physics and Materials Science, Faculty of Science, Chiang Mai University , 50200 THAILAND*sukarin.supakun.78z@st.kyoto-u.ac.jp

Abstract
A 1.6 cell S-band BNL type photocathode RF-gun
has been installed for a compact seeded terahertz(THz) free-electron laser (FEL) amplifier system at
Institute of Advanced Energy, Kyoto University. The
RF-gun is used for generating an ultra-short, intenseand low emittance electron beam suitable for a THz
radiation generation by a short planar undulator. The
photocathode plug is exchangeable by a load locksystem and illuminated by a UV laser with thewavelength of 266 nm. In the initial stage, the RF
conditioning, the performance test and themeasurements of electron beam properties wasperformed with using the copper cathode. Thedetailed of the RF-gun, test and measurement results
will be presented in this contribution.

electron laser (MIR-FEL) [2]. The system also
installed in the same accelerator room of MIR-FEL.
The schematic diagram of the compact seeded THz
FEL system is shown in Fig.1.
The installation of the photocathode RF-gun had
been started since the end of year 2013. Then, theinstallation of photocathode RF gun was completed
and the photoelectron from the gun was successfully
generated in the end of May 2015. The beamdiagnostic system was set up and beam properties
have been measured. The details of the photocathodeRF gun system, measurement set up and results are
explained in this proceedings.
Solenoid

INTRODUCTION

TripletQuadrupoles
Undulator

A photocathode RF-gun is well known as a one of
the best electron sources with many advantages. The
photocathode RF-gun is able to produce an electron
beam with a high-intensity, short-pulse, and lowemittance. This gun is also completely free from thebackbombardment effect [1] that allows higher bunchcharge and longer macro-pulse duration. The spatial
and temporal electron distribution can be manipulated
by controlling the laser distribution. By theseoutstanding features, the photocathode RF-guns are
suitable for a free-electron laser system and widelyused in many advance accelerator systems.
A 1.6-cell photocathode RF-gun has been installed
as a new electron source and will be used for acompact seeded terahertz free-electron laser (THzFEL) system at Institute of Advanced Energy, Kyoto
University. The target radiation wavelength of thissystem is from 300 to 800 μm which extend theradiation range of the current mid-infrared freeRF-gunChicane

Figure 1: Schematic view of the compact THz-FEL at
Institute of Advanced Energy, Kyoto University.

PHOTOCATHODE RF-GUN SYSTEM
The photocathode RF-gun system for the compact
seeded THz-FEL is mainly composed of an emittancecompensation solenoid magnet, the RF-gun cavity, aload-lock system, a laser port, vacuum components
and an evaporation chamber. The photocathode plugcan be exchange by the load-lock system attached tothe gun backplane. This system allow to exchange
and transport the plug without breaking the vacuum
condition. Two ionization pump and two nonevaporative getter (NEG) pump are used and provide
the total pumping capacity of 380 L/s. In this study, acopper cathode was used for the conditioning of the- 386 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP002

gun cavity and the beam property measurements. The
cut view of the photocathode RF-gun system is shownin Fig.2.
Translational
stage

RF-gunSolenoidmagnetLoad locksystemLaser
port
VacuumpumpFigure 2: Cut view of the photocathode RF-gunsystem.

Photocathode RF-gun
The 1.6-cell S-band BNL-type photocathode RFgun is the improved model of KEK and wasmanufactured in 2008 [3]. This gun has been tested
and succeeded in generation of high brightness
electron beam. The RF-gun performances have beenstudied by numerical simulations in [4]. The
conditioning of the cavity was performed with the RF
pulse duration of 2 μsec and the RF repetition rate of
10 Hz. The vacuum pressure and temperature of thegun was a few 10-8 Pa and 30 oC, respectively.

efficiency of the optical transportation was measured
to be 82-92%.

RF system
The photocathode RF-gun shares the RF powersource with the existing 4.5 cells thermionic cathodeRF-gun of the MIR-FEL. This RF source has the peak
power of 10 MW and the repetition rate of 10 Hz. The
waveguide RF switch, which is installed downstream
the circulator, is used for switching the RF power
between these two RF-guns. The phase different
between the RF and laser system can be adjusted by
the electronics phase shifter or the manual phaseshifter. The block diagram of the RF and laser system
is shown in Fig. 3.
CircuRFlator switch

Electronics PFN
Phase shifterMasterOscillator
ManualPhaseshifter

Pulse
GeneratorFunction
Generator
Clock
Synchronizer

Laser
Oscillator

PCPC

Alignmentcontrol

Oscilloscope

Oscilloscope
Ampli-fier

SHG
FHG

Function
Generator

Laser system
The photocathode RF gun is driven by a UV laser
with the wavelength of 266 nm [5]. The laser system
consists of a mode-locked Nd:YVO4 oscillator, abeam alignment control, two amplifiers and two nonlinear crystals (SHG and FHG). The oscillator
provides lasers at the wavelength of 1064 nm with therepetition frequency of 89.25 MHz and the pulse
duration of 7.5 ps at the full width at half maximum(FWHM). The beam alignment feedback control isused for the beam position and angle stabilization.
After that, the lasers are amplified by the two laser
diode pumped amplifiers. Finally, the pair of nonlinear crystals (SHG, FHG) converts the fundamental
wavelength into the fourth harmonic wavelength at
266 nm. The laser beam is transported to the gun byseveral UV enhanced aluminium mirrors. TheFigure 3: Block diagram of the RF and laser system
for the photocathode RF-gun.
MEASUREMENT AND RESULTS
To investigate the beam properties, we have set
up the beam diagnostic system consisted of a triplet
quadrupole magnet, a bending magnet, a steering
magnet, a Faraday cup, an extracting window, twofluorescence screens and two CCD cameras. The
schematic diagram of the beam diagnostic system isshown in Fig.4. The images of dark current and first
photoelectron of the focused beam on screen no.1 areshown in Fig. 5.

- 387 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP002

Solenoidmagnet

Laserport

TripletScreenno.1

Extracting
window

Charge (pC)

Steering
RF gun magnet

Bending Screenmagnet
no.2

Figure 4: Schematic diagram of the beam diagnostic
system for the photocathode RF-gun (top view).

1,600
1,400
1,200
1,000
800
600
4002000350 µJ
285 µJ
222 µJ
150 µJ

020 40 60 80 100 120 140 160 180
Laser injection phase (degree)

Figure 5: Images of the dark current (left) and the first
photoelectron (right) of the focused beam on thescreen no.1.
Bunch Charge and Dark Current
Bunch charge was measured by the carbon
Faraday cup placed out of vacuum and next to thebeam extracting window which is made from analuminum coated polyimide film with thickness of 8micron. The input RF power and the solenoid
magnetic field gradient were set at 8.6 MW and 1926
G, respectively. The laser was injected with a single
pulse. The measured bunch charge versus the laser
injection phases at different laser energies are shownin Fig. 6. The bunch charge increase proportionally
until peak at the laser injection phase around 70 to 80
deg. Then, the bunch charge decrease rapidly at thelaser injection phase over 80 deg. Because someelectrons were not accelerated due to the RF
deceleration phase. Fig. 7 shows the dark current as afunction of the solenoid magnetic field at the input RF
power of 9.1 MW. The maximum dark current is 70
pC at the solenoid magnetic field of 1779 G.

Dark current (pC)

Figure 6: Bunch charge as a function of the laser
injection phase at the different laser pulse energy.
80706050403020
1000500 1000 1500 2000 2500 3000 3500
Solenoid magnetic field (G)Figure 7: Dark current as a function of the solenoidmagnet field at the input RF power of 9.1 MW.
Beam Energy
The rectangular bending magnet was used as anenergy spectrometer. The magnet has the thickness of65 mm, and the pole gap of 30 mm. The screen no.2
is placed behind the bending magnet with the distancefrom the magnet yoke of 100 mm and offset from thebeam center of 60 mm. The center of bended beams
on the screen was observed by the CCD camera. The
laser energy was set at 180 µJ per three pulse to getclear image on the screen. The solenoid magnet wasadjusted to obtain a focused beam on screen no.2. The
dipole magnet current as a function of laser injection
phase is shown in Fig. 8. Then, the beam energy wasdetermined by comparing the beam trajectory withthe simulation result. It should be noted that themeasured energy should be lower than actual
accelerated energy because of the energy loss by theextracting window and air. The beam energy at theinput RF power of 9.1 MW is shown in Fig. 9.

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Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP002

The unnormalized rms emittance as a function of thesolenoid magnet current and the laser energy is shownin Fig. 10 and 11, respectively.

43.5

2.5

3

9.1 MW
6.7 MW
4.6 MW

2.528 MW5.6 MWrms emittance(mm mrad)Dipole current (A)4.5

0 10 20 30 40 50 60 70 80 90 100
Laser injection phase (degree)

1.51

4.5

115

125
135
145
Solenoid current (A)155

Figure 10: Unnormalized rms emittance as a functionof the solenoid magnet current at the bunch charge of53 pC and the injection phase of 40 deg.

43.533Measurement result

2.5
2y-axisx-aixs0.50

Simulation result020406080
100
Laser injection phase (degree)

Figure 9: Simulation (cross mark) and measurement(circle mark) results of the beam energy at the input
RF power of 9.1 MW as a function of the laser
injection phase.
From the Fig. 8, the beam energy is nearly
constant at the low laser injection phase, while thebeam energy decrease rapidly at the high laser
injection phase. From Fig. 9, the simulation result of
the RF gun with accelerating gradient of 50 MV/m
has not good agreement with the measurement result.
It may cause by accelerating field ratio between thefull- and half-cell of the RF-gun of the simulation andthe actual one are different. The further study of thenumerical simulation model is needed.
Transverse EmittanceTransverse beam emittance was measured by aquadrupole scan technique. The last magnet of thetriplet which has the effective length of 55 mm wasused for the measurement. The distance from centerof the magnet to the screen no.1 is 140 mm and fromthe cathode surface to the screen no.1 is 1275 mm.
The images on the screen no.1 were captured by a
CCD camera. Then, the beam sizes were calculated.

rms emittance(mm mrad)Beam energy (MeV)

Figure 8: Dipole current as a function of the laser
injection phases at the different input RF power.

22.5
21.5
10.5
0050

100 150 200
Bunch charge (pC)250300Figure 11: Unnormalized rms emittance as a functionof the laser energy at the solenoid gradient of 1926 G
and the laser injection phase of 40 deg.
CONCLUSION
A 1.6-cell photocathode RF gun has been
installed at the Institute of Advanced Energy, Kyoto
University. The photocathode RF gun successfully
generated the photoelectron with the maximum
measured beam energy around 4.4 MeV at the input
RF power of 9.1 MW. The bunch charge can reach up
to 1.4 nC at the laser injection phase around 70 degree
and the laser energy of 350 μJ. The unnormalized rms
emittance at the bunch charge of 53 pC is lower than
1 mm mrad in both horizontal and vertical.
For the further study, we will measure a precise
beam energy by in vacuum measurement in thechicane section. The numerical simulation model also
will be investigated in order to have a better
understanding of the cavity condition.

- 389 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP002

REFERENCES[1] T. Rao et al., “An Engineering Guide toPhotoinjector”, ISBN: 978-1481943222, 2013.
[2] H. Zen et al., “Present Status of Mid-Infrared
Free Electron Laser Facility in Kyoto
University”, in Proc. 4th Int. Particle Accelerator
Conf., Shanghai, 2013.
[3] N. Terunuma et al., “Improvement of an S-band
RF gun with a Cs2Te photocathode for the KEKATF”, Nucl. Instr. and Meth. A, 2010;613.
[4] H. Ohgaki et al., “Numerical Evaluation ofOscillator FEL with Multi-bunch Photo-cathodeRF-Gun in Kyoto University”, in Proc. 29th Int.
Free-Electron Laser Conf., Novosibirsk, 2007, pp390-393.
[5] H. Zen et al., “Development of Photocathode
Drive Laser System for RF Guns in KU-FEL”, inProc. of 36th Int. Free-Electron Laser Conf.
Basel, Switzerland, 2014.

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