The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Development of High Duty Operation RF Photoinjector
Fumio SAKAI, ****Xijie WANG, Hideyuki KOTAKI, Kazuhisa NAKAJIMA,*TakahiroWATANABE,*Keniti KINOS HITA,Shuji KONDO, Masaki KANDO,Hideki DEWA , *Toru UEDA, *Koji YOSHII,*Mitsuru UESAKA, **Atsushi OGATA, **Hiroshi NAKANISHI, ***Masakazu WASHIO, ***Akira ENDO,****IIan BEN-ZVI, ****John SKARITKA, ****Martine WOODLE
Japan Atomic Energy Research Institute, Tokai, lbaraki, Japan.
*Nuclear Engineering Research Laboratory, The University of Tokyo, Tokai, Ibaraki, Japan**KEK, Tsukuba, lbarak:i, Japan***Sumitomo Heavy Industries, Ltd., Tanashi, Tokyo, Japan****Brookhaven National Laboratory, Upton, NY, U.S.AAbstract
A RF photoinjector for the high duty operation wasdesigned and constructed by the BNLIKEK/SHI international
collaboration and installed at the S-hand linear acceleratorof the University of Tokyo. Subpicosecond electron bunches
compressed by a magnetic compression is planned to be used
for the subpicosecond X-ray generation and the laseracceleration. Preliminary experiments of the RF photoinjectorwere performed. The measured value of the Cu cathode's
quantum efficiency was 5xl0-5 •l Introduction
Technologies to obtain high brightness electron buncheswith a low emittance and a short duration have been studied
with photocathode RF guns using short pulse lasers in order
to apply for new technologies, laser accelerator, FEL, X-ray
generation by Compton scattering and so on 1),z). More than ten
years photocathode development and recent short pulse laser
development, the photocathode is emerging as a standard
laboratory tool for high brightness electron beam applications.
Laser wakefield accelerator (LWFA) expected as acompact high energy accelerator with high gradient particle
acceleration has been studied. JAERI, KEK and the Universityof Tokyo have studied LWFA using a linac with a thermal
electron gun, a magnetic compressor, and a TW laser withlOOfs pulse length3 >. In order to get the low emittance andshort bunched electron beam and also to accelerate higher
efficiently, a RF photoinjector as a electron source wasinstalled at this time.
RF photoinjector technologies has been integrated inBrookhaven National Laboratory{BNLi>. Recently, 1.6 cell
RF photoinjector has
been
developed by theBNLISLAC/UCLAcollaboration4>.
Basedonthisdevelopment, the improvement for high duty operation wasperformed by the BNLIKEK/SHI collaboration. Major
improvements were water cooling channels removing RF heatand RF seal structure of a cathode plate. In the following
section, the thermal analysis of the RF photocathode and theother systems such as the solenoidal magnet, a laser anddiagonostic system is reported. The results of preliminary
experiments are also reproted.
2 RF photoinjector system
The RF photocathode injector system shown in Fig.lconsists of a photocathode, a solenoidal magnet, diagnostic
system with a faraday cup and , a beam profiler; and aadjustable basement. The RF photoinjector developed byBNLISLAC/UCLA collaboration was installed aloinost one
year at Brookhaven Accelerator Test Facility (ATF). The datashows that the basic design for the low emittance and the shortbunch was confirmed. Improvements for high duty operation
are reported as follows.

Fig. I Rf photoinjection system
2.1 Thermal analysis
Major improvements for high duty operation are the seal
structure of the photocathode plate and the water coolingchannels.

-473-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Our RF design power is a 6MW peak with a 3.5 J.1 pulse
width and a 50Hz repetition rate. The RF power loss
distribution on the Gun surface was calculated by aSUPERFISH code. Thermal analysis shown in Fig.2 wasperformed by a program code FLUENT. Considering the heatbalance of the gun surface , the minimum cell to cell couplingchange and the structure of the gun which has several ports,such as laser ports, waveguide and vacuum port, the positionof cooling channels were decided. As a results of thermalanalysis, temperature difference of coupling iris is within 3 C
so that coupling change is less than 2%.

2.3 Laser
A Nd:YLF all solid laser was constructed for a driver ofthe photocathode. A oscillator is LW131 manufactured byLightwave Electronics. A regenerative amplifier manufactured
by Sumitomo Heavy Industries, Ltd. is pumped by two diodelaser. All solid laser allows for compactness and stability. The
forth harmonics of fundamental of Nd:YLF is illuminated on
the cathode. The FWHM pulse width of the second harmonic
was measured to be 25ps which implys that one of the forthharmonic was l8ps.
2.4 Diagnostic
A faraday cup, a phosphor profile monitor and a AI
mirror for the observation of the cathode surface are installed
as diagnostic tools. Charge signals from the faraday cup madeof was measured by an oscilloscope directly and calibrated bya current moniter. The phospher screen was used for themeasurement of the bunch length. Prompt light from theDesmarquest phosphor screen is measured by a femtosecond
streak camera(FESCA) with a 200fs resolution.

Fig.2 Temperature distribution

3 Preliminary experiment

2.2 Solenoidal magnet
Single emittance compensation magnet was constructed.
The measurement of the magnetic fields at the center and I em
away from the center shown in Fig. 4 shows the flat field at theoff-axis. The maximum magnetic field is 3 k-gauss and themagnetic field at the cathode surface is 15 gauss. Theemittance growth estimated is 0.15 n mm · mrad at themaximum field. A bucking coil is usually used to cancel themagnetic field at the cathode surface. Considering theconstruction of the bucking coil and the space for changing thecathode plates, it was decided that the bucking coil was notneeded in our case.
3.5
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• centerrc right(10mm)"'upper(10mm) r-"
II

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0

100

200

300

400

Distance from Cahtode Surface(mm)Fig.3 Magnetic field of solenoidal coil

500

Preliminary experiment of the RF photo injector system was.
performed using the diagnostic system between the RF
photoinjector and the a linac.
I IThe 79MHz RF for the mode locked Nd: YLF laser and the2856MHz RF for the linac were generated from the 119MHz
RF of a master oscillator. Trigger signals synchronizing thelinac RF pulses and the FESCA with illuminating light were
picked up from the laser.
i IUV light illuminating the photocathode was quadrupledfrom the 1057nm fundamental of the Nd: YLF laser by two
BBO nonlinear crystals. UV light was injected to thephotocathode with an oblique angle, because P-polarization
component is well known to enhance the quantum efficiency.
! IThe quantum efficiency versus the laser injection phaseand polarization angle was measured by the faraday cup. Pulse
length was also measured to use the prompt fluorescence fromthe Desmarquest by the FESCA,
3.1 QE measurement
The electron charge versus the laser injection phase isshown in Fig.4. The laser energy was 40 J.1 J. The phase at
maxinmm charge indicates the peak field, because theelectrons emitted at the high field is enhanced by theSchottkey effecf). The quantum efficiency at the peak charge
is5xto·5•-474-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

0.5

e I

0.4

region. The bunch length measurement in the subpicosecond
region with the prompt fluorescence and the Cherenkov
radiation will be performed after accelerating by the linac.

••

0-5 0.3~~
0...-..50
tnc.

•0.2.J: 40

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0.1~
c:

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0~30

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30

060

90

120

150

I

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1800 Sill!Je•••ID~ 100c:

l 0.2 1---+--+--- -..,----+--- .T----t

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_!

,-In

0The electron charge emitted from the cathode is dependent
on the polarization of the illumination light. The electroncharge versus the polarization of light is shown in Fig.5. The
polarization of light was changed with a 1/2 11. plate. It can beseen that available charge is maximized at a polarizer angle of97 degree which corresponds to P-type polarized light on thecathode.

j0~ 0Electron charge emitted from the cathodeversus laser injection phase._e100shotsl.l:
0Laser Injection Phase(degree)

Fig.4

H00
60
30
Laser lr!iection Rlase{de!Jee)120Fig.6 Electron bunch length vs. laser injection phase
4. Summary

The RF photocathode injector designed and constructedby the BNLIKEK/SHI collaboration was installed in theUniversity of Tokyo. It was colifirmed that the system of theRF photocathode injector was working. At next steps,
electron emitted from the photoinjector is accelerated with theS-hand Iinac, so more characteristics of the RF photoinjector
such as the energy, the emittance, and the bunch length, willbeen studied.

Ill.l:0Referencel]B.E.Carlsten et al, : Micro Bunches Workshop, Sep-.2830(1995)
2] X.J. Wang et al,: 1\lucl. Instr. And Meth. A356 (1995) 159
3]M.Kando et al: PAC97, 7V48 (1997)
4]D.T.Palmer, et al,: Proc. Part. Accel. Conf. , (1995) 982
5]T. Srinivasan-Rao et al: J.Appl.Phys.,77(3) (1995)10.1

60

90

150
120
Polarizer angle(degree)180

Fig.5 charge emitted from the cathode vs.
Polarizer angle
3.3Pulse length

The pulse length measurement was performed by theFESCA using the prompt fluorescence from the Desmarquest
phosphor screen. The bunch length versus the laser injection
phase is shown in Fig.6. The data of 100 shots accumulated
and single shot are shown. It can be seen that the jitter ofsystem such as laser fluctuation is around 1Ops. Due to thelaser power limitation, this experiments was not performed
with the electron charge constant, but the laser power constant.
The charactristics of the prompt fluorescence from theDesmarquest has not been recognized well, however,it wasconfirmed that this method was a convinient tool in this bunch-475-