The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Performance Test of a Photocathode RF GunFumio SAKAI, Jinfeng YANG, Masafumi YOROZU, Yasuhiro OKADA, Akira TSUNEMI, Yasushi AOKI, Akira ENDO,Takahiro KOZAWA*, Youichi YOSHIDA*, Tamotsu YAMAMOTO*, Seiichi TAGAWA*
Sumitomo Heavy Industries, Ltd (SHI)
2-1-1 Yatochou, Tanashi-shi, Tokyo, 188-8585, Japan*The Institute of Scientific and Industrial Research, Osaka University
8-1, Mihogaoka, Ibaraki, Osaka, 567-0047, JapanAbstract
A photocathode RF gun fior the high duty operation wasdeveloped by a BNUKEK/SHI collaboration and operated
at the linac facility of the University of Tokyo, successfully.
On the basis of this design, we constructed a gun systemand performed preliminary experiments at ISIR of Osaka
University. The experiments were performed using S-band
RF of l.SMW peak power. The quantum efficiency ofelectron emission was obtained to be 1o- 5 •1 Introduction

High brightness electron with low emittance and shortbunch length is expected interesting to apply to newtechnologies, such as FEL, laser acceleration, generation ofshort X ray pulse by Compton scattering, and so on. The
photocathode RF gun being capable of high duty operations
was developed by BNUKEK/Sumitomo heavy Industries,
Ltd (SHI) collaboration as a source of high brightness
electron bunches [ 1]. On the other hand, a stable picosecond
laser illuminating the cathode is a key component to obtaina stable electron generation. Our group has developed alaser diode (LD) pumped picosecond laser by a joint projectwith Time-Bandwidth Product, Ltd.[2].
For a preliminary experiment, the photocathode RF gun
and the picosecond laser were installed at the Institute ofScientific and Industrial Research of Osaka University [3].
The photocathode RF gun was set instead of a thermal gun
and connected to three linacs accelerating electrons up to120MeV. We report the fundamental characteristics ofelectron bunch from the photocathode RF gun and linacs.
2 Experimental apparatus

The photocathode RF gun system consists of the gun
with a Cu cathode, dipole magnets, and a solenoid magnet.
The LD pumped picosecond laser for illuminating cathodewas set to the side of the gun. Laser light was transportedwith a mirror system. The gun system was connected tolinacs by a beam transport system with the magnets andbeam diagnostic equipment. The whole system is shown inFig. I.
2. 1 Photocathode RF gun

The 1.6 cell photocathode RF gun has a Cu cathode,which has a long lifetime. Although Quantum efficiency(QE) of the Cu cathode is low (I o- 4 ), laser energy isincreased to get InC charge per bunch. For high duty
operation, the gun has three cooling channels to remove theRF heating and control the temperature of the gun within

Picosecond laser

Figure.! Experimental system. Ml-4: Charge andscreen monitor, T: Quadrupole Magnet, B: BendingMagnet, D: Dump
0.1 degree. The design condition of RF input is 6MW -peakpower with 4 f..lS duration and 50Hz-repetition rate.
However, the RF pulse of 1.5 MW peak-power with 3f..lSFWHM duration was used in this experiment. The repetition
rate was 1OHz.
2.2 Picosecond Laser for the Photocathode RF gun

The all solid LD-pumped Nd: YAG laser consists of apassive mode-locked oscillator with a semiconductor
saturable absorber mirror (SESAM), a regenerativeamplifier and a power amplifier. The frequency of theoscillator is 119 MHz being to equal to 1/24 of 2856 MHz
S-band radio frequency. A timing stabilizer measures thephase and the frequency offset between the laser pulses,
which is detected by a photo-diode, and the reference
119MHz RF signal, and adjusts the cavity length of theoscillator to maintain the constant phase relation between
the two signals. Pulses from the oscillator are picked up bya Pockels Cell. The regenerative amplifier is operated with
1-1OOHz repetition rate. The 1064nm fundamental with
about 2mJ pulse energy is quadrupled to 266nm UV
radiation with about 100 p J pulse energy by two non-linear
crystals. The stability of system, such as timing jitter,energy fluctuation and pointing stability is very important
for the photocathode RF gun. The specifications of the laser
are shown in Table 1. UV light from the laser wastransported by the mirror system and injected to the cathodeat oblique angle. The laser beam position on the cathodewas adjusted by a remote controlled mirror.

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120Ta bl e 1 Ssoec1'fi1cat10ns ofth e Iaser
Laser material
Nd:YAGwavelength
266nm (4 1h harmonics)_Pulse duration
23ps @I 064nm
Pulse energy
I OOJ,LJ/pulse@266nm
Repetition rate
1-lOOHz
Energy stability<2%
Timing jitter<0.5ps•••,......., 100(I)

Ill:Jc. 80'uc.

••

•
60(I)

....

2.3 Synchronisation system

••Ol

•

«J.s:: 40uA block diagram of the synchronisation system for
operation of the gun is shown in Fig.2. A master oscillator
supplies 119 MHz radio frequency. This RF signal is
divided into two. One drives the oscillator laser. The otheris multiplied in frequency by 24 to 2856MHz, to drive thegun and the accelerator. A synchroniser in the laser
generates a trigger for a Pockets Cell. As a result, the I OHz
laser outputs synchronised with the RF fields for electron
accelerator the accelerators are obtained. A phase shifter isused to adjust the laser injection phase. The timing jitter
between the master oscillator and laser output is controlled
within 0.5 ps by a timing stabiliser. However, the timing
jitter between the laser outputs and 2856MHz RF fields
increases actually by the other sources a frequency
multiplier, Klystron, and the change of circumstances.

• ••• ••

20 050100 150 200 250 300

Polarization angle of laser I ight
Figure.3 Bunch charge vs. Polarization angle. The
angle with the maximum charge is correspondent
to P-polarization.

120

••

<D'IllI 00• •:J~80u

•

c.

60

•

CDOl;a 40.s::u20•• •• ••

oLy~~--~··~~~~~~u·~~u-60

060

120

Laser Injection Phase180[degree]Figure.4 Emission bunch charge vs. laser injection
phase. The zero point is set at charge zero phasewhich corresponds to the zero axial field.

Figure.2 Block diagram of the synchronisation system
3 Experimental results

3.1 Electron emission
The yield of electron emiSSion is a key factor forapplications of electron pulses. Laser light was injected withthe oblique angle, so QE was dependent on the polarizationof light [4]. The electron charge was measured as a functionof the direction of the polarization using by a half wave
polarizer. A Faraday cup was set at about 1m from thecathode. Current of solenoid magnet was adjusted to get
maximum charge. The relation between the polarization andthe electron charge is shown in Fig.3. The maximum
electron charge was obtained at the P-polarization being
correspondent to the maximum energy absorption by thecathode. QE dependence on the laser injection phase wasmeasured under the fixed polarization and the current of thesolenoid. The zero degree of the injection phase isdetermined at the onset of the current, which corresponds tothe zero field at the cathode. In Fig.4, it can be seen that themaximum electron charge was obtained to be 115ps at the40degree. When the laser pulse energy with 266nm was65J,LJ, QE was 8x10-6 • This QE is about 1/20 of the reported
value for the same structural gun [5].
The emission charge from the cathode should bemaximum at 90 degree, because the surface field on thecathode is maximum. However, the electrons emitted at 90
degree have lower energy and high energy-dispersion, sothe collection efficiency at the Faraday cup should be low.
If higher RF power is supplied to the gun, the higher
electron charge enhanced by Schottky effect [6] is obtained
at near 90 degree.

3.2 Electron energy from the gun-195-

Energy of electron emitted from the cathode was

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

measured with a dipole magnet and a beam profiler at thedownstream of the gun. The measured electron energy
versus laser injection phase is shown in Fig.5. The
maximum energy was obtained from 10 to 40 degree. The
field gradient estimated from the energy data is about
35MV/m.

obtain the maximum transmittance. The charges at the gun
exit and at the downstream of linac were 90pC and 30pC,respectively. Low emittance was obtained due to thereduced space charge effect of low transmittance beam. We
also measured the emittance with other conditions, othermagnetic field of the solenoid magnet and the other laser
injection phase. The emittance ranged from 0.4 to 0.8 1tmmmrad, however, the transmittance was changed under eachcondition. As a result, the dependence of emittance on theseparameters was not analyzed in detail.

rf f

4 Conclusion

f f

The preliminary experiments using the photocathode RF
gun and the stable picosecond laser were performed.
Although RF power was low and RF conditioning time was
not enough, QE was measured to be about 10"5• The
normalised emittance was obtained to be 0.6 1tmm-mrad.
We start to construct the acceleration system up to 10
MeV in our facility to demonstrate the X-ray generation by
Compton scattering. The performance of the gun with 50Hz
high duty operation and the stability of electron bunches,such as timing jitter and pointing stability will be measured.

Laser Injection phase [degree]

This work was supported by the New Energy andIndustrial Technology Development Organization (NEDO).

Figure.5 Electron energy from the gun

5 Reference

3.2 Beam acceleration and emittanceElectron bunches from the gun were accelerated to117MeV with 1.2% energy dispersion by three linacs.
Transmittance through the three linacs was about 30%. The
low transmittance was due to misalignment, because it wasdifficult to adjust electron beams with the low field magnet
designed for a thermal cathode gun.
Emittance was measured at the downstream of linacs by
the standard quadrupole scan technique. The relation
between the square of beam diameter on a phosphor screen
and the current of a quadrupole magnet is shown in Fig.6.
The normalised RMS emittance was 0.6 1tmm-mrad. Laser
injection phase was 40 degree, and the magnetic field of thesolenoid magnet was 0.8 kG, which were determined to,....,20[1] F. Sakai and et at.: "Development of High Duty
operation RF Photoinjector", Proc. of the 11th Sympo.
on Accel. Sci. and Tech., Harima, PB47, (1997) 473[2] F. Sakai and et al.: "All-Solid-State Picosecond Laser
System for Photocathode RF Gun", Proc. of Parti. Accel.
Conf., New York, (1999), in press.
[3] J.Yang and et at.: "Experimental Results of a Laser
Photocathode RF Gun", Pros. Of the 24th Linear Accel.
Meeting , Sapporo, (1999) 170[4] T. Srinivasan-Rao, and et at.,: "Photoemission from Mg
irradiated by short pulse ultraviolet and visible lasers", J.
Appl. Phys., 77(3), (1995)1275[5] J.Yang and et al.: "Performance tests ob BNL/KEK/SHI
Photocathode RF Gun", Pros. Of the 24th Linear Accel.
Meeting , Sapporo, (1999) 128

I"'EE......,5V):IEa:clO"'X

516202428

Current of 0 [A]Figure 6 Square of beam diameter vs. Current of aquadrupole magnet. Solid line: Least square fittingwith the 200 order polynomial.

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