The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Multi-bunch generation by Photo-cathode RF gun
M. Kuriki *, K. Hirano, R. Kuroda a , M. Matsuda b, T. Nakanishi c, M. Nomura, S. Okumi c, I. Sakai d,M. Takano d, N. Terunuma, K. Hasegawa e, H. Hayano, J. Urakawa, Y. Yamazaki, M. Washioa ,and ATF collaboration
High energy accelerator research organization, Tsukuba,Ibaraki(a) Waseda university, Shinjuku, Tokyo(b) Yokohama national university, Yokohama, Kanagawa(c) Nagoya university, Nagoya, Aichi(d) National institute of radiation science, Chiba, Chiba(e) Graduate university for advanced studies, Hayama, Kanagawa
Abstract
KEK-ATF is doing R&D establishing high-current andlow-emittance multi-bunch electron beam for the future linear collider, GLC(Global Linear Collider) project. In 1.5
GeV injector linac, a thermionic gun with multi-pulse grid
pulser had been used as the electron source. In 2002, wehave modified our pre-injector by introducing the S-band
photo-cathode RF gun. We use the Nd:YVO4 357 MHzmode lock laser to generate the multi-bunch beam with 2.8
ns bunch spacing. Because of the limited laser power, several m J typically, CSTe high Q.E. ( more than 1%) cathodewas introduced to generate sufficient beam current, morethan 1.6nC. In this article, we report the new injector system and basic beam properties.

INTRODUCTION
KEK-ATF is a test facility to study the low-emittancemulti-bunch electron beam and beam instrumentation technique for GLC project. That consists from 1.5 GeV S-bandlinac, a beam transport line, a damping ring (DR), and adiagnostic extraction line.
In the linac, the beam instability and the beam loss at theinjection to DR had been an issue. It has been consideredthat the long beam tail in longitudinal direction made theenergy tail which was lost at the injection. In addition, asmall fluctuation on the bunching RF (two SHBs and a TWbuncher) caused the beam instability.
We have examined a photo-cathode RF gun in summer
2001 as the injector of ATF to solve the problem. The RF
gun cavity was 1.6 cells BNL gun IV, same type as that wecurrently used. The laser was Nd : YLF UV laser with 10ps bunch length, single shot. The cathode was a conventional copper surface. The electron beam obtained fromthis RF gun was therefore single bunch, but the stability
was excellent. The transmission from the gun to DR was
almost 100%, i.e. all of the electron bunch was successfully stored in DR without any significant loss. The intensity fluctuation was however up to 10% due to the power
and pointing jitter of the laser. This study demonstrated
that the basic performance of the RF gun was marvelous as* masao.kuriki@kek.jp

ATF injector. The intensity jitter was not a big difficulty
because the thermionic gun system had intensity jitter almost same. The laser jitter would be improved in future bytechnological innovations anyhow.
The important issue is the multi-bunch generation. The
multi-bunch electron acceleration is one of the essence torealize the large luminosity in GLC. The purpose of ATF isto demonstrated that the low-emittance multi-bunch beam.
Therefore, the multi-bunch electron beam is necessarily inATF.
To make the multi-bunch electron beam generation byRF gun, we have performed a test experiment again inSummer 2002. In this study, we introduced “multi-bunchlaser”. This laser can generate continuous laser pulse with
2.8 ns spacing, several m J /pulse power. We have introduced a new cathode material, CsTe that has high Q.E. toproduce enough current with this limited laser power.
The combination of the multi-bunch laser and CsTe cathode make the multi-bunch generation from RF gun possible
and the test in Summer 2002 was successfully done. Wehave concluded that we abandoned the thermionic gun andswitched to the RF gun officially. As the permanent system, the photo-cathode RF gun injector has operated sinceOctober 2002.

BEAM LINEThe RF gun complex consists of RF gun cavity,
Nd:YVO 4 mode lock UV laser, and CsTe cathode system.
The cavity is made from Oxygen free copper. The resonance frequency is tuned to be S-band central frequency,
2856MHz. The cavity has two cells operated in p mode,so the electric field in these cells is opposite to each other.
Because the first cell is electrically terminated at the centerof the cavity by the cathode plate, the electric field on axis
becomes very strong and right perpendicular to the cathodesurface. Due to this field, the emitted electron is immediately accelerated up to relativistic momentum.
This gun cavity was fully made in KEK including themachining, brazing, and welding. The cavity inner surface
was finished by super-fine lathe with diamond bite. This
technology has been originally established by X-band accelerating structure R&D group in KEK to suppress the RFThe 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Laser System(262nm)
I.P
200l/sG.V

Pre-injector diagnostic section
To StreakCamera

Faraday Cup& ScreenSolenoid∼ 15MW I.P 200l/s
RF-inICT∼ 47MW
RF-in

BPM

RFGun

Screen-6dB directional
coupler

Klystrondark current (nA)discharge break down in the accelerating structure. Fig. 2
shows dark current emitted from RF gun as function of input RF power. “new” and “old” guns were processed withthe fine and conventional lathes. This figure shows this finemachining has a big advantage to reduce the dark current
more than 1/10.

Figure 2: Darkcurrent
fromthe gun cavity
as function ofinput RF (1.6 m sduration and 12.5Hz reputation).
The current gun
is shown as “newgun”.

old gunnew gun101

1010-1-2

1

2

3

4

5

6
7
8
9
input power(MW)

Nd:YVO4 1064nm (IR) seed laser with 357 MHz mode
locking generates continuous laser pulses with 2.8 ns spacing. The power is 410mW CW. By fast switching of thetransparency of Pockels cell, a part of the continuous pulse
train is clipped out. A couple of Pockels cell assemblies
are used. The fast Pockels cell that has less than 1ns
rise and fall time mainly decides the bunch number in atrain. Another Pockels cell, slow one, has less than 3ns
rise time. This cell has a roll to suppress the amplitudeof the laser pulse cut by the fast one further. As a result, the clearly clipped bunch train is obtained. By shifting the clipping timing, the bunch number can be varied
from 1 to 1001 . This number is currently limited by theradiation safety regulation. This clipped laser pulse train
is then amplified by four amplifiers pumped by Nd:YAG
followed by SHG(Second Harmonic Generator) and FHG(Fourth Harmonic Generator) crystals converting the laserfrom IR (1064 nm) to UV (266 nm).
Laser power and pulse length are several m J /pulse and12ps in FWHM typically. This laser light is then introduced
to Linac tunnel and passes through a telescope assembly todecrease its spot size to 1/6. The laser spot size on thecathode depends on the laser condition and it is expected
between 1-3 mm on cathode.
The laser is injected to the gun cavity in nearly right angle to cathode. The final mirror is mounted in a special
1 20 for DR injection mode

ScreenQuadrapole
magnets

BPM

3m-Accelerating Tube

BPM

ScreenOTR

Analyzer
magnetWire
ScanerTo LinacFigure 1: Layout of ATF preinjector. RF power from a klystronis distributed to the RF gun and theaccelerating structure. Many instrumentation devices are implemented.
In actual system, the cathode loadlock system is connected directly tothe gun cavity.

vacuum chamber as shown in Fig. 1.
To get enough beam current with the limited laser power,
we use CsTe high Q.E. cathode. Since this material is easily
damaged by exposing to air, the evaporation of Cs and Te
on the base material (Mo), the transferring the cathode, andsetting the cathode plug to the gun cavity are all performed
in vacuum. We constructed so called “load-lock” system
to perform these procedures. CsTe cathode marked morethan 10% as initial Q.E., but it rapidly dropped down toless than 5% by feeding RF power into the cavity. Q.E.
was then slowly decreased in time, but it was kept morethan 0.5% for several weeks [1].
The schematic layout of the injector system is shown inFig. 1. The laser system and the load lock system are omitted because of their complications. Up to 15 MW of 62
MW S-band RF power made by Toshiba E3712 klystron isfed to the RF gun through 6dB directional coupler. Rest ofthe power is distributed to the first accelerating structure,
L0.
The RF gun is followed by a solenoid magnet that suppress the emittance growth by the space charge effectand RF deflection. Faraday cup is set to measure theemitted current. ICT(Integrated Current Transformer) andWC(Wall Current monitor) are also implemented to measure the beam current.
Three Quadrapole magnets stand side by side at downstream of L0 to make the b matching with the following
regular accelerator section. They are also used for emittance measurement by Q-scan method with the wire scanner beam profile monitor.
The beam momentum is measured with the analyzer
magnet. The beam energy was typically 80 MeV.
Optical Transition Radiation, OTR monitor is implemented to measure the bunch length. OTR light emittedfrom the inserted SUS plate is observed by a streak camerathat can measure the signal with sub-pico second resolution.

BEAM PROPERTYFig. 3 shows the beam profile in time measured by Wallcurrent monitor. These profiles were taken with numberof bunch set to 1, 10, and 20. From these figures, themulti-bunch electron beam is generated successfully. The
bunch clipping was marvelous, so the contrast between theclipped and removed part was very good. The intensity flatness in a train depends on the noise condition of the Pockels

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Figure 3: Beamprofile in timemeasured by Wallcurrent monitor.
From left, thebunch numbers
are set to 1, 10,and 20.

emittance (pi mm.mrad)

cell driver circuit.
The laser power was 2 - 3 mJ / bunch , and the cathode Q.E. was around 1%, so that more than 4.0 e +10 electron / bunch could be obtained. Even though, themaximum current is limited up to 2.0e + 10electron / pulse
by the radiation safety regulation. The current is therefore
adjusted to be 1.0e+ 10electron / bunch or less by reducing
the laser power.
To decide the operation condition, we measured thebeam current as function of the RF phase. The beam current was maximized at some phase, but the optimum phaseis 20 deg. early for shorter bunch and lower emittance. For
more details of the operation condition of RF gun, pleaserefer to [2].
The energy spread at the injector was measured by momentum analyzer system to be 1.9% . That was obtainedby optimizing the system to the lowest energy spread, butthere was “trade off”s to the beam current, the beam emittance, etc. Usually, the system is operated around 2-3%
energy spread.
141210864

single bunch
multi bunch

2000.20.4

0.6 0.8
11.2 1.4
Bunch current (1E+10 electron)

Figure 4: Beamemittancedata
are plotted asfunction of bunch
current.
The
triangle and circle
symbolshow
the single- andmulti-bunch data
respectively.

The transverse beam emittance was measured by thewire scanner with Q-scan method. This device scans beamwith a thin tungsten wire (30m m diameter) and observes
scattered gamma rays with a photo- tube. The measured
profile is then “projected”, so the emittance is “projected”
too. By plotting the gamma ray flux as function of the wire
position, the beam profile is reconstructed. Taking several
beam profiles with different phase advance (different Qmagnet current), we extracted the twiss parameter and theemittance. The analysis was done with SAD beam optics
analysis suite. Fig. 4 plots the normalized emittance asfunction of bunch current. The triangle and circle sym-

bol show data of single- and multi-bunch respectively. The
data are scattered, but the emittance looks larger for larger
bunch current. Any significant difference was not observed
between single- and multi-bunch data. The laser spot size
was guessed as 1-3 mm, but it was not controlled well.
By observing OTR light with a streak camera, the bunch
length was measured to be between 6.4 and more than 15
ps in FWHM.
The intensity jitter of the gun emission could be less than
3%, but it may be up to 10%. It depends on strongly thelaser condition.

SUMMARY
We have developed RF gun complex to generate multibunch electron beam with 2.8 ns bunch spacing. It wasimplemented by 1) Nd:YV0 4 357 MHz mode lock laser
with pulse clipping, 2) CsTe high Q.E. cathode with theload-lock system, and 3) RF gun cavity manufactured in
KEK with state-of-the-art technology. The excellent multibunch electron beam was obtained from 1 to 20 bunches.
The beam property was measured and confirmed that thebasic requirements in ATF are satisfied.
To improve the beam quality more, controlling and monitoring the laser condition is very important. Currently, wecan examine the laser condition only off line, i.e. the measurement disturbs the beam operation. We are currently
planning to introduce a real time laser monitoring devices.
To take the laser profile, we will introduce a real-timelaser profile monitor. A part of laser is cut by the l / 4 crystal to adjust the laser power. We re-use this laser light. The
profiler is taken by a CCD camera with UV converter. The
spatial resolution is expected to be 100 m m .
The laser power will be also measured in real time. Thelaser sampled with a wedge plate is measured its power byPIN photo-diode.
With these devices, we could monitor the laser condition
continuously in operation, and we could study the stability
including the laser system.

REFERENCES[1] N. Terunuma et al., Linac conference proceedings, p162-164,
2003[2] M. Kuriki et al., “Properties of the electron beam generated
by photo-cathode RF gun”, 13th Accelerator science symposium proceedings, 2001