Multi-bunch beam generation by Photo-cahode RF gun

M. Kuriki , K. Hirano, R. Kuroda , M. Matsuda , T. Nakanishi , M. Nomura, S. Okumi , I. Sakai ,
M. Takano , N. Terunuma, K. Hasegawa , H. Hayano, J. Urakawa, Y. Yamazaki, M. Washio,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. We have developed a new injector with theS-band photo-cathode RF gun. We use the Nd:YVO  357
MHz mode lock laser to generate the multi-bunch beamwith 2.8 ns bunch spacing.  cathode was introduced
because of the high Q.E. more than 1%. In this article, wereport the schem of the injector and basic beam properties.

INTRODUCTION
KEK-ATF is a test facility to study the low-emittancemulti-bunch electron beam and beam instrumentation technique for the future linear collider. That consists from 1.5
GeV S-band linac, a beam transport line, a damping ring(DR), and a diagnostic 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 RFgun cavity was 1.6 cells BNL type, same type as that wecurrently used. The laser was  UV laser with 10
ps 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 wasalmost 100%, i.e. all of the electron bunch was successfullystored in DR without any significant loss. The intensity
fluctuation was however up to 10% due to the power andpointing jitter of the laser. This study demonstrated that thebasic performance of the RF gun was marvelous as ATF injector. The intensity jitter was not a big difficulty because
the thermionic gun system had intensity jitter that was not
significantly small. In addition, the laser jitter would beimproved in future by technological evolutions anyhow.
The important issue is the multi-bunch generation. The masao.kuriki@kek.jp

multi-bunch electron acceleration is one of the essence torealize the large luminosity in linear collider. Therefore,
the multi-bunch electron beam is necessarily in ATF R &Ds.
To demonstrate the multi-bunch electron beam generation by RF gun, we have performed a test experiment again
in Summer 2002. In this study, we introduced “multi-bunchlaser”. This laser can generate continuous laser pulse with
2.8 ns spacing, several  /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. We
have 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 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  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, blazing, and welding. The cavity inner surfacewas finished by super-fine lathe with diamond bite. Thistechnology has been originally established by X-band accelerating structure R&D group in KEK to suppress the RFdischarge 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.

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

Pre-injector diagnostic section
To StreakCameraFaraday Cup& Screen

ICTBPM

RFGun

Ø47MW
RF-in

Solenoid

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

Screen-6dB directional
couplerdark current (nA)Ø15MW I.P 200l/s
RF-in

Klystronold gunnew gun1011010-1-2

12

34

5

678
9
input power(MW)

Nd:YVO 1064nm (IR) seed laser with 357 MHz mode
locking generates continuous laser pulse 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 variedfrom 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). The UV laser is finallyobtained.
Laser power and pulse length are several  /pulse and12ps in FWHM typically. This laser light is then introducedto 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, but it is expected
between 1-3 mm on cathode.
At first, the laser was injected oblique to the cathode with
67.5 degree. This is an easy way, but the laser spot is expanded in the horizontal direction more than twice. The
pointing instability is also enhanced in this direction. Then
we have changed the injection scheme in this April to headon. As we mentioned later, this modification improved thebeam property.
1 20 for DR injection mode

ScreenQuadrapolemagnets

BPM

3m-Accelerating Tube

BPM

ScreenAnalyzer
OTR magnet

WireScaner

To LinacFigure 1: Layout of the injector. RFpower from a klystron is distributed
to the RF gun and the acceleratingstructure. Many instrumentation devices such as screens, etc. are implemented. In actual system, the cathode load-lock system is connected
directly to the gun cavity.

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 toperform these procedures. CsTe cathode marked more than
10% as initial Q.E., but it rapidly dropped down to less than
5% by feeding RF power into the cavity. Q.E. was thenslowly decreased in time, but it was kept more than 1% forseveral weeks. For details of the cathode performance andthe load-lock system, please refer to Terunuma’s article [1]in this proceedings.
The schematic layout of the injector system is shown inFig. 1. The laser system and the load lock system are omitted because they are too complicated to draw in a picture.
The RF gun is placed at the first end of the beam line. Up to
15 MW of 62 MW S-band RF power provided by Toshiba
E3712 klystron is fed to the RF gun through 6dB directional coupler. Rest of the power is distributed to the firstaccelerating structure, L0.
The RF gun is followed by a solenoid magnet thatsuppress the emittance growth by the space charge effect. Faraday cup is set to measure the emitted current.
ICT(Integrated Current Transformer) and WC(Wall Current monitor) are also implemneted to measure the beamcurrent.
Three Quadrapole magnets
 stand side by side at downstream of L0 to make the 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 generated
by the interaction of the beam and the inserted SUS plateis observed by a streak camera that can measure the signal
with sub-pico second resolution.

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

emittance (pi mm.mrad)

clipped and removed part was very good. The intensity flatness in a train depends on the noise condition of the Pockels
cell driver circuit.
The laser power was 2 - 3  , and the cathode Q.E. was around 1%, so that more than    !!#" could be obtained. Eventhough, themaximum current is limited up to $%     &'( ! by the radiation safety regulation. The current is thereforeadjusted to be  )   *'+!! #" 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. That was 4.9% FWHM byoblique laser injection, but it was improved to be 1.9% byhead-on laser injection. 1.9% energy spread 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.

ysis was done with SAD beam optics analysis suite. Fig.
4 plots the normalized emittance as function of bunch current. These data were all taken with head-on laser injection. The emittance measured with the oblique laser injection was not analyzed well, but that was even larger
than 10 .,/,0 ,1#2  . The triangle and circle symbol show
data of single- and multi-bunch respectively. The data arescattered, 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. Although this performance is even enoughfor ATF specification, the stability is rather important andit was not investigated yet. Possible sources making thisinstability are the laser jitter. Currently we do not have
any real time monitoring device for laser, that should be
introduced for further study.
The intensity jitter of the gun emission could be less than3%, but it may be up to 10%. It depends on strongly thelaser condition. Also in this view, controlling and monitoring the laser is very important.

SUMMARY
141210864

single bunch
multi bunch

20

0

0.2

0.4

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 (50-, diameter) and observes
scattered gamma rays with a photo- tube. By plotting thegamma ray flux as function of the wire position, the beamprofile is reconstructed. Taking several beam profiles withdifferent phase advance (different Q-magnet current), we
extracted the twiss parameter and the emittance. The anal-We have developed RF gun complex to generate multibunch electron beam with 2.8 ns bunch spacing. It wasimplemented by 1) Nd:YV0  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. We are currently planning to introduce a real time laser monitoring
device such as power monitor, profile monitor, etc.

REFERENCES[1] N. Terunuma et al., WP-10 in this proceedings[2] M. Kuriki et al., “Properties of the electron beam generated
by photo-cathode RF gun”, 13th Accelerator science symposium proceedings, 2001