The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

Photocathode RF Gun Study for SPring-S Linac
Tsutomu TANIUCHI, Kenichi YANAGIDA, Shinsuke SUZUKI, Akihiko MIZUNO,Hiroshi YOSHIKAWA, Toshihiko HORI, Takao ASAKA and Hideaki YOKOMIZOAccelerator Division, Japan Synchrotron Radiation Research Institute
Kamigori, Hyogo, 678-12 JAPAN
Abstract
A photocathode RF gun has been studied for theupgrade of SPring-8 Linac. The beam characteristics
were simulated using MAFIA code and a high powermodel cavity was designed and under fabrication. Ahigh power test of this cavity is scheduled in next year
to study the fundamental characteristics of the RF gun
and compare with the simulation.
1 Introduction
A photocathode RF gun has been studied for an optional injector of the SPring-8 Linac[l]. This gun will
be used for the future applications[2] of the linac suchas a single pass FEL based on the self-amplified spontaneous emission which require smaller emittance andhigher peak current than the present one. The simulation study has been done by using a computer code
MAFIA[3]. In parallel with the simulation, the preparation of the experimental study is started. The purposesof this experiment are (i) to confirm a stable operation of
the RF cavity under the high gradient field environment,(ii) to confirm the effectiveness of surface treatments forthe reduction of the dark current and (iii) to accelerate photoemitted electron beam and compare the beamcharacteristics to those obtained from the simulation.
For this experiment, a single cell cavity was designed
and under fabrication. A single cell cavity was chosenbecause the field distribution is more simple than thatof multi cell cavities and preferable for the comparison
with the simulation. The surface of the cavity (Cu) is
used as a photocathode. The high gradient accelerating field such as lOOMV /m is required to minimize thespace charge effect and produce smaller emittance beam.
Furthermore, higher gradient up to 200MV jm is needed
to check the simulation because the simulation predicts
the emittance growth due to the RF field at the field
gradient of more than 150MV /m[4]. On the other hand,the dark current becomes large as the field gradient becomes higher and causes RF breakdown in the cavity.
Therefore, a high pressure ultra pure water rinsing andTiN coating are tested as the surface treatment for thereduction of the dark current.

characteristics was surveyed by changing the initial RFphase at which the bunch is on the cathode. It was
found that the emittance was minimized for the initial
RF phase of 30 degree. Using an optimum cavity shapedescribed above, the precise simulation was performedchanging with the field gradient on the cathode. Parameters used in this simulation are listed in Table 1.
Table 1
Parameters used for MAFIA simulation.
1[nC]
Bunch charge
Flat Top
Transverse Profile
1[mm]Spot size on Cathode
Flat Top
Loguitudinal Profile
10[psec]
Bunch Length[1rmm mrad] 0Initial Emittance30[deg]
Initial RF phase
10000
Number of Macro Particles
Fig. 1 shows the normalized rms emittance as a function of the distance from the cathode. The beam energyis 2.2MeV at the gun exit in the case that the electricfield on the cathode is 100MV /m. As seen from the figure, the emittance growth due to the space charge effectnear the cathode is reduced as the field gradient becomes
higher. However, the emittance at the gun exit becomeslarger as the field gradient is more than 250MV jm.

--100MV/m- - ·150MV/m- - -200MV!m----- 250MV!m-···-300MV!m

2.5.......

"dro1-.1s

2.0

•

~~w

1.5

r::
1.0

2 Simulation0

A simple single cell cavity is considered to be preferable for the high gradient acceleration because the gradient near the round corner of the gun exit can be smaller
and the probability of the discharge will be smaller thanthe multi-cell cavity. The beam acceleration in a single
cell cavity was simulated by using MAFIA code. In order to obtain an optimum shape of the cavity, the beam

0.02

0.04

0.06

0.1

0.08

Distance from Cathode(m]Fig. 1 The normalized emittance in the cavity for different field gradients on the cathode-203-

The bunch length along the beam line is shown inThe lith Sympo,sium on Accelerator Science' and Technology, Harima Science Garden City, 1997

Fig. 2. The bunch lengthening is significant below thegradient of 150MV /m on the cathode. From these results, the optimum gradient is around 150MVfm.

as against 0.55mm in the case of one port as shown in
Fig. 4.

1. 005o.55mm~--lOOMV/m- - ·lSOMV/m- - -200MV/m---··250MV/m--···300MV/m

3.5uCJ)!/)

0..

1.000
N~

1jCJ)

0.995

N

3.0

....
I.at:)

I---- ------- -----.

r
0.02

0n1s~

.

0.04

0.06

..,::..::.:..,=~ ..,: . : .

0.08

Distance from Cathode

0.990

0.-----------------t?•-~=~-•=~-.,=~ .,:..:.~ .,==-"'::..::.~

2.5·.-I.-I

z-6

0.1-4-20Y[m]Fig. 2 The bunch length in the cavity for different
field gradients on the cathode. The bunch
length is shown as the standard deviation of
the longuitudinal particle distribution.
3 High Power Model Cavity

3.1 RF Design
For the purposes mentioned above, a single cell Shand cavity was designed. The schematic drawing of
this cavity is shown in Fig. 3. The accelerating gap of
the cavity was determined so that the emittance became
minimum in MAFIA TS2 simulation.

Fig. 3 The cross section schematic of high powermodel cavity. There are two windows for thelaser injection. The incident angle is 24 and 90
degree.
There are two coupling port in this cavity to improve
the field symmetry and shorten the filling time. The
field symmetry is important to avoid the transverse kickeffect due to the dipole component of the accelerating
field. The displacement of the field center to the cavity center was calculated by MAFIA and it was 0.13mm

246[mm]Fig. 4 Asymmetry of accelerating field in RF gun cavity. In the case of 1 port, the center of field
shifts 0.55mm to the coupling port side. For
the 2 port cavity, this is reduced to 0.13mm.
The shorter filling time enables the higher field gradient or more stable operation. By connecting a dummyload to one coupling port, the Q value of the cavity canbe reduced and the filling time becomes shorter. In thisdesign, it is shorter than one fourth of the one port case.
Furthermore, the matching condition of the input port
is kept in this cavity even if the beam loading of thedark current becomes larger. This allows the absenceof the circulator between the cavity and klystron. Thecouplings between the cavity and the waveguides weredetermined by a simulation of Slater's tuning method using MAFIA. At first, the external Q for the output port,
Qextl, was calculated, then the external Q for the input
port, Qext2, was tuned so that l/Qext2=1/Qo+l/Qext1·
The calculated value agreed with the measured one
within lOThe cell radius, by which the resonant frequency of the accelerating mode was tuned, was roughly
estimated by using MAFIA and finally determined by ameasurement of a low power model cavity. The discrepancy between MAFIA calculation and the measurement
was within 2MHz that is about 30mm in the cell radius.
The maximum field gradient appears near the rounded
corner at cavity exit. The ratio of the maximum gradientand that on the center of the cathode is 1.09. The accelerating field gradient can be lager as this ratio is smaller.
In the single cell cavity, it becomes smaller than that ofdisk-loaded cavities. The relevant parameters are shownin Table 2.

3.2 Surface Treatment-204-

The reduction of the dark current ts needed for a

The II th Symposium on Accelerator Science and Technology, Harima.Science Garden City, I997

Table 2
Design parameters of high power model cavity.
Frequency
Number of cells
Accelerating gap
Bore diameter
Intrinsic Q value
External Q value for output port
External Q value for input port
Loaded Q value.
Filling time
Shunt impedance (for b=l)
Emax I Ecathode
Incident angle of laser

[MHz]
(mm]
(mm]

(J.tsec]
(Mn]
(deg]

study the beam characteristics and phenomena in a highgradient field environment. Two kinds of surface treatment are adopted to reduce the· dark current. RF pro-cessing of cavities will start next year and beam production by introducing laser pluses will start in the secondquarter. Three dimensional simulation has been started
to investigate the beam dynamics in a more realistic cavity field. In parallel with these studies, an RF gun system which achieves the beam characteristics required for
the SPring-S linac will be studied.

2856
Single
28
20
13000
3684
2786
1414
0.16
1.16
1.09
90 I 24

References

stable operation in a high field gradient. Furthermore,
the dark current produces undesirable gamma-ray background for the inverse Compton scattering. The cavities
are machined from OFHC copper blocks in which micro
pores are reduced by Hot Isostatic Pressing (HIP). It
was demonstrated that the high pressure ultra pure water rinsing and use of Ti were effective for the reductionof the dark current(5]. To confirm the. effectiveness ofthese treatment, three types of cavity with different surface treatments, normal treatment, high pressure ultra
pure water rinsing and TiN coating, are under fabrication. The magnitude of the dark current depends onthe cleanliness of the cavity surface. A high pressure
ultra pure water rinsing removes dusts and impurities.
We will use a water rinsing systerp. with the pressureof 80-90kglcm2 and the electric resistivity of 18Mflcm.
After the rinsing, the cavity is assembled as a closed
vacuum system in a class 1 clean room to avoid contamination. Use of Ti for the cavity surface is also effective
for the reduction of the dark current since its secondary
field emission coefficient is less than unity. The technique demonstrated before was to braze Ti on the top ofdisk, however, we will adopt TiN coating on the surface.
Only the surface area which is perpendicular to the electric field is coated and the thickness of TiN is controlled
less than 100 nm to reduce the surface loss as small as
possible.

(1] H. Yoshikawa et al., "Injector Linac of SPring-S"~
Proc. of the 1996 Int. Linac Conf., Geneva, August,1996.
(2] S. Suzuki et al., "Upgrade Plan of SPring-S Linac",Proc. of the 1996 Int. Linac Conf., Geneva, August,1996.
(3] M. Bartsh et al., Comp. Phys. Comm. 72, 22-39(1992).
(4] T. Taniuchi et al., "Study of an RF gun for theSPring-S linac", Proc. of 18th Int. FEL conf., Rome,August, 1996.
(5] H. Matsumoto, "Dark Currents", Proc of the 1996
Int. Linac Conf., Geneva, August, 1996.

3,.3 High Power Experiment

A high power test station with a 35MW klystronand a modulator will be built in the Machine Experiment Hall at SPring-8. The stage for the cavity andmonitor system is located in a clean booth of class 1000
to avoid the dust contamination during the assembly ofcavity and waveguides. As a first step, the beam monitor
system consist of a profile monitor, an energy analyzing
magnet, a slit and Faraday cups to measure the darkcurrent and its energy spectrum. The laser pulse will
be introduced from out of shield and injected through aquartz window.
4 Conclusion

High power model cavity for RF gun was designed to-205-