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

FIRST RESULTS OF THE CAESIUM TELLURIDE PHOTO-CATHODE
FOR ATF RF-GUN
N. Terunuma, M. Kuriki, H. Hayano, J. Urakawa, M. Nomura, K. Hirano, Y. Yamazaki
High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Ibaraki, JapanK. Hasegawa, Graduate University for advanced studies, Hayama, Kanagawa, Japan
M. Takano, National Institute of Radiation science, Chiba, Chiba, Japan
T. Nakanishi, S. Okumi, Nagoya University, Nagoya, Aichi, Japan
M. Washio, R. Kuroda, Waseda University, Yokohama, Kanagawa, Japan
Abstract
Injector system of the Accelerator Test Facility (ATF)
in KEK was changed from ordinal thermionic gun andbuncher system to the laser driven RF gun system.
Caesium telluride (Cs-Te) was chosen as the cathodematerial for ATF RF gun because of its higher quantum
efficiency. Photo-cathode preparation system is designed
to have an evaporation unit of Cs-Te and a loading unit ofcathode into the RF gun. All of actions in this system aredone in UHV without any vacuum breaks. It minimizes
the any chances of damage on the Cs-Te cathode causedby oxygen. In this paper, we describe the results obtained
through the recent ATF operations.

the Cs-Te cathode load-lock system for ATF RF gun as
shown in Picture 1. This system consists of two units, theevaporation unit of Cs-Te and the cathode-loading unit
into the RF gun. Different from other RF gun systems[2][3], all of our components are connected as a vacuum
system in situ. It means that there is no chance to have acontamination of air or oxygen after forming the Cs-Tecathodes. We could reproduce a cathode quickly without a
UHV startup time that is required to install the cathodes
formed in another place. This advantage was actually
powerful for the ATF experiments done in the lastoperation period.

INTRODUCTION
Studies for the multi-bunch beam are performed in theATF damping ring to establish the technologies requiredin the GLC project [1]. This multi-bunch beam is consistof 20 bunches in a train with a 2.8 ns bunch spacing.
Designed intensity of each bunch is 2x1010 electrons atmaximum. However, the operational beam intensity waslimited less than half of design’s because of the limitationof the radiation level. Beam loss was caused by a longer
energy tail and an energy jitter, caused by the oldthermionic-gun and buncher system. In order to fix theseissues, we decided to change the injector system to a laser
driven RF gun system. The multi-bunched laser pulses
will generate multi-bunch beam. Typical laser intensity of
our system is around a few µJ/bunch. Therefore, thequantum efficiency of the cathode material should haveabout 1% to generate the ATF beam. We selected thecaesium telluride because of its higher quantum efficiency,
around a few %, and the recent results done by CERN [2]and DESY [3] linear collider R&Ds. Materials used inother RF guns, such as Copper or Magnesium, may not be
used because their quantum efficiency is less than 1/100of Cs-Te’s. If we use these materials, we have to prepare
the very expensive huge power laser system and it may
not be stable.
To keep the higher quantum efficiency of Cs-Te, wehave to keep it in UHV. An oxygen exposure loses thequantum efficiency of Cs-Te dangerously. We constructed

Picture 1: Cs-Te photo-cathode load-lock system at theRF gun section in ATF.

CATHODE FORMATIONAs shown in Fig. 1, we have a hole at the center of thehalf-cell’s end plate. Cathode plug is installed to close it.
Therefore Cs-Te cathode should be formed on the surfaceof the plug faced in the half-cell cavity.
The Cs-Te cathode formation in our system will bedone as follows.
(1) Surface cleaning by 5 keV of Ar ion beam.
(2) Te layer formation by evaporation.
(3) Cs layer formation on the Te layer by evaporation.
The surface cleaning will be done when we install new
plugs into the system or to clean up the previously formed

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

Cs-Te layer. The sputter depth is about 50 nm. Tellurium
is evaporated from small pieces in a tungsten heater, andthe thickness on the plug is monitored as the same amounton an equivalent target. Typical thickness of Te is about 5
nm in ATF. Caesium is evaporating by heating a caesium
dispenser [4] and its amount is controlling by monitoring
the increase of the quantum efficiency [5].

RF GUN
Cathode Guide

Initial Quantum Efficiency
Fig. 2 shows an example of the quantum efficiency
growth during the caesium evaporation on a molybdenum
plug. Quantum efficiency was quickly increased anddropped sharply. It caused by a faster Cs evaporationcompare to the Cs-Te formation and it made a caesiumrich surface at a moment. Quantum efficiency was still
increased after stopping the Cs evaporation. This means
Cs rich surface was disappeared by increasing the Cs-Teformations. Finally the quantum efficiency of Cs-Tecathodes has a quite higher value, more than 15%.
Cs evaporation on the Mo Plug
20BeamCathode PlugEnd PlateFigure 1: Cross section of the ATF RF gun. Cs-Tecathode plug is loaded from left side.

QE(%)

15105

Base material of cathode, Cu and Mo
We have tested copper [2] and molybdenum [3] as abase material of the Cs-Te cathode. Quantum efficienciesof the formed Cs-Te layers are 5% and 15% for copper
samples and molybdenum samples, respectively. Inaddition, we have found damages on the surfaces of both
samples. They may be caused by the RF break down andlook like craters with a size of several microns. Copper
samples seem to have bigger craters than that of themolybdenum’s. Chemical boundary of the copper surface
seems to be softer and we guess it will affect the purity of
the Cs-Te compositions [6].
As a result of these initial experiments, we decided touse the molybdenum plugs for the standard ATF beam
generations. Picture 2 shows the Mo plug with the Cs-Te
cathode on the surface.

Picture 2: Mo plus with Cs-Te cathode.

Cs Off000.5

11.5

2

Elapsed Time (Hours)

Figure 2: Growth of the quantum efficiency at the CsTe formation.

BEAM OPERATION
We have tested Cs-Te cathodes for the electron beam
generation. These beams were used both for the RF gun
studies and for experiments with the ATF damping ring.

Quantum Efficiency with RF fields
The dropping of the quantum efficiency with RF fields
was observed as shown in Fig. 3. Quantum efficiency wasdecreased about a day to a stable level around 1%.
This behaviour was observed for all of samples after
the Cs-Te formation. Cathode transfer may affect thevacuum purity. So we repeated the transfer between theCs-Te evaporation chamber and the RF gun but it leads
no such decrease. This behaviour is actually caused by RFfields. Vacuum pressure in the RF gun was not changed
during the RF operation, even if it was in an initial period
or a stable period.
From March 2003, we had a chance to use a cathode forfive weeks of ATF operation. It shows no significant
decrease of the quantum efficiency after reaching a stablelevel.

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

contactor mounted on the cathode head-side insured theelectric contact between the cathode and the end plate.
We have no data for dark current with a simple end platefor comparison however we can say the gap is not an
issue at present.

Cs-Te with Mo plugs
2015Q.E. %SUMMARY
10Te 5nm: Q.E. ~ 3%

5

Te 10nm: Q.E. ~ 3%

Te 1nm: Q.E. ~ 1.5%0050

100150Elapsed Time (hours)

Figure 3: Dropping of the quantum efficiency with RFfields.

Dark CurrentThe dark current of the RF gun is an issue. We
measured it by using a faraday cup installed just after thesystem. Fig. 4 shows the history of the dark current. Dark
current decreases by increasing a RF operation time.
About a hundred hours after, dark current is not hard for
ATF operation. Compare to the RF gun experiments donein 2001, it was a single bunch beam generation with aused RF gun from Tokyo University, present RF gun has
1/10 of old one. This is explained as a result of finemachining of the RF gun surface with the X-bandstructure technologies.

aging~1
aging18(hours)

Dark current (nA)aging34+beam13
aging34+beam58
aging34+beam83

0.6

0.4

0.2

02

34

5

ACKNOWLEDGEMENTS
We would like to thank other members of ATF group.
We also would like to express our gratitude to Professors
Y. Kamiya and S. Enomoto for their continuousencouragement.

REFERENCES[1] GLC road map report, February 2003.
[2] CERN, CTF3 Note 020, September 2001.
[3] P. Michelate et al., EPAC96, p1510.
[4] http://www.saesgetters.com/[5] F. Furuta et al., AIP conference proceedings, vol. 570,p1015.
[6] A. di Bona et al., J. Appl. Phys. 80-5 (1996) p3024.

10.8

Multi-bunch beam generation by the Cs-Te photocathode with a laser driven RF gun was performed at ATF
since October 2002.
We constructed a Cs-Te cathode preparation system andcould reproduce a cathode quickly without any vacuum
breaks. It was a powerful advantage to perform ATF
studies in the last beam operation.
The quantum efficiency of Cs-Te on the Mo plugs
reaches about 15% when it was formed. It was also foundthat the above higher quantum efficiency is decreased
with the RF field and dropped into a stable level about 1%.
We confirmed this stable quantum efficiency was notchanged at least five weeks operation.
The beam quality of ATF was improved by installing
the RF gun system. The beam tail was reduced so thebeam loss was reduced. We can operate the multi-bunch
beam with designed beam intensity. We use this RF gun
system as an operating accelerator component.

678

RF Input Power (MW)Figure 4: Dark Current of the ATF RF gun.
As described before, the end plate of the half-cell cavity
has a hole to install a photo-cathode. There is a ring gap
with 0.5 mm distance after installing a cathode plug. A