Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Electron emission characteristic of CsTe photo-cathode in RF gun
KURIKI Masao , URAKAWA Junji, Pavel KARATAEV, TERUNUMA Nobuhiro,

HAYANO Hithosi, HIRANO Koichiro, YAMAZAKI Yoshio, OKUMI Shoji ,NAKANISHI Tsuyosi , YAMAMOTO Masahiro TAKANO Mikio HASEGAWA Katsushi
and the ATF collaboration
High energy accelerator research organization, Tsukuba,Ibaraki(a) Nagoya university, Nagoya, Aichi(b) National institute of radiation science, Chiba, Chiba(c) Graduate university for advanced studies, Hayama, Kanagawa
Abstract
As the electron source, CsTe cathode is used in theroutine operation of KEK-ATF (Accelerator Test Facility).
The stable operation is realized by the good performance ofthe cathode, i.e. a high quantum efficiency, QE more than
1% and a long life time more than a month, at least. QE isusually measured off line, at low surface electric field withXe UV-lump flushing, but QE is much influenced by thesurface field known as Shotkky effect. Therefore, QE measured off line may be much different from that in operation.
We measured QE of CsTe cathode in the real operation byvarying the laser power. Variation of QE as function of surface field was also observed by changing the laser injected
RF phase. As predicted by several theories, QE in the operation was much higher than that off line.

INTRODUCTION
KEK-ATF was established for R & D of Global Linear
Collider, GLC project. The purpose of ATF is to prove
the super-low emittance multi-bunch beam with a damping
ring. To generate the multi-bunch electron beam in ATF
Linac, the photo-cathode RF gun is employed.
CsTe is originally developed by CERN[1] as the high QE
cathode material. KEK-ATF and Nagoya university collaborated to apply this material as the cathode in the photocathode RF gun. In ATF, this cathode is routinely used as
the primary electron source. The beam property generated
by the RF gun meets the requirements as ATF injector. For
details, see ref. [2].
QE of the cathode was usually more than 10% initially,
but it decreased rapidly and converged around 1%.[3] It
was confirmed that this 1% QE was kept more than twomonths. Therefore, after the initial drop, the cathode life
time is not able to be defined, that means it looks infinite.
QE is usually measured off line, at low surface electricfield with an Xe-UV lump flushing. QE is, however, muchinfluenced by surface field known as Shotkky effect. Therefore, the off-line QE may be much different from that in theactual operation. The purpose of this article is to extract QE
at the operation and connect it to that off line. To do that, masao.kuriki@kek.jp

the QE behavior have to be understood, especially on thesurface field dependence.
Based on a basic formalism, the emission from a photocathode material , is expressed as;[4]    



(1)





where is thermionic emission constant, is temperature,is energy of a laser photon, is work function, andis Boltzmann constant. If the Shotkky effect caused by an
external field is introduced, the work-function is corrected
to be a function of the field as&

!#"%$  &')( *,+ 46"58-/7 .1& 032



(2)

+

where is the work-function at zero surface field, is thefield enhancement factor that is determined by the surface
condition, is amplitude of the surface field, and is thephase of the oscillated field. Assuming these equations, QE
as function of the surface field is expressed as,"

2

9 ":;"%$  9 " & <>=@ ? ( *,+ 4C"58-/7 .A& 0B2 D

9 " &



(3)

where
is QE at zero surface field.
If we take logarithm of Eq. 3, it gives

E 0 9 "F#"%$  E 0 9 "'&  ? ( 4C*,58+ 7 &G "-/.1032
 (4)E 0 9 "F;"%$ is expressed as a straight line functhat meanstion"-H.A0Bwith
2 . respect to root of the effective surface field,
THE SYSTEMThe RF gun system of KEK-ATF is schematically shownin Fig. 1. The RF gun cavity is based on the 1.6 cells BNL
gun IV. The cavity is modified to set the cathode plug on
the end plate. The cathode plug is made from Mo. Cs and
Te are evaporated on its surface to form CsTe as the cathode material in a different vacuum chamber. The evaporation chamber and the gun cavity is connected with vacuum356Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

the photo-electron effect. Empirically, it is known that QE
is generally a function of the emitted current. In that case,it would be better to use a alternative definition of QE in
derivative as;  RF gun cavity
Faraday cup

9 ":  $  ?

9 ":  $Figure 1: Schematic view of ATF RF gun system. BNL gun IV typecavity was modified to mount the cathode plug on the end plate. The gun
is followed by a solenoid magnet to control the beam size and emittance.
For the emission measurement from the gun, a Faraday cup is inserted
onto the beam line.

pipes so that the cathode plug is transported into the gun
cavity without air exposure. It is known that CsTe is affected by air exposure, that is why this system is used. For
the evaporation and transported system called by Load-lockchamber, see ref [3].
The multi-bunch beam is produced by illuminating
multi-pulse laser with 2.8 ns spacing. The laser wave
length is 266nm in UV. Each pulse has typically power of
2-3, 10 ps pulse width. Number of pulses is determined by timing of the pulse clipping by Pockels cell. The
number of bunches is then varied by changing the clipping
timing from 1 to 20. The maximum bunch number, 20 islimited by the radiation safety regulation. The laser power
is controlled by the laser power regulator that is composedfrom a rotational  plate and a polarized splitter. The
transmission efficiency of the splitter is varied by the laser
polarization determined with the plate angle.
The phase and power of input RF to the gun cavity iscontrolled by a mechanical phase shifter and attenuator.
The power is usually 7 - 8 MW that corresponds to 90
MV/m surface field at the maximum phase.
The emission current is measured by the Faraday cup,
FC inserted onto the beam line at need. The solenoid focusing is originally employed to correct and optimize thebeam emittance, but it is used to control the beam size tocatch all of the beam in this study.



4

QE MEASUREMENTS AND ANALYSISQE is defined as ratio of numbers of emitted electrons
and injected laser photons. QE in % is expressed with theconventional parameters as[4];

9 "   ?         

 

(5)

where is emitted current of the electron
beam in nA,  iswave length of the laser light in nm, is power of the laser. This expression assumes a linear response oflight in



(6)

4

4

Helm 120A
Helm 100A
Helm 80A
Helm 60A

3.5
3
2.5

F.C. current (nC)

Cathode plugwhere the units are same as in Eq. 5 In this article, QE atzero emission, i.e.
 is employed as the reference.
To extract Q.E, the laser power and the emission current
have to be measured. The laser power is initially calibrated
by a laser power meter at the position of the laser beamline closest to the cathode with 100% transmission of thelaser power regulator. The
 actual laser power is estimated where is the calibrated laser power andto be is the laser transmission that is determined by the angleof the  plate in the laser power regulator. The ! can becontrolled and monitored in our integrated system.
The emission beam current is measured by FC. To measure the emission current correctly, all of the electrons haveto be captured. Because the optimized strength of thesolenoid focusing depends on the beam energy, the optimized solenoid current is functions of the input RF power
and its phase because the beam energy depends on not only
the input RF power, but also the RF phase when the laser isilluminated. The signal from FC is sent to ADC to digitize
its charge. The data is then corrected and recorded.
F.C. current (nC)

Solenoidmagnet

 



4
3.5
3
2.5

2

2

1.5

1.5

1

1

0.5

0.5

0

-40 -20 0 20 40 60 80 100120
RF gun phase (deg)Helm 120A
Helm 100A
Helm 80A
Helm 60A

0-40 -200

20 40 60 80 100 120
RF gun phase (deg)Figure 2:

Emission current Figure 3:
Emission current
as function of the laser injection as function of the laser injection
RF phase with different focusing RF phase with different focusing
strengths. The laser power was strengths. The laser power was(' ) +* .
(' ) +* .
0.45"$#&%
0.75"$#&%

 

 

<

Fig. 2 and32 3 show the emission current with 0.45 and0.75 -,./10
laser power as function of RF phase withrespect to the laser injection timing. Data sets with various focusing strength of the solenoid magnet are drawn
together. The error bars are statistics only. From these figures, the emission depends on not only the laser power, butalso the RF phase. This phase dependence can be explained
as several effects as discussed later .
On the same RF phase, the measured current also depends on the focusing strength as expected. Since with the
optimized strength the total current is supposedly captured
by FC, the maximum data point among those by variety
of focusing strengths is taken as its representative on thatphase. Fig 4 shows these representative data as function of
RF phase with
32 three different laser power, 0.75, 0.45, and0.15 - /10
.



357

<

F.C. current (nC)

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)
40.75uJ
0.45uJ
0.15uJ

3.5
3

Figure 4:

The emission current as function of the RF phase.
Among those by the various
solenoid currents the maximum
current is plotted as the representative on that phase.

2.5
2
1.5
1
0.5
0

0

20

40 60 80 100
RF gun phase (deg)4

FC current (nC)

FC current (nC)

From these data QE was extracted. Fig 5 and 6 show
examples the emission as function of the laser power at 10
and 70 degree laser injection phase respectively. The fit
curve is evaluated by assuming a second order polynomial
function with a constraint on zero emission at zero laser
power. According to Eq. 6, the QE at zero emission is
obtained as its derivative, i.e. the coefficient of the firstorder of the fit curve.
3.5
3
2.5

4
3.5
3

2

1.5

1.5

1

1

0.5

0.5

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Laser power (uJ)

0

4



?C?

2.5

2

a saturation of the emission current with an increase of theemission. Most of this effect could be removed by taking
QE at zero emission. It is also true for the space charge
limit, in addition, this effect is expected to be very small atsuch high electric field.
The last candidate is the dynamics in RF field. This effect is that a part of the beam could not reach to the exit of
the gun. As long as the surface field is in the ordinal direction, the electron can be escaped from the inside of thematerial to the outer space. Even though, if the beam isemitted on some phase, the RF field is reversed before allof the beam is transmitted into the second cell. In that casea part of the beam is decelerated and will be lost resulting
a lower emission.
This effect is dominant in phase where the surface fieldis small. The region where the data are not fit well, canbe considered as such region. To suppress this effect onto
the fitting, the fit is actually performed with the data points
from 10 to 80 degree.
was evaluated
QE at the zero surface field limit,from this fit to be 
. As a biproduct, the was determinedabsolute RF phase origin givingin the current phase scale.
to be The obtained
was inconsistent to that measured under the low surface field. This inconsistency is not understood yet. It may be related to some inner structure of
CsTe because a simple metal like structure is assumed inour study. Anyway, It is one of the future issues.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Laser power (uJ)

9 "'&

$
H
? .A032 

9 " &Figure 5: The emission at 10 deg. Figure 6: The emission at 70
laser injection RF phase as function degree laser injection RF phase asfunction of the laser power.
of the laser power.

QE (%)

Fig. 7 shows QE as function of the laser injection RFphase. The error bars show the ambibuity on the QE evalution with the fitting. The curve is obtained by fitting thedata to Eq. 3. The maximum QE of 3.0% was recoreded
4
3.5
3
2.5

Figure 7: QE as function of2

the laser injection RF phase. The
maximum QE was obtained as
3.0% around 20 degree.

1.5
1
0.5
0

-20

0

20

40

60 80 100
RF phase (deg)around 20 degree. This is much higher than that measured
under the low surface field environment, 1.0% as expected.
One can also see easily on this figure that the data are notconsistent to the theoretical curve at the lower and higher
phase region.
Basically, QE may be influenced by the surface field,
surface charge limit effect, space charge limit effect, dynamics in RF field, etc. In the theory used in this fitting,
only the effect of the surface field known as Shotkky effectis assumed. The surface charge limit will be appeared as

SUMMARY
QE of CsTe cathode in RF gun at the operation condition was evaluated as the derivative of the emission current
with respect to the laser power. For the QE at the zero
emission limit, the results are much higher than that measured under the low surface field as expected. The surface
field dependence of QE can be explained with the simple
model including the Shotkky effect where the surface fieldis high, but some inconsistency was observed where thesurface field was relatively low.
QE at the  zero surface field limit was evaluated
to be



that was not consistent to  , measured
off line with Xe lump. It is a future issue to understand thisinconsistency.

4

?

$

?

REFERENCES[1] F. Chevallay, G. Suberlucq, H. trautner, “Production of ahigh average current electron beam with Cs-Te Photocathodes”,CTF3 Note20, 2001
[2] M. Kuriki et al., “Properties of the electron beam generated
by photo-cathode RF gun”, 13th Accelerator science symposium proceedings, 2001
[3] N. Terunuma et al., Linac conference proceedings, p162-164,
2003[4] M. Kuriki, “Electron source”, Seminar on high energy accelerator - Oho2002 Textbook,2002

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