1. High brightness electron sources, e-beam qualities and diagnostics
Development and Production of Photocathodes
for the CLIC Test Facility
G. Suberlucq
CERN - Geneva
CTF experiment
Participants of photocathode measurements
Recap of QE measurements
Cs2Te photocathodes
CsI + Ge Photocathodes
DC and RF gun GaAs tests
The photocathode production
Conclusion
References

2. CTF layout     drive beam probe beam Drive beam probe beam n 48 1q
[nC] 21
0.1  t
[ps]
<2
<3
RF structure E z
[MV/m]
LAS:
LIL section 10
HCS:
high charge sect. 60
CTS:
CLIC transfer sect. -4
CAS:
CLIC accel. sect. 80 3 
GHz 3+  
GHz 3-  
GHz
45MW
45MW
45MW 
7.8MHz
bunch compressor
and matching
CLIC modules (2 initial, 10 final)
RF gun
100 MV/m
CTS qf qd qf
CTS qf qd qf
HCS1
HCS2
8 MeV
drive beam
45 MeV
RF gun
70 MV/m qd qf qd qf
LAS
CAS
CAS
CAS
CAS
4 MeV
40 MeV 
T=46 MeV 
T=46 MeV
probe beam
laser
train generator
12.6 m
1.41 m
1.41 m

3. Reported results are from or with the collaboration of :
Participants
Reported results are from or with the collaboration of :
K. Aulenbacher, G. Benvenuti, A. di Bona, R. Bossart, A. Braem, H. Braun, D. Carminati, E. Chevallay, F. Chautard, J. Clendenin, M. Comunian, R. Corsini, R. Cosso, J.M. Dalin, J. Durand, S. Hutchins, J. Madsen, G. Paic, G. Rossat, G. Suberlucq, S. Schreiber, M. Wurgel

4. Recap of QE measurements 1/2

5. Recap of QE measurements 2/2
QE and lifetime of some alkali cathodes

6. Cs2Te photocathodes 1/3 Photocathode preparation
Substrate : Cu, Mo, Mg
Te = 10 nm ; Cs  °C optimized at =266nm
Vacuum transportation
Electro-optic parameters of Cs2Te layer :
Relative dielectric constant : r  10 kHz
Resistivity :   1011  : Is an insulator (Te = 10 nm , Cs  15 nm)
Specular reflectivity :7 %  Rs  nm as a function of the thickness layer and the substrate properties.
Photoemission parameters
Photoemission threshold : E0  3.5 eV
QE  nm
DC and RF gun measurements
QEmean  6 266nm and 8MV/m
A few weeks lifetime : At the beginning a fast drop during the first days, followed by a lower slope during few weeks
No visible charge or current limitations, on metallic substrate, up to 35 nC in 13 ps
Fast response time < a few picosecondes (limited by the streak-camera resolution)
High electric field operation : up to 100 MV/m, between 100 and 127 MV/m some breakdown voltage.
QE depends on the electric field : possible Schottky effect

7. Cs2Te photocathodes 2/3 RF gun lifetime measurements
Cath. Nb. 36 : 113 MV/m  E  127 MV/m
Cath. Nb. 37 : 92 MV/m  E  115 MV/m
QE versus electric field

8. CsI + Ge photocathodes 1/2
Photocathode preparation
Produced by A. Braem and D. Carminati (CERN-PPE div.)
mechanical and chemical cleaning
Deposition of : Al =150 nm , CsI = 350 nm and Ge = 2 nm
Baked out at 150 °C
Air transportation
Conditioning process
Difficult in the DC gun
In the RF gun, with high electric field and laser beam, it takes about 10 hours
The vacuum must be monitored
Main results
Maximum electric field : 70 MV/m
Maximum laser fluence : 1mJ/cm2
QE  0.1 % at  = 262 nm and E = 70 MV/m
No charge limitation up to 4.3 nC/cm2 , but charge limitation and pulse lengthening at 22 nC/cm2
Less than a few picoseconde response time without charge limitation
Lifetime not yet measured

9. CsI + Ge photocathode tests 2/2
Charge limit and time response
QE measurement for different cathodes
All QE measurements were done in the DC gun
Maximum laser fluence : 1 mJ/cm2

10. DC and RF gun GaAs tests
Dark current measurements

11. The Cs2Te photocathode production
The beam duty factor is typically 30 %

12. Conclusion Photocathode plug and RF spring
They worked, but with arcing and burning process during the RF operation. The RF contact should be improved to save the rear part of the gun, the plug, and probably, the photocathode layer.
The Cs2Te photocathodes (vacuum transportation )
All CTF specifications were fulfilled (more than 1% for more than one week, fast response, no charge limitation, 100 MV/m operation)
A Mo or a Mg under-layer with heating and fresh cesium gave the best results
But, the homogeneity of the layer, the QE and the lifetime reproducibility's, should be improved.
The CsI + Ge photocathodes (air transportation)
QE was as expected but the lifetime was not fully tested.
These cathodes showed a charge limitation, but higher than the probe beam charge. Without charge limitation, the response time is fast.
Different behavior from cathode to cathode
Electric field seems limited to 60 or 70 MV/m
Improvements and new cathode developments should be pursued
The GaAs photocathodes (polarized electron sources)
Encouraging first results were obtained in high electric field environment.

13. Photocathode developments
For the drive beam
Substrate vacuum cleaning by argon ion bombardment
Mg under-layer deposition, in the preparation chamber
Fresh cesium with special package
QE optimization at 355 nm
Improve QE measurements in the CTF drive beam
Better laser energy measurement, the closest as possible of the photocathode
Charge measurement better define, in electric field, spot size, RF phase etc....
Investigation in QE electric field dependence
European Research Network HCM - High Current Photoemission and Bright Injectors.
For the probe beam
Investigation on conditioning process and QE electric field dependence
Improve QE measurements in the CTF probe beam
Informal collaboration with CERN-RD 26 and the Weizmann Institute of Science (Rehovot, Israel) - A. Braem
CsI+LiF , CuI+MgF2 , Cs2Te+MgF2 , Al+CsF
GaAs photocathodes
Collaboration with KEK, Nagoya University and SLAC : under definition

14. References
H.H. Braun, K. Aulenbacher, R. Bossart, F. Chautard, R. Corsini, J.P. Delahaye, J.C. Godot, S. Hutchins, I. Kamber, J.H.B. Madsen, L. Rinolfi, G. Rossat, S. Schreiber, G. Suberlucq, L. Thorndahl, I. Wilson, W. Wuensch, Results from the CLIC Test Facility, CLIC Note 310,
A. di Bona, F. Sabary (CEA), S. Valeri (INFN-Modena), P. Michelato, D. Sertore (INFN-LASA), G. Suberlucq (CERN), Auger and X-ray Photoemission Spectroscopy Study on Cs2Te Photocathodes, Under publication by CEA
G. Suberlucq, Développement et production de photocathodes pour le CLIC Test Facility, CERN CLIC Note 299, may 1996
R. Bossart, H. Braun, M. Dehler, J.C. Godot, A 3GHz Photoelectron Gun for High Beam Intensity, CLIC Note 297
K. Aulenbacher. R. Bossart, H. Braun, J. Clendenin (SLAC), J.P. Delahaye, J. Madsen, G. Mulhollan (SLAC), J. Sheppard (SLAC), G. Suberlucq, H. Tang (SLAC) RF Guns and the Production of Polarized Electrons, CLIC Note 298, NLC-Note 20, 05/03/1996