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

CTF layout     drive beam probe beam Drive beam probe beam n 48 1 q [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

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

Recap of QE measurements 1/2

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

Cs2Te photocathodes 1/3 Photocathode preparation Substrate : Cu, Mo, Mg Te = 10 nm ; Cs  15 nm @ 110°C optimized at =266nm Vacuum transportation Electro-optic parameters of Cs2Te layer : Relative dielectric constant : r  50 @ 10 kHz Resistivity :   1011  : Is an insulator (Te = 10 nm , Cs  15 nm) Specular reflectivity :7 %  Rs  15 % @ 266 nm as a function of the thickness layer and the substrate properties. Photoemission parameters Photoemission threshold : E0  3.5 eV QE  10 % @ 266 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

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

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

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

DC and RF gun GaAs tests Dark current measurements

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

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.

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

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, 24-06-1996 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