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High brightness electron sources, e-beam qualities and diagnostics

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 Cs 2 Te photocathodes

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High brightness electron sources, e-beam qualities and diagnostics

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  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. Drive beam probe beam RF structure E [MV/m] z LAS: LIL section 10 n 48 1 HCS: high charge sect. 60 b q [nC] 21 0.1 CTS: CLIC transfer sect. -4 b  [ps] <2 <3 CAS: CLIC accel. sect. 80 t CTS qf qd qf CTS qf qd qf CAS CAS CAS qd qf qd qf CTF layout   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 HCS1 HCS2 8 MeV 45 MeV drive beam RF gun 70 MV/m LAS CAS  T=46 MeV  T=46 MeV 4 MeV 40 MeV probe beam laser train generator 12.6 m 1.41 m 1.41 m

  3. 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  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

  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 andGe = 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, 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

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