High brightness electron sources, e-beam qualities and diagnostics

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

2
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
?
T46 MeV
?
T46 MeV
4 MeV
40 MeV
probe beam
train generator
laser
1.41 m
12.6 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 _at_ 110C optimized at
  ?266nm
• Vacuum transportation
• Electro-optic parameters of Cs2Te layer
• Relative dielectric constant ?r ? 50 _at_ 10 kHz
• Resistivity ? ? 1011 ??? Is an insulator (Te
  10 nm , Cs ? 15 nm)
• Specular reflectivity 7 ? Rs ? 15 _at_ 266 nm
  as a function of the thickness layer and the
  substrate properties.
• Photoemission parameters
• Photoemission threshold E0 ? 3.5 eV
• QE ? 10 _at_ 266 nm
• DC and RF gun measurements
• QEmean ? 6 _at_ 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 lt a few picosecondes (limited
  by the streak-camera resolution)

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

9
CsI Ge photocathode tests
2/2

• Charge limit and time response
• QE measurement for different cathodes

10
DC and RF gun GaAs tests

• Dark current measurements

11
The Cs2Te photocathode production
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
• CsILiF , CuIMgF2 , Cs2TeMgF2 , AlCsF
• 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