1. Lee Jones Accelerator Physics Group Accelerator Science and Technology Centre STFC Daresbury Laboratory

2. Accelerator Physics Group
Lee Jones
Accelerator Physics Group
GaAs Photocathode R&D: Energy spread measurements and the nature of the activated p-GaAs(Cs,O) activation layer
Overview of the TESS experimental system
The nature of the p-GaAs(Cs,O) activation layer

3. Characterising photocathode sources:
Transverse & Longitudinal Energy Distribution

4. Characterising photocathode sources:
Transverse & Longitudinal Energy Distribution
Brighter electron beams require reductions in the transverse and longitudinal energy distributions (TEDC & LEDC) of photocathode electron sources
ASTeC have constructed a system to measure these
Flexible experimental system designed and commissioned
Various light sources
Liquid nitrogen photocathode cooling loop
Degradation / lifetime studies
Connection to vacuum suitcase – various cathodes
System connected to our III-V Photocathode Prep. Facility

5. TESS – The Transverse Energy Spread Spectrometer
Characterising photocathode sources:
Transverse & Longitudinal Energy Distribution
Brighter electron beams require reductions in the transverse and longitudinal energy distributions (TEDC & LEDC) of photocathode electron sources
ASTeC have constructed a system to measure these
Flexible experimental system designed and commissioned
Various light sources
Liquid nitrogen photocathode cooling loop
Degradation / lifetime studies
Connection to vacuum suitcase – various cathodes
System connected to our III-V Photocathode Prep. Facility
TESS – The Transverse Energy Spread Spectrometer

6. TESS – The Transverse Energy Spread Spectrometer
Increasing beam brightness: The intrinsic energy of NEA GaAs photocathodes
TESS – The Transverse Energy Spread Spectrometer

7. Increasing beam brightness: The intrinsic energy of NEA GaAs photocathodes
Retarding-field MCP electron multiplier
Mu-metal magnetic shield (cutaway)
Independent control of source & grid potentials
Ability to change the source-detector spacing
Inclusion of leak valve to ‘poison’ the cathode
Cathode
Phosphor screen
Microchannel Plate (MCP)
Laser beam path
Cryogenic reservoir
Cryogenic cooling pipes
Phosphor screen
Retarding grids
TESS – The Transverse Energy Spread Spectrometer

8. Increasing beam brightness: The TESS experimental setup
laser
grids
2-stage MCP
ND filter
lens
luminescent screen
CCD
camera
GaAs(Cs,O)-photocathode
d = 10 to 53 mm
Uacc = 0 to 1000 V
Umcp = 900 V
Uscr = 3.5 kV
laser spot Ø ~ 100 mm
Iph on the fA scale d
lens
Umcp
Uacc
Uscr

9. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
40 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

10. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
40 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

11. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
40 V
60 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

12. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
40 V
60 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

13. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
60 V
40 V
82 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

14. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
60 V
40 V
82 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

15. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
131 V
40 V
82 V
60 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

16. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
131 V
40 V
82 V
60 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

17. 17.06.2013 p-GaAs(Cs,O) photocathode
TESS Commissioning:
TEDC measurements at different accelerating voltages
245 V
131 V
40 V
82 V
60 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

18. TESS Commissioning:
TEDC measurements at different accelerating voltages
Screen luminescence profiles
245 V
131 V
82 V
60 V
40 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

19. TESS Commissioning:
TEDC measurements at different accelerating voltages
Screen luminescence profiles
245 V
131 V
82 V
60 V
40 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

20. TESS Commissioning:
TEDC measurements at different accelerating voltages
Screen luminescence profiles
245 V
131 V
60 V
40 V
82 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
r2 = r × (Uacc/40)1/2
p-GaAs(Cs,O) photocathode

21. TESS Commissioning:
TEDC measurements at different accelerating voltages
Screen luminescence profiles
245 V
131 V
60 V
40 V
82 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
r2 = r × (Uacc/40)1/2
p-GaAs(Cs,O) photocathode

22. TESS Commissioning:
TEDC measurements at different accelerating voltages
Screen luminescence profiles
245 V
131 V
60 V
40 V
82 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
r2 = r × (Uacc/40)1/2
p-GaAs(Cs,O) photocathode

23. TESS Commissioning:
TEDC measurements at different accelerating voltages
Screen luminescence profiles
245 V
131 V
60 V
40 V
82 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
r2 = r × (Uacc/40)1/2
p-GaAs(Cs,O) photocathode

24. TESS Commissioning:
TEDC measurements at different accelerating voltages
Transverse energy distribution profiles
245 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

25. TESS Commissioning:
TEDC measurements at different accelerating voltages
Transverse energy distribution profiles
245 V
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

26. TESS Commissioning:
TEDC measurements at different accelerating voltages
Transverse energy distribution profiles
245 V
~ exp (-etr/e0)
e0 = MTE = 45 meV
5 mm
d = 33 mm l = 635 nm
T = 300 K
p-GaAs(Cs,O) photocathode

27. 21.06.2013 p-GaAs(Cs,O) photocathode d  43 mm T = 300 K
TESS Commissioning:
TEDC measurements
with different wavelengths of light
l = 532 nm
l = 635 nm
Uacc = 60 V
Uacc = 230 V
5 mm
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

28. TESS Commissioning: Summary of TEDC results
Screen luminescence profiles
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

29. TESS Commissioning: Summary of TEDC results
Screen luminescence profiles
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

30. TESS Commissioning: Summary of TEDC results
Screen luminescence profiles
Transverse energy distribution profiles
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

31. TESS Commissioning: Summary of TEDC results
Screen luminescence profiles
Transverse energy distribution profiles
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

32. TESS Commissioning: Summary of TEDC results
Screen luminescence profiles
Transverse energy distribution profiles
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

33. TESS Commissioning: Summary of TEDC results e0 = 100 meV
Screen luminescence profiles
Transverse energy distribution profiles
e0 = 100 meV
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

34. TESS Commissioning: Summary of TEDC results e0 = 100 meV e0 = 45 meV
Screen luminescence profiles
Transverse energy distribution profiles
e0 = 100 meV
e0 = 45 meV
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

35. TESS Commissioning: MTE635 = 45  7 meV MTE532 = 100  15 meV
Summary of TEDC results
Screen luminescence profiles
Transverse energy distribution profiles
e0 = 100 meV
e0 = 45 meV
MTE635 = 45  7 meV
MTE532 = 100  15 meV
p-GaAs(Cs,O) photocathode d  43 mm T = 300 K

36. Accelerator Physics Group
Lee Jones
Accelerator Physics Group
GaAs Photocathode R&D: Energy spread measurements and the nature of the activated p-GaAs(Cs,O) activation layer
Overview of the TESS experimental system
The nature of the p-GaAs(Cs,O) activation layer
Part 2:

37. p-GaAs (Cs,O): Activation Layer Model
The growth of GaAs photocathodes and photoelectron scattering phenomena in this material are well-understood, and as such the bulk parameters of a p-GaAs-layer can be optimized for any specific photocathode application
In contrast, the atomic and electronic properties of the p-GaAs(Cs,O)-vacuum interface, which strongly influence the photoelectron escape probability, energy and angular distributions of emitted electrons, photocathode lifetime, etc., remain the focus of much research and discussion

38. p-GaAs (Cs,O): Activation Layer Model
The growth of GaAs photocathodes and photoelectron scattering phenomena in this material are well-understood
The atomic and electronic properties of the p-GaAs(Cs,O)- vacuum interface layer remain the focus of much research
Several layer models proposed, with two likely candidates:
The Dipole Layer (DL) model, applicable to ‘thin’ layers
The Hetero Junction (HJ) model, applicable to ‘thick’ layers
p-GaAs (Cs,O)-photocathodes with reasonably high Q.E. can be created with (Cs,O)-layers conforming to both models
The domains of validity for these models have not been defined
This has hindered our understanding of the interrelations between the intrinsic characteristic parameters of the (Cs,O)-layers and the performance characteristics of p-GaAs(Cs,O)-photocathodes
The growth of GaAs photocathodes and photoelectron scattering phenomena in this material are well-understood, and as such the bulk parameters of a p-GaAs-layer can be optimized for any specific photocathode application
In contrast, the atomic and electronic properties of the p-GaAs(Cs,O)-vacuum interface, which strongly influence the photoelectron escape probability, energy and angular distributions of emitted electrons, photocathode lifetime, etc., remain the focus of much research and discussion

39. p-GaAs (Cs,O):
Activation Layer Model

40. p-GaAs (Cs,O): Activation Layer Model
Photocathode grown by MOCVD on a (001) GaAs substrate
Activation under near-equilibrium conditions with illumination at 635 nm
First Cs peak reached in 25 minutes. Deposition rate is 0.02 ML/min
O2 applied when photocurrent (Jph) fallen to 98% of maximum value

41. p-GaAs (Cs,O): Activation Layer Model
Activation interrupted periodically to measure the longitudinal energy spread (LEDC), nei (elon) at positions i = 1 to 11 using a parallel plate analyser to record photocurrent (Jph) as a function of retarding voltage (Uret)

42. p-GaAs (Cs,O): Activation Layer Model
Activation interrupted periodically to measure the longitudinal energy spread (LEDC), nei (elon) at positions i = 1 to 11 using a parallel plate analyser to record photocurrent (Jph) as a function of Uret
elon varies according to Q.E.(t), but is constant after measurement #7

43. p-GaAs (Cs,O):
Activation Layer Model

44. p-GaAs (Cs,O): Activation Layer Model
Effective electron affinity, c* = evac – ecb, calculated for points 1 to 11
evac coincides with the electron energy where dne(elon)/delon is maximum
ecb = e·Uret where d2Jph(Uret)/dUret2 has its minimum value

45. p-GaAs (Cs,O):
Activation Layer Model

46. Electron affinity reaches its minimum value after 250 minutes.
p-GaAs (Cs,O):
Activation Layer Model
Electron affinity reaches its minimum value after 250 minutes.
The data show that the energy diagram for the p-GaAs(Cs,O)-surface remains un-changed for t > 250 min, as predicted by the HJ model.

47. p-GaAs (Cs,O): Activation Layer Model
Evolution of the oxygen content (Cox) within the (Cs,O)-layer during activation. The dotted lines are linear extrapolations of the measured (solid) curves.
The data show that the energy diagram for the p-GaAs(Cs,O)-surface remains un-changed for t > 250 min, as predicted by the HJ model.

48. p-GaAs (Cs,O):
Activation Layer Model

49. p-GaAs (Cs,O): Activation Layer Model
……… for the full story, download the open access paper.

50. TESS :
Planned future work

51. TESS : Planned future work
Continue to work with the Institute of Semiconductor Physics
Repeat the activation layer experiment at Daresbury measuring the transverse energy spread with the TESS
Construction of a vacuum suitcase (nearly complete)
Building a collaboration with the source group at CERN
Studies of the energy spread from NEA GaAs photocathodes subjected to programmed degradation
Energy spread measurements from cooled GaAs photocathodes
Integration of a broadband light source with the TESS system
Studies of energy spread from metal photocathodes

52. TESS : Planned future work
Continue to work with the Institute of Semiconductor Physics
Repeat the activation layer experiment at Daresbury measuring the transverse energy spread with the TESS
Construction of a vacuum suitcase (nearly complete)
Building a collaboration with the source group at CERN
Studies of the energy spread from NEA GaAs photocathodes subjected to programmed degradation
Energy spread measurements from cooled GaAs photocathodes
Integration of a broadband light source with the TESS system
Studies of energy spread from metal photocathodes

53. TESS : Planned future work
Continue to work with the Institute of Semiconductor Physics
Repeat the activation layer experiment at Daresbury measuring the transverse energy spread with the TESS
Construction of a vacuum suitcase (nearly complete)
Building a collaboration with the source group at CERN
Studies of the energy spread from NEA GaAs photocathodes subjected to programmed degradation
Energy spread measurements from cooled GaAs photocathodes
Integration of a broadband light source with the TESS system
Studies of energy spread from metal photocathodes

54. Thank you ! The ASTeC photocathode team: The ISP photocathode team:
Dr. Boris Militsyn
Sonal Mistry (Ph.D. student)
Dr. Tim Noakes
Paolo Pizzol (Ph.D. student)
Dr. Lee Jones
Stuart Wilde (Ph.D. student)
Dr. Keith Middleman
Bruno Camino (Ph.D. student)
Dr. Reza Valizadeh
Dr. Mark Surman
Ryan Cash (engineering)
Prof. Elaine Seddon (consultant)
Barry Fell (engineering)
Prof. Alexander Terekhov
Dr. Konstantin Toropetsky
Dr. Heinrich Scheibler
Dr. Vasiliy Bakin