1. High Intensity Polarized Electron Sources
Evgeni Tsentalovich
MIT
07/18/2006

2. Progress over past two decades
15 years ago
Now
Unreliable guns at development stage
Dreams to exceed 40% polarization
Routinely operated productive quality guns (SLAC, JLAB, Mainz, Bates…)
Strained, superlattice crystals with polarization approaching 90%
New photocathode materials
New gun concepts
07/18/2006

3. New requirements Very high current Very high polarization
New generation of accelerators (eRHIC, ILC) demand polarized injectors with extreme parameters
Very high current
Very high polarization
Low emittance
Another application: Energy Recovery Linac (ERL)
Very high current
No polarization
Very low emittance
07/18/2006

4. GaAs photocathodes Requirements: high QE and polarization
Remains the only material for polarized electron guns
Very high QE
Very high polarization
But ! Very demanding technology ( Ultra-high vacuum requirements)
07/18/2006

5. Semiconductor band structure
Conductingband
Band gap
Valence band
- low
- medium
- high
Doping (Z, Be) is used to control
the concentration of carriers:
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6. Band structure of GaAs Conductingband E k 3 1 1.6 eV 0.3 eV 3/2 1/2
-1/2
-3/2 3 1
1.6 eV
0.3 eV k
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7. Strained crystal E k 1 3 1.6 eV 0.3 eV -1/2 1/2 -3/2 3/2 -1/2 1/2 -1/2
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8. GaAs-based photocathodes
Superlattice GaAs:
Layers of GaAs on GaAsP
QE ~ 0.15%
Pol ~ 75%
Strained GaAs: GaAs on GaAsP
100 nm
Bulk GaAs
100 nm
14 pairs
QE ~ 0.8%
Pol ~ 85%
High QE ~ 1-10%
Pol ~ 35-45%
07/18/2006

9. Negative electron affinity
Most (but not all!) electrons reaching the surface are thermolized E
Conductive band
Vacuum level
Cs, O(F) deposition
Band gap (forbidden zone)
Valence band
surface x
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10. Photocathodes degradation
Poisoning by residual gases
Ion bombardment
Oxygen- and carbon-containing species are more harmful
Hydrogen and noble gases are more tolerable
This degradation can be healed by heat-cleaning at moderate temperatures (<550 C)
Most harmful
Only high-temperature (~600C) heat cleaning restores QE, and only partially
Effect is proportional to pressure in the chamber and to average current
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11. Charge saturation E
Vacuum level
surface x
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12. Charge saturation High doping →low polarization ! (SLAC data)
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13. High gradient doping Works very well
High ( )doped layer ~ 5 nm
Works very well
The high-doped layer is thin enough to preserve high polarization
Charge saturation is highly suppressed (at least for fresh crystals)
The top layer can survive only few high-temperature (~600 C) activations
Might be problematic for high-current guns
Superlattice
Buffer
Substrate
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14. DC gun design r' r Cylindrical symmetry Cathode Anode Emittance:
Normalized emittance doesn’t change with acceleration
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15. DC gun design
Infinitely small beam spot, no space charge, no nonlinear transverse forces r' r
Emittance:
Cathode
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16. DC gun design
Finite beam spot, no space charge, no nonlinear transverse forces r' r
With perfectly linear transverse forces only thermal emittance remains
Emittance:
Cathode
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17. Neglecting thermal emittance
Nonlinearity in the gun optics may introduce the emittance growth.
Cathode
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18. Space charge Cathode Anode
Space charge may change the beam profile and increase the beam emittance J r J r J J J J r r r r
Emittance growth might be suppressed by shaping the laser profile
07/18/2006

19. Space charge
Space charge effects are strongest when electrons have low energy (no space charge effects for relativistic beam)
Accelerate as fast as possible – high gradient in the gun
Accelerate as high as possible – high gun voltage, to reduce space charge effects between the gun and the accelerator
07/18/2006

20. Worst case scenario: large emitting spot AND high current density
Space charge
Worst case scenario: large emitting spot AND high current density
Child’s law:
- microperveance; d – distance between cathode and anode
Space charge influence:
Very strong
Strong
Weak
Space charge effects could be reduced by
Increasing gun voltage
Reducing cathode – anode gap
Increasing the emitting spot
Limited (breakdowns)
Non-linear transverse forces
07/18/2006

21. Emittance: Thermal GaAs cathode (room temperature) ~0.2 mm·mrad ·R(mm)
Thermal Cu, Cs2Te cathodes ~1.2 mm·mrad ·R(mm)
Real gun with small emitting spot (JLAB) ~ 5 mm·mrad
Real gun with large emitting spot (Bates) ~15 mm·mrad
Beam after RF chopping/bunching ~ mm·mrad
Estimations for RF (SRF) gun ~ 1-5 mm·mrad
ILC requirements ~ .05 mm·mrad
07/18/2006

22. Polarized electron guns:DC RF
Approved technology (at least for ~ 100 kV)
No working GaAs-based RF gun yet
Require RF chopping/bunching
Beam from the gun is bunched
RF bunching could be avoided with appropriate laser system
High acceleration rate, high electron energy from the gun
Low energy beam (space charge! )
Better suited for large emitting spot
BEST FOR CONVENTIONAL APPLICATIONS OR WHEN VERY HIGH CURRENT IS NEEDED
BEST FOR APPLICATIONS WITH VERY HIGH BRIGHTNESS AND LOW EMITTANCE
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23. DC Guns: Mainz
V = 100 kV
Active spot .25 mm
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24. DC Guns: JLAB
V = 100 kV
Active spot 0.2 mm
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25. DC Guns: Bates
V = 60 kV
Active spot 12 mm
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26. DC Guns: SLAC
V = 120 kV
Active spot 15 mm
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27. DC Guns: Nagoya
V = 200 kV
Active spot 18 mm
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28. DC Guns: Cornell
V = 500 kV (800 ? )
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29. RF guns The only practical experience: BINP (Novosibirsk)
Good vacuum conditions with RF on and unactivated GaAs crystal installed
Activated GaAs crystal survived just several RF cycles
Severe back-bombardment resulted in a very short life time
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30. RF guns (SLAC) 1.6 cell pill box Higher Order Mode (HOM) single cell
More open structure
No internal irises
More effective vacuum pumping
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31. RF guns (BNL & AES)
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32. RF guns: Warm SRF Much easier to do Better chances of success
Very expensive and untested technology
Significant practical experience
Unclear if GaAs-based cathode will survive RF gun conditions
Best vacuum possible
New, more robust cathode materials may appear (GaN)
Wide open apertures (eliminates back bombardment)
Much easier to do
Better chances of success
07/18/2006

33. Laser development Fiber lasers:
Very short pulses
Mode – locked, but rep. rate limited to MHz
Wavelength 1030 – 1500 nm, but could be frequency-doubled
Reliable
Relatively expensive
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34. Laser development Elliptical beams (SLAC)
Suppression of non-linear space charge effects
Maximizing brightness
Might be very useful for RF guns
Very challenging task
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35. ILC gun DC or RF gun could be used
ILC emittance requirements are so high that even RF gun is unlikely to meet them without dumping ring
Although dumping ring is still required for RF gun, it might be of much simpler design, saving millions
Conclusion: RF gun would be a better option, but it requires significant R&D and the success is not guaranteed
07/18/2006

36. eRHIC gun (ring-ring)
Modest intensity and emittance requirements
Regular DC gun is well suited for the task
Two options: mode-locked laser or RF chopper/buncher
Polarized electron gun for ring-ring eRHIC version is based on proven technology and doesn’t require any significant R&D
Mode-locked laser:
Simplifies injector
No emittance growth in chopper
RF chopper/buncher:
Complicates injector
Emittance growth in chopper
Beam compression reduces peak current demand from the gun
07/18/2006

37. eRHIC gun (linac-ring)
Extremely high current demand !!!
I(average) ~ 500 mA
I(peak) ~ 200 A
High polarization → strained GaAs → QE ~ 0.1%
Average laser power ~ 800 W
Such lasers do not exist. Possible solutions:
a) array of diode lasers
b) dedicated FEL – almost unlimited laser power, tunable
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38. Problems without known solution
Heat load (800 W on the cathode)
HEAT
t=1 mm
ACTIVE COOLING
GaAs
With a conventional cathode stalk system, the cathode would heat up to stellar temperatures, but, fortunately, melt first.
New problem: dynamic cooling (gun off !)
07/18/2006

39. Problems without known solution
Peak current (~200 A)
For DC gun :
Larger cathodes? Ring-like cathodes ?
Emitting spot :
What about emittance ???
07/18/2006

40. Can we relax the requirements?
With I(average) ~ mA the luminosity is the same as in ring-ring version
40-50 mA gun is still a very difficult task, but it is a LOT easier than 500 mA
Heat load and perveance problems go away
Life time of the cathode is still a major problem
07/18/2006