# Presentation: 162719

## High Intensity Polarized Electron Sources

*Evgeni Tsentalovich*
*MIT*

## Progress over past two decades

- 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

*15 years ago*

*Now*

## New requirements

- Very high current
- Very high polarization
- Low emittance

*New generation of accelerators (eRHIC, ILC) demand polarized injectors with extreme parameters*

*Another application: Energy Recovery Linac (ERL)*

- Very high current
- No polarization
- Very low emittance

## 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)

## Semiconductor band structure

- Conductingband

- Valence band

- Band gap

- E

- Doping (Z, Be) is used to control
- the concentration of carriers:

## Band structure of GaAs

- 0.3 eV

- E

- 1.6 eV

- k

- Conductingband

## Strained crystal

- 0.3 eV

- E

- 1.6 eV

- k

- 3/2

- 1/2

- 1/2

- 1/2

- -1/2

- -1/2

- -3/2

- -1/2

## GaAs-based photocathodes

- High QE ~ 1-10%
- Pol ~ 35-45%

- Bulk GaAs

- Superlattice GaAs:
- Layers of GaAs on GaAsP

- QE ~ 0.8%
- Pol ~ 85%

- 14 pairs

## Negative electron affinity

- Band gap (forbidden zone)

- Valence band

- Conductive band

- Vacuum level

- E

- x

- Most (but not all!) electrons reaching the surface are thermolized

- surface

## 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

## Charge saturation

- Vacuum level

- E

- x

- surface

## Charge saturation

- High doping →low polarization !

- (SLAC data)

## High gradient doping

- Substrate

- Buffer

- Superlattice

-  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

## DC gun design

**Cathode**

**Anode**

*Cylindrical symmetry*

- Normalized emittance                      doesn’t change with acceleration

## DC gun design

- r

- r'

- Emittance:

*Infinitely small beam spot, no space charge, no nonlinear transverse forces*

- Cathode

## DC gun design

- r

- r'

- Emittance:

*Finite beam spot, no space charge, no nonlinear transverse forces*

- Cathode

## Neglecting thermal emittance

- r

- r'

- Emittance:

- Cathode

## Space charge

**Cathode**

**Anode**

- J

- r

- J

- r

- J

- r

- J

- r

***Space charge may change the beam profile and increase the beam emittance***

**Emittance growth might be suppressed by shaping the laser profile**

## 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

## Space charge

- 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

- Worst case scenario: large emitting spot AND high current density

## 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               ~ 20-100 mm·mrad
- Estimations for RF (SRF) gun                       ~ 1-5 mm·mrad
- ILC requirements                                            ~ .05 mm·mrad

## Polarized electron guns:

- DC

- RF

- Approved technology (at least for ~ 100 kV)

- Better suited for large emitting spot

- No working GaAs-based RF gun yet

- RF bunching could be avoided with appropriate laser system

- Require RF chopping/bunching

- Beam from the gun is bunched

- High acceleration rate, high electron energy from the gun

- BEST FOR CONVENTIONAL APPLICATIONS OR WHEN VERY HIGH CURRENT IS NEEDED

- BEST FOR  APPLICATIONS WITH VERY HIGH BRIGHTNESS   AND            LOW EMITTANCE

- Low energy beam (space charge! )

## DC Guns: Mainz

- V = 100 kV
- Active spot .25 mm

## DC Guns: JLAB

- V = 100 kV
- Active spot 0.2 mm

## DC Guns: Bates

- V = 60 kV
- Active spot 12 mm

## DC Guns: SLAC

- V = 120 kV
- Active spot 15 mm

## DC Guns: Nagoya

- V = 200 kV
- Active spot 18 mm

## DC Guns: Cornell

- V = 500 kV (800 ? )

## 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

## RF guns (SLAC)

- 1.6 cell pill box

- Higher Order Mode (HOM) single cell

-  More open structure
-  No internal irises
-  More effective vacuum pumping

## RF guns (BNL & AES)

## RF guns:

- Warm

- SRF

- New, more robust cathode materials may appear (GaN)

- Very expensive and untested technology

- Significant practical experience

- Unclear if GaAs-based cathode will survive RF gun conditions

- Best vacuum possible

- Wide open apertures (eliminates back bombardment)

## 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

## Laser development

- Elliptical beams  (SLAC)

-  Suppression of non-linear space charge effects
-  Maximizing brightness
-  Might be very useful for RF guns
-  Very challenging task

## 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

## 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

- 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

- Polarized electron gun for ring-ring eRHIC version is based on proven technology and doesn’t require any significant R&D

## 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

## Problems without known solution

- Heat load (800 W on the cathode)

- New problem: dynamic cooling (gun off !)

## Problems without known solution

- Peak current (~200 A)

- Larger cathodes? Ring-like cathodes ?

- What about emittance ???

## Can we relax the requirements?

- With I(average) ~ 40-50 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