07/18/2006

High Intensity Polarized Electron Sources

Evgeni Tsentalovich

MIT

07/18/2006

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

07/18/2006

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

07/18/2006

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

Semiconductor band structure

Conductingband

Valence band

Band gap

E

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

- low

- medium

- high

317 cm10

318 cm10

319 cm10

07/18/2006

0.3 eV

Band structure of GaAs

E

1.6 eV

k

3/21/2

1/2

1/2

-1/2

-1/2-3/2

-1/2

31

jm

2/1S

2/3P

2/1P

1mlight j

%5013

13Pmax

Conductingband

07/18/2006

0.3 eV

Strained crystal

E

1.6 eV

k

3/21/2

1/2

1/2

-1/2

-1/2-3/2

-1/2

31

jm

2/1S

2/3P

2/1P

1mlight j

%100Pmax

07/18/2006

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

Bulk GaAs

QE ~ 0.15%Pol ~ 75%

Strained GaAs: GaAs on GaAsP

100

nm

Superlattice GaAs: Layers of GaAs on GaAsP

QE ~ 0.8%Pol ~ 85%

100

nm

14 pairsGaAs-based photocathodes

07/18/2006

Negative electron affinity

Band gap (forbidden zone)

Valence band

Conductive band Vacuum level

E

x

Cs, O(F) deposition

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

surface

07/18/2006

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 (~600C) heat cleaning restores QE, and only partially

• Effect is proportional to pressure in the chamber and to average current

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Charge saturation

Vacuum level

E

x

surface

07/18/2006

Charge saturation

318 cm105 319 cm102

High doping →low polarization !

(SLAC data)

07/18/2006

High gradient doping

Substrate

Buffer

Superlattice

High ( )doped layer ~ 5 nm19105~

• 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

07/18/2006

DC gun design

Cathode Anode

r

r'

rdrd

Emittance:

Cylindrical symmetry

Normalized emittance doesn’t change with accelerationn

07/18/2006

DC gun design

r

r'

0rdrd Emittance:

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

Cathode

07/18/2006

DC gun design

r

r'

0rdrd Emittance:

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

Cathode

With perfectly linear transverse forces only thermal emittance remains

thermal

RC thth

07/18/2006

Neglecting thermal emittance

r

r'

0rdrd Emittance:

Cathode

r

0rdrd Emittance:

r'

r

0rdrd Emittance:

r'

Nonlinearity in the gun optics may introduce the emittance growth.

07/18/2006

Space chargeCathode Anode

J

r

J

r

J

r

J

r

J

r

J

r

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

07/18/2006

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

Space chargeChild’s law: )V(UP)A(I 2/3

max

2cathode

d

S3.2P

- microperveance; d – distance between cathode and

anodeSpace charge influence:

Very strong Strong WeakmaxI~I 10/I~I max 100/I~I max

Space charge effects could be reduced by

• Increasing gun voltage

• Reducing cathode – anode gap

• Increasing the emitting spot

Limited (breakdowns)

Non-linear transverse forces

Worst case scenario: large

emitting spot AND high current

density

07/18/2006

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

07/18/2006

Polarized electron guns:

DC RFApproved 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! )

07/18/2006

DC Guns: Mainz

V = 100 kV

Active spot .25 mm

A50~I

07/18/2006

DC Guns: JLAB

V = 100 kV

Active spot 0.2 mmA120I

07/18/2006

DC Guns: Bates

V = 60 kV

Active spot 12 mm

mA30~Ipeak

A120~Iaverage

07/18/2006

DC Guns: SLAC

V = 120 kV

Active spot 15 mm

A10Ipeak

A5~Iaverage

07/18/2006

DC Guns: Nagoya

V = 200 kV

Active spot 18 mmA3Ipeak

07/18/2006

DC Guns: Cornell

V = 500 kV (800 ? )

07/18/2006

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

07/18/2006

RF guns (SLAC)

1.6 cell pill box Higher Order Mode (HOM) single cell

• More open structure

• No internal irises

• More effective vacuum pumping

07/18/2006

RF guns (BNL & AES)

07/18/2006

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)

Much easier to do Better chances of success

07/18/2006

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

sec10~ 13

21010

07/18/2006

Laser development

Elliptical beams (SLAC)

• Suppression of non-linear space charge effects

• Maximizing brightness

• Might be very useful for RF guns

• Very challenging task

07/18/2006

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

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

07/18/2006

eRHIC gun (linac-ring)Extremely high current demand !!!

I(average) ~ 500 mA

I(peak) ~ 200 A

High polarization → strained GaAs → QE ~ 0.1%

124/(%)QE)W(P)nm()mA(I laser

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

07/18/2006

Problems without known solutionHeat load (800 W on the

cathode)

With a conventional cathode stalk system, the cathode would heat up to stellar temperatures, but, fortunately, melt first.

HEAT

t=1 mm

ACTIVE COOLING

GaAs

o35Sk

tPT

oCcm

W75.k

2cm3S

New problem: dynamic cooling (gun off !)

07/18/2006

Problems without known solutionPeak current (~200 A)

2

2/3cathode

maxd

)V(US3.2)A(I

For DC gun :

2cathode cm3S

cm6d MV1U

Larger cathodes? Ring-like cathodes ?

Emitting spot :

What about emittance ???

07/18/2006

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