09/10/2007

Polarized Source for eRHIC

Evgeni TsentalovichMIT

09/10/2007

OUTLINE• Introduction• Existing gun review• Gun for eRHIC Linac-ring version:

– Peak current– Average current– Heat load

• Work plan• Conclusion

09/10/2007

Ring – ring version

AGS

BOOSTER

TANDEMS

RHIC

2 – 10 GeV e-ring

e-cooling

2 -10GeV Injector

LINAC

Proven technology. No major R&D required.

Luminosity limited to 1232 scm1021

09/10/2007

Linac – ring version

PHENIX

STAR

e-cooling

Two e-beam passes: 6.8 and 10 GeV

e+ storage ring5 GeV1/4 RHIC circumference

Main ERL (3.2 GeV per pass)

Allows higher luminosity. Requires development of Energy Recovery Linac (ERL) and high intensity Polarized Electron Source (PES)

09/10/2007

Bicycle Space flights

ERL MP3

Lasers

Birth control

Popcorn TVPES for

eRHIC

Perpetuum Mobile

ImmortalityHuman cloning

Interstellar travel

Invisibility

Antigravitation

Computers MODERN TECHNOLOGY

09/10/2007

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

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

Average laser power ~ 80 W (fresh crystal)

Hundreds Watts might be needed as crystal loses QE

Luminosity ~ I(average) ~ 250 mA

I(peak) ~ 100 A

High polarization → strained GaAs → QE ~ 0.5%

1233 scm101

09/10/2007

Existing guns: SLAC

V = 120 kV

Active spot 15 mm

A10Ipeak

A5~Iaverage

09/10/2007

Existing guns : Nagoya

V = 200 kV

Active spot 18 mmA3Ipeak

09/10/2007

Existing guns : Cornell

DESIGN:

V = 750 kV , I =100 mA

Achieved:

V = 300 kV , I =5 mA

No polarization, operated with blue =525 nm light

09/10/2007

Existing guns : Bates

V = 60 kV

Active spot 12 mm

mA30~Ipeak

A120~Iaverage

09/10/2007

Existing guns : JLAB

V = 100 kV

Active spot 0.2 mm

A120I )A300toup(

Gun for FEL

V=350 kV

I=9 mA (green light, no polarization)

Tests with green light (no polarization)

I > 1 mA

09/10/2007

Existing guns : MainzV = 100 kV

Active spot .25 mm

A50~I

Recent results on a test bench:

Bulk GaAs (P~ 40%)

Active spot ~ 2 mm

I=0.52 mA (up to 11 mA)

Lifetime ~ 70 C (20 hours at 1 mA)

09/10/2007

Existing polarized gunsI(peak) I(average) Beam Polarization

SLAC 10 A 5 A 15 mm High

Bates 30 mA 120 A 12 mm High

JLAB 120 A .2 mm High

Mainz 50 A .25 mm High

Mainz 2 mA 2 mm Low

09/10/2007

Main challenges

High average current – cathode damage by ion bombardment

High peak current – surface charge saturation (QE drops at high light intensity); space charge saturation

High heat load on the cathode – tens or hundreds of Watts of laser power

Solution:

Cathode with very large area

09/10/2007

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

09/10/2007

Average current (~250 mA)Current of ~ 1mA with lifetime ~ 20 hours has been achieved with the active spot of ~ 2 mm. If we increase spot to 2 cm, will we get 100 mA with the same lifetime ?

residual gas

cathodeIonized residual gas strikes photocathode

anode

Ion damage distributedover larger area

09/10/2007

Damage locationElectrons follow electrical field lines, but massive ions have different trajectory. Usually, they tend to damage central area of the cathode.

Laser spot

Cathode Damage groove

JLAB data

Ring-like cathodes ?

Emittance ??

Beam losses?Attractive option, but requires

serious investigation

09/10/2007

Additional opportunityMultiple gun approach

(BNL idea)

E

RF combiner

This approach may reduce the average current requirements by order of magnitude. Perhaps average current of ~ 50 mA will be sufficient to satisfy luminosity requirements.

09/10/2007

Peak current (~100 A)(Not absolutely necessary, duty factor could be increased with RF pulse compressor after the gun… But for the price of the emittance growth)

2

2/3cathode

maxd

)V(US3.2)A(I

For DC gun :

2cathode cm3S

cm6d kV640U

Space charge saturation

Surface charge saturation

0

5

10

15

20

25

0 10 20 30 40 50

Laser Power, Watts

Pea

k cu

rren

t, m

A

0

0.05

0.1

0.15

0.2

0.25

QE

,%

09/10/2007

Charge saturation

Vacuum level

E

x

surface

09/10/2007

Charge saturation

318 cm105 319 cm102

High doping →low polarization !

(SLAC data)

09/10/2007

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

Large area cathode solves the problem

09/10/2007

Heat load (80 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

o5.3SktPT

oCcmW75.k

2cm3S

( for average current of 250 mA )

Large area cathode improves the situation

09/10/2007

Tasks

• In order to design a gun with large cathode area very detailed calculations must be performed (especially beam losses)

• Even more complicated calculations are needed for ring-like cathode geometry

• Experimental measurements require cathode with active cooling

09/10/2007

Work plan

• Phase I:o Gun simulation (including ring geometry)o Design of the cathode with active cooling

• Phase II:o Design and construction of the guno Design and construction of the beam lineo Lifetime measurements at different currents

09/10/2007

Phase I - calculations• Achieve the required current of at least 50 mA, up to 250 mA• Calculate ion trajectories and optimize gun geometry to minimize

ion damage• Ensure that no beam scraping takes place in the gun vicinity• Estimate and optimize the emittance of the beam after the gun • Design the electron optics of the beam line following the gun to

transport the beam from the gun to a beam dump

The calculations will be conducted with a 3-D code that includes space charge effects.

09/10/2007

Phase I – cooling system design• Cooling system attached to the rear side of the photocathode• Design should allow crystal replacements without braking vacuum• Good thermal connection to the crystal• UHV compatibility • High (~100 kV) voltage compatibility• Temperature monitoring

Test chamber will be built, and the cathode will be heated by diode laser with cathode temperature monitored. HV and UHV compatibilities will be tested.

09/10/2007

Phase II – prototype gun and beam line design

• Prototype gun will be designed and built according to the results of the simulations in Phase I

• Cathode cooling system designed in Phase I will be incorporated • Load lock and preparation chamber• Beam line: two 90 turns, beam dump, lenses, steering coils • Pumps: ion pumps and NEGs• Diagnostics: flip screens, toroidal pickups, later – wire scanners

for emittance measurements

09/10/2007

Gun with beamline

Prep.Ch.

Tr.Vessel

Man

.Manipulator

Sol

Dipole

Sol

Dump

Sol

Dipole

GUN

SolSol

09/10/2007

Conclusion

• MIT-Bates in collaboration with BNL will study the possibility to build a very high intensity polarized electron gun

• Large area cathode will be implemented• Ring-like cathode geometry will be

investigated• Active cooling of the cathode will be used