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09/10/2007 Ring – ring version AGS BOOSTER TANDEM S RHIC 2 – 10 GeV e- ring e-cooling 2 -10GeV Injector LINAC Proven technology. No major R&D required. Luminosity limited to
Citation preview
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