Transcript Status of GaAs Photoemission Guns at JLab M. Poelker, P. Adderley, M.

Status of GaAs Photoemission Guns at JLab
M. Poelker, P. Adderley, M. Baylac, J. Brittian, D. Charles,
J. Clark, J. Grames, J. Hansknecht, M. Stutzman, K. Surles-Law
CEBAF:
• Gun lifetime
• Parity violation experiments
• Commercial Ti-sapphire
lasers
• Superlattice Photocathodes
• Atomic H/D exposure
• Ion Pump supply with nA
current monitoring
JLab IRFEL:
• 10 kW output power
• 350 kV DC GaAs gun
• 9 mA CW ave current
New Photoinjectors:
• 10’s of mA ave current at
high polarization.
• Obstacles?
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, PESP2004, Oct.7-9, Mainz, Germany
CEBAF 2-Gun Photoinjector
New cleanroom
for lasers
Gun Charge Lifetime Measured over 2001 -2004
M. Baylac
Gun Charge Lifetime
Steadily Decreasing
10 NEG pumps surround cathode/anode gap
NEG pump
replacement Summer
2003 improves lifetime
Parity Violation Experiments at CEBAF
Experiment
Physics
Asymmetry
Max run-average
helicity correlated
Position Asym.
Max run-average
helicity correlated
Current Asym.
HAPPEX-I
HAPPEX-II
13 ppm
1.3 ppm
10 nm
2 nm
1.0 ppm
0.6 ppm
HAPPEX-He
8 ppm
3 nm
0.6 ppm
G0
Qweak
P-REx
2 to 50 ppm
0.3 ppm
20 nm
20 nm
1 nm
1.0 ppm
0.1 ppm
0.1 ppm
complete
partially complete
approved
G0 Forward Angle was a Difficult Experiment
Very long experiment!
 “commissioning” run: August 2002 – January 2003
 “engineering” run: October 2003 – February 2004
 “production” run: March 2004 – May 2004
31 MHz time structure caused problems;
 Homemade lasers could not generate 70 ps pulses
 Homemade lasers were not long-term stable
 High bunch charge caused beam handling problems
Managing helicity correlated position asymmetry was
painful, at least initially.
Injector acceptance 111 ps
8 times Higher Bunchcharge; ~ 2 pC/pulse
70 uA
40 uA
20 uA
8 uA
2 uA
Significant bunchlength growth
Beam profile distortion
when laser spot tightly
focused at photocathode
1 uA
Lots of beamloss at apertures
Current from gun
Current delivered to Hall
Aperture loss
G0 Exp used active feedback to control asymmetries
Helicity correlated differences integrated over a few hours.
- Pockel cell aligned using
spinning linear polarizer
-Rotating halfwaveplate sets
asymmetries close to zero
before turning feedback ON.
- IA cell (waveplate+pockel
cell+linear polarizer) used to
correct charge asymmetry.
- PZT mirror used to correct
position asymmetries.
Correlations:
- Helicity correlated charge
and position asymmetry seen
to converge to zero over the
course of a few hours.
- Position differences induce charge asymmetry through scraping.
- Charge asymmetry induces energy and position differences through rf cavity loading.
- Position differences “appear” as energy differences at the dispersive point.
Helicity Correlated Beam Specs for g0 Forward
Total of 744 hours (103 Coulombs) of parity quality beam with a 4 cut on parity
quality.
Beam
Parameter
Achieved
(IN-OUT)
“Specs”
Charge
asymmetry
-0.28 ± 0.28
ppm
1 ppm
x position
differences
6 ± 4 nm
20 nm
y position
differences
8 ± 4 nm
20 nm
x angle
differences
2 ± 0.3 nrad
2 nrad
y angle
differences
3 ± 0.5 nrad
2 nrad
Energy
differences
58 ± 4 eV
75 eV
All parity quality specs have been achieved!!
• Commercial Laser was critical to
success of experiment.
• Cavity length ~ 5 m
• Tunable over ~ 20 nm at 780 nm
and 850 nm.
• Etalons provide range of
pulsewidths from 10 p to 70 ps.
• More than 250 mW power
• 499 MHz modelocked Tisapphire lasers for high current
polarized beam experiments (~ 100
uA ave.)
• 300 to 500 mW output power
• 770 nm or 850 nm operation
• Stable phase-locked pulse train
• Maintenance required
HAPPEx Approach
Minimize asymmetries at outset via careful choice and alignment of
pockel cell. No position feedback.
In the Laboratory;
 Compare pockels cells from different vendors
 Choose cell with smallest birefringence gradient
In the Tunnel;
 Without cell, verify good optics and laser polarization.
 Align insertable halfwaveplate for 90 degree rotation.
 Align pockel cell
 Minimize optical beam steering by positioning laser on
geometric center of cell.
With Beam;
Verify laser table measurements; plot e-beam HC position and intensity
asymmetry vs rotating halfwave plate orientation for “ideal” cell voltages.
Asymmetries should be small.
Adjust rotating waveplate orientation and cell voltages to minimize beam
asymmetries. Use IA cell to apply charge asymmetry feedback.
Final beam quality numbers pending but preliminary results
indicate HC specifications were met;
From HAPPEx-H
• Gun3 lifetime was poor
• Frequent laser spot moves
were necessary
• QE holes caused HC
asymmetry variations
14 mm
Gun3 superlattice GaAs
Gun2 strained layer GaAs
Superlattice Photocathode from SVT
QE (%)
Polarization
here
Analyzing Power
here
here
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
From Hall A Compton Polarimeter
• The highest polarization yet
measured at CEBAF; ~ 85%
• QE 0.8% versus 0.15%
• Analyzing power 4 % versus 12%
photon
electron
SVT Superlattice Summary
• Highest polarization ever measured at JLab: P = 85%
• Measurements of many samples at test stand indicates this is no fluke.
• 5 times higher QE than strained layer GaAs material (not yet
demonstrated at tunnel).
• Smaller analyzing power should provide smaller inherent charge and
position asymmetry. (Recent HAPPEx results do not support this claim.)
•We suffered surface
charge limit. QE drops
with increasing laser
power. A concern for
high current
experiments like Qweak.
QE not constant
QE (%)
QE vs hydrogen cleaning
Typical H-dose to clean
anodized samples
Drawback:
Superlattice
material is delicate
Can’t clean with
atomic hydrogen
Makes it tough to
anodize edge of
photocathode
Hydrogen exposure time (min)
From M. Baylac et al., in press
Ion Pump Power Supplies with nanoA Current Monitoring
Designed and constructed by J. Hansknecht
Ion Pump Locations
“Free” pressure monitoring at
10^-11 Torr
Pumps detect bad orbit and beamloss
Gun chamber pump
Wien filter
Y-chamber
Laser chamber
Ave. Power (kW)
10 kW for 1 sec.
(2.5 kW ave power at ¼ D
Courtesy C. H. Garcia and JLab FEL team
The JLAB FEL Injector is driven by a
350 kV DC GaAs Photocathode Gun
Drive Laser
Photocathode
40 cm
Ball cathode
PHOTOCATHODE PERFORMANCE
•Photocathode QE~6% after initial activation at 532 nm
•Photocathode delivers ~200 C between re-cesiations
•Typical day of operations draws ~35 Coulombs
•About 96% of previous QE is recovered with each re-cesiation
•12 activated cathodes and close to 40 re-cesiations performed
on a single GaAs wafer in one year
Courtesy C. H. Garcia and JLab FEL team
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
FEL
Demonstrated DC Photocathode Gun
performance at JLab IR-FEL
• Macropulse operation at 8 mA, 16 ms-long pulses at
2 Hz repetition rate. RF microstructure within
macropulse.
• CW operation at 9.1 mA with 75 MHz microstructure
and 122 pC/bunch.
• Gun routinely delivers 5 mA in pulsed and CW
modes with 135 pC/microbunch.
• Vacuum environment: 4.0E-11 Torr, 99.9% H2
Courtesy C. H. Garcia and JLab FEL team
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
FEL
Continuing Trend Towards Higher Average Beam Current
JLab FEL program with
unpolarized beam
100
ELIC with
circulator ring
10
Series1
Series2
Series3
1
First low polarization,
then high polarization
at CEBAF
0.1
0.01
1970
1980
1990
2010
2000
2020
2030
2040
Year
First polarized beam
from GaAs photogun
Source requirements for ELIC less demanding
with circulator ring. Big difference compared
to past talks. Few mA’s versus >> 100 mA of
highly polarized beam.
M. Poelker, EIC Workshop, March 15-17, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
ELIC Layout
One accelerating & one decelerating pass through CEBAF
(A=1-40)
Electron
circulator
ring
IR
IR Solenoid
IR
3-7
3 -7 GeV electrons
30--1150
30
50 GeV
GeV(light)
light ions
ions
Electron Injector
CEBAF with Energy Recovery
Beam Dump
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Department Of Energy
Snake
ELIC e-Beam Specifications
Typical parameters;
• Ave injector gun current 2.5 mA (and then 25 mA)
• Micropulse bunch charge 1.6 nC
• Micropulse rep rate 150 MHz (and then 1.5 GHz)
• Macropulse rep rate ~ 2 kHz, 5 usec duration.
I
CCR/c
1/f c
~100 CCR/c
Injector
I
Circulator Ring
CCR= 1.5 km
t
t
M. Poelker, EIC Workshop, March 15-17, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Gun Issues for ELIC
• Need 80% polarized e-beam.
• Use SVT superlattice photocathode. 1% QE at 780 nm;
• 6.3 mA/W/%QE
• ~ 1 W provides 1/e operation at 2.5 mA
• Commercial Ti-Sapp lasers with CW rep rates to 500 MHz
provide 0.5 W. Homemade lasers provide ~ 2W.
• Injector micropulse/macropulse time structure demands
laser R&D.
• 25 mA operation requires more laser power and/or QE.
• Charge Limit? Yes, at 1.6 nC/bunch and low QE wafers.
• Lifetime? Probably wise to improve vacuum (more later)
• Gun HV ~ 500 kV to mitigate emittance growth.
• Must limit field emission.
M. Poelker, EIC Workshop, March 15-17, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Gun Lifetime
• CEBAF enjoys good gun lifetime;
 ~ 200 C charge lifetime (until QE reaches 1/e of
initial value)
 ~ 100,000 C/cm2 charge density lifetime (we
operate with a ~ 0.5 mm dia. laser spot)
• Gun lifetime dominated by ion backbombardment.
• So it’s reasonable to assume lifetime proportional to
current density.
• Use a large laser spot to drive ELIC gun. This keeps
charge density small. Expect to enjoy the same
charge density lifetime, despite higher ave. current
operation, with existing vacuum technology.
M. Poelker, EIC Workshop, March 15-17, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Gun Lifetime cont.
Lifetime Estimate;
• Use 1 cm diameter laser spot at photocathode.
• At 2.5 mA gun current, we deliver 9 C/hour, 216
C/week.
• Charge delivered until QE falls to 1/e of initial value;
100,000 C/cm2 *1 Wk/216 C * 3.14(0.5 cm)^2 = 360 Wks!
36 Weeks lifetime at 25 mA.
• Need to test the scalability of charge lifetime with
laser spot diameter. Measure charge lifetime
versus laser spot diameter in lab. (J. Grames presentation
this workshop)
M. Poelker, EIC Workshop, March 15-17, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy