Transcript Polarized Source Advances Matt Poelker (Group Leader) & Joe Grames (Deputy) • Introduction, Motivation and History of Polarized Beams at CEBAF • • • • Higher Polarization -

Polarized Source Advances
Matt Poelker (Group Leader) & Joe Grames (Deputy)
• Introduction, Motivation and History of
Polarized Beams at CEBAF
•
•
•
•
Higher Polarization - Superlattice Photocathodes
Powerful Ti-Sapphire Lasers
Parity Violation Experiments
Load locked gun development
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations /SC-PAC2001-6.19.01
8-31-05
1
Motivation – Why Polarized Electrons?
 Approximately half of CEBAF experiments require polarized
electrons.
 High profile nuclear physics research at all halls
structure functions
nucleon form factors
parity violation (10 -> 1 -> 0.1 part-per-million)
 Past and present success due in part to reliable, high
polarization and high ave. current beam delivery from GaAs
photoguns
 New experiments are even more demanding: more current,
higher polarization, greater availability, tighter beam
specifications. Future success requires continued R&D focus
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Polarized Electron Tutorial
Electron energy levels
of bulk GaAs
Bare GaAs;
No electrons
Cs reduces
work function.
Some electrons
Cs + NF3 = “NEA”
Many electrons
P=
3-1
= 50%
3+1
Step 1: Put electrons into
conduction band
Step 2: Put electrons into the
accelerator
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Continuous Electron Beam Accelerator Facility
0.6 GeV linac
(20 cryomodules)
1497 MHz
“Synchronous
Photoinjection”
to prolong gun
lifetime
67 MeV injector
(2 1/4 cryomodules)
1497 MHz
A
B
RF separators
499 MHz
C
499 MHz,
Df = 120
B
A
Pockels cell
A
C
B
A
B
C
C
Gun
Double sided
septum
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
CEBAF Photogun Challenges
 Highest average current polarized beam in the
world
 Long periods of uninterrupted operation: 24/7
for more than 30 weeks/yr. This places
tough demands on gun vacuum and lasers
 Simultaneous beam delivery to three halls with
large beam current dynamic range (100 pA to
120 mA)
 Demanding Beam Specifications: small energy
spread, tight beam profiles, no cross talk
between halls, small helicity correlated beam
asymmetries, etc.
 Today’s achievements = Tomorrow’s Standard
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
History of Polarized Electrons at JLab
First Gun (1997)
Second Gun (1998)
1999 to Present
 Vacuum greatly
 Z-style spin manipulator
 Two gun design (one
improved with NEGs
 Bulk GaAs with 35%
spare).
 Photocathode edge
polarization
 Eliminate 90 bend.
anodizing
 Relatively poor vacuum
 Large bore vacuum
 Wien spin manipulator tube with NEG coating.
limits gun lifetime
 Shorter beamline
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
History of Polarized Electrons at JLab
First Laser (1996)
 Gain switched seed
laser and diode
amplifier (MOPA)
 One laser provides
beam to all halls
Second Laser (1998)
Third Laser (2000)
Three diode MOPAs,
One laser for each hall
for more flexibility.
 Technology
transferred to Mainz
Microtron
• Invented new
technique to modelock
Ti-Sapphire laser at
499 MHz.
• More powerful than
diode lasers.
• Wavelength tunable
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Noteworthy Achievements
 Highest average current polarized beam in world
 Longest demonstrated operational charge lifetime
 First to demonstrate synchronous photoinjection
 Results of first parity violation experiment
published within 6 months of run.
 Many new techniques and technologies have been
transferred to other labs: hydrogen cleaning, edge
anodizing, NEG pumped guns, gain switched diode laser
and amplifier.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
History of Polarized Electrons at JLab
P2I = Figure of Merit (P is polarization and I is beam current) We can do
more experiments and more difficult experiments when P2I is high.
80
Superlattice GaAs
and Ti-Sapp lasers
70
60
P2 I
50
40
Strained GaAs and
Ti-Sapp lasers
Three diode lasers
at 499 MHz
30
20
One diode laser at
1497 MHz and bulk GaAs
10
0
FY97
FY98
FY99
FY00
FY01
FY02
FY03
FY04
FY05
FY06
Fiscal Year
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Photocathode Material
100 nm
100 nm
High QE ~ 10%
Pol ~ 35%
“conventional” material
QE ~ 0.15%
Pol ~ 75%
Superlattice GaAs:
Layers of GaAs on GaAsP
14 pairs
Bulk GaAs
Strained GaAs:
GaAs on GaAsP
No strain relaxation
QE ~ 0.8%
Pol ~ 85%
Both are results of successful SBIR Programs
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Superlattice Photocathodes
100 kV Mott scattering
at Test Stand
From Hall A Compton Polarimeter
Typ. Polarization from
conventional strained GaAs
photon
electron
Wavelength (nm)
• Beam polarization ~ 85% - the highest polarization ever measured at JLab
• High QE, 0.8% versus 0.15% for “conventional” strained GaAs
• Small analyzing power should help parity violation experiments
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Superlattice Photocathodes
Learning to use this material has been challenging
 In particular, it cannot be hydrogen cleaned, an
essential follow up step of the original edge
anodizing procedure
 Protective Arsenic cap layer developed with
vendor
 Anodization procedure modified to minimize
contamination of inner portion of photocathode
sample
 Problems solved. High polarization, high QE and long
lifetime for HAPPEx parity violation experiment
 Ready to switch to superlattice material as new
CEBAF standard. Users can expect 85% instead of
75% polarization. Superlattice photocathodes in
both guns by October 2005
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Powerful Mode-locked Ti-Sapphire Lasers
• 1st commercial laser w/ 499 MHz rep rate
• Higher power compared to diode lasers
• Wavelength tunable for highest polarization
• Feedback electronics to lock optical pulse
train to accelerator RF
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Powerful Mode-locked Ti-Sapphire Lasers

Over the course of ~ two years we have purchased 4 TBWP lasers

One Ti-Sapphire laser with 31 MHz pulse repetition rate for
g0 experiment

Three Ti-Sapphire lasers with 499 MHz repetition rate for
conventional experiments at high current halls A and C
 Two lasers in the tunnel and one hot spare
Drawbacks (compared to diode lasers):
 Complicated lasers need maintenance every ~ 2 weeks.
 Complicated laser table (more optical elements required).
 Laser jock required.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
New Laser Clean Room
Clean laser environment,
Temperature Stable,
Safe for other injector
occupants
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Parity Violation Experiments at CEBAF
Helicity-correlated asymmetry specifications (achieved)
averaged over the experiment
1999
2010?
Experiment
Beam
Properties
(µA, %)
Physics
Asymmetry
Position
Asymmetry
Current
Asymmetry
HAPPEx-I
(100, 35)
13 ppm
10 (10) nm
1.0 (0.4) ppm
HAPPEx-II
(40, 85)
1.3 ppm
2 (8) nm
0.6 (2.6) ppm
HAPPEx-He
(40, 85)
8 ppm
3 (3) nm
0.6 (0.08) ppm
G0
(40, 70)
2 to 50 ppm
20 (4) nm
1 (0.14) ppm
P-REX
(100, 85)
0.5 ppm
1 nm
0.1 ppm
Qweak
(180, 85)
0.3 ppm
40 nm
0.1 ppm
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Preliminary Results from HAPPEx-He
Charge (C)
Gun Current (uA)
Daily Gun Current and Charge Plot
Time (hr)




Steady beam delivery since July.
40 uA and beam polarization ~ 85%. HAPPEx receives ~ 3 C/day
Run appears to be successful but position asymmetry could be smaller
Helicity magnets installed
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Load Locked Gun Development
No more gun bakeouts! Photocathode replaced in 8 hours versus 4 days.
•Multiple samples,
•No more
photocathode edge
anodizing,
•Smaller surface area
and no more venting
means:
•Better gun vacuum,
•Longer photocathode
lifetime
Installation at CEBAF January, 2006
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Conclusions
 Extremely good beam quality and knowledgeable staff
at CEBAF
 Much of CEBAF success can be attributed to Polarized
Source Program.
 Many Photogun Achievements and “World’s Firsts” :
highest ave current polarized beam, longest charge
lifetime, novel lasers and techniques, etc.
 CEBAF – on our way to becoming a production parity
violation lab
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05
Future Challenges
 Beam specifications typically grow more demanding. For
example, consider Qweak: polarization 85%, current 180 uA,
run average helicity-correlated charge asymmetry 0.1 ppm.
 Today’s achievements = Tomorrow’s standard
 Infrequent maintenance periods:
Lasers should be more reliable.
Laser table is very complicated
Gun lifetime (and therefore vacuum) – always room for
improvement.
 Apply CEBAF photogun technology to future NP machines
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Joe Grames CEBAF Operations 8-31-05