Transcript JLab Polarized Source Group • We are: Phil Adderley, Maud Baylac, Joshua Brittian, Daniel Charles, Jim Clark, Joe Grames, John Hansknecht, Matt Poelker,

JLab Polarized Source Group
• We are: Phil Adderley, Maud Baylac, Joshua Brittian,
Daniel Charles, Jim Clark, Joe Grames, John
Hansknecht, Matt Poelker, Marcy Stutzman
• 1 student, 4 technical staff, 4 scientists
• Source Group Hall of Famers: Charlie Sinclair,
Bruce Dunham, Larry Cardman, Scott Price, Peter
Hartmann, Michael Steigerwald, Tony Day, Kim
Ryan, Danny Machie, John Hogan, Bill Schneider,
Reza Kazimi, Paul Rutt, Ganapati Rao Myneni
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
GaAs Photoemission Guns
• An invention nearly 30 years old.
• Spin-dependent electron scattering tells us something
about the structure of the target (e.g., molecule, atom,
proton, neutron).
• Good tool for Condensed Matter and Atomic Physics
research.
• Great tool for High Energy and Nuclear Physics
research.
• Relatively new application; Unpolarized electrons for
Light Sources.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
The First GaAs Photoemission Gun
P=
3-1
= 50%
3+1
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Photoemission from GaAs
Bare GaAs surface;
Big work function.
No electrons
Cesium reduces
work function.
Some electrons
Cesium + Oxygen
“Negative Electron Affinity”.
Many electrons
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
First High Voltage GaAs Photogun
Electrons into the accelerator Dec., 1977
Collaboration announces parity violation June, 1978
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Continuous Electron Beam Accelerator Facility
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Continuous Electron Beam Accelerator Facility
0.6 GeV linac
(20 cryomodules)
1497 MHz
67 MeV injector
(2 1/4 cryomodules)
1497 MHz
Gain switched
diode lasers
499 MHz,
Df = 120
A
B
RF separators
499 MHz
C
B
A
Pockels cell
C
A
Gun
B
C
Double sided
septum
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Gun Issues Today and Tomorrow
• Lifetime at High Current (vacuum issues)
• Adequate photocathode QE and polarization
• Adequate Laser Power with RF pulse structure
• Control of Helicity Correlated Systematics
• Beam handling (space charge induced
emittance growth)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
A good day when…
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Photocathode Lifetime
Vacuum not perfect. QE degrades via ion backbombardment
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Lifetime vs Laser Position on Photocathode
Ionized gas is attracted toward the center of photocathode
Solution; position laser spot off axis
2.4 mm off axis
1.6 mm off axis
0.8 mm off axis
Center
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Limiting Active Area via Anodization
5 mm dia.
Anodized photocathode
Electrons emitted from edge of
wafer hit vacuum chamber walls.
This is bad for vacuum.
15 mm
Normal photocathode;
“out of box”
Anodization eliminates inadvertent
photoemission from locations not
intentionally illuminated with laser
light
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Limiting the Photocathode Active Area
Location #1; center
5mm (red)
anodized
11 mm (green)
unanodized
Location #2; 0.8 mm off axis
Location #3; 1.6 mm off axis
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Gun Lifetime at CEBAF
Gun charge lifetime degrading with time.
Limited NEG pump lifetime/capacity?
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Better Vacuum = Longer Lifetime
~2.5E-11 Torr
~5.0E-11 Torr
>15.0E-11 Torr
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Vacuum Considerations
Ultimate Pressure = Outgassing Rate x Surface Area
Pump Speed
We need;
Smaller outgassing rate, Less surface area, More pump speed.
discrepancy
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
NEG Pump Speed Measurements
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Vacuum Considerations
304 Stainless Steel:
old chamber, baked extensively before tests
316L Stainless Steel:
vacuum fired 900ºC, 4 hours; electropolished
304 SS
316L SS
6061 T-6 Aluminum:
explosion bonded to stainless steel flanges
new for these tests
Aluminum
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Atomic Hydrogen Cleaning
Photocathode sits here,
inside vacuum chamber
Molecular hydrogen
dissociated with
RF inductive discharge
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Atomic Hydrogen Cleaning
Extremely high QE
from bulk GaAs
Reliable cleaning method;
No wet chemicals
In situ cleaning
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Photocathode Studies
Polarization w/o
Hydrogen cleaning
Polarization w/
Hydrogen cleaning
Wavelength
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Synchronous Photoinjection
A
B
C
A
B
C
1497 MHz
60 degrees
• Use pulsed light to extract electrons only when we need them.
• This prolongs operating lifetime of the gun.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Synchronous Photoinjection
Chopper
viewer for
three beams;
• Pulsed lasers and
prebuncher ON
• Pulsed lasers,
prebuncher OFF
Beam to Hall B
Beam to Hall A
Beam to Hall C
• DC laser
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Lasers at CEBAF
RF Gain Switching
0.5 GHz
1 GHz
Diode seed laser
1.5 GHz
Diode amplifier
2 GHz
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Lasers at CEBAF
Diode lasers and high
polarization
photocathodes can’t
provide 100 uA
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Lasers at CEBAF
Homemade Ti-Sapphire laser for GEp and GEn
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Modelocked Ti-Sapphire Laser
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Lasers at CEBAF
Modelocked TiSapphire lasers from
Time-Bandwidth. Very
reliable. High power
and high rep rates.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Laser Table Configuration Jan., 2004
We rearrange the lasers every ~ three months
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Built at
Univ. of
Illinois
Feb. 1995
through
Dec. 1998
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
Feb. 1998
through
Spring 1999
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
July 1999 to present
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004
JLab’s 100 kV load-locked gun at Test Cave
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
M. Poelker, U Va graduate student seminar, 3 Mar., 2004