Transcript Polarized Electron Sources Joe Grames, and many others… Source Group Members - Phil Adderley, Maud Baylac, Joshua Brittian, Daniel Charles, Jim Clark, Joe Grames,

Polarized Electron
Sources
Joe Grames, and many others…
Source Group Members - Phil Adderley, Maud Baylac, Joshua
Brittian, Daniel Charles, Jim Clark, Joe Grames, John Hansknecht,
Ken Law, Matt Poelker, Marcy Stutzman
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
JLAB GSA Pizza Seminar
September 15, 2004
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
A history 30 years and growing…
Polarized electron beams have wide application in studies which range
from materials science to nuclear and high energy physics:
 the latter has driven the development of polarized e- sources
Semiconductor sources introduced in 1975 via optical pumping of GaAs
First e- source on an accelerator (P ~ 35%) : PEGGY, at SLAC (1978)
Strained GaAs reaches higher polarization (P~75%) in early 90’s (SLAC)
Nowadays, many accelerator facilities have used GaAs sources:
SLAC, MAMI, ELSA, CEBAF, MIT-BATES
Relatively new => Unpolarized electrons for Light Sources and
development for high current (10’s mA) polarized beams.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Optical Pumping of GaAs
Optical pumping between P 3/
2
and S 1/2
S 1/2
= 1.42 eV
P 3/2
-
+
=0.34 eV
P 1/2
mJ
E ga < E  < E gap+
p
hc / l
3-1
Pe =
= +/- 50%
3+1
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Photoemission from GaAs
Bare GaAs surface;
Large work function.
No electrons
Alkalai (Cs) reduces
work function.
Some electrons.
Ea > 0
Ea  0
Cesium + Oxidant (O or NF3)
“Negative Electron Affinity”.
Many electrons
Ea < 0
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
First High Voltage GaAs Photogun
I ~ 1 mA to
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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Optical Pumping of strained GaAs
Split degeneracy of P3/2
& optical pumping between P3/2 and S 1/2
-
+
Pe = +/- 100%, with E
gap
< E < Egap+
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Strained layer GaAs photocathode
Bandwidth Semiconductor (formerly SPIRE)
0.1 μm
Strained GaAs
• MOCVD-grown epitaxial
250 μm GaAs1-x Px
spin-polarizer wafer
• Lattice mismatch
 split
degeneracy of P3/2
x=0.29
250 μm
GaAs1-x Px
0<x<0.29
600 μm
p-type GaAs
substrate
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Photocathodes
3” wafer cleaved into square photocathodes (15.5
mm) for mounting on a “stalk” using In and Ta cup.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Present JLab polarized gun design
Cathode Ceramic
Anode (GaAs) Insulator
-100 kV
Laser
eNEG coated
beampipe
Cs
NF3
Non evaporable getter pumps (NEG)
4,000 liter/s pump speed  5E-12 Torr
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
July 1999 through today…
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Built at UIUC (Feb. ’95 – Dec. ’98)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
(Feb. ’98 – June ‘99)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Summer shutdown ‘04
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Layout of 100 keV injector
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Polarized electrons, anyone? (Sept. ’04)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Gun Issues (Yesterday, Today & 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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,
f = 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
3 Laser System (diode)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Bottom line: charge delivered…
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
…and photocathode lifetime
Imperfect vacuum => QE degrades via ion backbombardment
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Lifetime vs. Beam Location
Ionized gas is attracted toward the electrostatic center of photocathode
Solution => position laser spot (e- beam location) 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Limiting Active Area via Anodization
5 mm dia.
Anodize photocathode in electrolytic
bath of weak phosporic acid.
15 mm
Photocathode “out of box”
Electrons emitted from edge of
wafer hit vacuum chamber walls.
This is bad for vacuum.
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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Anodized vs. Unanodized
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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Improve QE => 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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Atomic Hydrogen Cleaning – bulk GaAs
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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Hydrogen Cleaning – strained GaAs
Polarization wo/
Hydrogen cleaning
Polarization w/
Hydrogen cleaning
Wavelength (nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
QE & polarization (strained GaAs)
FOM  I P
2
Quantum Efficiency
0.2 % at 840 nm yields 1 mA/mW
1.0 % at 780 nm yields 6 mA/mW
Polarization
P ~ 75 % at 840 nm
e
P ~ 35 % at 780 nm
e
Diode lasers ~ 40 mW
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
High(er) Power Lasers
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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Modelocked Ti-Sapphire Laser
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Time-Bandwidth Ti:Sap
Passively mode-locked
(SESAM)
Very reliable
(turn-key)
High power
(300+ mW)
Wavelength tunable
(peak polarization)
RF repetition rates
G0 ~ 31.1875 MHz
HAPPEX ~ 499 MHz
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Laser Table ~ January `04
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
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
What’s new? Strained Superlattice GaAs
Be doping (cm -3 )
19
GaAs (5 nm)
5.10
GaAsP (3 nm)
14 pairs
GaAs (4
nm)
GaAs 0.64 P0.36 (2.5 μm)
17
5.10
18
5.10
GaAs 1-x P x , 0<x<0.36 (2.5 μm)
p-type GaAs substrate
SVT associates, per SLAC specs.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Higher Quantum Efficiency
QE ~ 1% versus 0.2%
from “traditional”
strained layer material
we operate
here
Wavelength (nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
Higher Beam polarization
Measurements
at Test Cave as
high as 85%
Accelerator
measurement
comparable
Wavelength for Good
QE and Polarization
Typical
polarization
from traditional
material ~75%
Wavelength (nm)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
What about the gun horizon?
Best Technology Load Lock Polarized Electron Gun
High voltage
Preparation
Of NEA surface
Load, heat and
hydrogen clean
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
100 keV load lock test stand
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004
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
2000
2010
2020
2030
2040
Year
First polarized beam
from GaAs photogun
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
J. Grames, JLab Grad Pizza Seminar, Sept. 15, 2004