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GaAs Photocathode Activities at
Daresbury Laboratory
Narong Chanlek
Ph.D Student,University of Manchester
Accelerator Science and Technology Centre
Science & Technology Facility Council, UK
Ultra Bright Electron Sources Workshop, 29th June 2011
GaAs photocathodes
High QE over the entire spectrum of visible light once it
is activated to the negative electron affinity (NEA) state.
Some issues
Lifetime
Response time
2
p-type GaAs
EVAC
Vacuum
ECBM
EgEVB
Vbb
Vbb
Vdipole
eff
EF
Wbb
Cs,Op-type GaAs
EVAC
ECBM
EVBEF
Eg
Vacuum
NEAPEA
29th June 2011
ALICE Gun
3
Anode plate
Ceramic Cathode Ball
GaAs cathode
Current ALICE cathode:
VGF Bulk GaAs cathode,
Zn doped, 31 mm diameter
and 625 m thick
Consistently achieving 3%
QE @ 532 nm
No cathode change
possible.
29th June 2011
Photocathode preparation facility (PPF)
4
Provide a rapid and reliable
way to exchange the
photocathode.
Prepare the photocathods
in a controllable and
reproducible way.
Remove the activation
process from the gun.
Gun
Photocathode Preparation Facility
500 kV power supply
29th June 2011
Photocathode preparation facility (PPF)
5
Loading chamber
Atomic hydrogen chamber
Preparation chamber
Dummy gun
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629th June 2011
RM GaAs photocathode
New epitaxial grown GaAs cathodes – 10 mm diameter:
Reflection mode (RM) bound to molybdenum substrate
Transmission mode (TM) bound to glass or sapphire
substrate
Different thickness’ of active layer to enable a fast
measurement of time response.
Photocathodes
729th June 2011
Cathode preparation
Chemically cleaned with HCl/IPA solution in
N2 purged glove box.
Transported to the loading chamber without
exposure to the air.
Heat cleaned and activated in the
preparation chamber.
Degraded photocathodes are heat cleaned
and re-activated.
When it is not possible to fully recover the
QE, it is cleaned by atomic hydrogen
cleaning.
8
Achieved QE and lifetime
29th June 2011
Consistently achieving 14 - 19% QE.
Lifetime in the order of 1000 hours.
0 10 20 30 40 50 60 70 80
0.0
1.0x10-10
2.0x10-10
3.0x10-10
Pre
ss
ure
(m
ba
r)
Elapsed time (minutes)
0
4
8
12
16
Photocurrent
Cs current
QE
(%
)
0
2
4
6
8
10
C
s c
urr
en
t (A
)
A typical activation curve with O2
0 100 200 300 400 5000.8
0.9
1.0
1.1
QE = e-t/688.81
R2 = 0.95
QE
(a.u
.)
Elapse time (hours)
Dark lifetime in the preparation
chamber (9 10-12 mbar)
1/e lifetime = 1/0.001 hrs= 1000 hrs
929th June 2011
Cathode preparation
QE vs the number of activations with O2 and NF3
1 2 3 4 5 6 7 80
4
8
12
16
QE
(%
) @
63
5 n
m
Number of heat clean and activation cycles
1 2 3 4 5 60
4
8
12
16
QE
(%
) @
63
5 n
m
Number of heat clean and activation cycles
Repeated heat cycles and
activations with O2 lead to a
degradation in achieved QE.
Cathode needs to be removed
for chemical cleaning in order to
achieve high QE again.
NF3 provides a more stable QE
after repeated activations.
1029th June 2011
Surface science analysis
Surface analysis
chamberPreparation
chamber
Loading chamberAtomic hydrogen
cleaning chamber
1200 1000 800 600 400 200
Initial surface
Cleaned surface
Cs 3d5/2
Cs 3d3/2
Binding energy (eV)
Activated surface
Inte
nsit
y (
C/s
)
As L3M
45M
45
As L2M
45M
45
C 1sGa L3M
45M
45
O 1sO KL3/2
L3/2
Cs M5N
45N
45
O KL1L
3/2
Ga 2p3/2
Ga 2p1/2
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Gas exposures after activation with O2 and NF3
0.00 0.05 0.10 0.15
0.0
0.2
0.4
0.6
0.8
1.0
QE
(a
.u.)
Exposure (L)
H2, CH
4 and N
2
CO
CO2
O2
0.00 0.05 0.10 0.15
0.0
0.2
0.4
0.6
0.8
1.0
QE
(a
.u.)
Exposure (L)
NF3 activation exposing to CO
O2 activation exposing to CO
NF3 activation exposing to CO
2
O2 activation exposing to CO
2
NF3 activation exposing to O
2
O2 activation exposing to O
2
O2 activation Compared with NF3 activation
12
Future work - Cold cathode energy
distribution measurements
29th June 2011
Future experiments are to use existing
systems to measure the 3D energy
distribution of a low energy (10-20V)
electron beam. The GaAs cathode will
be at liquid nitrogen temperatures.
Use existing PPF for cathode
preparation.
13
Future work - Cold cathode energy
distribution measurements
29th June 2011
Energy distribution measured as a function of QE decay.
Cathode port
Laser port
MCP drive
MCP
Cathode at LN2
temperature
14
Future work - 160 KV photoinjector for time response
and emittance measurements (cold cathode)
29th June 2011
Experiment will make use of GaAs cathodes with different thicknesses
of active layer for time response measurements.
Diagnostic line will be used for
emittance measurements.
The aim is to measure both
parameters as a function of QE
decay at LN2 temperatures.
15
Acknowledgements
29th June 2011
I would like to thank the following people for their
contributions to this work:
Daresbury Laboratory ISP Novosibirsk
Lee Jones Alex Terekhov
Ian Burrows Sergey Kosolobov
Barry Fell Heinrich Schreibler
Ryan Cash
Julian McKenzie
Adrian Hannah
Mark Pendleton
Keith Middleman
Boris Militsyn
Thank You!