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D. A. Orlov1, A.S. Terekhov2, C. Krantz1, S.N. Kosolobov2, A.S. Jaroshevich2, A. Wolf1
1 Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany2 Institute of semiconductor Physics, 630090, Novosibirsk, Russia
Motivation: Photocathode multiple recleaning technique. Reliable, closed cycle, QY recovering.
TSR target. Photocathode performance.
Atomic hydrogen cleaning.
Capillary AH source at TSR target.
Results: UV-spectroscopy
(H-treatment optimization).
Results: Multiple recleaning.
Outlook
Detectors (ions and neutrals)
Photoelectron e-target
Interaction section 1.5m
Electron gun withmagnetic expansion
≈10...90
Collector
Ion beam
e-
e-source
TSR
~0.2 ... 8 MeV/u
Long term operation of high quantum yield GaAs-photocathodes
at the electron target of the Heidelberg TSR using multiple recleaning by atomic hydrogen
2
Photocathode performance at the electron target (A)
Currents up to 1 mA (2 mA)
Lifetime - 24 h at 1 mA (2mA)
kT = 0.5-1.0 meV kT|| = 0.02 meV
Photoelectron target
Superconducting solenoid
Preparation chamber
Loading chamber Hydrogen chamber
Gun chamber
Manipulator
E-gun
E-gun
collector
Mer
ging
reg
ion
3
End of lifetime
Photocathode performance (B): Lifetime
1. Dark lifetime (RT) > weeks (UHV)
GaAs
H2OO2 CO2
2. Dark lifetime (LT): hour-weeks
(temperature)
GaAs
H2OO2 CO2
CO CH4
Cold Cryosorption! T > 130 K
(e-current, energy, pressure, geometry)
3. Operating high-current lifetime:
Ion back stream!
GaA
s
E B
e
CO+, CH4+…
Ion deflection, barrier!
Beam profiles (D=12 mm)
Start Degraded
4
Atomic hydrogen cleaning
H2
H
RF coil
GaAs
oven
2. Hot filament source.
GaAs
ovenH
2
Energetic particles from the source!Risk of photocathode damage!
Low efficiency!Cathode heating!
High partial pressure of W!
3. Hot capillary source
1. RF plasma discharge source.
GaA
s
oven
W-capillary
H2
Just good ;-).
ove
n
5
AH treatment at the TSR target. Hot capillary source.
EfficientNarrow angular distribution of H-atoms
Low capillary temperature (no W-contamination)
H2
samplefilament
oven
palladium tube
W-capillary
H2
palladium tube
Leak valve
Leak valve
manipulator
manipulatorH
GaA
s
oven
W-capillary
H2
1900 K
ove
n
P=1.0E-08 mbar
6
AH treatment at the TSR target. Hot capillary source.
H2
samplefilament
oven
palladium tube
W-capillary
Feeding pressure
mbar
Capillary conductance
cm3/s
Degree ofdissociation
%
Angular distribution
sr
H-flux
atoms/cm2/s
H-flux
L/sL
0.75 2.3 2.4 0.65 7.71014 0.380.5 2.6 2.6 0.57 7.11014 0.35
0.15 2.8 3.2 0.41 4.01014 0.160.05 3.3 3.8 0.33 2.01014 0.10
Leak valve
manipulator
GaA
s
oven
W-capillary
H2
1900 K
ove
n
P=1.0E-08 mbar
T=450o C
t=5-10 min
When heat-cleaning does not help (after 3-5 times)
H-treatment (typical): Tcathode=4500 CH-flux: 5E14 atoms/cm2/sExposure time: 5-10 minExposure: 50-200 L
In 5 min transfer the sample to Prep. ChamberHeat-cleaning at 400-4500 C for 30 min.
Based on the data: K.G. Tschersich, JAP 87, 2565 (2000)
AH cleaning: UV spectroscopy
QY
(e
lec
tro
n/p
ho
ton
), %
3.0 4.0 5.0 6.0
different H0-exposures
10 L
200 L
Photon energy, eV
Cs/O layer removing by H0:
H-dose optimization
Cs/O layer removing by H0:
Clean -> CsO -> H
2. Clean (HCL + ISO)
1. After 4 CsO activations + heat-cleaning
4. H-cleaning
3. Cs + heat-cleaningQY
(e
lec
tro
n/p
ho
ton
), %
3.0 4.0 5.0 6.0
Photon energy, eV
To remove Ga and As oxides the AH exposure of about 100 L is enough.
- Accumulation of Ga/As oxides after multiple reactivations.
- AH efficiently removes oxides.
- The small presence of Cs.
H0 dose, L
1.5 year of operation!
(21 AH treatment, > 80 activation, 120 heat cleaning)
QY
(el
ectr
on
/ph
oto
n),
%
H0 dose, L
Atomic hydrogen: multiple recleaning
0
5
10
15
20
25
30
35
40
0 500 1000 1500 2000 2500Q
Y (
elec
tro
n/p
ho
ton
), %
5
10
15
20
25
0 500 1000 1500 2000 2500 3000 3500 0
MOCVD grown
transmission mode photcathode
LPE grown
transmission mode photcathode
AH multiple cleaning works almost perfectly with only slow QY decrease for MOCVD grown photocathodes.
9
QY degradation: heat-induced?
1. Accumulation of oxygen? NO!
3. Heat-cleaning induced degradation of transmission mode cathodes (mechanical strain)? YES!
2. Arsenic vacancies defects? NO!
AFM-image of photocathode with “smooth” surface
RMS = 0.2 nm
AFM-image of photocathode after multiple recleaning
Outside of peaks RMS = 0.5 nm
Peaks height 30-50 nm
QY degradation: heat-induced dislocations?
Dislocation net
1. Accumulation of oxygen? NO!
3. Heat-cleaning induced dislocations at the substrate (sapphire)-heterostructure interface?
2. Arsenic defects (vacancies)? NO!
11
Multiple recleaning of high QY photocathodes – it works!
Slow QY degradation is probably due to heat-induced defects (dislocations at the sapphire-heterostrucrure interface).
Still can be improved.
Conclusions & Outlook
12
13
Acceleration section
Toroid section TSR
quadrupole
Interaction section
Collector section
Detectors
TSR dipole
1.5 m
Ion beamIon beam
Photocathodesetup
vertical correction dipoles
TSR electron target section - overview
14
Superconducting solenoid
Preparation chamber
Loading chamber Hydrogen chamber
Gun chamber
Manipulator
Photocathode section - overview
Closed cycle of operation with atomic hydrogen treatment
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120
QY, %
Cycling number (H-treatment, HCL, or heating)
Lifetime of N5 photocathode in the target setup
'N5-h2_June01_2008.dat' u 1:7
1.5 mbar x 10 min
(1st AH), 2280 ML
1 mbar x 10 min,
1520 ML
0.3 mbar x 10 min, 456 ML
HCL
0.1 mbar x 10 min, 152 ML
1 mbar x 10 min,
1520 ML
0.3 mbar x 5 min
228 ML
HCL
5 A
H
3 A
H
3 A
H
3 A
H
2 A
H
H2
samplefilament
oven
palladium tube
W-capillary
The evolution of QY UV spectra
for different AH-exposures
QY
(el
ect
ron
/ph
oto
n),
%
QY
(el
ect
ron
/ph
oto
n),
%
16
TSR photoelectron target
~0.2 ... 8 MeV/uDetectors
(ions and neutrals)
e-target
Interaction section 1.5m
Electron gun withmagnetic expansion
≈10...90
Adiabaticacceleration
Collector
TSR dipole
Movable ion detector
Neutrals detector
Ion beam
e-
e-source
Fig.3 The figure shows the “history” of the N5-photocathode in the Heidelberg target (>1 year). In total the sample experienced more than 100 heat-treatment. Each minimum correspond “Cs-activation” which typically goes after H-treatment, except of N=85, where no Cs-cleaning was used. Others intermediate points correspond to 1, 2, 3 or 4-th activation. The values of AH-exposure are also indicated on the figure.
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120
QY, %
Cycling number (H-treatment, HCL, or heating)
Lifetime of N5 photocathode in the target setup
'N5-h2_June01_2008.dat' u 1:7
1.5 mbar x 10 min
(1st AH), 2280 ML
1 mbar x 10 min,
1520 ML
0.3 mbar x 10 min, 456 ML
HCL
0.1 mbar x 10 min, 152 ML
1 mbar x 10 min,
1520 ML
0.3 mbar x 5 min
228 ML
HCL
5 A
H
3 A
H
3 A
H
3 A
H
2 A
H
Fig.1 The spectra was measured after HCL or H-treatment or after activation by Cs or Cs/O with subsequent heating. The steps are described in the picture and ordering goes from up to down (the first step “before HCL”, the last on – “Cs/O2 +6.5 A” for N5 and “7.0 A + Cs +6.5 A” for N6). Find on the next page detailed description of the steps.
Atomic hydrogen cleaning: UV
spectroscopy
19
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120
QY, %
Cycling number (H-treatment, HCL, or heating)
Lifetime of N5 photocathode in the target setup
'N5-h2_June01_2008.dat' u 1:7
Cryogenic photocathode source
Vacuum conditions:UHV (5∙10-12 mbar)H2O, O2, CO2 <10-14 mbar
High requirements for surface preparation
Atomic hydrogen cleaning:
Photocathode at 100 K
Photocathode setup
Quantum Yield vs
UV photon energy
QY
(e
lec
tro
n/p
ho
ton
), %
QY
(e
lec
tro
n/p
ho
ton
), %
1 year of operation!
120 cycles (23 AH treatment)
3.0 4.0 5.0 6.0
different H0-exposureslow
high
Photon energy, eV
Cs/O layer removing by H0
Number of steps (H0 or heat-cleaning)
20
Photocathode performance at the electron target (A)
T-control (heat cleaning, operation): Photoluminescence & IR transmission spectroscopes, photoelectron spectra
Surface cleaning quality: UV QY spectroscopy
Emission properties: 2D energy distribution
Currents up to 1 mA (2 mA)
Lifetime - 24 h at 1 mA (2mA)
kT = 0.5-1.0 meV kT|| = 0.02 meV
Photoelectron target