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