D. A. Orlov 1, A.S. Terekhov 2, C. Krantz 1, S.N. Kosolobov 2, A.S. Jaroshevich 2, A. Wolf 1 1 Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany 2 Institute of semiconductor Physics, , 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 with magnetic expansion  ≈ Collector Ion beam e-e- e-source TSR ~ 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  = meV kT || = 0.02 meV Photoelectron target Superconducting solenoid Preparation chamber Loading chamber Hydrogen chamber Gun chamber Manipulator E-gun collector Merging region

3 Photocathode performance (B): Lifetime 1. Dark lifetime (RT) > weeks (UHV) GaAs H2OH2O O2O2 CO 2 2. Dark lifetime (LT): hour-weeks (temperature) GaAs H2OH2O O2O2 CO 2 CO CH 4 Cold Cryosorption! T > 130 K (e-current, energy, pressure, geometry) 3. Operating high-current lifetime: Ion back stream! GaAs E B e CO +, CH 4 + … Ion deflection, barrier! Beam profiles (D=12 mm) Start Degraded

4 Atomic hydrogen cleaning H2H2 H RF coil GaAs oven 2. Hot filament source. GaAs oven H2H2 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. GaAs oven W-capillary H2H2 Just good ;-). oven

5 AH treatment at the TSR target. Hot capillary source. Efficient Narrow angular distribution of H-atoms Low capillary temperature (no W-contamination) H2H2 sample filament oven palladium tube W-capillary H2H2 palladium tube Leak valve manipulator H GaAs oven W-capillary H2H K oven P=1.0E-08 mbar

6 AH treatment at the TSR target. Hot capillary source. H2H2 sample filament oven palladium tube W-capillary Feeding pressure mbar Capillary conductance cm 3 /s Degree of dissociation % Angular distribution sr H-flux atoms/cm 2 /s H-flux L/s L     Leak valve manipulator GaAs oven W-capillary H2H K oven P=1.0E-08 mbar T=450 o C t=5-10 min When heat-cleaning does not help (after 3-5 times) H-treatment (typical): Tcathode=450 0 C H-flux: 5E14 atoms/cm 2 /s Exposure time: 5-10 min Exposure: L In 5 min transfer the sample to Prep. Chamber Heat-cleaning at C for 30 min. Based on the data: K.G. Tschersich, JAP 87, 2565 (2000)

AH cleaning: UV spectroscopy QY (electron/photon), % different H 0 -exposures 10 L 200 L Photon energy, eV Cs/O layer removing by H 0 : H-dose optimization Cs/O layer removing by H 0 : Clean -> CsO -> H 2. Clean (HCL + ISO) 1. After 4 CsO activations + heat-cleaning 4. H-cleaning 3. Cs + heat-cleaning QY (electron/photon), % 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.

H 0 dose, L 1.5 year of operation! (21 AH treatment, > 80 activation, 120 heat cleaning) QY (electron/photon), % H 0 dose, L Atomic hydrogen: multiple recleaning QY (electron/photon), % 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 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

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13 Acceleration section Toroid section TSR quadrupole Interaction section Collector section Detectors TSR dipole 1.5 m Ion beam Photocathode setup 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 1.5 mbar x 10 min (1 st 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 AH 3 AH 2 AH H2H2 sample filament oven palladium tube W-capillary The evolution of QY UV spectra for different AH-exposures QY (electron/photon), %

16 TSR photoelectron target ~ MeV/u Detectors (ions and neutrals) e-target Interaction section 1.5m Electron gun with magnetic expansion  ≈ Adiabatic acceleration Collector TSR dipole Movable ion detector Neutrals detector Ion beam e-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. 1.5 mbar x 10 min (1 st 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 AH 3 AH 2 AH

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/O 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 Cryogenic photocathode source Vacuum conditions: UHV (5∙ mbar) H 2 O, O 2, CO 2 < mbar High requirements for surface preparation Atomic hydrogen cleaning: Photocathode at 100 K Photocathode setup Quantum Yield vs UV photon energy QY (electron/photon), % 1 year of operation! 120 cycles (23 AH treatment) different H 0 -exposures low high Photon energy, eV Cs/O layer removing by H 0 Number of steps (H 0 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  = meV kT || = 0.02 meV Photoelectron target