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D. A. Orlov 1 , A.S. Terekhov 2 , C. Krantz 1 , S.N. Kosolobov 2 , A.S. Jaroshevich 2 , A. Wolf 1

Long term operation of high quantum yield GaAs-photocathodes at the electron target of the Heidelberg TSR using multiple recleaning by atomic hydrogen. D. A. Orlov 1 , A.S. Terekhov 2 , C. Krantz 1 , S.N. Kosolobov 2 , A.S. Jaroshevich 2 , A. Wolf 1.

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D. A. Orlov 1 , A.S. Terekhov 2 , C. Krantz 1 , S.N. Kosolobov 2 , A.S. Jaroshevich 2 , A. Wolf 1

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  1. Long term operation of high quantum yield GaAs-photocathodes at the electron target of the Heidelberg TSR using multiple recleaning by atomic hydrogen 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 ~0.2 ... 8 MeV/u • 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 TSR Detectors (ions and neutrals) Photoelectron e-target Collector e- e-source Electron gun with magnetic expansion ≈10...90 Interaction section 1.5m Ion beam

  2. Photocathode performance at the electron target (A) Superconducting solenoid Gun chamber E-gun Preparation chamber Manipulator Loading chamber Hydrogen chamber collector Photoelectron target Merging region • Currents up to 1 mA (2 mA) • Lifetime - 24 h at 1 mA (2mA) • kT = 0.5-1.0 meV kT|| = 0.02 meV E-gun

  3. GaAs GaAs Photocathode performance (B): Lifetime H2O O2 CO2 1. Dark lifetime (RT) > weeks (UHV) CO CH4 H2O O2 CO2 2. Dark lifetime (LT): hour-weeks(temperature) Cryosorption! T > 130 K Cold CO+, CH4+… 3. Operating high-current lifetime: Ion deflection, barrier! e Ion back stream! GaAs E (e-current, energy, pressure, geometry) B Beam profiles (D=12 mm) Start Degraded

  4. GaAs oven Atomic hydrogen cleaning RF coil 1. RF plasma discharge source. H2 H Energetic particles from the source!Risk of photocathode damage! GaAs oven 2. Hot filament source. H2 oven Low efficiency!Cathode heating! High partial pressure of W! 3. Hot capillary source W-capillary H2 GaAs Just good ;-). oven

  5. W-capillary 1900 K H2 GaAs oven oven AH treatment at the TSR target. Hot capillary source. Efficient Narrow angular distribution of H-atoms Low capillary temperature (no W-contamination) Leak valve Leak valve palladium tube palladium tube P=1.0E-08 mbar H2 manipulator W-capillary H manipulator filament sample oven H2

  6. oven AH treatment at the TSR target. Hot capillary source. W-capillary 1900 K H2 GaAs oven T=450o C t=5-10 min Based on the data: K.G. Tschersich, JAP 87, 2565 (2000) Leak valve palladium tube P=1.0E-08 mbar When heat-cleaning does not help (after 3-5 times) H2 W-capillary H-treatment (typical): Tcathode=4500 CH-flux: 5E14 atoms/cm2/sExposure time: 5-10 minExposure: 50-200 L manipulator filament sample oven In 5 min transfer the sample to Prep. ChamberHeat-cleaning at 400-4500 C for 30 min.

  7. AH cleaning: UV spectroscopy Cs/O layer removing by H0: Clean -> CsO -> H Cs/O layer removing by H0: H-dose optimization different H0-exposures 10 L 200 L 1. After 4 CsO activations + heat-cleaning QY (electron/photon), % QY (electron/photon), % 4. H-cleaning 3. Cs + heat-cleaning 2. Clean (HCL + ISO) 6.0 5.0 4.0 3.0 6.0 5.0 4.0 3.0 Photon energy, eV Photon energy, eV - Accumulation of Ga/As oxides after multiple reactivations. - AH efficiently removes oxides. - The small presence of Cs. To remove Ga and As oxides the AH exposure of about 100 L is enough.

  8. Atomic hydrogen: multiple recleaning 25 40 1.5 year of operation! (21 AH treatment, > 80 activation, 120 heat cleaning) 35 20 30 25 15 20 QY (electron/photon), % QY (electron/photon), % 10 15 LPE grown transmission mode photcathode MOCVD grown transmission mode photcathode 10 5 5 0 0 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 H0 dose, L H0 dose, L 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! 2. Arsenic vacancies defects? NO! 3. Heat-cleaning induced degradation of transmission mode cathodes (mechanical strain)? YES!

  10. QY degradation: heat-induced dislocations? 1. Accumulation of oxygen? NO! 2. Arsenic defects (vacancies)? NO! 3. Heat-cleaning induced dislocations at the substrate (sapphire)-heterostructure interface? AFM-image of photocathode with “smooth” surface AFM-image of photocathode after multiple recleaning Peaks height 30-50 nm RMS = 0.2 nm Dislocation net Outside of peaks RMS = 0.5 nm

  11. Conclusions & Outlook 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.

  12. Detectors Photocathode setup Collectorsection Acceleration section TSR dipole Interaction section vertical correction dipoles Toroid section TSR quadrupole 1.5 m Ion beam TSR electron target section - overview

  13. Superconducting solenoid Gun chamber Preparation chamber Manipulator Loading chamber Hydrogen chamber Photocathode section - overview

  14. 1 mbar x 10 min, 1520 ML 1 mbar x 10 min, 1520 ML HCL HCL 5 AH 2 AH 3 AH 3 AH 3 AH 1.5 mbar x 10 min (1st AH), 2280 ML 0.1 mbar x 10 min, 152 ML 0.3 mbar x 5 min 228 ML 0.3 mbar x 10 min, 456 ML palladium tube H2 W-capillary filament sample oven Closed cycle of operation with atomic hydrogen treatment QY (electron/photon), % QY (electron/photon), % The evolution of QY UV spectra for different AH-exposures

  15. TSR photoelectron target Detectors (ions and neutrals) ~0.2 ... 8 MeV/u Neutrals detector Movable ion detector e-target Collector Electron gun with magnetic expansion ≈10...90 TSR dipole e-source Adiabatic acceleration e- Interaction section 1.5m Ion beam

  16. 1 mbar x 10 min, 1520 ML 1 mbar x 10 min, 1520 ML HCL HCL 5 AH 2 AH 3 AH 3 AH 3 AH 1.5 mbar x 10 min (1st AH), 2280 ML 0.1 mbar x 10 min, 152 ML 0.3 mbar x 5 min 228 ML 0.3 mbar x 10 min, 456 ML 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.

  17. Atomic hydrogen cleaning: UV spectroscopy 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.

  18. 1 year of operation! 120 cycles (23 AH treatment) QY (electron/photon), % Cryogenic photocathode source Cs/O layer removing by H0 Photocathode setup different H0-exposures low high QY (electron/photon), % Quantum Yield vs UV photon energy Photocathode at 100 K 6.0 5.0 4.0 3.0 Photon energy, eV Atomic hydrogen cleaning: Vacuum conditions: UHV (5∙10-12 mbar) H2O, O2, CO2 <10-14 mbar High requirements for surface preparation Number of steps (H0 or heat-cleaning)

  19. Photocathode performance at the electron target (A) Photoelectron target 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

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