V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Atomic Hydrogen Cleaning of Super Lattice photo cathodes

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Introduction Long term storage problem of photo cathodes Hydrogen Cleaning QE/Polarization investigations Conclusion

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany MAMI overview beam parameters 855MeV max. 100  A cw current  h =8 nm rad ca. 6000h – 7000h operation / year MAMI B beam parameters MeV max. 100  A  h =10 nm rad MAMI C

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Our ‘standard’ way from cathode package to work position in source: 1. sample from wafer is unpacked under nitrogen atmosphere, inserted in cathode holder and placed in transport vessel. 2. Transport vessel is connected to load-lock chamber and pumped to below torr. 3. Cathode holder is transferred through valve from load-lock chamber to preparation chamber. 4. Preparation chamber at ~a few torr. Cathode is heat cleaned and NEA-activated. 5. Activated photo cathode be placed into source (at probably even lower pressure).

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany MAMI standard photocathodes For test used: bulk GaAs (Wafer Tech. LTD., England, U.K.) For the beam production strained layer (up to 2004) Since then: Super lattice cathodes (Sankt Petersburg State Technical University, Russia) Example: ‘S-45’ piece of wafer SL5-998 As cup GaAs Highly doped with Be 6 nm 30 alternating layer of In 0.16 Al 0.2 Ga 0.64 As Al 0.28 Ga 0.72 As Form super lattice structure nm Buffer – Layer Al 0.4 Ga 0.6 As 1250 nm GaAs(100) Substrate0.5 mm

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Preparation. Q.E. – Trend: History of the super lattice cathode S-45  nm  final state worse than it looks….

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany New DBR and non-DBR photocathodes As coating Cap GaAs doped with Be6 nm 2x12+1=25 alternating layer of In 0.2 Al 0.19 Ga 0.61 As // Al 0.4 Ga 0.6 As Form super lattice structure 92 nm Buffer Al 0.35 Ga 0.65 As 580 nm Buffer GaAs12 nm Buffer GaAs ( to DBR )20 nm 2x22=44 alternating layer of Al 0.19 Ga 0.81 As // AlAs Form DBR structure 2830 nm GaAs(100) Substrate 0.5 mm As coating Cap GaAs doped with Be6 nm 2x12+1=25 alternating layer of In 0.2 Al 0.19 Ga 0.61 As // Al 0.4 Ga 0.6 As Form super lattice structure 92 nm Buffer Al 0.35 Ga 0.65 As 580 nm Buffer GaAs12 nm GaAs(100) Substrate 0.5 mm DBR type Non-DBR-type 7-395

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany ‘non-DBR’-wafer SL Data measured directly after wafer production at SPSTU

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Disappointment: QE is much too small, unpleasant nonlinearity. First results of new SL’s at our installation

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Oxide problem? Probably due to insufficient As-passivation of surface Deterioration seems to appear after typical storage times of months Possible Reason: oxide transfer As  Ga. Maybe not avoidable even if sample is stored under vacuum. Investigated,e.g., by D. A. Allwood et al. for GaAs ‘epi- ready’ surfaces. (Thin solid films, 412 (2002) 76-83) Allwood suggests slowing down oxide transfer by cooling to -20C: too late for our stock. Oxides not removed by conventional heating. Attempted solution: Atomic Hydrogen cleaning

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Atomic Hydrogen Sources 1. Radio frequency source. Atomic hydrogen cleaning of polarized GaAs photocathodes was successfully applied to strained GaAs cathodes used for producing highly polarized electrons. (see for example T. Maruyama et al. APL, 82,23 (2003) 4184) 2. Thermal cracking atomic beam sources are used successfully to remove native oxidation from GaAs and provide extremly good surface quality. See for example V. Andreev et al: Proc. Spin 2000, p.901. Open question: Polarisation after super lattice treatment? Note: 6 nm thin functional structure in SL top layer

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Hydrogen Cleaning: HABS H 2 dissociation typically 80-98% depending on operational conditions Atomic hydrogen flux density up to 1*10 16 /(cm 2 s) No high-energy neutrals or ions Low power consumption (P < 200 W) Integrated water cooling, low thermal load on other experimental equipment Hydrogen Atomic Beam Source (commercial system by Dr. Eberl MBE-components GmbH)

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Atomic Hydrogen Cleaning Installation Preparation Chamber UHV transport vessel Atomic Hydrogen Source

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Preparation with / without AHC =680nm

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Cathode transport from HABS to test source PKAT Preparation Chamber UHV transport vessel Photo: E. Riehn

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Results SL Improves *5 at low intensities+absence of saturation! (*50 improvement for high intensities at MAMI)

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Structure SL H: second activation 17.7% at 802nm  difference insignificant. (other 395 H sample achieves 85±3 % of Polarisation at MAMI)

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany DBR Structure SL 7-396H Extended P, QE datasets measured directly after production by Y. Yashin, SPSTU Highest Q.E values ever measured at high polarization in our lab (1.2%) Stands 5 times more incident power than conventional GaAs cathode (preliminary!)

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Conclusion GaAlAs / InAlGaAs Super lattice photo cathodes are the standard type at our facility. ‘Storage’ problem present in some (not all) wafers Atomic hydrogen cleaning by thermal cracker results in dramatically improved surface condition no significant polarization loss. Typical quantum efficiency 3-6 μA/mW at working point of high polarization (P=85%), Operation at accelerator started, now observing long term behaviour Promising first results from hydrogen cleaned DBR super lattice (7-12 μA/mW at max P).

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany The End

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Appendix

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Cathode lifetime under different operating conditions. Mode of operation and currentLife time, [hours] No operation, Stand by (valves closed) 1200 A2 Collaboration Operation, I=0.05 µA850 A1 Collaboration Operation, I=12.0 µA720 A4 Collaboration Operation, I=30.0 µA520 High Current Test, I=200 µA160

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Muster Title Based on GaAs strained layer – muster text Quantum Efficiency Wave length And go on.

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Story of MAMI Project studies about Microtrons 1975 Proposal of a Race Track Microtron 1979 First stage 14 MeV beam of MAinz MIcrotron MAMI 1983 Second stage at 183 MeV energy, maximal beam current 30µA 1990 Third stage 855 MeV beam of MAMI B 1991 Beam from distant upstairs polarized electron source 1992 First acceleration of polarised electrons to full energy 1999 Approval of the 1.5GeV Harmonic Double Sided Microtron (HDSM) as a fourth stage of MAMI Dec. 19, 2006, Beam through HDSM ! 1508MeV reached ! Feb. 23, 2007 until Mar. 05., 2007 Start the first production beam time with 10µA polarized beam polarization 84% at 1.508GeV Feb.27, 2007 performed a high current test and with reasonable radiation level in the HDSM halls 50µA beam current (75.4kW beam power) Oct. 5, 2007 Inauguration ceremony of MAMI C.

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany EKAN PKAT PKA2 PKA1

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Vacuum chamber handling All vacuum components from 304 stainless steel, Vacuum and beam line downstream to differential stage bake able to 250 °C Using continuously bake-out procedure. Heating elements – taps and special ordered jackets Heating 200 °C during one week. One of test source (PKA2) is coved now by NEG, under investigations. For example of chamber handling in CEBAF: Stutzmann et al. NIM A 574 (2007)

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany Preparation. Example Cathode S Cooling 45 min 3. Switch on Cs 1. Thermal heating 30 min, P~100 W, T~ °C 4. Waiting ~10 min., before photocurrent 5. Let in O 2 pressure ~2x10 -9 torr 6. Control maximum rise velocity of current 7. Stop after ~ 45 min

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany PKA1 Main Source of MAMI ChamberHigh voltage insulator NEG Pump Load Lock Chamber Preparation Chamber Manipulator Alpha Magnet Differential Stage Cathode Position Ion Pump Spin Rotator

V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany 2005: 6140h operation, 68% with polarised beam 2006: 5950h operation, 65% with polarised beam 2007: 7100h operation, 50% with polarised beam 2008: yet more then 50 % with polarised beam MAMI overview. Polarised electron source Polarised beam produced by photo cathodes based on A3B5 semiconductors by illuminating by circular polarised laser light. Activation by Cs:O Layers. Polarised beam means: 1.High quantum efficiency. 2.High degree of the polarisation