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Ultra Cold Electron Sources on the Basis of GaAs Photocathodes

MPIK-K. Ultra Cold Electron Sources on the Basis of GaAs Photocathodes. Dmitry Orlov. 2 cm. Ale Bakker, Emilie Kernen PHOTONIS Netherlands B.V., 9301 ZR Roden, The Netherlands. MCP. Photocathode. Injection. ~0.1… 8 MeV/u. Heidelberg Test Storage Ring.

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Ultra Cold Electron Sources on the Basis of GaAs Photocathodes

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  1. MPIK-K Ultra Cold Electron Sources on the Basis of GaAs Photocathodes Dmitry Orlov 2 cm Ale Bakker, Emilie Kernen PHOTONIS Netherlands B.V., 9301 ZR Roden, The Netherlands MCP Photocathode Injection ~0.1… 8 MeV/u Heidelberg Test Storage Ring Claude Krantz, Andrey Shornikov, Andreas Wolf Max-Planck-Institut für Kernphysik, 69117Heidelberg, Germany 10 m Photocathode E-target

  2. MPI-K Outline ■Physics of GaAs-photocathodes ■ Know-what-why-how of GaAs-photocathode and of cold magnetized e-beams ■ Sub-meV resolution at MPI-K GaAs photocathode target (examples) ■ Experimental set-up (E-target) for mA high brightness dc beams ■ Photocathode performance ■ Lifetime issue.. From 1 mA to 100 mA ■ Alternative photoemission sources ■ Outlook

  3. Physics of GaAs photocathodes S0=104 cm/s (diffusion barrier) Seff=107 cm/s hIN De=40cm2/s, Le=3.5 m REF Non-RR via surface states Conduction band Pe=0.2-0.6 Glass substrate with AR-layer TR hIN hIN Non-RR Surf.states Fermi level Valence band Vacuum p-GaAs p-AlGaAs Cs+O 1 ML S0=104 cm/s Transm.: Pe=1 QY=20-50% Pulses: ~ 1 ns (L=2 m) ~10 ps (L=0.2 m) Polarization – 30-50 % – appr. 100 % (strained) S0=9.5x104 cm/s Refl. Pe=1 1/Pe scaled, Pe=0.58

  4. kT=110-120 meV Evac EF “3-D” energy distribution of photoemitted electrons vacuum T=1300-1500 K Cold photoelectrons from NEA GaAs: Know-what-why-how. Photocathode kTC = 10 meV Thermocathode kTC > 100 meV kT=7 meV Ec Evac Suppression Suppression EF Suppression Ev (CsO) GaAs vacuum T= 80 K Suppression Strong energy and impulse relaxations Energy spreads of about kT E-current: up to few mA Lifetime : >24 h Fully activated cathode: QY= 15-35% Suppression:QYeff=1 % To get 1 mA: 0.15 W @ 800 nm D.A. Orlov et al., APL, 78 (2001) 2721; E-beam: diam. I=1mA, kT=7 meV: Diam 1 mm: norm~0.1 mm mrad, Bn~0.02 A/(mm mrad)2 Diam. 0.1 mm norm~0.01 mm mrad, Bn~2 A/(mm mrad)2

  5. E ΔE U0 ΔE v║ Δv' Δv Cold electrons: How to make them colder Reduction of kT 1. Magnetic expansion B0 (high field) Bguide(low field) Photocathode kT = 0.5 meV  = 20 2. Acceleration Reduction of kT|| Phase-space conservation kT|| = 0.02-0.1 meV

  6. Cold electron beam: How to keep the beam cold? High magnetic field is required high current + magnetic field high current low current e 1. To avoid beam divergence e e rc e 2. To suppress TLR keeping dT|| / dZ < 5 μeV/m : B ne-1/3 ne-1/3 e rc << λc e 3. To provide adiabatic transport Typical transition lengths R=100 mm R λc << R B

  7. E 1s2 2p3/2 En ΔEcore (1s2 2p3/2 nℓ'j )J 1s22s Eres = 1.0 meV T┴ T|| ~ 0.02 meV Core excitation energiesΔE (2s–2p) TSR = 44.30943(20) eV (±0.2 meV, 4.6 ppm) (<1% few body QED) 50 meV n = 10 (2p3/210d5/2)J = 4 (2p3/210d3/2)J = 2 MPI-K Electron target (2p3/210d3/2)J = 3 Photocathode 45Sc18+ TSR – 4 MeV/u PRL, 100, 033001 (2008)

  8. Rotational resolution (DR rate) ) -1 s 3 TSR e-target with cryo-photocathode cm -9 kT ~ 2 meV ^ (CRYRING data) Recombination rate coefficient (10 Model cross section with kT ~ 0.6 meV, kT ~ 0.025 meV ^ || Electron collision energy (milli-eV) direct & indirect process Dissociative recombination of HD+ TSR - e B* A + + e - (AB+)*+ nℓ = AB ** AB+ HD+ (1sσ, v = 0, J ) + e → HD** (1sσ nℓλ , v'J' ) → H(n) + D(n' ) Vibration v=0 -> 1 0.15 eV Rotation j=0 -> 1 4.5 meV PRL 100, 193201 (2008)

  9. MPI-K electron target Superconducting solenoid Gun chamber E-gun Preparation chamber Manipulator Hydrogen chamber Loading chamber ~0.1… 8 MeV/u collector GaAs-photoelectron target Merging region Heidelberg Test Storage Ring • Currents up to 1 mA • Lifetime - 24 h at 1 mA • kT < 1.0 meV kT|| = 0.02 meV E-gun

  10. 1 year of operation! 120 cycles (23 AH treatment) QY (electron/photon), % Photocathode vacuum setup: closed cycle of operation Cs/O layer removing by H0 Photocathode setup different H0-exposures low high QY (electron/photon), % Quantum Yield vs UV photon energy 6.0 5.0 4.0 Photocathode at 100 K 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)

  11. 1 year of operation! 120 cycles (23 AH treatment) QY (electron/photon), % PL-image system AH-treatment: NIR camera (850-950 nm) 650nm LEDs QY slow-degradation due to dislocation net developed, caused by multiple heat-cleaning.! GaAs-glass bonding can be improved ! For reflection cathode it should be better! Sample position PL-image (diam.25 mm) Not-degraded image Dislocation map is developed due to “wrong” heat-treatment Number of steps (H0 or heat-cleaning)

  12. GaAs GaAs GaAs-photocathode: Lifetime H2O 1. Dark lifetime (RT) > months (UHV) years (closed tubes) O2 CO2 CO CH4 H2O O2 CO2 2. Dark lifetime (LT): hour-weeks in open systemyears (closed tubes) Cold Cryosorption! Keep T > 130 K 3. Operating high-current lifetime: (e-current, energy, pressure, geometry) CO+, CH4+… a) Ion back stream! Ion deflection, barrier! e b) Ion-stimulated desorption GaAs c) Photocathode heating E B d) Laser induced desorption (UV, refl.) For I ≥ 1mA Lifetime ~1/In, n>1 (n~2) Beam profiles (D=12 mm) MPI-K target performance Start Degraded

  13. Alternative photoelectron sources: Photoemission from metals Photoionizaton of atoms hIN ~4-5 eV Metal Vacuum metal Short pulses: 1-10 fs QY=10-4 (1 eV above threshold) QY=10-5 (~100 meV above threshold) To get 1 mA: ~500 W @ 260 nm Short pulses: ~ fs Spread < 1 meV for pA, but not realistic to get mA-currents Extraction field ~ E=1kV/mm –> focusing at 1 m to get 1 mV spread -> laser power -> atomic beam density

  14. Alternative photoelectron sources Alkali photocathodes (Cs3Sb) PEA GaAs/GaAsP photocathodes hIN (532 nm?) Le~100 nm Conduction band hIN (532 nm?) Conduction band cooler cooler Non-RR Surf.states Fermi level Valence band Valence band Vacuum Cs3Sb p-GaAs Vacuum Cs+O Short pulses: few ps Le~100 nm, Eg=… eV, ~10 % absorption (532 nm) QY =10-2 (532 nm) Energy spread > 100 meV (NEA), van be reduced but dropping QY (10-3 ?). BULK material provides NEA -> LIFETIME: should be much longer, high resistance to ion sputtering (to be studied) Short pulses: 1-0.1 ps QY =10-2 – 10-3 (10-100 times better than for metal) Energy spread ~ 50 meV (ћ depended) Lifetime: should be much better that for NEA, but has to be studied

  15. MPIK-K OUTLOOK Ultra cold beams from GaAs photocathodes, - 1 meV energy spread - few mA dc-beams Currents 10-100 mA, lifetime issue Alternative photocathode (resisted to ion sputtering) have to be studied (e.g. Cs3Sb)

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