Photoinjectors and Photocathode Activity at Daresbury Laboratory

Transcription

1 Photoinjectors and Photocathode Activity at Daresbury Laboratory Tim Noakes Accelerator Physics Group, ASTeC STFC Daresbury Laboratory

2 DL Accelerators CLARA Proposed FEL test facility VELA RF gun and electron beam exploitation facility ALICE ERL test facility

3 Momentum (MeV/c) VELA ALPHA-X Gun All Data Sets (crest) 7 6 y=(ax 0.5 ), std-err:0.189 a=1.78 Run 3 Random Data Run 2 Random Data Run 1 Random Data Run 1 Experiment 2 Run 1 Experiment cell S band gun donated by Strathclyde University! Generally working well but some potential issues missing power? Effective shunt impedance 23 M /m compensation of main solenoid field on cathode with the bucking coil (or additional source of coupling?) Forward - Reflected RF Power (MW) CST Microwave Studio modelling and RF measurements being carried out to try to understand what is happening

4 High Repetition Rate Gun 1.5-cell gun to operate at 100 MV/m at up to 400 Hz repetition rate. Photocathode exchange system to accept plugs of the INFN/DESY type An RF probe is included for active monitoring and feedback Symmetric H-shaped coupler for precise control of the phase of the incoming RF along both feeds. Gun is being manufactured by RI, expected delivery December 2015.

5 Cathode Transfer for HRRG HRRG designed with transferable cathode pucks (INFN design) INFN puck design Can test fully characterised thin film photocathodes in VELA/CLARA FE ESCALAB-II surface analysis system being modified INFN puck transfer system New preparation chamber for cathode growth

6 CLARA Front End ALPHA-X gun transferred to CLARA FE for commissioning HRRG commissioned in original VELA position Eventually guns swapped back over for 400 Hz operations on CLARA When 2 guns available, ~5MeV on VELA as at present will still be available! Spectrometer for CLARA and VELA gun characterisation To BA2 ~25 MeV To BA1 ~50 MeV ~15 MeV in VB mode

7 Photocathode Research Projects 5 inter-related projects currently underway! Project Material Aims Collaborations 1 Transverse energy spread spectrometer GaAs, other III-Vs, metals, alkali antimonides/ tellurides Precise transverse and longitudinal measurements of energy distribution and quantum efficiency at low energy ISP, Novosibirsk Liverpool University (CI) 2 Metal photocathodes Cu, other metals and metal alloys? Surface preparation, quantum efficiency measurements, lifetime and degradation mechanisms Loughborough University 3 Theoretical modelling of photocathodes Cu, other metals Understanding factors effecting cathode performance, screening new photocathode materials Imperial College 4 Characterisation of PEA photocathodes CsSb, CsTe Using TESS and multi-probe capability to characterise photocathodes grown at CERN CERN 5 Novel photocathode materials Iron oxides, Fe filled Carbon nanotubes, topological insulators Evaluate novel materials for photocathodes, including spin polarised sources Manchester University (CI)

8 Transverse Energy Spread Collaborations Spectrometer Institute of Semiconductor Physics, Novosibirsk Design of the apparatus, data processing, collaborative experiments Liverpool University Electrostatic modelling of the spectrometer, developing longitudinal measurements Experiments Characterisation of GaAs(Cs,O) photocathodes Controlled degradation experiments (under O exposure) Other III-V materials

9 GaAsP photocathodes GaAsP photocathodes typically achieve 2-3% QE (lower than GaAs) 3 Langmuir oxygen exposure does not completely kill QE QE seen to recover when left under vacuum Longitudinal data at 635 nm Transverse data still being processed! GaAsP significantly more robust than GaAs!

10 Metal Photocathodes Collaborations Loughborough University Equipment ESCALAB-II Multi-probe ESCALAB Mk.II UV laser Analyser XPS Gun Kelvin Probe Off-line near-field optical microscopy, electron microscopy X-ray photoelectron spectroscopy (XPS) surface chemistry Atomic force microscopy (AFM) topography Kelvin probe work function UV Laser and pico-ammeter QE

11 Bulk v. Thin Film Copper Thin film Cu deposition QE as received QE after treatment Bulk 5 x x 10-5 Thin Film 1.5 x x 10-4 CPS x 10 4 Thin film Cu annealed 250 C Bulk Cu sputter and annealed 250 C Cu 2 O Cu Untreated bulk and thin film have low QE Sputtering and annealing bulk sample gives nominally clean Cu with good QE Annealed thin film has even higher QE despite residual O at the surface (Cu 2 O film) 2 Thin film Cu Bulk Cu Binding Energy (ev) CuO CuO

12 Theoretical Modelling of Photocathode Materials Collaboration Imperial College Use density functional theory to model photocathode materials and explain performance Spicer three step model of photoemission Light adsorption/electron generation Electron transport to the surface Electron escape from the material Geometry Atom types IN DFT OUT Energy levels Optical matrix elements PDOS IN OptaDOS OUT QUANTUM EFFICIENCY ANGLE OF EMISSION SPECTRAL DISTRIBUTION

13 Adsorbates on Cu(111) Oxygen Hydrogen cu_111_6_li_all_other cu_111_6_h_all_other cu_111_6_o_all_other e (ev) e (ev) Lithium 0 e (ev) Clean M -10 M -10 M G M K -6 G M K G Effect of single monolayer of adsorbate evaluated Calculate band structure and spectral response Work function plays a large part but not the only factor! M K

14 Alkali Antimonides/Tellurides Collaboration CERN (CTF3 PHIN Gun team) Vacuum suitcase developed to transfer alkali antimonide/telluride samples from CERN to Daresbury Testing starts in January 2016! Vacuum suitcase TESS CERN photocathode puck Multi-probe

15 Novel Photocathode Materials Collaboration Manchester University Iron oxide thin films XPS of Fe filled CNTs Fe filled Carbon Nanotubes Topological Insulators Surface characterisation (XPS, AFM, etc) Spin-polarised measurements

16 Next Steps (1) TESS Low temperature measurements Developing white light source to extend the range of materials which can be studied (metals, CsTe, multi alkalis) Metals More work on thin films, single crystal (to compare with theory) Modifying the ESCALAB-II and using sample transfer system to prepare and test photocathodes in the high repetition rate gun on VELA/CLARA

17 Next Steps (2) Theory Stepped surfaces, partial coverages of adsorbates Extension of code to model transverse and longitudinal energy spread Move from explaining behaviour to designing photocathodes! Alkali antimonides/tellurides First experiments with CsSb samples grown at CERN Developing an alkali antimonide/telluride growth capability at Daresbury Novel photocathode materials Characterisation of Fe filled carbon nanotubes Further measurements on other materials including spin polarisation µ-mott polarimeter

18 Acknowledgements STFC - Boris Militsyn, Lee Jones, Ryan Beech (ASTeC, AP), Reza Valizadeh, Keith Middleman (ASTeC, VS), Mark Surman (ASTeC, MARS), Louise Cowie, Phillipe Goudket (ASTeC, RF), Tom Jones, Barry Fell, Ryan Cash (TD) ISP SB RAS, Novosibirsk - Alexander Terekhov, Heinrich Schiebler University of Liverpool - Carsten Welsch, Oleg Karamyshev, Lee Devlin Loughborough University - Mike Cropper, Sonal Mistry Imperial College - Nic Harrison, Bruno Camino CERN - Valentin Fedosseev, Eric Chevallay, Christoph Hessler, Irene Martini Manchester University - Elaine Seddon Lancaster University Graeme Burt