1. Photocathode Research and Development at Daresbury Laboratory Tim Noakes 6 th June 2016

2. Motivation CLARA VELA ALICE Support for on- site accelerators Future UK facilities – UK FEL Participation in international projects- AWAKE, LHeC (no current involvement) Contributing to the International Effort in this area Conference contributions Research papers

3. Photocathode Properties and Materials ClassMaterialQEWaveleng th GunApplication Normally conducting metals Cu, Mg10 -5 -10 -4 UVNC-RFLow rep rate FELs (VELA, CLARA) Super- conducting metals Nb, Pb10 -5 -10 -4 UVSC-RFHigh rep rate FELs Positive electron affinity semiconductor CsTe, CsSb, etc0.1-0.2Visible - UV NC-RF, DCFELs, ERLs Negative electron affinity semiconductor GaAs, etc0.1-0.35IR- VisibleDC (XHV)Polarized sources, ERLs (ALICE) Interested in all materials, historical bias towards NEA semiconductors, but now increasingly focused on metals

4. Photocathode Research Projects Project (Equipment)MaterialAimsStatus 1Transverse and Longitudinal energy distribution measurement (TESS and PPF) GaAs, other III-Vs, metals, antimonides and tellurides Precise measurement of the quantum efficiency and energy distribution at low energy System operational, experiments underway 2Metal photocathodes (Multi-probe surface analysis system) Cu, other metals, single crystals, thin films, coatings Cathode preparation, quantum efficiency measurements, composition and topography measurements Commissioning complete, experiments underway 3VELA support/in- accelerator testing (ESCALABII) Cu, other metals, etc Surface preparation, quantum efficiency measurements, work function measurements Being converted for use with HRRG cathodes 4Alkali antimonides and tellurides Cs 3 Sb, Cs 2 TeCollaboration with CERN to analyse their samples using multi-probe and TESS equipment Underway 5Theoretical simulation of photocathode performance Cu, other metals and alloys, (semiconductors?) Understanding factors effecting cathode performance, screening new photocathode materials/ideas Project underway 6Novel Photocathode materials Carbon nanotubes, topological insulators, etc Horizon scanning of alternative materials for photocathodes in accelerator applications Preliminary studies initiated 7Cathode and optics laser induced damage experiment (COLIDE) Cu, other metals/materials Assessment of the effect of high power UV laser on the photocathode surface Being installed

5. Photocathode Preparation Facility (PPF) Originally designed as an ALICE upgrade Fast entry load lock system Hydrogen cleaning vessel Activation chamber, 6 sample carousel, Cs deposition and gas dosing (O, NF 3 ) XHV (10 -12 mbar) base pressure Precision leak valve for controlled degradation experiments Precise QE measurements Currently attached to TESS Allows entry of ex-situ grown samples Cleaning facilites still applicable (H cleaning, annealing) Extension of QE measurements to mapping and use of variable wavelengths

6. Transverse Energy Spread Spectrometer (TESS) Collaborations Institute of Semiconductor Physics, Novosibirsk – Design of the apparatus, data processing, collaborative experiments Liverpool University – Electrostatic modelling of the spectrometer, developing longitudinal measurements Particles approaching mesh with same initial energy Blue: Detected Red: Not detected Green: Particle collided with mesh Original (retarding field) technique - resolution ~ 4 V Modified (modulated laser) technique - resolution < 250 meV Comsol model of TESS structure used to calculated accurate field data

7. TESS – Progress and Next Steps Experiments Transverse and longitudinal measurements from GaAs Oxygen degradation studies GaAsP Next Steps White light source with monochromator to extend the range of materials Metals, particularly Cu Cs 3 Sb, Cs 2 Te, multi-alkalis GaN ? Low temperature measurements White light source with dual grating monchromator

8. Multi-probe facility for Metal Photocathode Research Built and commissioned as part of EuCARD2 X-ray Photoelectron Spectroscopy (XPS) Atomic Force Microscopy (AFM) Ultra-violet Photoemission Spectroscopy (UPS) Low Energy Electron Diffraction (LEED) Quantum Efficiency measurements UV laser Analyser X-ray gun AFM LEED Load lock

9. Multi-probe facility for Metal Photocathode Research Progress Survey of alternative bulk metals Evaluation of various cleaning procedures Further study of O plasma cleaning Growth of thin film samples and comparison with bulk Next Steps More thin film work Single crystal studies (to compare with theory) MgO and other overlayers Vacuum suitcase being commissioned to transfer samples between multi-probe and TESS 970965960955950945940935930925 Binding Energy (eV) x 10 4 0 2 4 6 8 10 CPS Bulk Cu Thin film Cu Bulk Cu sputter and annealed 250°C Thin film Cu annealed 250°C CuO Cu Cu 2 O

10. VELA Support/In-Accelerator Testing of Photocathodes ESCALAB Mk.II system High resolution XPS Low resolution Scanning Electron Microscope Quantum efficiency measurements Kelvin Probe Being modified to take High Repetition Rate Gun (HRRG) photocathode pucks New deposition chamber for thin film growth on VELA photocathodes Sample cleaning Magnetron sources INFN style ‘puck’

11. VELA Support/In-Accelerator Testing of Photocathodes Load lock system for HRRG also developed and being installed Progress Under installation Next Steps Testing of the transfer system Commissioning of HRRG Evaluation of alternative cathodes in VELA Possible Trans-national access to VELA via ARIES (EuCARD follow-on grant)? Transfer Arms High Repetition Rate Gun Vacuum Suitcase Heating Stage Gate valves

12. Alkali Antimonides/Tellurides Collaboration – CERN (CTF3 PHIN Gun team) Multi-probe TESS CERN photocathode ‘puck’ Sample transfer system developed to transport alkali antimonide/telluride samples from CERN to Daresbury TESS sample holder Multi-probe sample holder Based on existing sample transfer between TESS and Multi-probe

13. Alkali Antimonides/Tellurides – Progress and Next Steps Progress Transfer system designed and manufactured In-vacuum testing underway Next Steps Experiments on Cs 3 Sb, Cs 2 Te and multi-alkali photocathodes Analysis in multi-probe and TESS Development of an alkali antimonide/telluride growth capability at Daresbury Vacuum suitcase

14. 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 Energy levels Optical matrix elements PDOS OUT IN QUANTUM EFFICIENCY ANGLE OF EMISSION SPECTRAL DISTRIBUTION DFT OptaDOS IN OUT

15. Theoretical Modelling – Progress and Next Steps Progress QE determination for clean Cu surfaces and other metals Effect of adsorbates Next Steps Stepped surfaces, partial coverages of adsorbates Extension of code to model transverse and longitudinal energy spread Thin films, alloys, semiconducting overlayers Move from explaining behaviour to designing photocathodes! ħω θ e-e- e-e- e-e- e-e- z y x e-e- Φ l1l1 l2l2 l3l3 l4l4 l5l5

16. Novel Photocathode Materials Collaboration – Manchester University Fe filled Carbon Nanotubes Iron oxide thin films Topological Insulators Surface characterisation (XPS, AFM, etc) Quantum efficiency and spin polarisation measurements

17. Novel Photocathodes – Progress and Next Steps Progress Preliminary measurements of Fe filled carbon nanotubes Longer term Further measurements on other materials Spin polarisation measurements using Mott polarimeter µ-Mott polarimeter XPS of Fe filled CNTs

18. COLIDE Purpose built chamber to take high power UV from the VELA photoinjector laser Evaluation of damage on photocathodes and optics Cathode and Optics Laser Induced Damage Experiment (COLIDE)

19. COLIDE Progress Chamber designed, manufactured and being installed Next steps Experiments to evaluate the effect of laser parameters on cathode damage Ex-situ measurements with optical interferometry and scanning electron microscopy

20. Summary Several inter-related projects on photocathode research underway Main focus on metal photocathodes for use in VELA/CLARA accelerator Maintain/extend expertise on other popular photocathode materials (III-Vs, alkali antimonides/tellurides) ‘Horizon scanning’ of potential new materials using both simulation and experiment Impact Playing a key role in supporting existing and future accelerator projects in the UK (e.g. VELA/CLARA) Contributing to the large international effort on photocathode research

21. Photocathode Research and Development Team Accelerator Physics Group Vacuum Science Group Magnets and Radiation Sources Group Loughborough University Imperial College Tim Noakes Reza Valizadeh Boris Militsyn Lee Jones Elaine Seddon Keith Middleman Sonal Mistry Mark Surman Bruno Camino Mark Roper Technology Department Barry FellRyan Cash Ryan Beech