Commissioning of Photocathode Research Facilities at Daresbury Laboratory and First ResultsDr Tim NoakesAccelerator Physics Group, ASTeCDr Reza ValizadehVacuum Science Group, ASTeC
Photocathode Research at STFC Daresbury LaboratoryPhotocathode research dates back to commissioning of the first photoinjector on the ALICE accelerator in 2006350 kV DC photoinjector using GaAs (Cs,O) photocathodes More concerted effort to improve performance including development of PPF in 2009ALICE (and EMMA)Photocathode Preparation Facility (PPF)
Metal Photocathode ResearchInterest in metal photocathodes stems from the establishment of VELA (and in the future CLARA) at DL VELA Accelerator Area2.5 cell RF gun with polycrystalline Cu photocathodeExperimental equipment for metal photocathode research being established!
Photocathode Research EquipmentThree systems currently in use for photocathode researchSAPI - Surface Analysis/Preparation Installation (more usually called the Multi-probe!)ESCALAB-II Surface analysis facilityTESS Transverse Electron Energy Spread spectrometer
Multi-probe system for metal photocathode researchX-ray Photoelectron Spectroscopy (XPS) - Compositional and chemical analysisAtomic Force Microscopy (AFM) - Surface roughness evaluationLow Energy Electron Diffraction (LEED) Crystalline ordering (single crystal only)Kelvin Probe Contact potential difference work function measurementsQuantum Efficiency measurements UV LED light source and calibrated photodiodeCommissioning this instrument is first milestone of sub-task 12.5.3 (RF photocathodes) for EuCARD2 project!
Commissioning DataAFM images of polycrystalline CuRoughness and composition likely to be key parameters in determining photocathode performanceWide Energy XPS scan of clean Ag
ESCALAB-II InstrumentExperiments on Cu and other metal photocathodes underwayUHV modified ESCALAB-II instrumentHigh resolution XPSLow resolution Scanning Electron MicroscopeQuantum efficiency measurements (UV LED and calibrated photodiode)Kelvin Probe (temporary)
Cleaning Cu for the VELA photocathodeThree treatments: Ar ion bombardment, O plasma cleaning, HCl etchingXPS DataO plasma treatment provides thin protective oxide (can be transported through air)Annealing in-situ produces clean surfaceInitial Q.E. estimates for VELA ~2x10-5 (many assumptions made!)
SampleQuantum EfficiencyAs-loadedNo photocurrentAr bombarded4.5 x 10-5O plasma treatedNo photocurrentPlasma treated/annealed2.0 x 10-5HCl etchedNo photocurrent
Alternative Metal Photocathode MaterialsScreen a selection of metalsCommonly used for photocathodesWidely used in acceleratorsLow work function
Measure XPS, QE and As loadedAfter Ar ion bombardmentAfter heating to 200OCAlso re-measure QE and after XPS (before heating)XPS data for polycrystalline Ag
Preliminary DataNot all data collected yet (problems with heating and Kelvin probe equipmentXPS data for ion bombarded samplesQE not always higher when drops can change after XPS (with time!)Adsorption of residual gasses?Morphological changes?
Theoretical Modelling of Photocathode PerformanceASTeC Photocathode Research Team, STFC Scientific Computing, Imperial CollegeDensity Functional Theory modelling of the electronic band structure of CuUse Spicer three step model to calculate photocathode propertiesInvestigate effect of adsorbatesExtend to other metals/alloys
Further Work on Metal PhotocathodesO Plasma cleaning of alternative metalsSingle crystal Cu (and other) photocathodesThin film and metal alloy photocathodesModification of the ESCALAB-II to accept INFN style photocathode pucksTransverse and longitudinal energy measurements from metal photocathodes
Transverse Energy Spread Spectrometer (TESS)Spectrometer to make transverse and longitudinal energy measurementsCryo-cooled photocathode holderRetarding field analyserLaser light source
Commissioning DataFirst images obtained from GaAs(Cs,O) photocathodes prepared in PPF ~2% Q.E.No voltage dependence instrumental function very smallShorter wavelength light - electrons promoted from deeper within the valence band - higher Q.E., larger energy spread
Work presented at FEL13 in New York and ERL 13 in Novosibirsk
Metal Photocathodes on TESSSample transfer system between Multi-probe and PPF/TESS
UV light source (266 nm Crylas Laser system)
Currently being commissioned on ESCALAB-II!
Attached to PPFVacuum suitcaseMagazineCut-through
SummarySuite of equipment being developed at Daresbury for the preparation and characterisation of metal photocathode materialsMulti-probe (SAPI)ESCALAB-IITESSFirst results Cleaning of Cu for use in the VELA RF gunScreening of alternative metal photocathodesFuture plansFurther work on metals (plasma cleaning, single crystals, alloys)Adapting the ESCALAB-II for use with INFN cathode pucksTransverse and longitudinal energy spread measurements
AcknowledgementsASTeCDeepa Angal-Kalinin, Boris Militsyn, Lee Jones, Joe Herbert, Reza Valizadeh, Keith Middleman, Mark SurmanDaresbury Technology DepartmentRyan Cash and Barry FellLoughborough UniversityMike Cropper, Sonal MistrySTFC Scientific Computing DepartmentMartin Lueders, Barry SearleImperial CollegeNick Harrison, Bruno Camino