Commissioning of Photocathode Research Facilities at Daresbury Laboratory and First

ResultsDr Tim Noakes

Accelerator Physics Group, ASTeCDr Reza Valizadeh

Vacuum Science Group, ASTeC

Photocathode Research at STFC Daresbury Laboratory

Photocathode research dates back to commissioning of the first photoinjector on the ALICE accelerator in 2006

350 kV DC photoinjector using GaAs (Cs,O) photocathodes

More concerted effort to improve performance including development of PPF in 2009

ALICE (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 Area

2.5 cell RF gun with polycrystalline Cu photocathode

Experimental equipment for metal photocathode research being established!

Photocathode Research Equipment

Three systems currently in use for photocathode research

SAPI - 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 research

X-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 photodiode

AFM

LEED

X-ray Gun

Analyser

UHV Vacuum System

Commissioning this instrument is first milestone of sub-task 12.5.3 (RF photocathodes) for EuCARD2 project!

Commissioning Data

AFM images of polycrystalline Cu

Roughness and composition likely to be key parameters in determining photocathode performance

0

2000

4000

6000

8000

1000 800 600 400 200 0

O 1s

C 1s

Ag 4p

Ag 3p1/2

Ag 3p3/2

Ag (MNN)

Ag 3d5/2

Ag 3d3/2

Binding Energy (eV)

Co

un

ts

Wide Energy XPS scan of clean Ag

ESCALAB-II Instrument

Experiments on Cu and other metal photocathodes underway

• UHV modified ESCALAB-II instrument

• High resolution XPS• Low resolution Scanning

Electron Microscope• Quantum efficiency

measurements (UV LED and calibrated photodiode)

• Kelvin Probe (temporary)

Cleaning Cu for the VELA photocathode

Three treatments: Ar ion bombardment, O plasma cleaning, HCl etching

XPS Data

Sample Quantum Efficiency

As-loaded No photocurrent

Ar bombarded 4.5 x 10-5

O plasma treated No photocurrent

Plasma treated/annealed 2.0 x 10-5

HCl etched No photocurrent

O 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!)

Alternative Metal Photocathode Materials

Screen a selection of metals•Commonly used for photocathodes•Widely used in accelerators•Low work function

Measure XPS, QE and •As loaded•After Ar ion bombardment•After heating to 200OCAlso re-measure QE and after XPS (before heating)

XPS data for polycrystalline Ag

Preliminary Data

• Not all data collected yet (problems with heating and Kelvin probe equipment

XPS data for ion bombarded samples

• QE not always higher when drops• can change after XPS (with time!)

• Adsorption of residual gasses?• Morphological changes?

Theoretical Modelling of Photocathode Performance

ASTeC Photocathode Research Team, STFC Scientific Computing, Imperial College•Density Functional Theory modelling of the electronic band structure of Cu

• Use Spicer three step model to calculate photocathode properties

• Investigate effect of adsorbates• Extend to other metals/alloys

Further Work on Metal Photocathodes

• O Plasma cleaning of alternative metals

• Single crystal Cu (and other) photocathodes

• Thin film and metal alloy photocathodes

INFN puck design

• Modification of the ESCALAB-II to accept ‘INFN style’ photocathode pucks

• Transverse and longitudinal energy measurements from metal photocathodes

Transverse Energy Spread Spectrometer (TESS)

Spectrometer to make transverse and longitudinal energy measurements•Cryo-cooled photocathode holder•Retarding field analyser•Laser light source

Retarding fieldanalyser

Cryo-cooled photocathode mount

Laser path

High resolution camera

Commissioning Data

First images obtained from GaAs(Cs,O) photocathodes prepared in PPF ~2% Q.E.

60 V 230 V

• No voltage dependence – instrumental function very small

• Shorter wavelength light - electrons promoted from deeper within the valence band - higher Q.E., larger energy spreadWork presented at FEL’13 in

New York and ERL ‘13 in Novosibirsk

Metal Photocathodes on TESS•Sample transfer system between Multi-probe and PPF/TESS

•UV light source (266 nm Crylas Laser system)

Currently being commissioned on ESCALAB-II!

Attached to PPF

Vacuum suitcase

Magazine

Cut-through

Summary• Suite of equipment being developed at Daresbury

for the preparation and characterisation of metal photocathode materials

• Multi-probe (SAPI)• ESCALAB-II• TESS

• First results • Cleaning of Cu for use in the VELA RF gun• Screening of alternative metal photocathodes

• Future plans• Further work on metals (plasma cleaning, single

crystals, alloys)• Adapting the ESCALAB-II for use with INFN cathode

pucks• Transverse and longitudinal energy

spread measurements

Acknowledgements

ASTeCDeepa 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