1. Metal photocathodes for NCRF electron guns Sonal Mistry Loughborough University Supervisor: Michael Cropper (Loughborough University) Industrial Supervisor: Reza Valizadeh (ASTeC, STFC, Daresbury Laboratory) Boris Militsyn (ASTeC, STFC, Daresbury Laboratory) 1

2. Overview of presentation Research aims and motivation Photocathode properties Approach taken Measurements and techniques Results and conclusions 2

3. Research Aims Investigate metallic photocathodes for use in electron guns Study the dependence of quantum efficiency and energy spread of emitted electrons as a function of surface preparation and contamination during photocathode operational lifetime 3

4. Motivations for Research Interest in investigating alternative metal to copper with better properties Investigate applicability of metals to deliver ultra high beams VELA VELA (Versatile electron linear accelerator) Photoinjector consists of 2.5 cell S-band RF gun Cu photocathodes integrated onto copper cavity 4

5. Principal Photocathode Properties Quantum Efficiency (QE): Fraction of electrons emitted from the surface per incident photon Intrinsic Emittance: Angle of divergence of generated electrons Response Time: Time taken between absorption of light pulse and photoemission Lifetime of photocathode: Time taken for the QE to fall to 1/e of its initial value 5 Energy Metal Vacuum e-e- hνhν EFEF E VAC ɸ E photon = hν

6. Approach Study range of metals Study a range of bulk metal samples to identify candidate materials for photocathode Optimise surface preparation New methods and procedures will be devised for surface preparation and to control the influence of surface composition Investigate thin film photocathodes Compare and evaluate thin films and bulk metals Single and polycrystalline photocathodes Investigate differences between single and polycrystalline samples 7

7. Principal Equipment ESCALAB-II instrument used for: 1) Preparation of photocathodes Ion bombardment Annealing 2) Analysis of photocathodes High resolution XPS QE measurements Work function measurements 8

8. Measurements and Techniques Photoelectron 1s 2s 2p X-ray X-ray Photoelectron Spectroscopy E k = hν – E b – ɸ s Kinetic Energy X-Ray Energy Binding Energy Spectrometer work function 9 eV CPD = φ probe - φ sample Kelvin Probe: work function Sample Probe VsVs +++++++++++++++ - - - - - - - - -

9. QE measured using UV LED/ UV LASER QE measurements comprise a LED source- 265 nm (4.65 eV) light and a pico-ammeter to measure photocurrent. photodiode used to measure LED power QE suggests how much current can be extracted from a cathode and as such is an indication of the potential beam current. hνhν e-e- e-e- e-e- e-e- e-e- 11 Energy Metal Vacuum e-e- hνhν EFEF E VAC ɸ E photon = hν UV LASER source which offers higher intensity at 266 nm

10. A survey of a selection of bulk metals, chosen because: Widely used in accelerators Low  Experimental Procedure Zirconium Vanadium Titanium Lead Niobium Magnesium Aluminium Silver 12 Procedure: Routine XPS, QE and  measurements are: As received After argon ion bombardment After oxygen plasma treatment After annealing

11. MetalQEXPS (%) O 1s C 1s Measured  (eV) Al Received Ar + sputter 9.5E-6 2.2E-5 36.8 38.3 13.4 17.4 4.022 4.914 Ag Received Ar + sputter 8.5E-6 5.1E-5 0 59.4 0 0 5.088 5.111 Cu Received Ar + sputter 5.0E-6 1.1E-5 32.9 66.2 0 0 5.353 5.316 Mg Received Ar + sputter 6.0E-6 1.7E-3 35.2 52.3 40.0 0 3.390 3.372 Mo Received Ar + sputter 1.47E-7 2.48E-6 24.2 64.9 7.8 17.8 5.071 5.178 Results: Bulk metals Ar + sputter MetalQEXPS (%) O 1s C 1s Measured  (eV) Nb Received Ar + sputter 3.9E-7 1.9E-4 47.8 48.6 16.2 21.0 5.301 4.713 Pb Received Ar + sputter 2.9E-5 2.4E-4 43.9 34.8 0 0 4.573 4.660 Ti Received Ar + sputter 0 3.3E-4 39.2 53.6 14.6 16.8 4.746 4.467 V Received Ar + sputter 1.4E-6 2.2E-5 45.7 45.9 25.0 0 5.506 4.999 Zr Received Ar + sputter 3.88E-6 2.89E-4 48.4 44.1 14.4 0 4.434 4.263 15

12. MetalQEXPS (%) O 1s C 1s  (eV) Cu O 2 plasma Anneal 250°C (0.5 hr) 0 1.6E-4 80.1 3.1 76.7 6.5 5.653 5.716 Nb O 2 plasma Anneal 300°C (0.5 hr) 5.21E-7 1.34E-4 87.5 5.7 80.2 6.3 5.712 4.510 Pb O 2 plasma Anneal 160°C (0.5 hr) Anneal 200°C (0.5 hr) 3.47E-7 6.94E-6 1.67E-5 82.1 7.1 77.9 10.9 77.8 9.9 5.616 4.300 4.473 MetalQEXPS (%) O 1s C 1s  (eV) Ti O 2 plasma Anneal 250°C (0.5 hr) Anneal 250C (24 hr) 0 6.32E-5 1.16E-4 87.1 5.1 88.3 4.5 79.9 9.0 5.774 4.499 4.268 Zr O 2 plasma Anneal 250°C (0.5 hr) Anneal 250°C (24 hr) 3.82E-7 6.94E-5 1.35E-4 78.4 12.4 83.5 3.9 74.6 9.8 4.941 4.263 4.797 Mg O 2 plasma Anneal 200°C (0.5 hr) Anneal 200°C (4 hr) Anneal 200°C (24 hr) 3.82E-7 2.40E-5 4.90E-5 7.09E-5 83.5 3.2 76.8 3.5 67.8 9.4 66.7 3.8 4.396 3.869 3.737 3.627 Measurements for Ti, Zr, Mg, Cu, Nb, Pb:  O 2 Plasma Cleaned for 20 minutes  Annealed Results: Bulk metal O 2 plasma cleaned

13. Metal thin filmQEXPS (%) O 1s C 1s Measured  (eV) Cu Received Heated 250°C Repeat 1.47E-6 1.14E-4 1.17E-4 23.5 67.8 20.2 61.7 17.9 62.0 5.1435 4.914 4.68 Nb Received Heated 250°C Heated 300°C Ar + sputter 7.75E-7 2.45E-5 5.66E-6 2.64E-4 63.1 25.3 61.5 4.3 55.8 15.9 9.6 0 4.3533 4.8859 5.1401 4.7703 Results: Thin film metals Metal thin films deposited on silicon substrate by magnetron sputtering So far only Cu and Nb thin films have been produced 16

14. Copper thin film compared with bulk sample (½ hour heating at 250°C) Wide scan XPS Bulk (%)Thin film(%) O 1s6.117.943 C 1snone62.03 Cu 2p93.920.027 x 10 4 2 4 6 8 10 12 14 CPS 10008006004002000 Binding Energy (eV) Cu 2p 1/2 Cu 2p 3/2 Auger Lines Work function (eV) QE Bulk5.2341.70E-5 Thin Film4.681.17E-4 17

15. XPS Bulk (%)Thin film(%) O 1s16.249.57 C 1s20.98none Nb 3p62.7890.43 Niobium thin film compared with bulk sample (Ar + sputter) Wide scan O 1s Nb 3d 5/2 Nb 3d 3/2 Nb 3p 3/2 Nb 3p 1/2 Nb 3s Work function (eV) QE Bulk4.7131.9E-4 Thin Film4.77032.64E-4 18

16. Summary and Future Plans Most favourable bulk material in terms of QE is Mg Other materials that exhibit improved QEs are Pb, Ti, Nb and Zr Ion bombardment leaves rough surfaces ( detrimental to emittance) O 2 plasma cleaning and post annealing is effective Comparable QE results for Ti, Zr, Cu and Nb. Preliminary data for thin films yields better QE than bulk metals 19 Further work will focus on: the use of plasma cleaning thin film preparation photocathode degradation

17. Thank you for your attention 20