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)

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Overview of presentation

2

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

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

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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

 

Ener

gy

Metal Vacuum

e-

hν

EF

EVAC

ɸ

Ephoton = hν

Approach

• Investigate differences between single and polycrystalline samples

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

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Measurements and Techniques

Photoelectron

   

   

   

       

1s

2s

2p

X-ray

 

X-ray Photoelectron Spectroscopy

Ek = hν – Eb – ɸs

Kinetic Energy X-Ray

EnergyBinding Energy

Spectrometer work function

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eVCPD= φprobe - φsample

Kelvin Probe: work function

Sample

Probe

Vs

+++++++++++++++

- - - - - - - - -

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.

hν

e-

e-

e-e-e-

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Ener

gy

Metal Vacuum

e-

hν

EF

EVAC

ɸ

Ephoton = hν

• UV LASER source which offers higher intensity at 266 nm

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

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Procedure: Routine XPS, QE and measurements are:•As received•After argon ion bombardment

•After oxygen plasma treatment•After annealing

Metal QE XPS (%)O 1s C 1s

Measured (eV)

Al Received Ar+ sputter

9.5E-62.2E-5

36.8 38.313.4 17.4

4.0224.914

Ag Received Ar+ sputter

8.5E-65.1E-5

0 59.4 0 0

5.0885.111

Cu Received Ar+ sputter

5.0E-61.1E-5

32.9 66.2 0 0

5.3535.316

Mg Received Ar+ sputter

6.0E-61.7E-3

35.2 52.340.0 0

3.3903.372

Mo Received Ar+ sputter

1.47E-72.48E-6

24.2 64.97.8 17.8

5.0715.178

Results: Bulk metals Ar+ sputterMetal QE XPS (%)

O 1s C 1sMeasured (eV)

Nb Received Ar+ sputter

3.9E-71.9E-4

47.8 48.616.2 21.0

5.3014.713

Pb Received Ar+ sputter

2.9E-52.4E-4

43.9 34.8 0 0

4.5734.660

Ti Received Ar+ sputter

03.3E-4

39.2 53.614.6 16.8

4.7464.467

V Received Ar+ sputter

1.4E-62.2E-5

45.7 45.925.0 0

5.5064.999

Zr Received Ar+ sputter

3.88E-62.89E-4

48.4 44.114.4 0

4.4344.263

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Metal QE XPS (%)O 1s C 1s

(eV)

Cu O2 plasma Anneal 250°C (0.5 hr)

01.6E-4

80.1 3.176.7 6.5

5.6535.716

Nb O2 plasma Anneal 300°C (0.5 hr)

5.21E-71.34E-4

87.5 5.7 80.2 6.3

5.7124.510

Pb O2 plasma Anneal 160°C (0.5 hr) Anneal 200°C (0.5 hr)

3.47E-76.94E-61.67E-5

82.1 7.177.9 10.977.8 9.9

5.6164.3004.473

Metal QE XPS (%)O 1s C 1s

(eV)

Ti O2 plasma Anneal 250°C (0.5 hr) Anneal 250C (24 hr)

06.32E-51.16E-4

87.1 5.188.3 4.579.9 9.0

5.7744.4994.268

Zr O2 plasma Anneal 250°C (0.5 hr) Anneal 250°C (24 hr)

3.82E-76.94E-51.35E-4

78.4 12.483.5 3.974.6 9.8

4.9414.2634.797

Mg O2 plasma Anneal 200°C (0.5 hr) Anneal 200°C (4 hr) Anneal 200°C (24 hr)

3.82E-72.40E-54.90E-57.09E-5

83.5 3.276.8 3.567.8 9.466.7 3.8

4.3963.8693.7373.627

Measurements for Ti, Zr, Mg, Cu, Nb, Pb:O2 Plasma Cleaned for 20 minutes Annealed

Results: Bulk metal O2 plasma cleaned

Metal thin film QE XPS (%)O 1s C 1s

Measured (eV)

Cu Received Heated 250°C Repeat

1.47E-61.14E-41.17E-4

23.5 67.820.2 61.717.9 62.0

5.14354.9144.68

Nb Received Heated 250°C Heated 300°C Ar+ sputter

7.75E-72.45E-55.66E-62.64E-4

63.1 25.361.5 4.355.8 15.99.6 0

4.35334.88595.14014.7703

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

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Copper thin film compared with bulk sample (½ hour heating at 250°C)

Wide scan XPS Bulk (%) Thin film(%)O 1s 6.1 17.943C 1s none 62.03Cu 2p 93.9 20.027

x 104

2

4

6

8

10

12

14

CPS

1000 800 600 400 200 0

Binding Energy (eV)

Cu 2p1/2

Cu 2p3/2

Auger Lines

Work function (eV)

QE

Bulk 5.234 1.70E-5

Thin Film 4.68 1.17E-4

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x 103

2

4

6

8

10

12

14

16

18

20

CPS

600 500 400 300 200 100 0Binding Energy (eV)

XPS Bulk (%) Thin film(%)O 1s 16.24 9.57C 1s 20.98 noneNb 3p 62.78 90.43

Niobium thin film compared with bulk sample (Ar+ sputter)

Wide scan

O 1s

Nb 3d5/2

Nb 3d3/2

Nb 3p3/2

Nb 3p1/2

Nb 3s

Work function (eV)

QE

Bulk 4.713 1.9E-4

Thin Film 4.7703 2.64E-4

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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)

• O2 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

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• Further work will focus on:

• the use of plasma cleaning

• thin film preparation

• photocathode degradation

Thank you for your attention

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