A photoemission model for low work function coated metal surfaces and its experimental validation

Jensen, Kevin L.; Feldman, Donald W.; Moody, Nathan A.; O’Shea, Patrick G.

doi:doi:10.1063/1.2203720

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Photocathodes are a critical component many linear accelerator based light sources. The development of a custom-engineered photocathode based on low work function coatings requires an experimentally validated photoemission model that accounts the complexity of the emission process. We have developed a time-dependent model accounting for the effects of laser heating and thermal propagation on photoemission. It accounts for surface conditions (coating, field enhancement, and reflectivity), laser parameters (duration, intensity, and wavelength), and material characteristics (reflectivity, laser penetration depth, and scattering rates) to predict current distribution and quantum efficiency (QE) as a function of wavelength. The model is validated by (i) experimental measurements of the QE of cesiated surfaces, (ii) the QE and performance of commercial dispenser cathodes (B, M, and scandate), and (iii) comparison to QE values reported in the literature for bare metals and B-type dispenser cathodes, all for various wavelengths. Of particular note is that the highest QE for a commercial (M-type) dispenser cathode found here was measured to be 0.22% at 266 nm, and is projected to be 3.5 times larger for a 5 ps pulse delivering 0.6 mJ/cm² under a 50 MV/m field.

Article Outline

INTRODUCTION
EMITTED CURRENT AND QUANTUM EFFICIENCY
1. The modified Fowler-Dubridge photocurrent model
2. Thermal emission current
3. The evaluation of quantum efficiency
TIME-DEPENDENT ELECTRON AND LATTICE TEMPERATURE EVALUATION
1. The heat diffusion equations
2. Absorbed laser power
3. Electron and lattice specific heat
4. Scattering rates and thermal conductivity
5. Numerical solution of thermal equations
6. Quantum efficiency and bare metals
7. Macro-time-scales and multiple pulses
WORK FUNCTION VARIATION AND SURFACE COVERAGE EFFECTS
1. Macroscopic versus microscopic variation: The patch model
2. Modified Gyftopolous-Levine model
3. Comparison of modified GL to experiment of cesium-coated tungsten
4. Experimental setup to measure QE of Cs on W
5. Comparison of modified GL theory to experimental data
DISPENSER CATHODES
1. Field and intensity variation
2. The quantum efficiency of dispenser cathodes
CONCLUSION

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KEYWORDS and PACS

Keywords

photoemission, work function, photocathodes, coatings, reflectivity, current distribution, laser materials processing

PACS

79.60.Bm

Clean metal, semiconductor, and insulator surfaces
85.60.Ha

Photomultipliers; phototubes and photocathodes
73.30.+y

Surface double layers, Schottky barriers, and work functions
81.65.-b

Surface treatments
42.62.-b

Laser applications

ARTICLE DATA

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http://link.aip.org/link/doi/10.1063/1.2203720

PUBLICATION DATA

ISSN:

0021-8979 (print)

1089-7550 (online)

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American Institute of Physics

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