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Graduate Student Seminar - 09/24/2010

"Dispenser Photocathodes for Free Electron Lasers:  Let's Get Fresh!"

by Zhigang Pan

Friday, September 24, 2010 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisor:  Professor Patrick O'Shea

Photocathodes, as electron sources for high power free electron lasers (FELs), emit electrons through the photoelectric effect in response to an incident laser pulse. High power FELs demand a bright, high current electron beam so the fraction of photons which result in an emitted electron -- the quantum efficiency, or QE -- must be high. However, high QE can come with the price of short lifetime because it relies on delicate surface coatings despite demanding operational requirements, cathode heating, and contamination. We seek to increase the lifetime of coated photocathodes by controllably dispensing a fresh surface coating in situ -- this is the UMD dispenser photocathode concept. UMD has already demonstrated extended lifetime of a cesiated metal photocathode; our present research involves the study of contamination effects and more complex high QE coatings. In this talk we will begin with dispenser photocathode operation and the effect of alkali metal coatings on the QE of a metal photocathode. We will next present results of contamination causing coating degradation and conclude with a view towards the fabrication and testing of very high QE, alkali metal-based, semiconductor cathodes which hold much promise if their lifetime can be extended.

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