| Paper | Title | Page |
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| THPPH068 | The Development and Application of a Photoemission Model for Cesiated Photocathode Surfaces | 744 |
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A photoemitter capable of in situ rejuvenation with a high quantum efficiency (QE) is a photocathode candidate of FELs and linear accelerators. Our program to develop a controlled porosity dispenser (CPD) photocathode* led to the development and validation of a theoretical model that accounts for low work function surfaces from submonolayer coverage of alkali (and alkali earth) metals. Here, we substantially modify our previous theoretical thermal photoemission model** by: first, considering the impact of electron-electron and electron-lattice scattering; second, by the development of an integrated absorption-transport-emission model developed to evaluate the moments of the electron emission distribution function; and third, by the inclusion of quantum effects and surface features on the emission probability***. For the experimental conditions to which we compare the resultant theory, there are no adjustable parameters. We discuss the performance of the model in the prediction of the QE of bare metals and coated surfaces, project the performance to regimes of interest to FELs, and use elements of the model to assess theoretical quantities of interest such as emittance.
* N. A. Moody, et al. (this conf.); ** K. L. Jensen, et al., (to be published in JAP99); *** K. L. Jensen, et al., APL88, 154105 (2006), ibid, 164105. |
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| THPPH069 | Experimental Progress Toward Low Workfunction Controlled Porosity Dispenser Photocathodes | 748 |
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High efficiency, long-lived photocathodes are crucial for continued development of high power FELs. Most photocathodes with high quantum efficiency in the visible range suffer from short lifetime, due to chemical instability of the photosensitive surface layer that evaporates or is contaminated during operation in a RF-injector environment. We propose a controlled porosity dispenser (CPD) concept as a means of rejuvenating photosensitivity and performance. In many high-QE photocathodes, degradation over time is due to the loss of a surface coating of cesium. In CPD cathodes, cesium is replaced at near-room temperature via diffusion, maintaining high QE and extending effective lifetime. Measurements of increasingly complex photoemitting surfaces were performed, yielding QE as a function of cesium coating thickness, temperature, and photon wavelength. These measurements validate a photoemission model* that allows calculation of emission distribution, photocathode performance, and surface conditions of the cathode. Techniques for room-temperature cleaning of the CPD surface were determined and a prototype CPD photocathode based on cesiated sintered tungsten was evaluated.
* K. L. Jensen, et al., (this conference) |