Physics of high-intensity nanosecond electron source

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Abstract
A new high intensity, short time electron source is now being used at the Stanford Lineal Accelerator Center (SLAC). By using a GaAs negative affinity semiconductor in the construction of the cathode, it was possible to fulfill the operation requirements, such as peak currents of tens of amperes, peak widths of the order of nanoseconds, hundreds of hours of operation stability, and electron spin polarization. The cathode is illuminated with high intensity laser pulses, and photoemitted electrons constitute the yield. Because of the high currents, some non-linear effects are present. Very noticeable is the so called Charge Limit (CL) effect, which consist of a limit on the total charge in each pulse, that is, the total bunch charge stops increasing as the light pulse total energy increases. In this paper we will explain the mechanism of the CL and how it is caused by the photovoltaic effect. The treatment is based on the Three Step model of photoemission. We will relate the CL to the characteristics of the surface and bulk of the semiconductor, such as doping, band bending, surface vacuum level, and density of surface states. We also discuss possible ways to prevent the CL to take place.

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