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abstract:
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There has been a long-standing question about the amount of charge extracted from the UCLA photo-injector as a function of incident laser energy (quantum efficiency). One expects the charge measured at the exit of the rf gun to be a linear function of applied laser energy to the photocathode. However, this is not the case fo the UCLA experiment thus far. However, the quantum efficiency data which measures the total extracted charge out of the rf gun as a function of laser energy, is very non linear. From zero nC to approximately 0.5nC the charge extracted follows a linar trend. From 0.5nC to 3nC the extracted charge falls off unexpectedly. Because of the highly dynamical behaviour of the electron beam near the cathode, a series of PARMELA simulations have been carried out to model the evolution of the beam in this region. These simulations give excellent agreement with the experimental data. Also, a simple model is put forth to explain the physical process which limits the emission of charge.
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