The theory and simulation of relativistic electron beam transport in the ion-focused regime.

Several recent experiments performed at the University of Michigan and elsewhere have demonstrated that efficient transport of relativistic electron beams can occur in a narrow window of background gas pressures. This pressure window is known as the ion focused regime (IFR) and now provides an alternative to conventional transverse beam focusing methods using electrostatic and magnetostatic lenses. This dissertation presents a comprehensive analytical and computational treatment of ion focused electron beam transport. One of the problems with stable IFR beam transport in linear and recirculating accelerators is emittance growth from gas scattering. A simple model of emittance growth is developed which establishes a scale length for emittance growth. For transport experiments less than this scale length, scattering can be ignored and the analysis is greatly simplified. Under these conditions, IFR equilibria are studied using three separate analytical models: the beam envelope equation, relativistic Vlasov-Maxwell equations, and a test particle description. Separately, these three descriptions provide valuable information about transverse beam confinement; together they provide a very clear physical picture of IFR transport. It is found that, if the gas pressure is carefully controlled, ions can provide sufficient confinement to allow for stable beam transport on time scales less than the ion bounce time. On the other hand, a wide variety of resonant unstable behavior can exist in the beam-plasma channel. Under stable conditions, small signal theory is used to predict time scales for several of the instability mechanisms which are expected to be important in IFR channels. To substantiate the theoretical analysis and obtain deeper understanding of the beam-plasma channel, a computer simulation of IFR beam transport experiments was designed using a 2-D particle-in-cell code called MAGIC. The results of these simulations are useful in making qualitative comparisons between theory and experiments, and provides a new and interesting view of IFR beam transport.PhDNuclear EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104249/1/9034525.pdfDescription of 9034525.pdf : Restricted to UM users only