Theoretical understanding of ATA laser-guided transport

Many laser-guided transport experiments have been performed on the ATA accelerator. The results of these experiments are dominated by the observed head to tail pulse radius and emittance variation. The primary mechanisms responsible for this behavior are spatially dependent focusing (axial and transverse), time dependent focusing, ion production and dynamics, and secondary electron effects. These mechanisms have been studied in axisymmetric calculations and non- axisymmetric calculations. The axisymmetric calculations have obtained from the DPC, ENV and ST computer codes. The non-axisymmetric calculations have been obtained from the BENDER code. Laser-guided transport relies on the ionization of a background gas (benzene is used in ATA) by a laser, and the subsequent formation of an ion channel caused by the expulsion of the plasma electrons responding to the passage of a relativistic electron beam. The focusing behavior of the ion channel is complicated by the fact that the ions are mobile on the time scale of the electron beam pulse length. Because the ions are primarily moving radially as a result of the electron beam radial electric field, the response time scale decreases as the beam current increases. Experiments have shown a current threshold for undesirable beam radius increase. To assist inmore » understanding this behavior, in addition to numerical calculations the secondary electron effects and basic ion radial oscillation can be studied with approximate analytic models. The main items of interest are the time scale of the first half cycle of radial ion oscillation and whether or not the secondary electrons are expelled, as intended. 4 figs.« less