EROSION AND EVAPORATION THEORY IN ION-FOCUSED ELECTRON-BEAM TRANSPORT

Three physical mechanisms that degrade the energy transport by electro n beams in the ion focused regime are examined. One of these, magnetic evaporation, affects the whole beam pulse and reduces the current pro pagated. It occurs when a transverse magnetic field enables beam elect rons with sufficient transverse energy to escape from the beam. A simp le model for this process is developed, and its predictions are shown to be closely matched by simulation results. The other two degrading m echanisms cause a steady erosion of the beam head, thereby shortening the pulse. When propagating along the plasma channel, beam electrons c ause electrostatic ejection of plasma channel electrons. The energetic cost of this process is the slowing of beam head electrons, which are eventually lost from the beam, i.e., eroded away. Inductive erosion i s the cylindrically symmetric aspect that takes place when there is no transverse magnetic field. An analytic model for this is developed an d compared with particle-in-cell code simulations and with experiments . A transverse magnetic field makes the electron displacement process asymmetric, with channel and beam electrons leaving on opposite sides of the channel. No reliable analytic model for this is available, so s imulation results are used to obtain an approximate dependence of eros ion rate on parameters. A simple argument is presented for dealing wit h the transition between inductive and magnetic erosion. The predictio ns correlate well with experimental results. (C) 1996 American Institu te of Physics.