Theory of electron and ion flow in relativistic diodes

The time dependent evolution of electron and ion flow in large aspect-ratio relativistic diodes is studied by analytic models and computer simulation. Four distinct phases are considered: (1) Pure electron flow at low voltages, (2) Weak pinching at large voltages, (3) Collapsing pinch due to time dependent ion emission from anode plasma and (4) Steady-state pinched electron flow and laminar ion flow. The physics incorporated in a new, time dependent, simulation code is outlined and the simulation results are compared to the analytic predictions. In particular, we find that in phase (4) the ion current may be larger than the electron current and present day accelerators may thus produce mega-ampere ion currents.