Theory of collective ion acceleration in the luce diode

The most successful collective ion acceleration to date has been observed when a relativistic electron beam has been discharged in the Luce diode (dielectric anode) geometry. In these experiments, the ratio of peak ion energy to beam kinetic energy has been as high as 30. The ion source is a plasma formed on the anode surface by electron impact ionization of the dielectric material.. The diode physics, which couples the electron beam generation to the number of ions extracted from the plasma, is an essential feature of the acceleration mechanism in the Luce diode. Two dimensional electromagnetic particle-in-cell simulations which are based on the Cornell University Luce diode experiments have been carried out. The simulations include an exact model of the electron beam generator1s diode and the drift space where the collective ion acceleration takes place. Ion acceleration is clearly due to trapping in a train of traveling waves. The source of the waves appears to be an ion-electron two stream instability. The instability growth rate, saturation length and coincident microwave production are consistent with experimental results.