Collective ion acceleration controlled by a gas gradient

Several schemes were studied for attaining collective ion acceleration by the injection of an intense relativistic electron beam along magnetic field lines into a low pressure (less than 1 Torr of helium) neutral gas. Positive, zero, and negative gas pressure gradient schemes were examined. A detailed study is presented of the case where the gas pressure decreases from the beam injection plane (the negative gradient scheme). In this case, as many as 1011 14 MeV doubly ionized helium ions were observed. A 1 MeV, 70 kA, 100 ns duration electron beam was used in conjunction with an applied magnetic field (with field strength up to 7 kG). A simple collective ion acceleration model is presented as part of this study. This model was originally proposed by Ryutov in order to explain ion acceleration in reflexing diode systems. The model is modified here to account for the electron energy distribution under the conditions where the ion energy is a maximum. The modified model is then used to predict the ion energy distribution. Good agreement has been obtained between the experimental results and the theoretical predictions. In addition, a particle code was developed to enable a study of the complete, self-consistent, initial condition problem. Preliminary results are presented which indicate a stagnating effect in the ion acceleration process