Theory of voltage decay in applied-B ion diodes

Summary Form only given, as follows. A recently proposed theory of applied-B ion diodes has proved very successful in predicting the diode voltage and current at peak power in several experiments and numerical simulations. For times after peak power, the basic theory is in good agreement with simulation but does not predict the experimentally observed voltage decay, suggesting a mechanism involving the anode plasma. A simple model of the flux penetration that consists of a constant density and velocity for the expanding anode layer, moderated by the erosion velocity of the ion beam, has been found to be in good agreement with a number of experiments for very reasonable layer densities and velocities. Examination of this system has revealed that, for a given layer density and expansion velocity, there exists a current density that maximizes the decay rate. Ion emission with a uniform current density that is less than the value that maximizes the decay rate has been shown to be unstable, leading to nonuniformities. Since the local variations in the ion emission result from nonuniformities of the flux penetration, this instability directly affects the ion beam focus.<>