Theory of instability-generated divergence of intense ion beams from applied-B ion diodes

Recent advances in the analytic theory and numerical simulation of applied-B ion diodes in general and, in particular, the diodes being tested experimentally on the Particle Beam Fusion Accelerator II at Sandia National Laboratories, Albuquerque, NM, are described. Numerical simulation of these diodes using the three-dimensional particle-in-cell code QUICKSILVER and analytic stability analysis have provided new insight into the physics governing instability-induced beam divergence. Experimental evidence of these instabilities is described and compared with the theory. When simulation parameters are chosen to model present experiments, QUICKSILVER calculates beam divergence near the measured value. A new understanding of the nature of the electromagnetic instabilities and their effects on beam divergence has led to suggestions for improved diode design. A simulation of one of these design concepts shows divergence levels in the vicinity of 10 mrad, a level considered adequate for ignition experiments