Ion divergence and the stability of magnetically insulated diodes

Cold fluid equilibria for magnetically insulated ion diodes are unstable¹). The interaction of these unstable waves with electrons leads to loss above a threshold wave amplitude²). The waves also interact with ions producing both divergence and energy spread. The magnitude of these effects depends strongly on the diode equilibrium and the wave parameters. Since, previous stability analyses ignored the virtual cathode and the charge-neutral region, both important features of Applied-B ion diode equilibria³), we have investigated diode stability with these features included. The nature of ion diode instability is changed dramatically. Ultimately one would like to control ion divergence and energy spread, which depend on the wave amplitudes. We present test particle calculations of wave-electron interactions within the diode. Comparison of our results with 3-D PIC simulations⁴), indicate that wave amplitudes late in the simulations are just large enough to produce electron loss. This suggests that a reduction in ion divergence might result from decreasing the wave amplitude required to induce electron loss. This and other possible approaches for reducing ion divergence are discussed.