Non-Adiabatic Ions in the Distribution Function from Self-Consistent Calculations of the Plasma Focus

A three-dimensional simulation in phase space of the ions in the plasma focus has demonstrated that in some modes of operation of the experiment, the ion distribution function has two components. Most of the ions are in a quasi-thermal equilibrium, but a small component of the distribution function is accelerated to high energies in agreement with experimental results to be reported at this conference by Hobby, Morgan, Peacock. The interaction of this high energy component with the background ions yields a high neutron emission and, in particular, gives rise to an anisotropy in the energies of the neutron emission. Previous two-dimensional magnetohydrodynamic simulations of the dense plasma focus have in many respects agreed quantitatively with experiment. The MHD-simulations, however, while describing a thermonuclear neutron emission, have not been able to account for the experimentally observed anisotropic neutron emission. The numerical experiments of a ''magnetohydrodynamic focus'' have shown that, at high bank energies and lower densities, the ions become collisionless in the plasma focus. This paper therefore reports the simulation of 20 000 ion particles, which interact in the self-consistent electromagnetic field of an electron fluid. The particles have co-ordinates of radius, radial velocity, and axial velocity. The singular ions are produced by non-adiabatic motion of ions with large Larmor radius about the fieldfree axis of the pinch. While these ions produce an anisotropic neutron emission, they effectively remove the high-energy tail of a Maxwellian distribution, and thereby limit a larger thermonuclear emission from the dense plasma focus. To minimize this effect, the escape of high-energy singular ions must be halted by ion-ion collisions. Hence, as the bank energy of the plasma focus is increased, the density of the plasma focus must necessarily be increased further. Thus on the basis of the simulations reported in this paper, scaling laws of the optimized neutron yield for the plasma focus to bank energies of a megajoule have been determined. Further applications of these computer simulations of non-adiabatic self-consistent ion motions include studies of the tail of the magnetosphere and reversed fields in the theta-pinch. (author)