Temporal and spatial evolution of carbon ion beam emission in plasma focus discharges

Abstract

Summary form only given. In plasma focus (PF) a high density, high temperature, short duration Z-pinch like plasma column is formed, following a radial compression phase. The PF is a well known source of energetic ion beams, of characteristic energy from hundreds of keV to tens of MeV. Several theoretical and computational models have been developed in an attempt to explain the ion production and acceleration mechanism in PF discharges. Despite this theoretical effort, a complete explanation of the ion emission mechanism is still not available. We have performed a detailed investigation on the time evolution and angular distribution of PF ion beams. We have previously characterized the main features of ion beam emission in a low energy, 1.8 kJ, 160 kA Mather type PF device, operating in methane. In the present investigation axial and radial biased Faraday cup ion probes, which are also sensitive to XUV radiation, were used simultaneously in conjunction with a radial X-ray detector to correlate the ion beam emission with the different stages in the PF dynamics. We have observed that the ion beams emitted radially are mostly associated with the radial compression phase of the PF discharge, prior to the formation of a Z-pinch like plasma column. At this stage the ion beams composition is dominated by C+2 ions. At later times, around the maximum compression phase, the axial emission becomes dominant and the ion beam composition is found to be mainly C+4 and C+5 ions. In general, the axial ion beam emission is found to be of much higher characteristic energy and flux than the radial emission. These results indicate that no unique mechanism for ion beam emission in PF can be assumed. Based on the time correlations and measured properties of the ion beams, the plasma conditions at the time of the different stages in the ion emission will be discussed, in order to assess the validity of current PF ion beam emission models, which account for different properties of the beams, such as characteristic energy scaling and angular distribution.