Acceleration and trapping of fast ions in self-organized magneto-plasma structures in the dense plasma focus

Abstract

Recent research at the PF-1000 Dense Plasma Focus facility strongly suggests that the early part of neutron emission is caused by fast deuterons with energy on the order of ∼100 keV, having approximately equal axial and radial velocity, temporally coinciding with the occurrence of self-organized, bounded magneto-plasma structures, which remain trapped within the reaction zone for tens of transit times. The experimental evidence, predominantly qualitative in nature, does not clarify the nature and origin of the accelerating electric field responsible for high ion energy and of the magnetic field that might be confining the ions to the reaction zone except for the suggestion that they have toroidal and poloidal magnetic field components whose presence is revealed by magnetic probes. Current theories, conjectures, and models of plasma focus find it difficult to accommodate three-dimensional features of ion motion and magnetic field revealed by multiple experiments within their scope. This paper revisits the relevant experimental evidence and introduces a model that is deliberately non-quantitative in order to accommodate the qualitative nature of the available experimental evidence. The model leads to a functional form for the 3-dimensional distribution of magnetic field associated with the spontaneously self-organized magneto-plasma structures. This enables the discussion of properties of 3-dimensional trajectories of ions accelerated by electric fields induced during their growth. Many qualitative observations about the nature of neutron emission in Dense Plasma Focus and the observed phenomenology of plasma evolution can be understood in terms of this model in a unified manner. The model also helps conceive a new generation of diagnostic schemes targeted at getting quantitative information that is out of reach of currently available diagnostics.