Critical analysis of Plasma Focus design based on the implications of an upper pressure limit

This letter deals with the influence of the upper pressure limit beyond which the neutron yield drops even when the focus current increases with the deuterium filling pressure in Plasma Focus (PF) devices. It is shown that such cut-off in the neutron production is connected with the current sheath energy available for ionization of the neutral gas swept up during the radial compression stage. It is demonstrated that this limit can be prevented in a design that is optimized with respect to the focus current. The treatment includes all electrode geometries used in Z-pinch devices: the well-known Mather and Filippov PF gun types, thus, result, in this analysis, as a natural consequence of the two possible ways of optimizing a design with respect to the focus current, avoiding the upper pressure limit. A theory on origin and consequences of such a limit is presented and used to explain various phenomena observed in PF discharges.