Investigations on the transition between column and micropinch mode of plasma focus operation

X-ray emission from pinch plasma devices with pinch currents ranging from 200 to 400 kA operated with pure high-Z gases are investigated with temporal spatial and spectral resolution. If operated using elements Z less than 18 (e.g., nitrogen or neon) K-shell emission is observed from column-like volumes several 100 micrometer in diameter and several mm in length (bulk or column mode). For Z greater than 18. (e.g., krypton or xenon) emission with hv grater than 1 keV is only observed from micropinches. For argon (Z = 18) both modes of operation can be observed. The occurrence of a specific mode depends on the initial gas pressure. In this paper the transition regime between column and micropinches is investigated with particular regard to argon. A criterion is proposed to decide whether column mode or micropinch mode is expected.