Investigation of initial stage of hybrid X pinches

Abstract

Summary form only given. The initial stage a hybrid X pinch (HXP) plasma formation has been studied using laser probing x-ray radiography and electrical measurements. This stage is especially interesting in HXPs in comparison with standard X-pinches because of the strong influence of the electrode plasmas on the process of neck development. Electrode material evaporated by strong UV radiation from the exploding wire can shorten the electrode gap before hot spot formation. The interaction of electrodes plasmas and the exploding wire plasma determines the discharge parameters required for proper HXP operation. The experiments in this work were performed on different pulsers with output current from 4 kA to 1.2 MA and current rise time from 50 ns to 340 ns. The low inductance super small pulse generator Micro based on ceramic capacitors and a flashover vacuum optically triggered switch was specially designed to study the initial phase of the HXP. This pulser with a peak current of 5 kA and current rise rate 100 A/ns reproduced very well the processes in HXPs on more powerful devices in first tenths nanoseconds of the discharge and enables us to study the details of plasma formation without powering big machines. The results obtained on the Micro pulser were compared with the results obtained in experiments on our older pulsers GVP (10 kA, 350 ns) and BIN (250 kA, 100 ns) at the Lebedev Institute, and XP (400 kA, 100 ns) and COBRA (1 MA, 100 ns) at Cornell University. It was shown that in HXPs from materials with low melting temperature and high core expansion rate (Al, Cu, Ag, Au) the wire core expands and fills inter-electrode gap and prevents fast diode shortening by the electrode plasma. That makes possible using relatively long pulse drivers for HXP.