Transient Hollow Cathode Effects and Z pinch Formation in a High Current Capillary Discharge with a Metal Plasma
The results from a series of experiments are presented whose purpose is to explore different schemes which may lead to the formation of pure metal plasmas in a capillary discharge with parameters appropriate for X ray lasing. The experiments were performed in ceramic wall capillary discharges at currents of up to 120 kA, with an available ID of between 3 and 8 mm and with lengths from 60 to 100 mm. Initial plasma conditions in the capillary exploit transient hollow cathode effects in a preionizing discharge. A laser focussed onto the back surface of the cathode initiates both beam activity in the capillary volume and plasma injection. To promote metal ablation into the pinch channel of elements other than the ceramic wall material, a number of graded ring schemes have been tried. The plasma is observed axially using both time and energy resolved soft X-ray pin hole images as well as from time resolved soft X-ray spectra. By varying the rate of rise of the current of the main discharge, and the preionizing conditions the diameter and the stability of the Z-pinch column are seen to be affected. The ratio of the species from the ablated wall material to the plasma formed from the graded ring structure is found to depend both on the capillary dimensions as well as the other operating conditions.