Wire core and coronal plasma expansion in wire-array Z pinches with small numbers of wires

Abstract

Wire core and coronal plasma formation and expansion in wire-array Z pinches with small numbers of wires have been studied on a 1 MA, 100 ns rise time pulsed power generator and a 500 kA, 50 ns generator. Two-frame point-projection x-ray imaging and three-frame laser optical imaging and interferometry were the principal diagnostic methods used for these studies. The x-ray images show that dense coronal plasma forms and is maintained close to each dense wire core in the array. A less dense, rapidly expanding ͑ϳ10 m/ns͒ coronal plasma, best seen in the laser images, surrounds the ϳ100 m radius dense corona. These results are in agreement with computer simulations and modeling carried out by Yu et al. ͓Phys. Plasmas 14, 022705 ͑2007͔͒. Results are also presented for the dependence of the wire core and coronal plasma expansion rates on the wire diameter, number of wires and current through individual wires and the overall configuration for Al, Cu, and W wire arrays. For example, the W wire dense core expansion rate increases with increasing initial wire diameter from 5.1 m ͑0.1 m/ns͒ to 12.7 m diameter ͑0.3 m/ns͒.

Wire number dependence of the implosion dynamics, stagnation, and radiation output of tungsten wire arrays at Z driver

“…31 When the wire number becomes larger than the optimum number ($375 6 25), the wire cores start merging very early during the prepulse stage of the load current pulse, the precursor plasma inside the array becomes negligible or non-existent, and the wire array pinches as a thin shell which is prone to instabilities. Applying this explanation to W and Al experimental results, we estimate wire core sizes and core expansion velocities close to those independently measured in Imperial College and Cornell University, 28,29,33,34 This finding further reinforces the plausibility of our speculations. In addition, analytical calculations 35 and 2D model simulations 26 suggest the enhanced Z-pinch stability due to plasma distribution inside the array prior the onset of wire motion.…”

“…However, taking into account the fact that our wires were shorter, 1 cm instead of 1.5 to 2 cm of Ref. 29, and the extremely small IWGs (0.1 mm versus 3-5 mm of Ref. 29) of our arrays, it is plausible to assume that the cathode electrode core broadening effect may have increased the core expansion velocities for almost the entire wire length bringing them closer to our estimated v op ¼ 0.09 cm=ls.…”

Ablation dynamics in wire array Z-pinches under modifications on global magnetic field topology

“…In addition, previous experiments with standard aluminium arrays performed in the Llampudken generator using a similar mass per unit length as that of the aluminium experiments reported here, the precursor temperature has been estimated in the range 30-50 eV. 24 However, in the experiments where the magnetic field topology changes in time, the precursor plasma is not stationary on axis, hence it is expected to have a temperature closer to the minimum estimation. Assuming a …”

“…Even though the differences on the number of wires within the array could modify the narrowing of the ablation streams, 13 experiments using four wires in standard wire array configurations have shown similar imploding dynamics to those performed using eight (or more) wires in standard arrays. 24 As a result, the effects of reducing the number of wires do not affect the ablation dynamics as importantly as using different wire diameters within the array. Therefore, the effects of using the appropriate mass per unit length for the generator should produce much more evident differences on the plasma dynamics due to the magnetic field topology variations than the overmassed experiments.…”

High resolution absorption spectroscopy of exploding wire plasmas using an x-pinch x-ray source and spherically bent crystal

“…13,14 This system is characterized by the existence of a cold, < 5 eV, dense, >10 21 cm −3 , wire core plasma that ablatively supplies mass to the surrounding relatively diffuse and higher temperature coronal plasma. The coronal plasma carries the vast majority of the current and is therefore subject to the J × B force which drives the plasma towards the geometric axis in wire array z-pinches.…”