Z-pinch discharges in aluminum and tungsten wires

Abstract

A series of experiments on Z-pinch plasmas, driven by a pulsed power generator that delivers 160 kA with a rise time (10%–90%) of 65 ns are reported. Tungsten wires of various diameters were used and results are compared with 15 μm diameter aluminum wire. The expansion of the pinch is studied as a function of wire diameter and material. Schlieren observations show that the coronal plasma of various diameters of tungsten wires expands with the velocity of (9.4±1.0)×103 m/s. The aluminum pinch expands at least a factor of 2 faster. The m=0 perturbations appear at about 8 ns for the aluminum compared with 20 ns for the tungsten pinch. The wavelength and diameter of the perturbations increase with time for both types of wires, and relatively faster for the aluminum pinch. The short wavelength perturbations (∼200 μm) persist for a longer time for larger diameter tungsten wires. Bright spots are seen to appear after 60 ns from the current start for tungsten wires, whereas for aluminum wires, bright spots appear after 40 ns. The decay time of bright spots is 40 ns for the smallest diameter tungsten wire compared with only a few nanoseconds for larger diameter wires. Hard x-ray emission above 6 keV was observed from tungsten wire pinches, but it was not observed from either bright spots or the plasma column for the aluminum pinch. However, hard x-ray emission from the anode due to an electron beam was observed for wires of both materials.

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.…”

“…These estimates are in reasonably good agreement with the independently measured core sizes for both materials. 28,[31][32][33][34] The above discussion suggests that the optimum IWG op is the gap where the wire coronas merge 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 ($50l for W and $220l for Al) shell. Analytical calculations 35 and 2D model simulations 26 support this assumption and suggest that the plasma distribution inside the array, prior to the onset of the wire motion, enhances the pinch stability.…”

“…The calculated v c for aluminum arrays by the heuristic model 27 The corona and core expansion velocities in the single wire measurements of Ref. 28 were as follows; for Al, the corona and core expansion velocities were measured to be, respectively, $2 cm=ls and 0.7 cm=ls. For the W case, the corona expansion velocity was found to be $1 cm=ls and core 0.15 cm=ls More recent measurements by Shelkovenko and coworkers 29 with x-ray backlighting techniques, which are not affected by the close to the core coronas, suggest core expansion velocities for our 8 to 9 lm wires (600 and 500 wire arrays) of the order of 0.06 cm=ls.…”

One-, two-, and three-dimensional modeling of the different phases of wire array Z-pinch evolution

“…Comparison of simulated laser schlieren images with those from the IMP experiments described in Ref. 13 indicate that the corona minimum and maximum radii and the instability wavelengths agree to within 20% throughout the first 50 ns. The expansion of the core is, however, somewhat faster in simulation than in experiment.…”

“…Experimental observations suggest that the bright-spots observed in x-ray streak photographs are coincident with the penetration of the core by the necks of the instability observed in laser schlieren photographs. 13 Despite the penetration of the core by the neck, the vast majority of the length of the core remains intact.…”