Suppression of the Ablation Phase in Wire Array Z Pinches Using a Tailored Current Prepulse

A new wire array configuration has been used to create thin shell-like implosions in a cylindrical array. The setup introduces a $\sim 5\mathrm{kA}$, $\sim 25\mathrm{ns}$ current prepulse followed by a $\sim 140\mathrm{ns}$ current-free interval before the application of the main ($\sim 1\mathrm{MA}$) current pulse. The prepulse volumetrically heats the wires which expand to $\sim 1\mathrm{mm}$ diameter leaving no dense wire core and without development of instabilities. The main current pulse then ionizes all the array mass resulting in suppression of the ablation phase, an accelerating implosion, and no trailing mass. Rayleigh-Taylor instability growth in the imploding plasma is inferred to be seeded by $\mu \mathrm{m}$-scale perturbations on the surface of the wires. The absence of wire cores is found to be the critical factor in altering the implosion dynamics.

Figure 1

Diagram of the array configuration with the resistive voltage divider applied.

Figure 2

Plots of (a) the $dI/dt$ of the total current and the current in just the load array and (b) the $dI/dt$ and the resistive voltage across a 17 mm diameter, $8\times 15\mu \mathrm{m}$ Al load array during the prepulse.

Figure 3

(a) Interferogram of a load array taken 2 ns before current switch with the measured Al line density ($n_{g}L$) shown in (b). (c) Interferogram of a load array obtained $t\sim 40\mathrm{ns}$ after current switch with (d) the measured electron line density profile. Red lines indicate initial positions of the wires.

Figure 4

(a) Radial optical streak photograph showing implosion of a 17 mm diameter, $8\times 10\mu \mathrm{m}$ Al load array. (b) Interferogram of a load array taken 76 ns after current switch showing no trailing mass between the original wire positions (red lines) and the imploding plasma.

Figure 5

(a),(b) XUV images showing development of MRT instability during implosion of the load array. Arrows show initial positions of the wires.