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

Abstract

A new wire array configuration has been used to create thin shell-like implosions in a cylindrical array for the first time. The setup introduces a ∼5 kA, ∼25 ns current prepulse followed by a ∼140 ns current-free interval before the application of the main (∼1 MA) current pulse. The prepulse volumetrically heats the wires which expand to ∼1 mm diameter leaving no dense wire core and without development of instabilities. The main current pulse then ionises 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 µm-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.Fast z-pinch plasma implosions driven by multi megaAmpere currents are very efficient at converting stored electrical energy into X-rays. Achieving high x-ray powers by releasing this energy in a short time requires a high degree of azimuthal symmetry of the imploding plasma and a low level of initial axial perturbations seeding the growth of magnetic Rayleigh-Taylor (MRT) instabilities. The highest x-ray powers in z-pinch implosions were achieved using cylindrical arrays made of large numbers of fine metallic wires (280-300 TW, >2 MJ at ∼20% efficiency on the Z facility at SNL [1] [2]). Experimental studies of wire array z-pinches have shown [3] [4] [5] thateven for large number of wires the arrays do not form a plasma shell imploding as a 2-D object. Instead, the wires remain as discrete, compact objects for the first 60-80% of the implosion and during this time only the coronal plasma, continuously ablated from the stationary cores, are accelerated towards the axis by the J×B force [4]. This ablation phase determines the initial conditions for the implosion in two ways. Firstly, the ablated plasma fills the interior of the array which contributes to the mitigation of the growth of the MRT instability. Secondly, the quasi-periodic modulation of the ablation rate along each individual wire occurring at the 'natural' wavelength (∼250 μm for W and ∼500 μm for Al) is responsible for the large level of axial perturbations at the start of the implosion phase. The existence of the ablation phase is observed in all known wire array z-pinch experiments (currents between ∼1 MA and 26 MA, and implosion times between ∼50 ns and ∼800 ns) and occurs because current is initially carried by the small amount of plasma formed on the wire surfaces, and not by the wire cores themselves.In this letter we report on experiments where, for the first time, the ablation phase in wire array z-pinches was suppressed, including dramatic suppression of the axial perturbations in the wires and suppression of the precursor plasma flow. This was achieved by using a short duration low-level current pre-pulse (∼25 ns, ∼5 kA) followed by a ∼140 ns current-free interval before the main (∼1 MA, 100 ns) current pulse driving the implosion is applied. Th...

Characterisation of the current switch mechanism in two-stage wire array Z-pinches

“…Therefore, by the end of the pre-pulse, the load likely exists as a liquid-vapour mixture. This observation is consistent with previously published 23 interferometric results that show the line integrated atom density (n a l) after a period of thermal expansion is slightly lower than would be expected given the number of atoms in the wire, suggesting the presence of liquid droplets that are not detected by the interferometer. Additional measurements concerning the detailed radial structure of preconditioned, expanded wire cores will be published separately.…”

“…23. Here, we present a much more detailed account and analysis of electrical measurements obtained during the current pre-pulse.…”

“…Previous work has shown 23 that preconditioning of the load array in this manner dramatically alters the ensuing load implosion dynamics; the ablation phase is eliminated, and no trailing mass remains at the initial array radius during the final implosion. The inverse array provides intermediate inductive energy storage to drive the load implosion.…”

“…The width of the voltage pulse ($15 ns) defines the timespan for energy deposition within the load array, despite the fact that a small amount of current continues to flow in the load, driven by inductive current division heavily biased toward the low inductance inverse array. 23,25 Energy deposition was determined from the experimental data by numerical computation of the following integral:…”

“…Two-stage wire array z-pinch loads have been proposed as a means to both sharpen the current profile and suppress or control the ablation phase. 23 A two-stage wire array zpinch consists of a parallel arrangement of two cylindrical arrays, as shown in Fig. 1.…”