Atomization and merging of two Al and W wires driven by a 1 kA, 10 ns current pulse

Abstract

Possibility of preconditioning of wires in wire array Z-pinch loads by an auxiliary low-level current pulse was investigated in experiments with two aluminum or two polyimide-coated tungsten wires. It was found that the application of a 1 kA, 10 ns current pulse could convert all the length of the Al wires (1 cm long, 15 μm diameter) and ∼70% of length of the W wires (1 cm long, 15 μm diameter, 2 μm polyimide coating) into a gaseous state via ohmic heating. The expansion and merging of the wires, positioned at separations of 1–3 mm, were investigated with two-wavelength (532 nm and 1064 nm) laser interferometry. The gasified wire expanded freely in a vacuum and its density distribution at different times could be well described using an analytic model for the expansion of the gas into vacuum. Under an energy deposition around its atomization enthalpy of the wire material, the aluminum vapor column had an expansion velocity of 5–7 km/s, larger than the value of ∼4 km/s from tungsten wires. The dynamic atomic polarizabilities of tungsten for 532 nm and 1064 nm were also estimated.

Wire Explosion in Vacuum

“…According to [178], the core expansion velocity at (d I/dt) av = 150 A/ns was (3-6) • 10 5 cm/s for bare wires made of Au, Cu, Ag, and Al and (0.4-1) • 10 5 cm/s for bare wires made of W, Mo, and Pt. Shi et al [19], Rousskikh et al [123], Zhao et al [172], Wu et al [173], [174], and Li et al [184] measured v core values similar to those reported in [168] and [178].…”

“…However, in these cases, the intrinsic impedance of the driver became comparable to the resistance of the exploded wire, and the actual rate of the wire current is decreased. For instance, Wu et al [174] report that for (d I/dt) av = 100 A/ns, the rate of the current in an exploded 13-μm tungsten wire was 47 A/s. Sarkisov et al [94], [152], [176]- [179] proposed a driver for WE experiments which combines an off-the-shelf voltage pulse generator capable of producing nanosecond-rise time currents and a segment of coaxial cable.…”

“…This type of voltage divider was proposed by Fletcher [183]. Voltage dividers of this type were used in the WE experiments reported in [19], [174], [175], and [184]. For a capacitive voltage probe, the response to a unit pulse can be described approximately as…”

“…Wu et al [174] and Hamilton et al [195] investigated the process of atomization of Al wires exploded in vacuum at (d I/dt) av = 100 and 160-200 A/ns, respectively. In both experiments, the linear density of Al particles in the discharge channel was estimated based on interferograms obtained using laser radiation (λ = 532 nm).…”

“…The interferograms were taken 200-300 ns after the onset of current flow through the wire. Wu et al [174] and Hamilton et al [195] used the dynamic polarizability of neutral Al atoms α = 10.8 • 10 −24 cm 3 for λ = 532 nm [200]. In the experiment [195], the diagnostics was arranged so that the fringes in the interference pattern were parallel to the axis of the discharge channel.…”