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

Abstract

In this paper, we describe the operation of a two-stage wire array z-pinch driven by the 1.4 MA, 240 ns rise-time Magpie pulsed-power device at Imperial College London. In this setup, an inverse wire array acts as a fast current switch, delivering a current pre-pulse into a cylindrical load wire array, before rapidly switching the majority of the generator current into the load after a 100–150 ns dwell time. A detailed analysis of the evolution of the load array during the pre-pulse is presented. Measurements of the load resistivity and energy deposition suggest significant bulk heating of the array mass occurs. The ∼5 kA pre-pulse delivers ∼0.8 J of energy to the load, leaving it in a mixed, predominantly liquid-vapour state. The main current switch occurs as the inverse array begins to explode and plasma expands into the load region. Electrical and imaging diagnostics indicate that the main current switch may evolve in part as a plasma flow switch, driven by the expansion of a magnetic cavity and plasma bubble along the length of the load array. Analysis of implosion trajectories suggests that approximately 1 MA switches into the load in 100 ns, corresponding to a doubling of the generator dI/dt. Potential scaling of the device to higher current machines is discussed.

Effect of the prepulse current with an adjustable time-delay on the implosion dynamics of two-wire Z-pinch

“…The suppression of the ablation stage in a cylindrical aluminum wire array Z-pinch experiment was achieved by introducing a prepulse current using a two-stage wire array configuration, by Harvey-Thompson et al in 2011 [16]. In this configuration, the bottom exploding array plays a role as a fast current switch that provides a current free interval of ∼100 ns before the start of the main pulse current [17]. However, the two-stage wire array configuration introduces an inevitable disturbance in the measurement of the current, while the bottom array can lead to additional inductance in the load region [17].…”

“…In this configuration, the bottom exploding array plays a role as a fast current switch that provides a current free interval of ∼100 ns before the start of the main pulse current [17]. However, the two-stage wire array configuration introduces an inevitable disturbance in the measurement of the current, while the bottom array can lead to additional inductance in the load region [17]. Moreover, the current free interval in the two-stage wire array is fixed owing to its dependence on the mechanism of the load configuration.…”

Preconditioned wire array Z-pinches driven by a double pulse current generator

“…After ∼100 ns the main current was switched into the preconditioned wires, allowing the implosion of all the mass of the load array. The precursor plasma and the trailing mass were greatly suppressed and the x-ray pulse width was reduced [11]. However, the use a two stage wire array configuration for wire preconditioning impose considerable restrictions on the range of wire array Z-pinch load configurations and on the time interval between the prepulse current and the main current.…”

“…After ~ 100ns the main current was applied to the preconditioned wires allowing the implosion of all the mass of the load array. The precursor plasma and the trailing mass were greatly suppressed and the X-ray pulse width was reduced 11 . However, this method by using a two stage wire array was very inconvenience, since it greatly depended on the load configurations.…”