MHD instabilities developing in a conductor exploding in the skin effect mode

Abstract

The results of experiments with exploding copper conductors, performed on the MIG facility (providing currents of amplitude of about 2.5 MA and rise time of 100 ns), are analyzed. With an frame optical camera, large-scale instabilities of wavelength 0.2–0.5 mm were detected on the conductor surface. The instabilities show up as plasma “tongues” expanding with a sound velocity in the opposite direction to the magnetic field gradient. Analysis performed using a two-dimensional MHD code has shown that the structures observed in the experiments were formed most probably due to flute instabilities. The growth of flute instabilities is predetermined by the development of thermal instabilities near the conductor surface. The thermal instabilities arise behind the front of the nonlinear magnetic diffusion wave propagating through the conductor. The wavefront on its own is not subject to thermal instabilities.

The influence of the solid to plasma phase transition on the generation of plasma instabilities

“…Thick exploding wires are chosen as loaded targets since the low, slowly rising heating current conveniently allows the study of plasma generation at the skin effect mode, where all phases of matter can exist simultaneously for relatively long times. Copper (Cu) is the material of choice here since it has well known physical properties, established strength material behaviour, its equation of state (EOS) is experimentally validated 21 and has been widely used for the study of the development of instabilities in the skin effect mode 10 .…”

“…different plasma devices) but the governing physics will remain the same as suggested by our simulations. Adaptation of our approach to known instabilities, such as the electrothermal instability (ETI) in the linear or nonlinear magnetic diffusion regime 6,7,10,14,17 that begins to grow immediately after the heating source is applied onto the solid target, will provide significant insight and is already being pursued (Supplementary Note 4). Indeed, early stage results 29 show that the inclusion of the material’s physical properties modifies the ETI into an instability, which for future reference we choose to call it ETM instability, having growth rates at least one order of magnitude larger than those found in the literature.…”

“…This fundamental interest is not only triggered by basic physics curiosity to understand plasma instabilities but also stems from important plasma applications, among others in inertial confinement fusion (ICF), either in magnetized pulsed power liners or in ICF capsule targets where strong currents or laser pulses are used to heat the target, respectively 7–10 . Plasma instabilities 2–6, 11, 12 are the result of any disturbance that may occur in a plasma parameter such as density, temperature, magnetic or electric field, or current.…”

“…Usually the information of the physical processes during the phase change from solid, liquid or gas to plasma is neglected 6,7,9,13 . In addition, the use of assumptions on the plasma conditions after surface plasma initiation deteriorates the efforts towards the necessity to explore methods for mitigating the plasma instabilities 10 . On the other hand, it is clear and widely recognized by the scientific community 6,9,13,14 that the complexity of such a multiphysics problem poses serious difficulty to successfully incorporate the realistic physical initial conditions of the targets into the simulations.…”

“…These uncertainties have been the trigger of dedicated investigation of the processes related to the initial target physical conditions 10,17,18 . Until now all these studies were limited to the coupling of properties of the solid, among others the resistivity, the heat conduction, the magnetic diffusion, the viscous damping and the stress tensors, into the system of the MHD equations.…”