Foil explosion in megagauss magnetic fields: Non-uniform expansion and instabilities

Abstract

This paper presents an analysis of the results of an experiment on the explosion of copper foils, which was carried out on the MIG generator that produced a current through the foil at a level of 2 MA with a current pulse rise time of 100 ns. The foil length (along the z axis) was 20 mm, width (along the x axis) 5 mm, and thickness (along the y axis) 250 μm. It was observed that when the foils were exploded in megagauss magnetic fields with the foil current flowing along the z axis, the expansion of the plasma was extremely nonisotropic. The expansion of the foil material along the x axis was suppressed, and it expanded only along the y axis. An analysis of the experimental data carried out using a radiation magnetohydrodynamic code has shown that the nonisotropy of the expansion was associated with an enhancement of the magnetic field at the narrow side of the foil. As a result of the field enhancement, the magnetic pressure at the narrow foil side significantly exceeded the magnetic pressure in the center of the foil surface. Using an HSFC Pro optical camera (providing a frame exposure time of 3 ns), large-scale instabilities with wavelengths of 0.2–0.5 mm were detected on the foil surface. In appearance, these instabilities were similar to those developing in an exploding rod. It was suggested that the most likely cause of the appearance of the plasma structure observed in the experiment was the growth of flute instabilities.