Study on the magneto-Rayleigh–Taylor instability of inverse exploding planar foils

Abstract

Experiments of the explosion of thin planar foils have been carried on the QiangGuang-I facility (∼1.4 MA peak current, ∼110 ns rise time) to study the evolution of the plasma instability. An inverse-field configuration was utilized with the current return post placed in the center and two parallel planar aluminum foils of 20  μm thickness, 1 cm width built on each side. The foil was ablated into a plasma slab with a particular width, and the inner surface and outer surface expanded in opposite directions and suffered from unequal magnetic pressure, resulting in divergent instability evolution. To alter plasma acceleration, multiple load configurations, including symmetric and asymmetric cases, were used to change the distance between the foil and back-post. The diagnostic system was fielded to provide a side-on view of exploding foils, including laser shadowgraphy, laser interferometry, and an optical framing camera. The characteristic structure, wavelength spectrum, and growth rate of the instability were compared for both sides. During the early magnetohydrodynamics instability developing stage, the inner side had a similar feature to the outer side surface, but with a larger ratio of the long wavelength in the spectrum. The anti-correlated relationship between the correlation coefficients of two surfaces appeared to be a kink instability mode. When the interface was subjected to deceleration, a transition from an early instability mode to a Magneto-Rayleigh–Taylor (MRT) instability mode was observed in studies, with the growth rate and characteristic wavelength growing fast. Because of the higher magnetic pressure and earlier retardation, the inner edge was more severely affected by MRT instability than the outer edge. A theoretical model was developed to interpret the experimental data.