No abstract

The fast Z-pinch plasma formation, exploding dynamics, and the evolution of the instability can be controlled experimentally by making special structures on metal surface layer to change the initial state of material, which is valuable for studying the Z-pinch physics. Experiments on the explosion of thin flat foils which have been etched into a periodical structure on surface are performed on the QG-1 facility (～1.4 MA peak current, ～100 ns rise time) in order to study the effects of different surface conditions on explosion and control the evolution of the instability in fast Z-pinch plasma. A kind of inverse load configuration is used in experiment in which the return current post is set at the central axial-position and two modified flat foils are strained outside symmetrically as the main load. So the corresponding <i> <b>J</b> </i> × <i> <b>B</b> </i> force directs outward from the return current post orthogonal to the foil plane, creating an acceleration and pushing the foil plasma away from the center in this configuration. Different surfaces of the foil are also investigated in different conditions because of the asymmetric magnetic field distribution which is useful to study the different evolutions of instability. The foils used in the experiment mainly are the 30-μm-thick aluminum foil. The wavelength of groove perturbations seeded on the surface is 2 mm wide and ～10 μm deep. The plasma explosion dynamic behaviors around conditioned area are diagnosed by laser shadowgraphy, laser interferometry, multiframe optical self-emission imaging and B-dot. It is found that the initially etched periodical structure on surface can control the plasma structure in exploding process which can be concluded as follows. Developing plasma structure shows a periodic character similar to the initial surface structure and the eigenwavelength of the Al is suppressed. In the meantime, the surface without etched perturbations is also influenced by the etched side, showing a similar instability structure but with a lower amplitude. The correlation between two surfaces turns stronger than the case of normal foils. A faster expanding rate occurs in the deep region of the initial periodical groove structure which causes a reverse structure to form. In the discontinuous area of the conditoned structure, a narrow stream of plasma jets perpendicularly from the metal surface which causes a half-wavelength to occur in spectrum analysis. The magneto-hydro-dynamic theory analysis shows that the change of electrothermal instabilities is caused dominantly by the modulation of current density flowing around the periodical structure.

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.