The electrical explosion of single wire occurs in many application fields, such as wire-array Z-pinch, synthesis of the nanopowder, high-intensity magnetic field source, etc. The initial stage of the electrical explosion of single wire has a critical influence on the stagnation and X-ray yield in the wire-array Z-pinch. The impressive result of X-ray yield from wire-array Z-pinch is a major motivation to promote the research in this field. Although numerous studies have been carried out to gain a deep insight into the physics of the electrical explosion of single wire, more experimental investigations are necessary to optimize the energy deposition and expansion rate. It is important to investigate the characteristics of the electrical explosion of single wire under the negative polarity pulsed-current, which is adopted in many Z-pinch facilities. In this paper, the electrical explosion of aluminum wire under negative polarity pulsed-current in vacuum is investigated. In the present experiments, the light emission is measured by the photomultiplier and streak camera. A laser probe EKSPLA-PL2251C (30 ps, 532 nm) is adopted to perform the shadowgraphy, schlieren and interferometry diagnostics. The radial knife-edge schlieren scheme is employed to translate the regions with plasma refractivity and gas-type refractivity. The interferometry is constructed based on Mach-Zehnder interferometer. The shadowgram, schlieren image and interferogram are recorded by Canon cameras. The typical waveforms of the voltage, current and light emission from the electrical explosion of 15 m-diameter, 2 cm-long aluminum wire are derived. The energy deposition at the instant of voltage collapse is about 2.4 eV/atom (vaporization energy is about 4 eV/atom). In order to increase the energy deposited into the wire, the 15 m-diameter, 2 cm-long aluminum wire with 2 m polyimide coating is exploded with the same electrical parameters. The energy deposition in the coated wire is about 5 eV/atom. From the shadowgram of the electrical explosion of uncoated aluminum wire, the expansion velocity of the high-density region can be estimated to be about 2.2 km/s. However, the expansion velocity of the high-density region of the polyimide-coated aluminum wire is about 5 km/s. The schlieren images show that the wire is exploded into a binary structure, i.e., a high-density core surrounded by the low-density corona. It should be noted that the energy deposition in the coated wire is larger than the vaporization energy, indicating that the aluminum wire is totally in gaseous state. Thus, the plasma region in the schlieren image of electrical explosion of coated wire is not distinct. The core-corona structure is depressed by the insulating coatings to a certain extent. The configuration of the parallel wire is adopted to estimate the expansion velocity of the plasma shell. The expansion velocity of the low-density plasma is about 5.8 km/s. Two-dimensional distribution of the phase shift is derived through the interferogram. The central part of the gas-type material with a radius of 0.1 cm exhibits a large positive phase shift, while the peripheral plasma shows a small negative phase shift. The three-dimensional atomic density distribution is reconstructed in the gas-type distribution area in which the contribution of electrons is negligible. In our experiments, the energy deposition of the electrical explosion of uncoated wire ranges from 2 to 4 eV/atom. This may be caused by the initial conditions of the wire surface and the connection between the wire and electrode. Further research should be carried out for a better understanding of this phenomenon.
