Interferometry and X-ray diagnostics of pinched helium plasma in a dense plasma focus with an Al-wire on the axis.

The paper describes the evolution of the ordered dense toroidal- and plasmoidal-like structures in a pinch column and the hard X-ray emission from mega-ampere dense plasma-focus discharges performed at the helium (He) filling. Some shots were carried out with an Al-wire of 270 μm in diameter, which was placed along the z-axis in the front of the anode face. The evolution of the considered structures was investigated by means of a multi-frame laser interferometer system as well as the X-ray diagnostics, and it was compared with their evolution observed earlier at the deuterium (D₂) filling. In He-plasma, the ions had a higher mass and Z-number, and at the same initial filling pressure, the velocity of plasma transformations was decreased, but the stability of the investigated structures, as well as the self-generated azimuthal current component and the soft X-ray radiation were increased. The distribution of the plasma electron density (determined from the interferometric images) made it possible to estimate closed currents during the quasi-stationary phases. It was found that the internal toroidal- and plasmoidal-currents reached the level of hundreds of kA. The plasma corona around the Al-wire (penetrating through the internal structures) impeded the formation of a small diameter of the pinch constriction, but it did not prevent the production of fast electron beams with energies above 100 keV, similar to those observed at the D₂-filling. [ABSTRACT FROM AUTHOR]