The Study of Plasma Dynamics and Short Wavelength Radiation in Axial Discharges (Plasma Focus and Vacum-Spark).

In this work two important characteristics of high density and high temperature plasmas formed in axial discharges are studied. In the first part an experimental investigation of the structure and the dynamics of the current sheath and its effects on targets in a 3.3 kJ plasma focus are presented. Shadowgraphs are recorded in the presence of a flat disc target placed downstream of the anode. These shadowgraphs reveal that the disc target does not affect the dynamics of the focus when it is at the distance of 10 mm (the radius of the anode in our case) from the anode end. However, the shadowgraphs recorded with the disc target at a distance of 7 mm exhibit the sequential focusing phenomenon, i.e., the current sheath moves over the disc and forms a pinch beyond it, with the disc acting as a new anode. This sequential focusing is also confirmed by recording the voltage probe signal, neutron pulses and neutron flux. The shadowgraphs are also recorded in the presence of a wire target (1-mm diameter) inserted along the pinch axis. The strong focusing of the current sheath onto the I-mm wire indicates that this small plasma focus machine can be used for focus-fiber experiments. In the second part the dynamics of a collapsing plasma column and the properties of micropinches formed in the collapsed plasma column in a low-inductance vacuum-spark discharge are studied. Time integrated single shot x-ray spectra of He-like and H-like ions and their corresponding n = 2, Li-like and He-like satellites of elements (S, Cl, Ca, Cr, and Fe) are recorded. The experimental spectra are compared with simulated spectra (using the code FL V), and the electron density and the temperature of the micropinch plasma are estimated. The dependence of electron density and temperature of the micropinch plasma on atomic number Z shows a fair agreement with the available data in the literature obtained using different diagnostic techniques. The electron temperature estimated from the spectra of H-like ions is higher than that from He-like ions which indicates that the plasma is ionizing. Vacuum ultraviolet spectroscopy of elements (Ti, Fe, and Mo) indicates that the transitions from the ionization stages (Ti XI, Ti XII, Fe XV, Fe XVI, Mo XIV, Mo XV, Mo XVI) are prominent. However, in the Ti spectrum strong transitions from lower ionization stages (Ti VII, Ti VIII) emerge when the spectrum is recorded in the presence of a small superposed axial magnetic field. The presence of this external magnetic field reduces the compression.