D–D fusion neutron emission and plasma compression in a small-sized plasma focus operating with D2 and D2+Ar gas mixtures

Research was conducted to determine the influence of adding a small amount of argon on plasma-focus electrical discharges in deuterium. The influence of the initial deuterium pressure on discharges was also verified. A 3 kJ Mather-type plasma focus device was used together with the following diagnostic systems: magnetic and electric probes, vacuum ultraviolet and soft x-ray pinhole camera, schlieren laser imaging, PIN photodetectors, silver activation counter, and scintillation probes. A total of 470 discharges were performed and examined for two constant initial deuterium pressures: 280 Pa and 340 Pa. The 280 Pa was the optimum pressure for discharges in pure deuterium. Discharges in D2 + Ar mixtures were performed with the following Ar fractions for 340 Pa of D2 (in relation to the total mass of the mixture): 0.9%, 2.8%, and 10.4%. And discharges for 280 Pa with: 1.1%, 3.4%, 9.6%, and 17.5%. Most experiments were conducted with pumping of the experimental chamber prior to discharge, and a smaller number without. A maximum increase in total D–D fusion neutron emission yield (Yn) of 22% was achieved for discharges in 98.9%D2 + 1.1%Ar compared to discharges in pure D2. Experiments with pre-discharge pumping proved to be significantly more beneficial in terms of Yn. Three spontaneously occurring modes of plasma pinch compression were identified, for well-compressed discharges: stable, partially-stable, and unstable. And another three, for discharges not so well-compressed. They differed significantly in terms of compression dynamics, electrical waveforms and fusion neutron emission. Discharges in stable mode were characterised by lower radial compression velocities, slower evolution, greater pinch stability and low Yn. In contrast, discharges in unstable mode were characterised by faster radial compression of plasma, strong presence of filamentary-like structures, low stability, the presence of hot spots and the highest Yn. The reasons for the changes in Yn turned out to be changes in discharge statistics.