D(3He,p)4He and D(d,P)3H fusion in a small plasma focus operated in a deuterium helium-3 gas mixture

A 3 kJ plasma focus was operated with a He-3-D-2 gas mixture, with partial pressures in the ratio of 2:1, corresponding to an atomic number ratio of 1: 1 for He-3 and D atoms. The fusion reactions D (He-3,p)He-4 and D(d,p)H-3 were measured simultaneously using CR-39 polymer nuclear track detectors placed inside a pinhole camera positioned on the forward plasma focus axis. A sandwich arrangement of two 1000 mu m thick CR-39 detectors enabled the simultaneous registration of two groups of protons with approximate energies of 16 MeV and 3 MeV arising from the D(He-3,p)He-4 and D(d,p)H-3 reactions, respectively. Radial track density distributions were obtained from each CR-39 detector and per-shot average distributions were calculated for the two groups of protons. It is found that the D(He-3,p)He-4 and D(d,p)H-3 proton yields are of similar magnitude. Comparing the experimental distributions with results from a Monte Carlo simulation, it was deduced that the D(He-3,p)He-4 fusion is concentrated close to the plasma focus pinch column, while the D(d,p)H-3 fusion occurs relatively far from the pinch. The relative absence of D(d,p)H-3 fusion in the pinch is one significant reason for concluding that the D(He-3,p)He-4 fusion occurring in the plasma focus pinch is not thermonuclear in origin. It is argued that the bulk of the D(He-3,p)He-4 fusion is due to energetic He-3(2+) ions incident on a deuterium target. Possible explanations for differing spatial distributions of D(He-3,p)He-4 and D(d,p)H-3 fusion in the plasma focus are discussed.