NUKLEONIKA 2006, 51(1):47-53

A 3 kJ plasma focus was operated with a ³He-D₂ gas mixture, with partial pressures in the ratio of 2:1, corresponding to an atomic number ratio of 1:1 for ³He and D atoms. The fusion reactions D(³He,p)⁴He and D(d,p)³H 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 mm 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(³He,p)⁴He and D(d,p)³H 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(³He,p)⁴He and D(d,p)³H proton yields are of similar magnitude. Comparing the experimental distributions with results from a Monte Carlo simulation, it was deduced that the D(³He,p)⁴He fusion is concentrated close to the plasma focus pinch column, while the D(d,p)³H fusion occurs relatively far from the pinch. The relative absence of D(d,p)³H fusion in the pinch is one significant reason for concluding that the D(³He,p)⁴He fusion occurring in the plasma focus pinch is not thermonuclear in origin. It is argued that the bulk of the D(³He,p)⁴He fusion is due to energetic ³He²⁺ ions incident on a deuterium target. Possible explanations for differing spatial distributions of D(³He,p)⁴He and D(d,p)³H fusion in the plasma focus are discussed.