Studies of plasma focus fusion zone geometry using proton coded aperture imaging

The spatial distribution of DD fusion in a 1.6 kJ plasma focus (PF) device is investigated using the coded aperture imaging (CAI) technique, enabling single shot fusion images to be obtained. Simultaneously, two beryllium fast-neutron activation detectors are employed to measure neutron yield and anisotropy. The coded mask patterns used are based on Singer cyclic difference sets with various open fractions. The physical mask patterns are laser-machined in stainless steel foil, and CR-39 nuclear track detectors record the coded image of the ~3 MeV DD protons emitted from the PF pinch region. In one series of experiments using pure deuterium, five CAI cameras were employed simultaneously: one positioned on-axis (0°) and four at 45° to the PF axis. For another series of experiments two higher-resolution CAI cameras were positioned at 90° to the PF axis, on opposite sides of the pinch. The PF was operated in either pure deuterium gas or deuterium-krypton admixtures of various concentrations
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. For single PF shots, the shape of the fusion emission zone is observed to change between high and low neutron-yield shots; high and low neutron-anisotropy shots; and between pure deuterium and deuterium-krypton admixtures. These results are discussed with regard to PF fusion mechanisms.