Coded Aperture Imaging technique for investigation of fusion source spatial distribution in Plasma Focus device

摘要

The Coded Aperture Imaging (CAI) technique, which was originally developed for x-ray and γ-ray astronomy, has been applied to investigate the spatial distribution of DD fusion in a 1.6 kJ (1−3×10⁸ n/shot) Plasma Focus device. The coded mask pattern is based on a Singer cyclic difference set with 341 open pixels in 91×15 array (25% open fraction). The physical mask was fabricated by laser-machining 300 µm square holes in stainless steel foil. As the plasma pinch is a semi-extended line source, the mask pattern rows (×15 dimension) were spaced by a factor of 6, to obtain an almost square mask and reduce the effective open fraction to about 4%, which enhances the image signal-to-noise ratio. The CAI cameras were placed at 90° to plasma focus axis, with CR-39 detectors used to register tracks of protons from the D(d,p)T reaction. A 75 µm Kapton film stopped all charged particles, other than the ∼3 MeV protons, from reaching the CR-39. A deconvolution algorithm was applied to the measured proton tracks coordinates to obtain images of the DD fusion source. The number of proton tracks registered per shot was typically (1−2)×10⁵. Two beryllium fast-neutron detectors and a plastic scintillator were employed simultaneously to measure the time-integrated and time-resolved neutron yield and anisotropy for each shot. The results clearly show that the CAI technique yields images with greater detail and considerably improved signal-to-noise ratio by comparison with conventional pinhole imaging.