Deuterium plasma focus measurements using solid state nuclear track detectors

Abstract

A camera obscura technique was used to investigate the spatial distribution of fusion protons emitted from a small deuterium plasma focus device operated in its neutron-optimized regime (14 kV, 3 kJ, and 400 Pa D2 gas). An indium-foil activation detector was employed simultaneously to measure the shot-to-shot neutron yield. The camera obscura was positioned on the forward axis of the plasma focus, 12 cm from the tip of the hollow copper anode, covering a conical field-of-view of half-angle slightly less than 20 degrees. The nuclear track detector material PM-355 was used in the camera focal plane to register protons from the 2H(d,p)3H reaction, hence imaging the spatial distribution of fusion occurring in the plasma focus. A kapton film covering the camera entrance pupil served to protect the detector from the hot plasma jet; it also stopped completely all energetic charged particles other than the (≃ 3 MeV) fusion protons. Following chemical etching of the detector plates, the track parameters-size, circularity, average-grey-value and (x-y) position-were measured by an automated scanning system comprising an optical microscope equiped with a CCD camera, motorized stage and focus-adjust, and interfaced to a PC computer. From the accumulated track data, representing 119 plasma focus shots, the imaged fusion density profile was obtained. A detailed Monte Carlo simulation program was used to interpret this profile. The results indicate that the beam-target mechanism, in which energetic deuterons emitted in a forwardly directed cone from the plasma focus pinch interact with the cold deuterium gas, accounts for 90 to 96% of the fusion production for our device. There is however clear evidence for a non-zero (4 to 10%) contribution emitted directly from the pinch region.