Evidence of thermal and non-thermal mechanisms coexisting in dense plasma focus D-D nuclear reactions

Dense plasma foci are the most efficient devices in the production of fast neutrons proceeding from deuterium nuclear fusion reactions. This work deal s with experiments in a small plasma focus machine and attempts to distingu ish neutrons of thermal origin from non-thermal neutrons, and the search fo r possible sources of both. Soft x-rays emitted by bremsstrahlung in a plasma focus were experimentally studied, using a multiple pin-hole camera with different aluminium absorbe rs in each hole. This method allows one to obtain the time-integrated soft x-ray image, as well as an estimation of the mean electronic temperature. T he time-resolved soft x-ray intensity is registered with a filtered p-intri nsic-n (PIN) diode detector, shielded with a beryllium sheet. The time-reso lved hard x-ray intensity (associated with particle acceleration) was regis tered using a plastic scintillator coupled to a fast photomultiplier tube. With a similar system, the time-resolved neutron emission is also registere d. From the soft x-ray photographic studies, bright points with temperature s two or three times higher than the bulk plasma temperature are observed. These bright points (one or two per focus) can reach temperatures of over 7 keV, and their formation seems to correlate with successive necking produc ed by m = 0 instabilities in the pinch column. Time-integrated and time-res olved measurements of neutron yield, performed in comparison with time-reso lved measurements of soft and hard x-ray radiation, show the different infl uence of thermal and non-thermal mechanisms in the nuclear fusion reactions .