No abstract

Dense plasma foci are the most efficient devices in the production of fast neutrons proceeding from deuterium nuclear fusion reactions. This work deals with experiments in a small plasma focus machine and attempts to distinguish neutrons of thermal origin from non-thermal neutrons, and the search for 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 absorbers 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. The time-resolved soft x-ray intensity is registered with a filtered p-intrinsic-n (PIN) diode detector, shielded with a beryllium sheet. The time-resolved hard x-ray intensity (associated with particle acceleration) was registered using a plastic scintillator coupled to a fast photomultiplier tube. With a similar system, the time-resolved neutron emission is also registered. From the soft x-ray photographic studies, bright points with temperatures 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 produced by m = 0 instabilities in the pinch column. Time-integrated and time-resolved measurements of neutron yield, performed in comparison with time-resolved measurements of soft and hard x-ray radiation, show the different influence of thermal and non-thermal mechanisms in the nuclear fusion reactions.

The average angular distribution of neutron emissions has been measured in the Fuego Nuevo II (FN-II) dense plasma focus device (5 kJ) by means of CR-39 plastic nuclear track detectors. When pure deuterium is used as the filling gas, the data can be adjusted to a Gaussian function, related to anisotropic emission, superposed on a constant pedestal, related to isotropic emission. When deuterium-argon admixtures are used, the anisotropic contribution is best represented by a parabola. The same analysis is applied to previously reported results, for fewer shots, in pure deuterium from the PACO device, which is similar in size to the FN-II. In both devices the anisotropic component is smaller than the isotropic one, but with different features. In PACO the anisotropic component is concentrated on a large narrow beam around the axis, but its contribution to the total neutron yield is significantly smaller than in the FN-II, where the anisotropic component spreads over a wider range. The neutron flux per shot is monitored in both devices with calibrated silver activation detectors, at 20˚and at 90˚from the axis. The average values of the neutron flux at these two angles are used, along with the angular distributions obtained form the track detectors, in order to estimate the absolute neutron yield of both the isotropic and the anisotropic contributions. From examining different groups of shots, it is found that the shape of the angular distribution is important in the estimation of anisotropy, and that the value usually reported, as the ratio of neutron counts head-on and side-on, as measured by activation counters, may be misleading.

The spectrum of neutrons from the 1-MJ Plasma Focus of Frascati, in the 90° direction, has been analysed. The spectra have been determined by the time-of-flight method, with a resolution of about 0.12 MeV (∼5%). The presence of secondary peaks, at both sides of a 2.45-MeV peak, constitutes new evidence that a relevant fraction (∼0.5) of the neutron yield is due to non-thermal processes. The interpretation of the secondary peaks by means of beam-target models requires complex beam configurations, such as one or more circular loops or a pattern of divergent beams.