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

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.

Study of Neutron From a Dense Plasma Focus Paco Instrument by Means of Nuclear Tracks Detectors

“…In particular, the neutron pulse duration (~100 ns) and the observed neutron anisotropy provide compelling evidence that supra-thermal mechanisms play an important role. However, in earlier work we found that there are both isotropic and anisotropic contributions in the neutron angular distribution, the former being the most important [4,5]. Although it has been established that there is more than one fusion mechanism, a detailed understanding of them is still lacking.…”

A Portable Pulsed Neutron Generator

“…Moreover, the average energy of neutrons in the axial direction was found to be greater than the energy of thermonuclear neutrons (2.45 MeV), providing more evidence that there is a non-thermal component contributing to the total neutron yield. 3 Later, it was…”

Study of ions and neutrons from a dense plasma focus instrument by means of nuclear tracks detectors

“…However, the mechanism that gives rise to such reactions is still a source of controversy. It has been found that there are actually two separate and well defined pulsed of neutrons in DPF machines, and some authors have proposed that they might have different origins: Thermonuclear mechanisms and beam-target mechanisms [3]. Whether the thermonuclear mechanism is the only explanation for isotropic neutron emission may still be open for discussion.…”

“…In general, SSNTDs seem to be particularly suitable for time-integrated experimental studies of different nuclear reaction products, such as 3 He and 4 He nuclei, protons, fast neutrons and heavy impurity ions. Fast neutrons can be detected with the polycarbonate CR-39 (C 12 H 18 O 7 ) through the tracks of recoil protons originating in the detector material itself, or in radiator foils such as polyethylene placed in front.…”