J. Rangel

Isotropic and anisotropic components of neutron emissions at the FN-II and PACO dense plasma focus devices

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.

Neutron anisotropy and X-ray production of the FN-II dense plasma focus device

The FN-II Dense Plasma Focus is a small (< 5kJ ) Mather type device, where the dependence of neutron yield and its anisotropy, in terms of deuterium filling pressure, and the neutron emission angular distribution have been studied. Two diferent electrode configurations have been tested, showing that their geometry plays an important role both on neutron yield and anisotropy. Time integrated anisotropy has been measured with silver activation counters, on a shot to shot basis. CR-39 nuclear track detectors are used to determine the angular distribution of neutrons, averaged over tens of shots, showing that an isotropic pedestal accounts for 70% of the emission, while the anisotropy component accounts for the remaining 30%. The neutron yield shows a tendency to increase with anisotropy, as well as with the emission of hard X-rays observed on-axis. Scintillator- photomultiplier detectors show a slight Doppler shift in the neutron energy at backward angles, supporting the beam-target mechanism. Additional information has been obtained from time inte- grated X-ray diagnostics, which include filtered multi-pin-hole cameras.

Angular distribution of fusion products and x rays emitted by a small dense plasma focus machine

Time integrated measurements of the angular distributions of fusion products and x rays in a small dense plasma focus machine are made inside the discharge chamber, using passive detectors. The machine is operated at 37kV with a stored energy of 4.8kJ and a deuterium filling pressure of 2.75torr. Distributions of protons and neutrons are measured with CR-39 Lantrack® nuclear track detectors, on 1.8×0.9cm2 chips, 500μm thick. A set of detectors was placed on a semicircular Teflon® holder, 13cm away from the plasma column, and covered with 15μm Al filters, thus eliminating tritium and helium-3 ions, but not protons and neutrons. A second set was placed on the opposite side of the holder, eliminating protons. The angular distribution of x rays is also studied within the chamber with TLD-200 dosimeters. While the neutron angular distributions can be fitted by Gaussian curves mounted on constant pedestals and the proton distributions are strongly peaked, falling rapidly after ±40°, the x-ray distributions show...

High contrast radiography using a small dense plasma focus

Radiographs are obtained with a small (4.6kJ) dense plasma focus machine, using the x-rays which cross a 300μm aluminum window on the axis. Contrast is improved by inserting an iron needle on the tip of the electrode. Measurements with TLD-100 dosimeters have shown the average dose to increase from 0.077±0.006mGy/shot, when a hollow cathode is used, to 0.11±0.01mGy/shot, with the needle. The spectrum is estimated using aluminum filters and the effective equivalent energy is found to be in the range of 20–25keV.

Neutron yield and pressure evolution during a dense plasma focus device shot series

Adsorption and desorption of the filling gas, by the electrodes, the insulator and chamber materials of plasma focus devices, have been suggested as probable causes for the fluctuations in their neutron yield. This work describes analysis of data, aimed at looking for evidence to support this hypothesis. Before starting each series of discharges, a vacuum around 10−6 Torr is achieved. The filling gas, pure deuterium, is maintained under static conditions. A sudden fall of the initial pressure, around 5%, is systematically observed after the first shot in each series, before creeping back at an almost constant rate, in successive shots. On the other hand, for the first shot with fresh filling gas, the neutron yield is always low and systematically increases for the second one. The pressure evolution for the following shots shows no correlation with the neutron yield fluctuations. Thus, except for the first two shots, we find no evidence to support the hypothesis that the neutron yield fluctuations are related to an adsorption–desorption process. It is also observed that a tendency exists for the last shots of each series to yield a larger number of neutrons but with a larger dispersion. This study has been done with both solid and hollow anodes, showing qualitatively similar results in both cases.

Spectroscopic Study of Electrical Glow Discharges in Gases

The variation of the power of the light emitted in a Glow Discharge in Gases of low pressure (GDGLP) excited by a DC source was studied. A lack of dependency of the kind of gas used and the pressure it is located at was obtained. This is comparable to the potential drop which takes place in the discharge by inelastic collisions such as ionization, recombination, excitation, relaxation, etc.

Cross-Calibration of Neutron Detectors for the Dense Plasma Focus FN-II Time of Flight Analysis

The Fuego Nuevo II (FN‐II) dense plasma focus machine is a small device (∼5kJ @ 37 kV). In order to have an adequate picture of the neutron pulse with temporal resolution, a set of five similar scintillation‐photomultiplier detectors have been assembled, using BC‐400 scintillators, 50 mm high and 120mm radius, coupled to photomultipliers. These detectors are shielded by a lead wall, and a copper foil 50 mm and 3 mm thick, respectively. This work shows some preliminary results, in which a response of the five systems is shown, when they are placed 7.5 m away from the neutron source. A comparison between the time integrated neutron pulses and the neutron yield, measured with silver activation counters, is made.