Measurements are made of high-energy electron trajectories in a plasma focus as functions of position, time, energy, and angle of emission. The spatial resolution of the X-ray emission shows that low-energy X-rays are emitted from the anode surface. It is also suggested that the highest energy X-rays originate from a small region on the axis. The so-called shadow technique shows that the electron beam is perpendicular to the anode surface. Polar diagrams of medium and high-energy X-rays agree with the bremsstrahlung emission from a relativistic electron beam, the current of which is several 100 A.

https://iopscience.iop.org/article/10.1088/0032-1028/20/2/001/pdf

Trajectories of high energy electrons in a plasma focus

The dense plasma focus (DPF), a coaxial plasma gun, utilizes pulsed high current electrical discharge to heat and compress the plasma to very high density and temperature with energy densities in the range of 1-10 ? 1010 J/m3. The DPF device has always been in the company of several alternative magnetic fusion devices as it produces intense fusion neutrons. Several experiments conducted on many different DPF devices ranging over several order of storage energy have demonstrated that at higher storage energy the neutron production does not follow I4 scaling laws and deteriorate significantly raising concern about the device's capability and relevance for fusion energy. On the other hand, the high energy density pinch plasma in DPF device makes it a multiple radiation source of ions, electron, soft and hard x-rays, and neutrons, making it useful for several applications in many different fields such as lithography, radiography, imaging, activation analysis, radioisotopes production etc. Being a source of hot dense plasma, strong shockwave, intense energetic beams and radiation, etc, the DPF device, additionally, shows tremendous potential for applications in plasma nanoscience and plasma nanotechnology. In the present paper, the key features of plasma focus device are critically discussed to understand the novelties and opportunities that this device offers in processing and synthesis of nanophase materials using, both, the top-down and bottom-up approach. The results of recent key experimental investigations performed on (i) the processing and modification of bulk target substrates for phase change, surface reconstruction and nanostructurization, (ii) the nanostructurization of PLD grown magnetic thin films, and (iii) direct synthesis of nanostructured (nanowire, nanosheets and nanoflowers) materials using anode target material ablation, ablated plasma and background reactive gas based synthesis and purely gas phase synthesis of various different types of nanostructured materials using DPF device will discussed to establish this device as versatile tool for plasma nanotechnology.

https://iopscience.iop.org/article/10.1088/1742-6596/591/1/012021/pdf

Dense Plasma Focus - From Alternative Fusion Source to Versatile High Energy Density Plasma Source for Plasma Nanotechnology

In order to gain insight into the possible neutron production mechanism in sub-kilojoule miniature plasma foci, the anisotropy in neutron and hard x-ray emissions from a newly developed fast miniature plasma focus device (2.4??F, 27?nH, T/4 ~ 400?ns, 12?15?kV, 170?270?J, total mass ~25?kg) is investigated. The system produces an average neutron yield of (1.2?0.2) ? 106?neutrons/shot into 4? sr at ~80?kA peak discharge current and 5.5?mbar deuterium operation, as measured by a 3He proportional counter. Relative measurements of radiation yields, to gauge the emission anisotropy, have been performed using a pair of fast scintillator?photomultiplier detectors placed along the axial (0?) and radial (90?) directions. The average forward to radial neutron yield anisotropy is found to be 1.3?0.2. The average peak neutron energy for the axial and radial directions is estimated to be (2.9?0.3)?MeV and (2.6?0.1)?MeV, respectively. The observed higher fluence along the anode axis and the coincidence between occurrence of maximum neutron yield and anisotropy at 5.5?mbar deuterium filling gas pressure, suggest that the neutron production mechanism in the sub-kilojoule range miniature plasma focus device FMPF-1 may be predominantly beam?target in nature.

https://iopscience.iop.org/article/10.1088/0741-3335/51/7/075008/pdf

Experimental study of neutron emission characteristics in a compact sub-kilojoule range miniature plasma focus device

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 two maxima around the axis, presumably as a result of the collision of a collimated electron beam against the inner electrode, along the axis.

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

Neutron detectors have been developed and applied to a variety of pulsed plasma sources with intensities ranging from 102 to 1012 neutrons/pulse. For yields as small as 102 neutrons, a 6Li-glass-scintillator detector is used. A rhodium-activation counter has been developed for yields greater than 106 neutrons. These detectors are used for total yield and angular distribution measurements. Neutron energies are determined primarily by the time-of-flight technique. Limitations on this technique for neutron yield and energy measurements are discussed. Improved methods are needed to measure the spatial extent of these neutron sources.

Neutron Diagnostics for Pulsed Plasma Sources

The fluence of neutrons from a plasma focus was measured by gamma spectrometry of an activated silver target. This method results in a significant increase in accuracy over the beta‐counting method. Multiple detectors were used in order to measure the anisotropy of the fluence of neutrons. The fluence was found to be concentrated in a cone with a half‐angle of 30 deg about the axis, and to drop off rapidly outside of this cone; the anisotropy was found to depend upon the total yield of neutrons. This dependence was strongest on the axis. Neither the axial concentration of the fluence of neutrons nor its dependence on the total yield of neutrons is explained by any of the currently proposed models. Some other explanations, including the possibility of an axially distributed source, are considered.

K. H. Kim

Papers

Anisotropy of the neutron fluence from a plasma focus.