Small Plasma Focus Device Research Articles

Effect of preionization on the dynamics of current sheath in a small plasma focus device

In the dense plasma focus (PF) device, the dynamics of the current sheath is an important parameter for good focusing or pinching. In this paper, we present the effect of preionization on current sheath dynamics, its pinching evidence and details of its filamentary structure by measuring the axial run-down velocity, confinement time and focus amplitude in a miniature PF device using a Rogowskicoil and high-voltage (HV) probe. For this purpose, the discharge is produced in deuterium gas with deposited energies lying in the range of 3.3 kJ. The Rogowski coil and HV probe signals reveal the formation of an axisymmetric parabolic current sheath propagating down the coaxial tube, and longer confinement time of it during the collapse phase and increase in focus amplitude with preionization source. The axial run-down velocity of the current sheath is increased from 2.1 to 3.2 cm/μ s. The focus duration is from 45 to 72 ns and the focus amplitude is from 1.8, which increases to 2.9 with preionization at the optimum pressure. The effect of the working pressure on the current sheath dynamics and its spatial evolution are also studied.

Potentiality of a small and fast dense plasma focus as hard x-ray source for radiographic applications

Currently, a new generation of small plasma foci devices is being developed and researched, motivated by its potential use as portable sources of x-ray and neutron pulsed radiation for several applications. In this work, experimental results of the accumulated x-ray dose angular distribution and characterization of the x-ray source size are presented for a small and fast plasma focus device, ‘PF-400J’ (880 nF, 40 nH, 27–29 kV, ∼350 J, T/4 ∼ 300 ns). The experimental device is operated using hydrogen as the filling gas in a discharge region limited by a volume of around 80 cm3. The x-ray radiation is monitored, shot by shot, using a scintillator–photomultiplier system located outside the vacuum chamber at 2.3 m far away from the radiation emission region. The angular x-ray dose distribution measurement shows a well-defined emission cone, with an expansion angle of 5°, which is observed around the plasma focus device symmetry axis using TLD-100 crystals. The x-ray source size measurements are obtained using two image-forming aperture techniques: for both cases, one small (pinhole) and one large for the penumbral imaging. These results are in agreement with the drilling made by the energetic electron beam coming from the pinch region. Additionally, some examples of image radiographic applications are shown in order to highlight the real possibilities of the plasma focus device as a portable x-ray source. In the light of the obtained results and the scaling laws observed in plasma foci devices, we present a discussion on the potentiality and advantages of these devices as pulsed and safe sources of x-radiation for applications.

Statistical characterization of the reproducibility of neutron emission of small plasma focus devices

The purpose of this work is to discuss the techniques related to the detection of fast pulsed neutrons produced in plasma focus (PF) devices, the statistical analysis of the corresponding data, and the methodologies for evaluation of the device performance in low emission neutron sources. A general mathematical framework is presented for the assessment of the reproducibility of the neutron emission of small PF devices given the shot-to-shot distribution and detector efficiency. The effect on the reproducibility in case of using two independent detectors is also discussed. The analysis is applied to the neutron emission of the plasma focus device PF-50J operating in repetitive mode (0.1–0.5 Hz and 65 J bank energy).

Prospects for 13N Production in a Small Plasma Focus Device

In this paper, the feasibility of 13N radioisotope production by a small plasma focus device for using in positron emission tomography (PET) has been studied. A large quantity of experimental data on the deuteron beam emission in dense plasma focuses are summarized and has been used in estimation of deuteron energy spectrum, intensity and angular distribution. The induced activity of 13N by 12C(d,n)13N reaction in an external solid target is calculated for different ‘m’ values (the power in energy distribution function of deuterons), and for a repetition rate plasma focus. A small plasma focus can produce 13N radionuclides in the order of 10 kBq in one shot, and it can be increased to few 10 MBq in a rep rate working mode with f = 10 Hz after 600 s operating time. Whereas a typical PET scan in myocardial blood perfusion assessment requires about 4 GBq radiopharmaceutical of 13N, it is concluded that a small plasma focus device, even with repetition frequency of f = 10 Hz can’t produce adequate 13N activity for this special PET imaging. Nonetheless, higher producible activities in higher energy PF devices and by endogenous production methods (i.e., nuclear reactions are induced inside the pinch itself) maybe result to introduction of an optimized repetitive high energy plasma focus as an alternative for cyclotrons in this special application.

Beryllium neutron activation detector for pulsed DD fusion sources

A compact fast neutron detector based on beryllium activation has been developed to perform accurate neutron fluence measurements on pulsed DD fusion sources. It is especially well suited to moderate repetition-rate (<0.2 Hz) devices, such as the plasma focus or Z-pinch. The detector comprises a beryllium metal sheet sandwiched between two large-area xenon-filled proportional counters. A methodology for calculating the absolute response function of the detector using a “first principles” approach is described. This calibration methodology is based on the 9 Be(n,α) 6 He cross-section, energy calibration of the proportional counters, and numerical simulations of neutron interactions and beta-particle paths using MCNP5. The response function R ( E n ) is determined over the neutron energy range 2–4 MeV. The count rate capability of the detector has been studied and the corrections required for high neutron fluence measurements are discussed. For pulsed DD neutron fluencies >3×10 4 cm −2 , the statistical uncertainty in the fluence measurement is better than 1%. A small plasma focus device has been employed as a pulsed neutron source to test two of these new detectors, and their responses are found to be practically identical. Also the level of interfering activation is found to be sufficiently low as to be negligible.

Short-Lived PET Radioisotope Production in a Small Plasma Focus Device

High-energy deuterons from a small plasma focus device (1.7 kJ) have been used for short-lived radioisotope production. For the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> C(d, n) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> N reaction, 40 kBq of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> N was obtained by bombarding a graphite target at a 1-Hz shot repetition rate for 30 s. This could be increased to 10 MBq at a 16-Hz repetition rate. Plasma focus devices operating with higher bank energies should produce substantially higher isotope yields.

Enhancing Neutron Emission From a 500-J Plasma Focus by Altering the Anode Geometry and Gas Composition

The plasma focus (PF) could be a valuable neutron source for Homeland Security applications if high repetition rate with reproducible output is demonstrated. Increasing the neutron yield from a small PF device would contribute to producing a PF neutron source than can compete with other electronic neutron generators for Homeland Security applications and neutron radiography. Two approaches to increase the neutron yield were examined. The first was to alter the anode geometry from a conventional cylinder (20-mm diameter) to a cone (20-5 mm) over a 20-mm length. The neutron-yield increase was 5× in pure D2. The second was to mix inert gases with the deuterium. When 2%, 4%, and 8% Ar and Kr were added to D2, the cylindrical anode showed a maximum 3.6× increase in neutron yield while the conical anode did not show any improvement in neutron yield. However, the operating mass density (pressure) range was increased for the conical anode when inert gases were added.

Multiple pinch formations in small plasma-focus devices

Multiple pinch formations in small plasma-focus devices

Neutron Production at the Small Plasma-Focus Device With Antianode

Neutrons from deuteron-deuteron fusion reactions were used for the study of pinch dynamics in a modified small plasma-focus device at a current peak of 200 kA. In front of the inner electrode (anode), an antielectrode was placed. Neutron yield reached 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> - 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> per shot. The time-resolved neutron signals were registered with two scintillation detectors positioned end-on and side-on at the same distance of 150 cm from the interelectrode gap. They enabled the comparison of difference in energies and estimation of directions of fast deuterons producing neutrons. The XUV frames imaged the pinch configuration at the moment of X-ray and neutron emission and differed in shots with high and low neutron yields. Interferograms enabled the evaluation of distribution of plasma density. The presence of the antianode decreased the neutron yield and influenced the distribution of the plasma density in front of the anode.

Nuclear activation measurements of High energy deuterons from a small plasma focus

High-energy deuterons from a small (1.7 kJ) plasma focus device were studied by nuclear activation of a boron-carbide target. The ratio of 10B( d , n ) 11C and 12C( d , n ) 13N yields indicates a deuteron spectrum decreasing rapidly between 400 keV and 1 MeV. This spectrum could take the form of d N d / d E ∝ E − n with n ≈ 9 .

Characterization of neutron emission and measurement of the electronic density in a plasma focus device of 400J

Experimental results of anisotropy in the distribution of neutron flux and his mean energy in the radial direction (90°), and measurements of the electron density in a very small plasma focus device, PF-400J (880 nF, 30 kV, 120 kA, 400 J, 300 ns time to peak current, dI/dt∼4 × lO11 A/s) are presented. The following diagnostics were applied in deuterium: Time of Flight (ToF) to estimate the neutron mean energy. An array of CR-39 (C12H16O7) nuclear track detectors covered with polyethylene located radially at several positions (between -90° to 90°) in order to determinate the angular distribution of the neutron emission. The measurements of the electron density were realized in hydrogen, and the plasma discharge was synchronized with a pulsed Nd-YAG laser (∼6ns FWHM at 532nm) in order to obtain interferometric diagnostics. Discharges were performed with a charging voltage of 30±2 KV (∼400J).

Efficiency of Energy Transfer in a Small Plasma Focus Device

A study of energy transfer in a small plasma focus device has been carried out during its axial phase. The snow-plough model has been used in the simulation as a basic model for the calculation of plasma dynamics. The energy transferred to the plasma is calculated by considering the work done by the electromagnetic piston during the axial phase. It was found that the plasma energy calculated by this model agrees well with the experimental data within the pressure range of 1 mbar to 4 mbar if the mass shedding effect is included in the model. According to the present computation, the energy transferred into the plasma, in the case of a plasma focus with 2.3 kJ initial energy operated with nitrogen gas within the pressure range of 1 to 4 mbar, is between 224 J to 250 J. This corresponds to energy transfer efficiency of 9.6% to 10.7%. The mass shedding factor decreases from 0.23 to 0.069 with increasing pressure. Correspondingly, the energy transfer efficiency changes slightly at a higher pressure.

Imaging of Fusion Protons from a 3 kJ Deuterium Plasma Focus

This paper reports a study of the proton emission from a 3 kJ, 14 kV plasma focus device operated with deuterium gas at 400 Pa. A filtered pinhole camera with a 1.8 mm diameter hole is placed axially downstream of the plasma focus, and images of the proton-emitting region are recorded using CR-39/PM-355 nuclear track detectors. The detector plates are scanned using an automated track measurement system and the spatial track density profile is acquired. The resulting density distribution is interpreted with the help of a simple pinhole imaging model that assumes the 2H(d, p)3H reaction protons are emitted from a conical region extending from the tip of the anode to a fixed distance downstream. Comparison of the experimental track density profile with the model calculations supports the view that the beam-target mechanism is the dominant fusion production mechanism in this small plasma focus device.

Deposition of titanium nitride thin films on stainless steel—AISI 304 substrates using a plasma focus device

A 3.3 kJ pulsed plasma focus device was used to deposit thin films of titanium nitride (TiN) at room temperature onto the stainless steel—AISI 304 substrates. The small plasma focus device, fitted with solid titanium anode instead of the usual hollow copper anode, was operated with nitrogen as the filling gas for deposition of TiN thin films. Films were deposited with different numbers of focus shots, at different distances from the top of the anode, and at different angular positions with respect to the anode axis. Deposited films have been characterized for their structure by X-ray diffractometry (XRD), surface morphology by scanning electron microscopy (SEM), elemental composition and distribution mapping by energy dispersive X-ray (EDX) analysis and hardness using a nanoindenter. XRD patterns show the growth of as-deposited polycrystalline TiN thin film. Diffraction patterns for films deposited along the anode axis show induction of a phase corresponding to iron chromium nickel on the film–substrate interface. SEM pictures confirm uniformly distributed TiN grains with hardly any crack over the film surface. Conglomeration of smaller TiN grains, to form bigger size grains, is seen to occur at the films deposited with higher total ion flux. The EDX spectra show the presence of expected constituent elements. EDX mapping confirms the uniform distribution of TiN on the film surface. The variation in structure, morphology, thickness and hardness of the deposited films with the variation of film deposition parameters is explained, qualitatively, on the basis of ion emission characteristics of the focus device. Polycrystalline, smooth and hard thin films of TiN are successfully deposited at room temperature stainless steel—AISI 304 substrates using the plasma focus device.

A plasma focus driven by a capacitor bank of tens of joules

As a first step in the design of a repetitive pulsed neutron generator, a very small plasma-focus device has been designed and constructed. The system operates at low energy (160 nF capacitor bank, 65 nH, 20–40 kV, and ∼32–128 J). The design of the electrode was assisted by a computer model of Mather plasma focus. A single-frame image converter camera (5 ns exposure) was used to obtain plasma images in the visible range. The umbrellalike current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher. The calculations indicate that yields of 104–105 neutrons per shot are expected with discharges in deuterium.

Generation Of High Energy Ion Beams from a Plasma Focus Modified for Low Pressure Operation

This paper reports on the modification of a small plasma focus device (UNU/ICTP PFF) to operate at low pressure for enhanced ion beam generation. For the present operation, the electrode geometry of the plasma focus device is modified to redirect the plasma motion at the end of the axial rundown phase so that normal plasma focus action will not occur. The redirection of the plasma leads to a rapid increase in the plasma resistivity and hence a high localized electric field will be established and as a result, high energy ion beam will be generated. With air as the working gas, pressure as low as 10-3 mbar has been tested. At such a low pressure, breakdown of the discharge must be initiated with the use of a set of twelve plasma injection cable guns. The modified plasma focus has been shown to be able to produce high energy ion beams of nitrogen, argon and hydrogen consistently.

Spatial structure and energy spectrum of ion beams studied with CN detectors within a small PF device

The paper reports the application of Solid-State Nuclear Track Detectors to study the pulsed plasma-ion streams emitted from plasma-focus (PF) type discharges, which were performed within a low-energy PACO device constructed at Instituto de Fisica Arroyo Seco. The PACO device was operated under static initial gas conditions or with dynamic gas puffing. Studies of the structure of the fast deuteron beams were carried out within an energy range from 80 keV to about 2 MeV . Studies of ion energy and an ion angular distribution were also performed. The measurements showed that the fast deuterons are emitted in many “narrow” micro-beams, as in other larger PF devices. The anisotropy of the deuteron angular distribution was explained by the stochastic character of the formation of local ion sources within the PF discharge column.

Measurement of magnetic field and velocity profiles in 3.6 kJ United Nations University/International Center For Theoretical Physics plasma focus fusion device

A Mather-type small plasma focus device was operated in air filling in the pressure range of 0.5 to 1.0 Torr and capacitor bank charging voltage of 13–15 kV. A strong focusing action was observed in this pressure range. Magnetic probe signals at various axial positions were used to estimate velocity of current sheet and axial magnetic field distribution profiles. It was observed that under the present experimental conditions the magnetic field remains constant at 0.72 T from z=0.0 cm to z=8.0 cm but falls rapidly to 0.52 T at about z=14.5 cm at a fixed radial distance of 2.65 cm. The magnetic field and velocity measurements indicate a current shedding effect—only 68.5% of the total injected current flows into the focus region. The rapid drop of the magnetic field at z=8.0 cm suggests that further (initial shedding is at the insulator) current and mass shedding in the focus tube is significant after this value of z. Experimental values of velocity of the current sheet are compared with those of the snow plough theoretical model.

Different modes of plasma sheath motion in the radial compression and pinch phases in plasma focus

Two distinct modes of plasma sheath (PS) motion, the stable mode and the instability mode, in the radial compression and pinch phases in a small Mather-type plasma focus device (20 kV, 18 kJ) have been observed using laser differential interferometry. The two modes occur randomly from shot to shot in consecutive discharges and their probabilities are much influenced by the anode end structures. The instability mode is characterized by the rapid radial movement of the sliding front of the PS along the anode (centre electrode) end surface. The two modes show very different sheath and pinch configurations according to their different ways of motion. They also have notable influences on the neutron yield, in that the instability mode always produces a lower one and the stable mode a higher one. The mean neutron yield for consecutive discharges is made low and largely deviated by the co-existence of the two modes. The instability mode is found to be caused by the anode end geometry with a small transition arc, whose nature is of the m = 0 MHD type but only with a half wavelength, occurring at the plasma - electrode layer. The instability can be either avoided by a hemispherical-ended anode with a larger transition arc or stabilized by the stagnation of the radial movement of the sliding front of the PS in the initial radial compression by the hollow edge and the stable mode formed. The magnetic field introduced in front of the PS by the anode end discharge that occurs from the very beginning of the main discharge is also found to act as a stabilization mechanism for the instability and to cause some of the stable mode.

Occupational exposure to X-rays from a small plasma focus

The occupational exposure to the hard X-rays emitted by a small plasma focus device is assessed using a plastic scintillator and photomultiplier. Personnel monitoring film badges are used to assess the radiation levels at various positions surrounding the plasma focus.