Mather-type Plasma Focus Device Research Articles

Experimental Study of Current Discharge Behavior and Hard X-ray Anisotropy by APF Plasma Focus Device

Amirkabir (APF) is a new Mather-type plasma focus device (16 kV, 36 μf, and 115 nH). In this work we present some experimental results as variation of discharge current signal respect to applied voltage at the optimum pressure, focusing time of plasma versus gas pressure, and variations of current discharge with different insulator sleeve dimensions. As we prospected optimum pressure tending to increase as we tried to higher voltage levels. The time taken by the current sheath to lift-off the insulator surface and therefore quality of pinched plasma depends on the length of the insulator sleeve. The results show that the insulator diameter can influence on pinch quality. Behavior of hard X-ray (HXR) signals with the pressure and also anisotropy of HXR investigated by the use of two scintillation detectors. The distribution of HXR intensity shows a large anisotropy with a maximum intensity between 22.5° and 45° and also between −22.5° and −67.5°.

X-ray Emission from Plasma Focus: Envisioned by Various Competitive Detectors

A study of X-ray emission from a Mather-type plasma focus device by simultaneously employing various X-ray detectors like silicon pin diode, photoconducting detectors (PCDs)—CVD-diamond and gallium arsenide (GaAs), plastic scintillator coupled with photomultiplier tube with and with out optical fiber is presented. The pin diode and PCDs are masked with 10 μm thick cobalt filter. The device is energized by 9 μF capacitor bank charged at 18 kV (1.45 kJ), giving a peak discharge current of about 175 kA, with hydrogen as the filling gas. The optical fiber coupling is found to be beneficial in minimizing the electromagnetic noise generated during the system operation.

Effective hard x-ray spectrum of a tabletop Mather-type plasma focus optimized for flash radiography of metallic objects

The effective spectrum of the hard x-ray output of a Mather-type tabletop plasma focus device was determined from attenuation data on metallic samples using commercial radiographic film coupled to a Gd2O2S:Tb phosphor intensifier screen. It was found that the radiation has relevant spectral components in the 40–150 keV range, with a single maximum around 60−80 keV. The radiation output allows for 50 ns resolution, good contrast, and introspective imaging of metallic objects even through metallic walls. A numerical estimation of the induced voltage on the focus during the compressional stage is briefly discussed.

The correlation of x-ray emission with pinch energy in a 1.5 kJ plasma focus

Correlation of x-ray emission with pinch energy from a 1.5 kJ Mather-type plasma focus device for Ag and Sn inserts at the Cu tapered anode tip is reported. The space and time resolved x-ray emission characteristics are investigated by using a simple pinhole camera with appropriate filters and a multichannel pin-diode spectrometer. High voltage probe and Rogowski coil signals are used to estimate the pinch energy. At optimum conditions, the maximum x-ray yield in 4π-geometry is found to be 9 and 8 J/shot with efficiency of 0.6% and 0.5% for Sn and Ag inserted anodes. This is despite the fact that input energy converted to pinch energy is lower at 8% for Sn insert compared with 15% for the Ag insert. An increase in x-ray yield with an increase in pinch energy is observed for Sn as well as Ag. Pinhole images reveal that x-rays of energy less than 5 keV are emitted from the focus region and the high-energy x-rays are emanated from the anode tip.

Nitrogen ion implantation of silicon in dense plasma focus

A low energy (1.45kJ) Mather type plasma focus device is used for nitrogen ion implantation in mono-crystalline silicon. The silicon specimens are exposed to different number of focus shots by placing the targets in front of the anode at a fixed distance. Raman spectroscopy and X-ray diffraction are employed to characterize the implanted specimens. The results indicate that mono-crystalline silicon transforms into amorphous structure through micro-crystalline phase on increasing the implantation dose. Further irradiation of the specimens results in the formation of amorphous Si3N4 layers. High temperature annealing in argon ambient transforms the amorphous Si3N4 into β-Si3N4.

Effect of the insulator length on Mather-type plasma focus devices

Abstract Plasma focus devices were studied in order to identify the design parameters and logical relationships that exist among device components. A quantitative parameter has been defined as the ratio of insulator sleeve length to the radial distance between the electrodes, “ L ins / ( b − a ) ”. It is demonstrated that the Mather-type plasma focus device, operated in the optimum regimes, has a remarkably low sensitivity to this parameter which relates the insulator length to the electrodes geometry. A modification factor for the axial velocity in different PF devices has been introduced to maintain the value of the “ q ” parameter within the range of zero to one.

The nitriding of aluminium by dense plasma focus

High current, short length nitrogen ion beam pulses emanating from a low energy (1.8 kJ) Mather type plasma focus device are utilized for room temperature surface nitriding of aluminium specimens. X-ray diffraction (XRD), scanning electron microscopy and microhardness are used to study the surfaces of untreated and treated specimens. The surface hardness of the ion implanted specimens is found to improve by 300%. XRD studies reveal that the increase in microhardness is caused by the formation of cubic aluminium nitride phase.

Dependence of neutron yield on the deuterium filling pressure in a plasma focus device

A Mather-type plasma focus device (32 µF, 4 kJ), called Hanyang University Plasma Focus device, is developed as a prototype device for the irradiation test of neutrons for electric probes and cables to be used in Korea Superconducting Tokamak Advanced Research with pure deuterium gas. The six different lengths of electrode are used in order to see the dependence of neutron yield on the deuterium filling pressure at given system conditions such as capacitance, currents and inductance, after optimizing the focusing condition in terms of electrode length versus filling pressure. The relationship between the pressure and electrode length is to be fitted well by the snow-plow model. The neutron fluence and angular distribution are measured at the angles of 0°, 25°, 60° and 90° by locating the bubble neutron dosimeter at a distance 30 cm from the inner electrode head at each electrode length. The anisotropic factor tends to increase with pressure and the total neutron yield is strongly dependent on the isotropic emission. The maximum neutron yield is estimated to be about 1.6 × 108 (n/shot) at a pressure of 3.4 Torr.

Comparative study of soft x-ray emission characteristics in a low energy dense plasma focus device

An investigation on the soft x rays emitted in a 2.2 kJ Mather-type dense plasma focus device using a multichannel diode spectrometer and a simple pinhole camera is reported. Emitted x rays associated with different shapes (hollow, solid, and hemispherical) of anode and in hydrogen/nitrogen gas medium are compared. The structure of x-ray emitting sites as well as x-ray yields were found to be strongly influenced by the shape of the anode and the filling gas pressure. The maximum yield of 2.2 J into 4π sr was obtained in the case of hemispherical anode in hydrogen gas medium. The x-ray pinhole images of the collapsed plasma with the hemispherical anode indicated spot-like structure having 500–800 μm in diameter. On the contrary, other anode shapes showed columnar pinched structure of 8–10 mm in length and 1–2 mm in diameter. Results indicated that an appropriate design of the anode could enhance the x-ray yield by more than tenfold in a conventional low energy dense plasma focus device.

Study of neutron emission in a low-energy plasma focus with β-source-assisted breakdown

The influence of pre-ionization around the insulator sleeve of plasma focus, by a mesh-type β-source 28Ni63 on neutron emission is investigated. The experiment is conducted on two low-energy (1.15 and 0.58 kJ) Mather-type plasma focus devices. The neutron emission is found to be increased by upto 25%. The results of this experiment suggest that pre-ionization may be helpful in designing a device as a sealed neutron source for different field applications.

Energy Dissipation in Run-Down Phase of Mather-Type Plasma Focus Discharges

The influence of the cathode structure on the focus discharges in a 7 kJ Mather-type plasma focus device was investigated by using two types of cathode electrodes, 1) bar and 2) tubular cathodes. I...

Anomalous hard x-ray emission mode in a plasma focus discharge with hydrogen-argon mixtures

We observed an anomalous discharge mode causing explosive surface evaporation and emission of hard x rays of 10–40 keV in the regime of the lower hydrogen partial pressures of hydrogen–argon mixture in a Mather type plasma focus device of 1.5 kJ. At the anomalous mode, the x rays were emitted from only the surface of the anode without emissions from a plasma column or hot spots, accompanying with explosive evaporations of the anode surface.

Measurement of Argon Ion Beam and x-ray Energies in a Plasma Focus Discharge

We investigated the characteristics of ion beam emission in a small 1.9 kJ Mather-type plasma focus device filled with argon gas, and obtained the energy spectra of argon ion beams using a time-of-flight (TOF) method with a capacitive current monitor and a well-designed Faraday cup. We also observed x-ray emissions, and obtained the time correlation between the x-ray emissions and the ion emissions. The maximum energy of the argon ions was over 800 keV, and the most common ion energy was 200 keV. Ion emission started before the x-ray emission, and was also observed when no x-ray emission was detected; however stronger ion emissions were observed when x-ray emissions were detected. The energy of soft x-rays was determined to be about 2.96 keV of Ar Kα line using a mass absorption method, and hard x-ray emissions of Cu Kα line was also observed. We compare our experimental results with several existing models of the charged particle acceleration in a plasma focus discharge.

Surface nitriding of graphite substrate by plasma focus device towards synthesis of carbon nitride coating

Circular graphite substrates have been nitrided in a plasma focus device. Nitrogen ions consisting of 100-ns pulses emanating from a Mather type plasma focus device were allowed to hit the surfaces of graphite substrates for 20 and 30 times (shots) to produce nitrided coatings. Microstructural characterization of the coatings carried out by XRD determined the d values of the nitride compound thus produced and four d values matched with those predicted by Cohen and co-workers and Wang et al. [Phys. Rev. B32 (1985) 7988; Science 245 (1989) 841; Phys. Rev. B41 (1990) 10727; Phys. Rev. B58 (1998) 11890] for carbon nitride. SEM and optical microscopic studies of the nitrided graphite surface reveal a rounded and island like morphology of the carbon nitride grains. Comparison of XPS results of the unnitrided and nitrided graphite shows the evidence for carbon–nitrogen bonding. IR spectra attribute the 1274.4 cm −1 absorption peak to C 3N 4 (sp 3). The clear improvements observed in microhardness values point to the growth of a hard carbon nitride phase.

Room temperature deposition of titanium carbide thin films using dense plasma focus device

Thin films of titanium carbide (TiC) were deposited on 304-stainless steel substrates using a dense plasma focus device. The conventional hollow copper anode of the 3.3 kJ Mather type plasma focus device was replaced by solid titanium anode for deposition of TiC thin films. The argon–acetylene admixture (in 7:3 ratio) was used as the filling gas at a pressure of 1.5 mbar. Substrates were placed along the anode axis at a distance of 11.5 cm from the top of the anode. Thin films of TiC were deposited using different numbers of focus shots, like 10, 20 and 30, at room temperature substrates. Deposited films were analyzed for their structure, surface morphology, surface profile, elemental composition and distribution using X-ray diffraction (XRD), scanning electron microscopy (SEM), surface profiling and energy dispersive X-ray spectroscopy (EDX). XRD patterns in all three cases are found to have diffraction peaks for (111), (200), (220), (311) and (222) TiC crystalline planes. Scanning electron micrographs show a smooth film surface with increasing clustering of TiC grains with the increase in number of focus deposition shots. EDX spectra confirm the presence of expected constituent elements with the gradual increase in TiK α peak (of the TiC thin film) relative to FeK α peak (of the substrate) with increasing numbers of focus deposition shots due to increasing film thickness. EDX mapping reveals a uniform distribution of TiC over the substrate surface. These results confirm the first ever deposition of TiC thin films using a dense plasma focus device.

Characteristics of Ion Beams Produced in a Plasma Focus Device

The characteristics of the ion beams produced in a plasma focus device were studied. In the experiment, a Mather-type plasma focus device was used which was prefilled with H2 at 2.3 Torr. Ion species and their energy spectra were evaluated with a Thomson parabola spectrometer. The particle pinhole image was obtained with an aluminum filtered particle pinhole camera. The ion current density was measured with a biased ion collector. From the measurement, the proton beam energy was found to be distributed from 0.15 MeV to 2 MeV. The particle pinhole image of the proton beam with an energy of more than 1 MeV has spotlike patterns. The ion current density of 2400 A/cm2 with a pulse width of 50 ns was observed on the axis at z=260 mm from the top of the anode. The peak power brightness of the proton beams was evaluated to be 18 GW/cm2/sr/keV, and the total power brightness was evaluated to be 20 TW/cm2/sr.

The Effect of High Axial Sheath Velocity on Neutron Yield in Plasma Focus

To study the effect of higher axial sheath velocities on neutron production, the geometry of the inner electrode (anode) of a 3-kJ Mather-type plasma focus device was modified. Double-stage stepped-anode configurations were tested with a speed-enhanced region long enough for a significant increase in speed but not long enough to allow the development of force-field flow-field separation at the end of the axial phase. Peak axial speeds up to 15 cm/μs were achieved without modifying the deuterium filling pressure and charging voltage. The neutron and soft X-ray productions were found to be dependent on sheath velocity. A new scaling law for the nonbeam component of the neutron yield was proposed.

Nitrogen ion spectrum from a low energy plasma focus device

The nitrogen ion spectrum generated from a low energy Mather-type Plasma Focus device is reported. The main species of ions present are N/sup +1/, N/sup +2/, and N/sup +3/. It is found that its relative concentrations at a distance of several cm from the ion source are in agreement with those predicted for an equilibrium charge-state of the beam, and hence the spectrum of the neutral nitrogen is also derived. By taking into account the lateral spread of the beam due to multiple elastic scattering with the background gas, the derivation of absolute values for the total ion spectrum within the investigated energy range (>170 keV), the total number and total energy of the fast particles has been obtained.

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.

Instability of the current sheath implosion in an 18-kJ Mather-type plasma focus.

An instability phenomenon of the current sheath implosion at the anode (center electrode) end, namely the unstable mode, in a Mather-type plasma focus device (DPF-40, 20 kV, 18 kJ) has been observed. The unstable mode is characterized by rapid slippage of the sliding front of the current sheath along the anode end surface and a "flat" sheath configuration caused by an instability phenomenon occurring at the sheath front-anode boundary layer. The unstable mode occurs randomly with respect to a stable mode from shot to shot in consecutive discharges, and significantly influences plasma pinching and neutron yield.