Plasma Focus Research Articles

Preliminary Results of a Novel Designed 7.7 kJ Plasma Focus Device with Hemisphere Electrodes

The Hemisphere Plasma Focus Device, a new construction of plasma focus devices is constructed and operated with total energy of 7.7 kJ. The discharge takes place between two concentric hemispherical electrodes. The plasma sheath is accelerated and confined by electromagnetic force. Rogowiski coil and potential divider are used to characterize the discharge current, the discharge voltage and obtain the pinch effect in shape of sharp crevice. The deposited energy is about 31.5% of the input energy for 6kV charging voltage. The total system inductance is about 285 nH with low resistance of about 23 mΩ. The plasma inductance has maximum value at the pinch time. The pinch time decreases by increasing input energy and also by decreasing the helium gas pressure. The average sheath velocity varies in range between 1.5 and 2.7 cm/µs which indicates that the charge state is about 2 and plasma temperature should mostly be over 11.2eV.

Design of a 16-channel fiber-optic coupled radiation detection array for radiation asymmetry studies in a 750-kJ dense plasma focus.

A 16-channel fiber optically coupled radiation detection array has been developed for studies of radiation asymmetries and emission histories from the Verus Research 750-kJ Dense Plasma Focus. Each detector in the array consists of a light-tight housing with a plastic scintillator coupled to a fiber optic that is fed into one channel of a multi-anode photomultiplier tube (PMT). The PMT and associated electronics are located in a remote electrically shielded control room. The detector head is configurable for using a Be-9 foil to take advantage of the 9Be(n,α)6He reaction as a fast neutron activation detector or with a bare scintillator alone to record the radiation emission history. Fiber optically coupling the detector head not only provides electrical isolation in the pulsed power environment but also allows the spatial footprint of the detector array to be reduced with concomitant flexibility in positioning each individual detector head. The array allows for spatially resolved neutron yield and radiation waveform measurements for fast z-pinches. The activation detector heads were calibrated for the total neutron yield against silver and indium activation counters for the total neutron yield. Fiber scintillation was found to contribute to the time-resolved detector head signals and was accounted for.

Ion Acceleration Parameter of the Plasma Focus Device

High-energy ion beam emission above megaelectronvolts from the plasma focus strongly correlated with the transient impedance developed across the plasma column. Ion acceleration parameter is formulated based on the experimental ion spectrum coupled to the pinch simulation, to estimate the mean ion energy E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mn</sub> from the accelerating potential V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> . The experimental plasma impedance of 200- 500 mΩ sets off E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mn</sub> =(3→ 8)V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> for sub-kJ to MJ-scale plasma focus devices. Adjustment of the parameter during the radiation phase speeds up or slows down E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mn</sub> and therefore alters the acceleration characteristics of the plasma focus ion emissions. Simultaneously, extensive measures through the magnetic spectrometry along with the space charge formulation on a diode ensure a reasonable level of the data quality on plasma focus ion beams.

Broad-energy oxygen ion implantation controlled magnetization dynamics in CoFeTaZr

In this paper, a novel pulsed broad energy spectrum ion-implantation technique, using the dense plasma focus device (DPF), for uniform oxygen-ion doping along the thickness of a ~250 nm thick magnetic CoFeTaZr layer is investigated. A new operational regime of the dense plasma focus – the off-focus mode – is explored to avoid the surface damage of the exposed sample by the high energy plasma streams/jets and instability accelerated ions, typically observed in conventional efficient-focus mode operation. The faraday cup measurements shows the increase in ion fluence from 3.83 × 1013 ion/cm2 for efficient-focus mode to 8.76 × 1013 ion/cm2 for off-focused mode operation in the broad-ion-energy range of 1–100 keV. The x-ray photoelectron spectroscopy (XPS) of the unexposed sample shows the presence of Co in Co0, Co2+ and Co3+, Fe in Fe0, Fe2+ and Fe3+, and Ta in Ta0 and Ta2+ oxidation states while Zr was observed with only metallic Zr binding energy peaks indicating the surface oxidation of the unexposed sample. The exposure to oxygen plasma in DPF device led to the increase in the higher oxidation states of Co, Fe and Ta with reduction in metallic binding energy peak and the deconvolution of oxygen XPS spectrum confirmed the bonding of oxygen to Co, Fe and Ta. The magnetization dynamics of unexposed and oxygen-ion doped samples was studied using magnetoimpedance measurements in the 1–2.5 GHz frequency range. Gilbert’s damping factor, in-plane anisotropy and effective magnetization of the magnetic substrate were calculated and it is found that these properties can be modulated with a lighter ion dosage using this novel pulsed broad-energy-ion implantation technique. It is concluded that the off-focus mode DPF operation can provide the ions of required energy and fluence to implant oxygen ions across the thickness of the CoFeTaZr magnetic thin film to modulate its magnetic properties.

Investigation of Electron Beam Characteristics Emitted From Various Gases Employing the Vlasov–Maxwell Equations With the Forward Semi-Lagrangian Method

The characteristics of the electron beams emitted from the hydrogen ( $H$ ), nitrogen ( $N$ ), neon (Ne), and argon plasma pinches have been numerically investigated and compared by solving the Vlasov–Maxwell system with the forward semi-Lagrangian approach. Several outcomes have been obtained. For examples, it was found that, in the case of 1-mbar H, the electron current density reached a value of about $1.431\times 10^{9} \,\,\text {A}\cdot \text {m}^{-2}$ due to turbulence, while it reached ${2.554} \times {10}^{7} \,\,\text {A}\cdot \text {m}^{-2}$ due to other phenomena. For 2-mbar $N$ , the calculated voltage drops across the $m = 0$ regions ranged from about 817 kV to 4 MV. Among these gases, Ne had the biggest upstream emission current of about 9.7 kA, while $N$ emitted the least electron beam current of about 4.1 kA. Computations showed that the total charge delivered by each gas beam was of order 0.1 mC. The method of modeling, presented in this article, can be efficiently used to study the dynamics of a plasma pinch. Since the estimated electron beam currents for various gases are in good agreement with the measured ones, the proposed method can be used for designing the plasma focus devices for the electron beam source purpose.

Characterization of carbonized magnesium formed in a low energy plasma focus device

Carbon-ion deposition on magnesium substrate was performed using a low energy Mather-type plasma focus device. Deposition process was carried out at 0, 15, 30 and 45° angular position with respect to the device anode axis. X-ray diffraction (XRD) confirmed presence of Mg2C3, Mg (OH)2 and graphite crystals on the produced films. Field emission scanning electron microscope (FE-SEM) coupled to elemental mapping analysis from surface and cross section of the samples was performed. The analysis showed the formation of a multi-layer thin film, in which carbon nano particles (CNPs) covered the composite layer. The CNPs were formed by graphite crystals while the composite layer formed by combination of Mg2C3 and Mg (OH)2 crystals. The band gap energy of indirect transition (⁓1.75 eV) of Mg2C3 was extracted from optical measurements of UV–Visible spectrum. XRD and Raman spectroscopy were used to investigate the graphite crystal structure of CNPs at each deposition angle. The study indicates that the graphite crystal structure of the samples is consisted of nano-crystalline and reduced nano-crystalline graphite structures. X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental composition of the surface structure. The presence of Mg2C3 and Mg (OH)2 in composite layer, nano-crystalline, and reduced nano-crystalline graphite in CNPs was identified by high resolution transmission electron microscopy (HR-TEM). Potentio-dynamic polarization test was used to analyze the corrosion resistance of the prepared thin films with respect to un-deposited substrate. The findings indicate that the thickness and the size of Mg2C3 crystals have direct effect on corrosion resistance parameters.

Plasma processed tungsten for fusion reactor first-wall material

Tungsten is one of the prime candidates for a first-wall material near the divertor area due to its high temperature strength, high thermal conductivity, low erosion rate and low tritium retention. The erosion resistance of tungsten to the edge plasmas and transient events are carefully investigated in a simulated fusion environment. Here, we use the dense plasma focus (DPF) device operated in a D2 as a source for pulsed fusion plasma. The tungsten (α-W) substrates with a preferential growth direction along (110) plane were used. These pristine-W samples were nanostructurized using a (i) low-temperature continuous nitrogen RF plasma system and (ii) coated with 60 nm tungsten film, using high-temperature argon plasma in a dense plasma focus (DPF) device. The low- temperature plasma treatment created mesh-like porous nanostructure on the surface of pristine-W with change in crystalline orientation to (200), while the DPF-based deposition resulted in a nanocrystal (30–50 nm) decorated surface with enhanced (200) orientation. The crack propagation and bubble formation during DPF D2 plasma exposure were significantly controlled by the surface modification of tungsten. The mesh-like structure was modified to form loosely bound spherical nanoparticles, while the nanocrystals remained tightly bound and grew in size with D2 plasma exposure. The better adhesion of the nanocrystals and controlled growth along the (200) direction resulted in least change in hardness measurements for the nanocrystal decorated samples. Thus, nanocrystal decoration of tungsten with a preferential growth direction of (200) can help reduce the fusion-induced damage in first-wall materials.

Influence of electrode geometry on X-ray emission, plasma inductance, voltage, and current derivative signals obtained from a plasma focus device

Plasma focus devices have been investigated as a pulsed radiation source. Different studies have shown that the shape of the anode affects strongly the plasma dynamic and X-ray emission that takes place in plasma focus devices. In the present work, a hybrid-type plasma focus device (6 μF, 16 ± 1 kV), named PF-2kJ, is operated at various pressures of hydrogen gas with five cylindrical anodes of the same lengths but different in shapes, without cathode bars. The effect of cathode bars on various electric parameters is studied for the anode that has the maximum X-ray emission. The electrical signals of Rogowski coil (current derivative), voltage divider (voltage at the anode), and photomultiplier tubes (X-rays) were analyzed for all the anodes. Inductance and the voltage at the time of the pinch were derived from the measured voltage and current derivative signals. The X-ray dose measurements were performed using thermoluminescent dosimeters (TLD-100). The PF-2kJ performance is not the best while using the cathode bars. Our results show that by changing the anode shape just over the insulator sleeve the plasma focus devices can be switched between close energies stored in the capacitor bank.

On the representation of dense plasma focus as a circuit element

The dense plasma focus is a plasma discharge powered by a capacitor bank. Standard diagnostics include the measurement of the time derivative of the current through and the voltage across its connections with the capacitor bank. Interpretation of this diagnostic data often involves some assumptions regarding the representation of the dense plasma focus as a time varying inductance. One of the characteristic features of the current derivative waveform is a relatively sharp dip and an associated sharp voltage spike. This has often been interpreted as a result of a rapid rise in the time varying inductance of the plasma. Sometimes, an anomalous plasma impedance is invoked. This Letter discusses instances where such interpretation creates conceptual difficulties. A first principles approach to the representation of the dense plasma focus as a circuit element reveals some fundamental problems with the traditional representation of plasma focus as a time varying inductance. The anomalous impedance is shown to be necessary to account for the difference in the motional impedance implied by a time-varying inductance in the circuit element representation and a first principles description based on Poynting's theorem. Dynamo effects that convert post-stagnation local motion of plasma into three-dimensional magnetic fields are shown to contribute to the effective inductance of the plasma focus and resolve the observed conceptual difficulties.

Magnetohydrodynamic simulations of a megaampere-class Kr-doped deuterium dense plasma focus

The addition of Kr dopant to a deuterium or deuterium–tritium dense plasma focus (DPF) is conventionally thought to enhance radiative cooling of the imploding sheath, resulting in a tighter pinch and, under optimized conditions, increased neutron yield [M. Krishnan, IEEE Trans. Plasma Sci. 40, 3189 (2012)]. In this work, 2D radiation magnetohydrodynamic (MHD) simulations are conducted of a DPF at peak current levels in the 2–3 MA range with Kr dopant concentrations of 0%, 0.1%, and 1.0% (by volume). Fully kinetic simulations are required to accurately model the pinch stagnation and accurately predict total neutron yield (thermonuclear + beam target), as MHD simulations cannot capture kinetic effects or beam-target neutron production. However, insights can be gained from following the evolution of the bulk dynamics of the sheath. The results show that sheath width narrows with increasing dopant concentration due to increased radiation. Thermonuclear neutron yields of ∼109−1010 are observed, which is in good agreement with experimental data [E. N. Hahn et al., J. Appl. Phys. 128, 143302 (2020)] and simulations [N. Bennett et al., Phys. Plasmas 24, 021702 (2017)] that measure yields of ∼1011 at ∼2 MA with ∼1% of that yield having thermonuclear origin. Scaling in excess of the conventional ∝I4 scaling is observed, though this should be confirmed with 3D and/or fully kinetic simulations of Kr-doped DPFs.

Method to Improve Initial Azimutal Uniformity of a Current Shell in Devices with a Plasma Focus

Method to Improve Initial Azimutal Uniformity of a Current Shell in Devices with a Plasma Focus

Effect of filling gas on hard x-rays and ion beam emission from a 2 kJ plasma focus device

ABSTRACT The effects of the atomic number of the fill gas on hard x-ray and ion beam emission from a 2.2 kJ plasma focus were studied. Hydrogen, Nitrogen and Argon gases were used in the experiment. The anode voltage and discharge current were measured to calculate plasma inductance and to study the pinching regime in the three gases. The results showed that the heavier gas in the machine increases the duration of x-ray radiation, ion beam emission, and results in larger pinching plasma inductance. The Lee model suggests that the increase in the pinching plasma inductance can be explained by the occurrence of radiation collapse in the pinch operated with Argon gas. It is postulated that the occurrence of anomalous plasma heating may stop plasma compression and prevent the plasma from a complete collapse to reach near-zero radius.

Dynamics of ion beam emission in a low pressure plasma focus device

The plasma that accelerates and compresses in the formation of the pinch in dense plasma focus devices has been found to be an abundant source of multiple radiations like ion beams and x-rays. In this work, the ion beam and x-ray emissions from a 2.7 kJ (13.5 kV, 30 µF) plasma focus device operated at pressure below 1 mbar were investigated. The time profile of the ion beam emission was analysed from the simultaneously measured ion beam, soft and hard x-ray signals using biased ion collectors, BPX 65 silicon PIN diode and a scintillator-photomultiplier tube assembly. Time resolved analysis of the emissions revealed that the emission of the ion beam corresponded to several different pinching instances. Two components of the ion beam were identified. An ion beam of lower energy but higher intensity was emitted followed by an ion beam of higher energy but lower intensity in the first plasma pinch. The ion beam emitted from the first plasma pinch also has higher energy than subsequent plasma pinches. The emission was found to be associated with the amplitude of voltage spike. The results from ion beam and electron beams suggest that they were emitted by the same localized electric field induced in the pinched plasma. The strongest plasma focus discharge indicated by sharp voltage spike of high amplitude and highest ion beam energy were both observed at 0.2 mbar. The average energy of the ion beam obtained is (53 ± 13) keV. At this optimum condition, the ions beam with the highest energy also led to the highest hard x-ray emission.

Development of an attachment for continuous step-by-step shock-wave irradiation of long-length superconducting tapes in the «Plasma Focus» setup

The article presents the results of the creation of an additional attachment placed on the “Plasma Focus” installation (PF), and designed for uniform movement of superconducting tapes through the working chamber of PF. This device allows step-by-step shock-wave impact at the long superconducting samples and tapes in the “Plasma Focus” installation in order to increase their superconducting and technological characteristics. The worth of this device that the process of irradiation of the tapes can be carried out without interrupting and completely turning the plasma installation off, which is usually necessary to displace the tape and make the strike to the next section of the tape. The use of attachment also allows to get an additional data integrated over the length of the processed tape and to study the effect of plasma shock-wave action to the physical and structural properties of tapes. The experiments carried out will make it possible to select and test the optimal plasma strike models and recommend further conditions of shock-wave treatment, which would make it possible to use this method for long tapes with the aim of using them in various devices.

Особенности повреждаемости и модифицирования поверхностных слоев алюминия и его сплавов мощными потоками энергии в установке Плазменный фокус

The results of the analysis of damageability and modification of the structural-phase state of the surface layers of aluminum and its alloys by powerful flows of fast high-energy ions and high-temperature plasma in Plasma focus devices, as well as using pulsed laser radiation. Pure Al, an alloy of the Al – Mg – Li system, a duralumin alloy, and a composition of a ceramic coating Al2O3 on an Al substrate are considered. It is shown that in the regime of Al irradiation with a power density of q ≈ 106 – 107 W/cm2 in the nano- and microsecond range of pulse durations, ultrafast crystallization of melted surface layer occurs with the formation of a wavy surface relief and the structural fragments of sub-microcrystalline and nanoscale size. After the action of deuterium plasma flows on a duralumin alloy tube located along the axis of the Plasma focus device a modification of the structural-phase state of the alloy is observed: the initial two-phase state of an αAl-solid solution of copper in aluminum and inclusions of the second phase of CuAl2 became fine-grained and single-phase due to the dissolution of CuAl2 particles in the melt. Irradiation of an alloy of the Al – Mg – Li system containing (wt %) 2 % Li and 5 % Mg at q = 5·106 W/cm2, t = 50 – 100 ns after four pulsed impacts of fast ions and deuterium plasma led to the modification the structural-phase state of the surface layer of the alloy, associated with an increase in the content of magnesium oxide and a decrease in the crystal lattice parameter of the Al-based solid solution. The formation of spherical cavities due to the evaporation of lithium into the internal micropores of the surface layer was also found. The low damage and structural stability of Al2O3 ceramics on an Al substrate under beam-plasma impacts in plasma focus device with a radiation power density q ≤ 108 – 109 W/cm2 in the nano- and microsecond range of pulse duration is noted. At the same time, the Al2O3/Al composition was unstable to pulsed laser radiation in the free-running mode (q = 105 – 106 W/cm2, t = 0.7 ms) and Q-switch mode (q = 107 – 108 W/cm2, t = 80 ns). In both cases the coating peeled off from the substrate.

1D kinetic study of pinch formation in a dense plasma focus: Transition from collisional to collisionless regimes

The pinch-formation stage of a deuterium dense plasma focus, and associated “shock-flash” neutron yield, is studied using 1D kinetic simulations considering a plasma column with initial pressure P, initial radius R, and the compression to be driven by a constant current I. The relative behavior of the compression is shown to be similar for fixed ratios of the characteristic ion mean free path to the radius of the plasma column at stagnation, λst/Rst. This dimensionless parameter is shown to scale like I4/(P3R5). The compression ratio, R/Rst, is found to be a minimum when λst/Rst≈1 and is the largest in the collisionless limit where λst≫Rst. This behavior is in contrast to the analogous planar pinch where R/Rst decreases from one constant for λst/Rst≪1 to a smaller constant for λst/Rst≫1. The yield in the collisionless regime is shown to fall between the two well-known I4 scaling laws. Furthermore, this regime exhibits qualities that potentially make it appealing for radiography applications, such as increased localization in time and space of the neutron formation.

Повреждаемость ниобия импульсными потоками ионов гелия и гелиевой плазмы

The damageability of niobium by pulsed fluxes of helium ions (HI) and helium plasma (HP) in the Plasma Focus (PF) setup was studied at a flux power density qi ~ 108 W/cm2 and qp ~ 107 W/cm2, respectively, and pulse duration τi ≈ 30 – 50 ns and τp ≈ 100 ns. In the implemented irradiation conditions the erosion of the material is observed associated with the evaporation of the surface layer (SL), which occurs somewhat more intensively in the central part of the irradiation zone under the action of the most high-energy fluxes of HI and HP. The typical features of the damageability of the niobium SL under the considered irradiation conditions are revealed. These include: melting of SL with the formation of a wavy surface relief and a large number of blisters of two types — gas-filled and with destroyed shells, as well as the presence of microcracks. The appearance of blisters was associated with the formation of complexes based on the combination of implanted helium atoms with vacancies and interstitial impurity atoms (C, O, N, etc.) and their subsequent growth and coagulation in the liquid phase under pulsed action of energy fluxes on the irradiated Nb surface. Some of the microcracks formed in the SL under the action of thermal stresses coincide with the sliding lines of the material that arise under the action of high-speed plastic deformation. A network of such microcracks creates a block structure on the Nb surface. In the irradiated surface layer of niobium, zones of columnar crystals and a cellular microstructure of the surface are found, in which the average cell size is ~ 100 nm. It was shown by the method of numerical simulation that in the indicated zones the process of solidification of the SL proceeded through directional crystallization at a high speed, which reached ~ 35 m/s near the irradiated surface.

Production of porous films of silicate glass using a plasma focus setup

The aim of the work was to obtain and study films of porous silicate glass. The proposed method for producing porous films is based on the rapid cooling of the melt on the surface of a glass plate after exposure to short pulses of argon plasma generated on an electrodischarge installation of plasma focus type. Silicate glass films obtained by such method have the properties of both porous glasses and foam glass. The specific volume density of these films is ∼ 0.4 g/cm3, their porosity and hygroscopicity correspondingly ∼ 0.3 and ∼ 30%

Влияние импульсного ионно-плазменного воздействия и электронного облучения на механические свойства сплава Инконель 718

The effect of pulsed ion-plasma irradiation in the Plasma Focus installation and irradiation with 20 MeV electrons at the Miсrotron-ST accelerator on the mechanical properties of the precipitation-hardening Inconel 718 alloy obtained by the method of selective laser fusion has been studied. It was found that ion-plasma and electron irradiation does not affect the properties of the alloy, which is explained by the small thickness of the modified layer during PF irradiation and insufficiently high electron fluence for the manifestation of radiation-stimulated processes of redistribution and dissolution of dispersed precipitates of the strengthening phases. The results obtained indicate a high resistance to crack propagation and the absence of a tendency of the studied alloy to embrittlement.

A Theoretical Study of the Possibility of Obtaining the Radioisotope 13N for Medical Applications Using Dense Plasma Focus Device

The aim of this research is to study the possibility of obtaining the short-lived radioisotope 13N by colliding with a deuterium ion beam emitted by an NX2 dense plasma focus device with a graphite target, as a technique alternative to the traditional technique for getting short-lived radioisotope (SLR) accelerators. Lee code was used to study the properties of the plasma pinch, which is the source of deuterium ion beams. Then, a function of distributing the beam energy and stopping power was found within the graphite using SRIM program. The reaction cross-section was also found from an EXFOR database and the radiative yield of the reaction was calculated. The radioactivity of 13N was calculated and found to be equal to A = 4.14×103 Bq. The effect of the change in deuterium gas pressure on the radioactivity was studied and it was found that the best value for the pressure is 3 Torr, where the number of deuterium ions is the greatest amounting to 3.12×1011 ions and thus the radioactivity increased to 5.616 ×103 Bq. The effects of repetition rate of the studied dense plasma focus device and the exposure time of the graphite target to deuterium ions were studied. The radioactivity was calculated at repetition rates (1,5,10,16 ) Hz and exposure times (30,60,600) sec., where we noticed an increase in the value of radioactivity at a repetition rate of 16 Hz and an exposure time of 600 sec., reaching to 28791 × 103 Bq. To obtain a radioactivity value for 13N suitable for use in PET technique, the radioactivity value was calculated at high repetition rates (50,100,500) Hz and exposure times (30,60,600) sec. We obtained the value of radioactivity 527 ×106 Bq at a repetition rate of 500 Hz and an exposure time of 300 sec. and this value is suitable for use in PET technique. However, there are technical difficulties in the studied dense plasma focus device to obtain a high repetition rate of this value. The radioactivity of 13N was calculated at very high times of exposure of the graphite target to deuterium ions (1000, 1500, 2000, 2500, 3000) sec. at the repetition rates of the device studied. The highest radioactivity value obtained was 55.79 ×106 Bq at an exposure time of 3000 sec. and this value is less than the required value for the required use in PET technique. Keywords: Dense plasma focus device NX2, Lee code, Radioactivity.