Current Sheath Research Articles

INVESTIGATION OF PLASMA VISCOSITY AND MAGNETIC FORCES IN COAXIAL PLASMA DISCHARGE DEVICE

This paper investigates the axial plasma current sheath, PCS viscosity force, and its correlation with other magnetic forces acting on it at three different radial positions (2.625, 3.625, and 4.125 cm) within the annular space between the coaxial electrodes system of plasma discharge device (12 kV-56 kA) with positive central electrode. The rate of change of PCS momentum was also investigated at these axial and radial positions. The discharge processes are carried out with an argon gas pressure of 0.8 Torr. Experimental results of azimuthal magnetic induction as a function of axial and radial positions and Navier Stokes equation were employed to compute exactly, the rate of change of PCS momentum, axial magnetic force, magnetic pressure gradient force, and viscosity force.

Symmetry and structure in the “Generalized Plasma Focus problem”

The “Generalized Plasma Focus problem” refers to a generic class of plasma propagation phenomena that share many features of a dense plasma focus device. Its recent theoretical development has been shown to predict some features of the pinch phase in PF-1000 and POSEIDON. The theory attempts to decompose the plasma propagation problem into two weakly interdependent subproblems. This is achieved by expressing every physical variable of an applicable continuum model of the plasma as the product of a scaling parameter, which contains device-related information and represents its numerical magnitude, and a scaled variable that is devoid of device-related information, is of order unity, and represents the spatiotemporal structure of that variable. The first subproblem seeks a traveling surface of revolution whose local normal velocity equals the scaling parameter for velocity and is aligned with the magnetic force density. Spatiotemporal distributions of all the scaled variables must move along with this reference surface by definition. This paper explores the resulting scaling theory and its symmetry properties. A new coordinate transformation results in a formula for the spatiotemporal distribution of the magnetic field of the curved and non-steady plasma sheath. New insights into the snowplow effect are obtained. A current sheath with a rear boundary exists only when the current is decreasing and only when the current carrying plasma is less dense than the fill gas. The current sheath thickness is the same for small and large devices. The geomagnetic flux compression problem has an exact solution.

Investigation of the Plasma Current Sheath Dynamics in the Run Down Phase of the Hemispherical Plasma Focus Devices Using Two-Dimensional Time Dependent Modeling

Investigation of the Plasma Current Sheath Dynamics in the Run Down Phase of the Hemispherical Plasma Focus Devices Using Two-Dimensional Time Dependent Modeling

Observation of plasma dynamics in a theta pinch by a novel method

A novel experimental method is proposed for observing plasma dynamics subjected to magnetic fields based on a newly developed cylindrical theta-pinch device. By measuring simultaneously the temporal profiles of multiple parameters including the drive current, luminosity, plasma density, and plasma temperature, it provides a basis for observing the plasma dynamics of the theta pinch, such as shock transport and magnetohydrodynamic instability. We show that the plasma evolution can be distinguished as three phases. First, in the radial implosion phase, the trajectories of the current sheath and shock wave are ascertained by combining experimental data with a snowplow model (Lee model) in a self-consistent way. Second, in the axial flow phase, we demonstrate that m = 0 (sausage) instability associated with the plasma axial flow suppresses the plasma end-loss. Third, in the newly observed anomalous heating phase, the lower-hybrid-drift instability may develop near the current sheath, which induces anomalous resistivity and enhanced plasma heating. The present experimental data and novel method offer better understanding of plasma dynamics in the presence of magnetic fields, thereby providing important support for relevant research in magneto-inertial fusion.

Deep genomic analysis of malignant peripheral nerve sheath tumor cell lines challenges current malignant peripheral nerve sheath tumor diagnosis

Malignant peripheral nerve sheath tumors (MPNSTs) are soft-tissue sarcomas of the peripheral nervous system that develop either sporadically or in the context of neurofibromatosis type 1 (NF1). MPNST diagnosis can be challenging and treatment outcomes are poor. We present here a resource consisting of the genomic characterization of 9 widely used human MPNST cell lines for their use in translational research. NF1-related cell lines recapitulated primary MPNST copy number profiles, exhibited NF1, CDKN2A, and SUZ12/EED tumor suppressor gene (TSG) inactivation, and presented no gain-of-function mutations. In contrast, sporadic cell lines collectively displayed different TSG inactivation patterns and presented kinase-activating mutations, fusion genes, altered mutational frequencies and COSMIC signatures, and different methylome-based classifications. Cell lines re-classified as melanomas and other sarcomas exhibited a different drug-treatment response. Deep genomic analysis, methylome-based classification, and cell-identity marker expression, challenged the identity of common MPNST cell lines, opening an opportunity to revise MPNST differential diagnosis.

Performance-enhanced clogging-free viscous sheath constriction impedance flow cytometry.

As a label-free and high-throughput single cell analysis platform, impedance flow cytometry (IFC) suffers from clogging caused by a narrow microchannel as mechanical constriction (MC). Current sheath constriction (SC) solutions lack systematic evaluation of the performance and proper guidelines for the sheath fluid. Herein, we hypothesize that the viscosity of the non-conductive liquid is the key to the performance of SC, and propose to employ non-conductive viscous sheath flow in SC to unlock the tradeoff between sensitivity and throughput, while ensuring measurement accuracy. By placing MC and SC in series in the same microfluidic chip, we established an evaluation platform to prove the hypothesis. Through modeling analysis and experiments, we confirmed the accuracy (error < 1.60% ± 4.71%) of SC w.r.t. MC, and demonstrated that viscous non-conductive PEG solution achieved an improved sensitivity (7.92×) and signal-to-noise ratio (1.42×) in impedance measurement, with the accuracy maintained and free of clogging. Viscous SC IFC also shows satisfactory ability to distinguish different types of cancer cells and different subtypes of human breast cancer cells. It is envisioned that viscous SC IFC paves the way for IFC to be really usable in practice with clogging-free, accurate, and sensitive performance.

New evidence about the nature of plasma filaments in plasma accelerators of type plasma-focus

In this work, new evidence and information about both the origin and evolution of filamentary structures observed in the current sheath of a small plasma focus (PF) discharge are reported. The experiments were carried out in a small generator of low energy (multipurpose generator) at the PF configuration, under different operating conditions. These include different anode and insulator geometries, without return bars and in a high-performance regime of the generator at high pressures (10 mbar). The evolution of the plasma structures is characterized by means of refractive optical techniques. The electrical behavior of the discharge, as well as its performance, are monitored with conventional electrical diagnostics and neutron and x-ray detectors, respectively. Plasma filaments of the same species are present in all tested configurations, however, in experiments with larger effective anode length (and smaller anode radii), the plasma region containing the filaments moves away from the anode surface and remains confined in a region of the plasma sheath, such as a toroidal plasma belt, without reaching the top of the anode nor participating in the radial compression phase. According to images of the plasma sheath in its early phase, the filaments originate and evolve from a precursor annular plasma formed in the lower part of the anode next to the insulator, during the process of electrical breakdown. The local character that these dense-filamentary structures acquire in the evolution of the plasma sheath would discard the current-filament hypothesis. On the other hand, experiments performed in deuterium gas with anodes of larger effective lengths and without filaments in the radial compression phase allowed the production of neutrons and x-rays at a high performance.

Dynamics of implosion phase of modified plasma focus studied via laser interferometry and electrical measurements

Dynamics of the implosion of the dense plasma focus play an essential role in converting electrical energy into the kinetic energy of the current sheath and subsequent production of accelerated electrons, ions, hard X-ray, and neutron emission. This paper presents the analysis of the implosion parameters, such as the implosion velocity and imploding mass, coupled with electrical parameters observed on the PF-1000 facility with a modified electrode system. The first two parameters are based on the 16-frame Mach–Zehnder interferometer, which provides the spatial distribution of electron density in a time sequence. Measurement of the total current, current derivative, and voltage enables us to evaluate the total inductance and kinetic energy driven by the capacitor bank. Then comparing the inductances and kinetic energies evaluated from the interferograms and electrical waveforms can provide more precise information on the current flowing in the imploding sheath. We present a possible way to deal with the fact that only part of the total current flows through the imploding layer. With the supposition that the rest of the current flows close to the insulator, we conclude that roughly 70% of the total current flows through the pinch, which is in good agreement with an input parameter of the Lee model, for example.

Study on optimization of laying mode of two parallel high voltage cables in the same phase of two circuits

In power lines, the proportion of parallel cables is gradually increasing, but the uneven current distribution and sheath circulation affect the safe and stable operation of parallel cables. Existing studies show that cable current and sheath circulation are obviously affected by cable laying mode. At present, the designed cable laying mode is as “pin” laying and horizontal “I” laying. Combined with the above engineering difficulties, this paper focuses on the parameter optimization scheme of 220 kV two circuit parallel cables in the same direction when the metal sheath is cross connected and grounded. Firstly, the operation model of 220kV two circuits in-phase two parallel cables is established. The numerical differences between the current distribution non-uniformity coefficient and sheath circulating current of the cables are analyzed and calculated under the condition of “pin” laying and horizontal “I” laying. Meanwhile, the influence of different phase sequence arrangement on the current distribution non-uniformity coefficient and sheath circulating current of the cables is compared. Secondly, the cable laying mode of “symmetrical pin” is designed, and the improvement effect of uneven coefficient of cable current distribution and metal sheath circulating current under “symmetrical pin” laying is compared and analyzed. Finally, the differences of the above parameters in different phase sequence arrangements are compared and analyzed. The research results show that the symmetrical pin laying can improve the uneven distribution of current and sheath circulating current, which has important engineering practical value.

Effect of tailored density profiles on the stability of imploding Z-pinches at microsecond rise time megaampere currents

To study the effect of the radial density profile of the material of a metal-plasma Z-pinch load on the development of magneto-Rayleigh–Taylor (MRT) instabilities, experiments have been performed at the Institute of High Current Electronics with the GIT-12 generator which produces microsecond rise time megaampere currents. The Z-pinch load was an aluminum plasma jet (PJ) with an outer plasma shell. This configuration leads to the formation of a uniform current sheath in a Z-pinch load upon application of a high-voltage pulse. It was successfully used in experiments with hybrid deuterium gas-puffs (Klir et al 2020 New J. Phys. 22 103036). The initial density profiles of the Z-pinch loads were estimated from the pinch current and voltage waveforms using the zero-dimensional ‘snowplow’ model, and they were verified by simulating the expansion of the PJ formed by a vacuum arc using a two-dimensional quasi-neutral hybrid model (Shmelev et al 2020 Phys. Plasmas 27 092708). Two Z-pinch load configurations were used in the experiments. The first configuration provided tailored load density profiles, which could be described as ρ(r) ≈ 1/r^s for s > 2. In this case, MRT instabilities were suppressed and thus a K-shell radiation yield of 11 kJ cm−1 and a peak power of 0.67 TW cm−1 could be attained at a current of about 3 MA. For the second configuration, the radial density profiles were intentionally changed using a reflector. This led to the appearance of a notch in the density profiles at radii of 1–3 cm from the pinch axis and to magnetohydrodynamic instabilities at the final implosion stage. As a result, the K-shell radiation yield more than halved and the power decreased to 0.15 TW cm−1 at a current of about 3.5 MA.

Magnetic field induction and magnetic force distribution profiles in plasma focus discharge device

We report a simple-to-perform technique to investigate the distribution of the azimuthal magnetic field induction, B θ , and the induced magnetic force acting on the plasma current sheath (PCS) in a plasma focus (PF) discharge. This in situ measurement technique can undoubtedly be beneficial when other fast-imaging techniques are not available. techniques are not available. Experimental work was conducted in the low-energy Mather-type EAEA-PF1 device operated in argon. The axial distribution (B θ ) z along the coaxial electrodes system was measured with a four magnetic-probe set technique at different radial distances (r = 2.625 × 10−2 to 4.125 × 10−2 m) within the annular space between the coaxial electrodes during the 1st and 2nd half cycles of the discharge current waveform, where inner electrode of coaxial electrode system has a +ve polarity and −ve polarity, respectively. Axial, radial and total magnetic force distribution profiles were estimated from B θ data. Investigation of PCS shape in terms of its inclination (curvature) angle, θ, along the axial rundown phase and the correlation between the magnetic forces per unit volume acting on the PCS, the inclination angle θ of the PCS, and the formation of a powerful PF action during the 1st and 2nd half cycles is carried out. Dependence of inclination angle, θ, on total magnetic force per unit volume acting on PCS axial motion was studied, separately, during the 1st and 2nd half cycles.

K-shell radiation and neutron emission from z-pinch plasmas generated by hybrid gas-puff implosions onto on-axis wires

Z-pinches have been explored as efficient soft x-ray sources for many years. To optimize x-ray emission, various z-pinch configurations were tested. This paper presents data obtained with a hybrid gas-puff z-pinch imploding onto on-axis wires on a microsecond, multi-megaampere GIT-12 generator. In our previous experiments, the hybrid gas puff, i.e., an inner deuterium gas puff surrounded by an outer hollow cylindrical plasma shell, was used to produce energetic protons, deuterons, and neutrons up to 60 MeV [Klir et al., New J. Phys. 22, 103036 (2020)]. The behavior of the hybrid gas-puff z-pinch on GIT-12 was interpreted as a high-density plasma opening switch with a microsecond conduction time, 3 MA conduction current, nanosecond opening, and up to 60 MV stand-off voltage. These properties can be employed to transfer the current into an on-axis load with a high rise rate. In the recent experiments on GIT-12, we therefore placed single or multiple aluminum wires on the axis of the hybrid gas-puff z-pinch. Before a current sheath arrived at the axis, a coronal plasma was seen around the wire. A rapid increase in x-ray radiation was observed when the coronal plasma imploded onto the axis. The coronal plasma implosion resulted in a long (2 cm), narrow (∼mm) column radiating in the Al K-shell lines. With the single Al wire of 80 μm diameter, the K-shell x-ray output reached 5.5 ± 0.8 kJ in a 0.6 ± 0.1 TW peak power and 7 ± 1 ns pulse. The higher K-shell yield of 12 ± 2 kJ and peak K-shell power of 0.7 ± 0.1 TW were achieved with four 38 μm diameter Al wires. (Their cross section formed the corners of a square with 1 mm side.) The presence of the wires on the axis significantly suppressed ion acceleration and neutron production. Deuterium-deuterium (DD) neutron yields of about 1.2 × 1011 were 20 times smaller than the yields produced in shots without any wire. The DD neutron yield was increased up to 4.5 × 1011 when the Al wire was replaced by a fiber from deuterated polyethylene. A characteristic feature of the experiments with the (CD2)n fiber was a rapid expansion with the velocity approaching 900 km/s.

Simulation of the inner electrode geometry effect on the rundown phase characteristics of a coaxial plasma accelerator.

A 2D computational model, incorporating the Snowplow approximation in the mass balance, is used to simulate the acceleration of an annular current sheath along two coaxial electrodes, with the inner one having either cylindrical or conical shape. The circuit, mass and momentum equations are simultaneously solved in 2D (r, z) considering initial breakdown along the insulator surface, ideal gas mass accretion by the current sheath (snowplow model) and distributed inductance along a coaxial transmission line short-circuited by the current sheath. Plasma density and electron temperature in the current sheath are estimated using standard planar shock theory. Numerical integration of the model’s equations for a given electrode geometry yields the temporal evolution of the current sheath parameters during the axial acceleration phase. In order to see the effect of the inner electrode shape on sheath parameters (i.e. transit time, kinetic energy, total mass, shape, etc.) and/or circuit properties (i.e. circuit inductance, voltage and current evolution, etc.), the portion of the inner electrode beyond the insulator was given a conical shape. By changing the cone slant in a range between ±5°, it was found that the current driven on the plasma sheath varies nonlinearly with the angle. The divergent (positive angle) electrode gives the sheath the highest kinetic energy, being twice the value corresponding to that of the straight inner electrode case, and the transit time is reduced from 1.34 to 1.20 µs. The estimates of plasma density and electron temperature indicate that the achievable ion densities are on the order of 4x1022 m-3, which corresponds to 30 % ionization, and typical temperatures at the end of the rundown phase are on the order of 8 eV. These values are comparable with those measured in experimental devices. The development of this tool will enable us to benchmark its results against an experimental installation currently close to being operational, and a future follow-up paper will be devoted to the comparison between the prediction of the rundown phase behavior and experimental results utilizing conical electrodes.

Implosion dynamics and radiation characteristics of preconditioned hybrid X-pinch driven by double pulse current

Preconditioned hybrid X-pinch (HXP) experiments have been performed on the Qin-1 facility to investigate the effect of the prepulse current on the implosion dynamics and radiation characteristics. HXPs with molybdenum wires of ∼1.2 mm in length and 25 to 50 μm in diameter have been tested. Driven by the prepulse current, the molybdenum wire is heated to a core-corona structure with a Joule energy deposition of 0.5–4.8 eV/atom, which is smaller than its atomization enthalpy. The radial expansion velocity of the wire core is 0.5–3 km/s. The main current of 450 kA/400 ns is applied ∼610 ns after the prepulse current. The implosion dynamics of the preconditioned HXP is characterized by two-shell structure implosion. Immediately after the main current starts, the current mainly passes through the corona plasma located in a large radius, and the implosion of corona plasma lasts for ∼100 ns until the corona plasma collides with the wire core. Then, the current distribution switches from the corona plasma to the wire core, which results in the onset of implosion of the wire core. The implosion of the wire core generates hot spots and x-ray radiation. The calculated inductance and radius of the current sheath agree with the experimental results. The corona plasma performs as a current switch, which allows the main current passing through the wire core after 100 ns. It indirectly contributes to sharpening the main current used for implosion of the wire core. A single hot spot with x-rays over 2.5 keV is measured using the preconditioned HXP.

MHD Simulation of Physical Processes in Spherical Plasma-Focus Chambers with Allowance for Neutron Generation

The results of the development of a two-dimensional MHD code for carrying out computational studies of the dynamics of plasma current sheath in spherical chambers with a plasma focus are presented. Equations of magnetohydrodynamics with allowance for magnetic field diffusion, thermal conductivity and plasma radiation are used in this work. An implicit scheme is used in the calculation of the magnetic field, which makes it possible to describe the motion of plasma in a low-density region behind the plasma sheath. The formulas that take into account the possible appearance of anomalous resistance in the plasma are used to calculate the plasma conductivity. The neutron yield is calculated with allowance for thermonuclear and beam–target neutron generation mechanisms. The effect of the minimum residual gas density behind the plasma sheath on the cumulation of the plasma sheath is studied. The effects of magnetic field diffusion, thermal conductivity and anomalous plasma resistance on the plasma sheath dynamics are considered. The calculations are performed for two spherical plasma-focus chambers operating with currents up to 1 and 2 MA and neutron yields to 1012 and 1.5 × 1013 DT neutrons, respectively. The comparison of the calculated dependences with experimental data on the current, voltage and neutron yield made it possible to refine the parameters used in the calculations and achieve a satisfactory agreement between the simulation and experiment.

Experimental investigation of a low-energy linear plasma propulsion device

This paper presents preliminary results for a new design of a pulsed linear plasma device that generates electromagnetic plasma propulsion. The current sheath dynamics and the plasma propulsion along an extension tube have been studied. Breakdown of helium gas occurs between coaxial electrodes when applying a high-voltage pulse. This paper presents measurements with a number of probes, as well as camera images of the plasma propagation. A plasma is ejected and fills a 40 cm length extension tube. Additionally, the measurements show that the length of the propelled plasma column and its intensity inside the expansion tube is increased by increasing the charging voltage, while the plasma propulsion current is decreased along the extension tube. It is noted that a lower critical time; 4.7 μs, is needed to ionize the gas and form the plasma current sheath.

Formation of double shell during implosion of plasma metal puff Z-pinches

This work presents the results of experimental and theoretical research of impact of tailored density profile and application of external axial magnetic field on initial spatial distribution of the plasma density in the plasma metal puff Z-pinch and on its implosion dynamics. It has been discovered that upon implosion of the plasma metal puff Z-pinch some stripes interpreted as the system of two coaxial shells appear on the optical images. With the help of numerical simulation, the formation of the plasma liner consisting of a mixture of carbon and bismuth ions and formed by the expansion of the plasma jet of the arc burning on the bismuth electrode has been considered in this work. It has been shown that the lightweight carbon ions facilitate formation of the density distribution smoothly decreasing with the increase in radius, that, in turn, leads to suppression of the Rayleigh–Taylor instability in the current sheath upon further implosion. It has also been demonstrated that availability of the two types of ions in plasma considerably different in mass leads to formation (in the compression phase) of a double shell with externally located heavy ions. It has also been revealed that the application of the external axial magnetic field leads to reduction in the plasma metal puff Z-pinch initial diameter.

BASINÇ VE VOLTAJIN KÜRESEL PLAZMA ODAK CİHAZI DİNAMİ ÜZERİNE ETKİSİ

A spherical plasma focus device with two concentric electrodes is simulated using equal amount of deuterium and tritium mixture as a working gas. The developed model is used to investigate the effect of gas pressure and charging voltage on dynamics of current sheath in spherical plasma focus. While charging voltage is varied from 15 kV to 30 kV to see the effect of voltage, gas pressure is increased from 1 Torr to 20 Torr for gas pressure study. In this work, it is found that there is a strong correlation between current sheath dynamics and investigated parameters which are gas pressure and charging voltage.

MHD Simulation of Hemisphere Plasma Focus Using Snowplow Model

The Hemisphere Plasma Focus (HSPF) device is a new construction of plasma focus (PF) devices wherein the discharge takes place between the inner and outer concentric hemispherical electrodes with total energy up to 3.4 kJ. The pinch effect appears in the shape of a sharp crevice in the discharge current signal. HSPF is simulated by utilizing the Snowplow Model depending on the circuit equation incorporated with the momentum equation. Some modifications were added to the model to improve the theoretical data in order to be consistent with the experimental results. The code includes a rundown phase starting from the equator point toward the antipodal point and the reflected shock phase at the axis. The model results are compared with experimental results, and the effect of discharge parameters such as the discharge voltage and helium gas pressure on the plasma parameters is studied. The total system inductance is about 285 nH with a resistance of about 23 mΩ. Furthermore, the plasma inductance has a maximum value at the pinch time, which is decreased by increasing the charging voltage or decreasing the gas pressure. The mass factor in the HSPF device is found to be relatively low compared to that of the coaxial PF device. The shock front and current sheath velocities are increased by increasing the drive factor while the pinch time is decreased. Also, the minimum pinch radius and the plasma inductance have a reverse trend as a function of the drive factor.

Scenario of a magnetic dynamo and magnetic reconnection in a plasma focus discharge

The paper discusses a possible energy transformation that leads to the acceleration of fast ions and electrons. In plasma-focus discharges that occur during deuterium filling, which have a maximum current of about 1 MA, the accelerated deuterons produce fast fusion neutrons and fast electrons hard X-ray emissions. Their total energy, which is of the order of several kilojoules, can be delivered by the discharge through a magnetic dynamo and self-organization to the ordered plasma structures that are formed in a pinch during the several hundreds of nanoseconds of the pinch implosion, stagnation, and evolution of instabilities. This energy is finally released during the decay of the ordered plasma structures in the volume between the anode face and the umbrella front of the plasma and current sheath in the form of induced electric fields that accelerate fast electrons and ions.