Pinch Formation Research Articles

Experimental study of the effect of circuit inductance on pinch-time in plasma focus MTPF2.5 KJ .

This paper investigates the effect of circuit inductance on pinch-time of plasma focus. The dense plasma focus is a plasma discharge powered by a capacitor bank. The Mather type Dense Plasma Focus is constructed in cylindrical geometry and is made up of coaxial anodes and cathodes with varying radiuses. An insulator sleeve separates the electrodes, and the cathode rode forms circles around the anode tube. Different parameters, such as geometry and structure of the anode bar, and decreasing inductance are involved in the optimization of plasma focus. One way to increase the gain and optimization of pulsed plasma systems is to reduce the inductance of the circuit. For this purpose, we investigate the effect of different values of total circuit inductance on pinch formation time. Experimental and simulation results are compared. the proposed system shows a decent functionality that if the inductance reduces, the efficiency of the system will increase.

Bayesian inference of spectrometric data and validation with numerical simulations of plasma sheath diagnostics of a plasma focus discharge

Plasma Foci are pulsed coaxial discharges with numerous radiation applications and interesting scientific phenomena. Although the physics answered much of the processes involved in these discharges, many related fundamental questions still remains doggedly unresolved. One of the obstacles to deeper knowledge is the scarcity of reliable experimental data. This work presents an elaborate experimental assessment of the electron density in the rundown phase of a 400 J Plasma Focus operating with hydrogen. The rundown of the plasma sheath is basically a hypersonic shock wave between two coaxial electrodes accelerated by the Lorentz force, and it is important to control the pinch formation. The electron density of the passing sheath is measured by means of the Stark broadened hydrogen alpha emission with spatial and temporal resolution. The experimental data is post-processed using Bayesian posterior probability assessment. The results are conflated with the numerical model CShock to construe an educated explanation of the sheath behavior during the rundown. In particular, it is possible to reckon the formation of a toroidal instability reported in previous experiments, and to estimate the plasma sheath temperature (4–20 eV) and velocity (62.5 km/s) at this stage.

Study of Interaction of Plasma Flows with Magnetic Field During Implosion of Cone-Cylindrical Nested Arrays

The results of the studies of the compression of the plasma of cone-cylindrical nested arrays of mixed composition at current flow through them of up to 3 MA at the Angara-5-1 facility are presented. The outer array of the nested array is a conical array of thin nylon fibers or tungsten wires, while the inner one is a cylindrical tungsten wire array. The implosion of current in this type of nested arrays is a unique opportunity for simulating the interaction of magnetized plasma flows with a strong magnetic field. In the space between the inner and outer arrays, the super-Alfven plasma flow from the outer array collides with the magnetic field of the discharge current through the inner array. As a result of this interaction in the space between arrays, different plasma flow modes are implemented: pre-Alfven (Vr VА), and a mode with the formation of the transition region—a shock wave (SW) between arrays, depending on the ratio of the plasma formation rates of arrays of nested array and on the ratio of their radii. The SW region formation in the space between arrays occurs near the inner array, where the kinetic pressure of the plasma flow from the outer array is balanced by the magnetic pressure of the discharge current of the inner array. New experimental data was obtained on the features of the SW region formation near the surface of the inner array and the nature of its change during the interaction. The use of an external conical array enabled us to obtain the SW position depending on the ratio of radii of the outer and inner arrays in one shot. It is shown that the interaction of plasma flows of the outer conical array with the magnetic field of the inner cylindrical array leads to a significant decrease in the zipper-effect at the final stage of plasma compression of the inner array and the pinch formation. The obtained experimental results are compared with the results of simulating the plasma motion between the arrays using the three-dimensional radiation-magnetohydrodynamic code MARPLE3D.

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.

Flow Z-pinch plasma production on the FuZE experiment

The Fusion Z Pinch Experiment (FuZE) investigates sheared-flow stabilization of classic m = 0 and m = 1 instabilities in Z pinches with an embedded axial flow. FuZE consists of a 100 cm coaxial plasma accelerator, where neutral gas is ionized and accelerated in a pulsed electrical discharge, followed by a 50 cm assembly region, where pinches are formed. Maintaining the pinch requires continuous plasma injection provided by a deflagration mode in the coaxial accelerator. Two discharge modes, with and without deflagration, are investigated on FuZE. Pinch formation is observed with deflagration only. Plasma velocities in the assembly region are found to match the E × B velocity estimated in the accelerator based on a 1D circuit model, indicating that a 1D MHD approximation may offer a valid description of the plasma in the accelerator channel. The velocity of magnetic field propagation is found to agree with the snowplow model based on momentum conservation, and the lifetime of the pinch is shown to be in agreement with constraints imposed by mass conservation.

Hydrogen ion emission studies in 4π plasma focus device space

Plasma focus devices (PFD) produce copious flux of ions in particular hydrogen ions in 4π space. To achieve dense plasma pinch and then the emission of hydrogen ions at a voltage of 13 kV, experiments have been carried out at various gas pressures inside the chamber. Discharge current signals were recorded by Rogowski coil to choose only discharges with a high degree of current sheath symmetry and plasma pinch formation. Wide-angle 3D images of hydrogen ion emissions were efficiently formed for the first time in our laboratory on novel position-sensitive panorama mega-size cylindrical polycarbonate image detectors (MS-CPID) processed in self-invented panorama mega-size single-cell electrochemical etching (MS-ECE) systems. Hydrogen ion emission images are simply observed and analyzed by the unaided eyes. Near 4π wide-angle hydrogen ion image detection, iso-fluence contours, fluence on polar angle distributions, and hydrogen ion energy estimation were studied. MS-CPID/MS-ECE methods are breakthrough ion image detection art and technology for hydrogen ion plasma mechanism studies with provision of 4π position-sensitive data matrix bank. Studies confirm 4π hydrogen ion emission from “anode top” and radial ion run-aways from “anode assembly” forming “hydrogen cathode shadows” on MS-CPIDs making 4π PFD space availability evident for full-scope large-scale advanced hydrogen ion mechanism studies and applications.

Study of the Compression of a Condensed Deuterated Target Installed on the Wire Array Axis

The implosion of combined loads consisting of an outer wire (fiber) array and inner cylindrical target was studied experimentally at the Angara-5-1 facility (3.5 MA, 100 ns) at currents of up to 3.5 MA. The experiments were carried out with 12- and 20-mm-diameter outer arrays made of 15-μm-diameter aluminum wires, composite arrays made of aluminum wires and 25-μm-diameter kapron fibers, and arrays made of kapron fibers with a 1-μm-thick aluminum coating. The number of wires varied from 10 to 40. The targets were made of agar-agar or low-density deuterated polyethylene. The parameters of the Z-pinch plasma were determined using the Angara-5-1 diagnostic complex, which included optical streak cameras, X-ray frame cameras, X-ray detectors, X-ray pinhole cameras, neutron detectors, and a mica-crystal X-ray spectrograph. It is established that the plasma compression dynamics and the formation of local plasma structures generating neutrons depend on the load configuration: the array diameter, the number of wires (fibers), and the diameter and density of the target. The most efficient compression and the highest plasma parameters (the compression ratio and plasma temperature), as well as the highest neutron yield, were achieved in experiments with 12-mm-diameter aluminum wire arrays inside which a 1-mm-diameter deuterated target with a mass density of 0.3 g/cm3 was installed As a result of collision of the bulk of the array mass with the inner target, a compact pinch with a diameter of ≈0.5 mm forms. The pinch formation is accompanied by the generation of a soft X-ray pulse. The development of MHD instabilities in the pinch plasma results in the formation of multiple hot spots (HSs) on the pinch axis with a typical size of 200–300 μm and an electron temperature of 0.4–0.7 keV. The HS formation is accompanied by emission of neutrons with a mean energy of 2.7 ± 0.2 MeV. The maximum neutron yield achieved in these experiments was 2.6 × 1010 neutrons/shot.

Pinch formation and the zippering effect in laser-triggered discharge plasma under various electrode separations

In applying a laser-triggered discharge-produced plasma (LT-DPP) as a light source, the most important issues are the supply of fuel and the suppression of a large-scale nonuniformity called the zippering effect. This paper reports pinch formation in LT-DPP under various electrode separations from 4 mm to 10 mm, which provides information for suppressing the zippering effect. Spherically expanding fuel ablated from the cathode becomes visible when the discharge current becomes large. By 100 ns after the laser trigger, the laser-ablated fuel extends 5 mm from the cathode. The width of the fuel reduces gradually with the increase of the current and forms a pinch at the current peak at 200 ns for the case of a 5mm electrode separation. When the electrode separation is larger, neck-like fuel distribution is observed and the pinched portion propagates toward the anode. The condition for reducing the zippering effect is discussed.

Dose Measurements in a 20-J Repetitive Plasma Focus

In this article, the results of X-ray dose measurements executed using thermoluminescent dosimeters in experiments with a very small (20 J) repetitive plasma focus device named SORENA-1 are presented and analyzed. The working gas in these experiments was Argon. Also, pinch formation in experiments with this device has been observed. This device has been designed and constructed in Plasma and Nuclear Fusion Research School of Nuclear Science and Technology Research Institute of Iran. From these results, it is concluded that we can do experiments with this device using Ar as working gas all over the working days of year, and a good symmetry for measured dose around the device has been seen.

Exploring X-ray lasing in nitrogen pinch plasma at very high and fast discharge current excitation

The possibility to demonstrate X-ray lasing in nitrogen at 13.4 nm through recombination scheme driven by capillary discharge excitation has been explored at a high discharge current of ~95 kA, with ~46 ns quarter period. The emission from nitrogen pinch plasma showed a fast pulse at the instant of pinch formation, overriding the long duration Bremsstrahlung emission. The spectroscopic study revealed dominant X-ray line emissions at 2.8 and 2.1 nm, apart from various X-ray line emissions at higher wavelengths. Line emission at 2.8 nm confirms the formation of NVI charge state of nitrogen. At lower pressures, faint emission of Ly-α line at 2.4 nm indicated formation of NVII ions by further heating. The favourable role of pre-pulse in the formation of higher charge states of nitrogen was also established beyond doubt. This study provides important inputs for future experiments towards demonstration of X-ray lasing at 13.4 nm.

Bending and growth of entrained air filament under converging and asymmetric rotational fields

Here we have proposed the increase of the entrainment rate by extruding an air filament under the action of convergent but asymmetric rotational field. By varying the source speed and the diameter of rotational fields, we showed the bending of an air filament towards the higher strength direction of the asymmetric inertia. Interfacial profiles like bubble ejection from the air filament and non-collapsible entrainment with air accumulation in a stagnant zone are obtained in finite volume based numerical simulations, on gradual increase of average rotational fields. Physical understanding of bent interface profile reveals the presence of multiple stages in filament growth depending upon the inertia of surrounding medium. Accumulation of air in the stagnant zone is found to be more prominent in case of rotational speed based asymmetry in contrast to its counterpart having diametric asymmetry of imposing sources. Relative comparison between these two methods of producing asymmetric field showed faster growth of filament upon varying the source diameter, while keeping the speed same. In case of extreme retardation and enhancement of rotational asymmetry, film pinch off and formation of bubble train have been reported. The shape of ejected bubbles is governed by the inertia of the surrounding medium, and bubbles have taken elliptical shapes with their major axis aligned parallel to the adjacent velocity field.

Experimental investigation of the effect of insulator sleeve length on the time to pinch and multipinch formation in the plasma focus facility

The length of insulator sleeve is varied to investigate its effect on the pinch formation in the plasma focus facility. In this paper, the effect of insulator length on the time to pinch at various pressures and working voltages in the 1.15 kJ Mather type plasma focus is investigated. The results show that with 4.5 cm insulator length the time to pinch at all pressures is minimum. Other results also confirm that with increasing of pressure the time to pinch is increased. Moreover, with increasing working voltage the time to pinch is decreased. Pictures, captured using a digital single lens reflex (DSLR) Canon EOS 7D system, show that multipinch phenomenon is formed.

Pinch and swell structures: evidence for strain localisation by brittle–viscous behaviour in the middle crust

Abstract. The flow properties of middle crustal rocks are commonly represented by viscous flow. Examples of pinch and swell structures found in a high strain zone at St. Anne Point (Fiordland, New Zealand) and Wongwibinda (N.S.W., Australia) suggest pinch and swell structures may be initiated by brittle failure of the more competent layer in conjunction with subsequent material softening. On this basis we develop a numerical model where Mohr–Coulomb constitutive strain localising behaviour is utilised to initiate pinch and swell structure development. Results show that pinch and swell structures develop in a competent layer in both Newtonian and non-Newtonian flow, provided the competent layer has sufficient viscosity contrast and can localise strain to form shear bands. The flow regime and strain localising characteristics of the surrounding country rock appear not to impact pinch and swell structure formation. The degree of material softening after the initial strain localising behaviour is shown to impact pinch and swell characteristics, while extensive material softening causes the formation of thick necks between swells by limiting the focused localisation of strain into shear bands. To aid analysis of the structures and help derive the flow properties of rocks in the field, we define three stages of pinch and swell development and offer suggestions for measurements to be made in the field. Our study suggests that Mohr–Coulomb strain localising behaviour combined with viscous flow is a viable alternative representation of the heterogeneous rheological behaviour of rocks seen in the middle crust. This type of mid-crustal rheological behaviour can have significant influence on the localisation of strain at all scales. For example, inclusion of Mohr–Coulomb strain localising behaviour with viscous flow in just some mid-crustal layers within a crustal-scale model can result in significant strain localisation, extending from the upper crust into the middle crust. This localisation also influences the development of near-surface structures.

Study of the implosion of foam-wire loads at the Angara-5-1 facility

Results are presented from experiments on studying the compactness of compression of imploding nested foam-wire loads at currents of up to 4 MA at the Angara-5-1 facility. The degree of pinch compression was estimated from the dynamics of the spatial distribution of the current (magnetic field) and the shape of the soft X-ray pulse. The load consisted of nested cascades, one of which being a wire array and the other being a hollow or solid low-density cylinder made of agar-agar foam with a wall thickness of 100–200 μm. In some experiments, one of the cascades was made of C20H17O6 solid-state organic acid foam. The radial distribution of the magnetic field inside the nested cascades of the imploding foam-wire load (both between the cascades and inside the inner cascade) was measured using tiny magnetic probes. The measured radial distributions of the magnetic field are compared with the magnetic field configuration calculated using a one-dimensional MHD code simulating the implosion of a nested foam-wire load. It is shown that the spatial structure of the current and magnetic field during the implosion of such a load is determined by the development of supersonic and subsonic magnetized plasma flows in its cascades. The specific features of pinch formation and methods for the compensation of the nonsimultaneous pinch compression between the anode and the cathode (the zipper effect) during the implosion of a nested foam-wire load are analyzed.

Study of the implosion characteristics of quasi-spherical wire arrays on the Angara-5-1 facility at currents of up to 4 MA

Results are presented from experimental studies of the spatial distribution of the density of matter in the central part of the discharge gap and the formation of the temporal profile of the X-ray power in the course of implosion of quasi-spherical wire arrays at discharge currents of up to 4 MA. The spatial distribution of the X-ray intensity in the central part of the discharge gap and the temporal profile of the X-ray power are used as implosion characteristics of quasi-spherical wire arrays. The quasi-spherical arrays were formed by the radial stretching of unstrained wires of initially cylindrical and conical wire arrays under the action of the electrostatic field. The temporal profile of the output X-ray pulse in the photon energy range of 0.1–1 keV is shown to depend on both the geometrical parameters of the quasi-spherical array and the longitudinal distribution of its mass. It is found that a 40% increase in the wire mass due to deposition of an additional mass in the equatorial region of a quasi-spherical array leads to a 15% increase in the average current radius of the pinch and a 30% decrease in the X-ray yield. Experiments with quasi-spherical arrays made of kapron fibers with deposited Al and Bi conducting layers were also carried out. It is demonstrated that application of such arrays makes it possible to control the profile and duration of the generated X-ray pulse by varying the mass, material, and location of the deposited layer. It is found that deposition of an additional mass in the form of a thin Bi stripe on tungsten wires near the cathode end of the array allows one to mitigate the influence of the cathode zipper effect on the pinch compression and formation of the X-ray pulse in tungsten arrays.

Experimental study of the structure of the plasma-current sheath on the PF-1000 facility

The results of studies of the plasma-current sheath structure on the PF-1000 facility in the stage close to the instant of pinch formation are presented. The measurements were performed using various modifications of the calibrated magnetic probes. Studies of the influence of the probe shape and dimensions on the measurements accuracy were done. The current flowing in the converging sheath at a distance of 40 mm from the axis of the facility electrodes was measured. In the optimal operating modes, this current is equal to the total discharge current, which indicates the high efficiency of current transportation toward the axis. In such shots a compact high-quality sheath forms with shock wave in front of the magnetic piston. It is shown that the neutron yield depends on the current compressed onto the axis. This dependence agrees well with the known scaling, Yn ∼ I4. The use of the total discharge current in constructing the current scaling, especially for facilities with a large stored energy, is unjustified.

Magnetic probe measurements of the current sheath on the PF-3 facility

Results are presented from experimental studies of the dynamics of the current sheath (CS) on the PF-3 plasma focus facility. The parameters of the sheath, including the current distribution in it, were measured using absolutely calibrated magnetic probes installed at different positions with respect to the facility axis and the anode surface. The CS dynamics in discharges operating in argon and neon was investigated, and the skin depth in different stages of the discharge was determined. One of the probes was installed at a distance of ≈2 cm from the facility axis, which made it possible to estimate the efficiency of current transfer to the region of pinch formation. Operating modes were obtained in which the current dynamics detected by magnetic probes at different distances from the axis agreed well with the dynamics of the total discharge current until the instant of singularity in the current time derivative. It is shown that shunting breakdowns can lead to the formation of closed current loops. The shunting of the discharge current by the residual plasma is directly related to the efficiency of snowplowing of the working gas by the CS as it propagates from the insulator toward the facility axis.

Effect of Wire Diameter and Addition of an Axial Magnetic Field on the Dynamics of Radial Wire Array $Z$-Pinches

The operation of radial wire array Z-pinches driven by a 1-MA 250-ns current pulse was studied. Variation in the cathode diameter and wire diameter does not affect the overall plasma dynamics but controls the time of wire breakage and the time of pinch formation. The measured times of full wire ablation at the cathode were used to determine the ablation velocity (V abl), and the results give a scaling V abl ~ (wire diameter)-0.46. Experiments with added axial magnetic field show an increase in the pinched plasma diameter, possibly due to the compression of the axial magnetic flux by the imploding plasma.

Incoherent effect of space charge and electron cloud

Trapping by resonances or scattering off resonances induced by space charge (SC) or electron cloud (EC) in conjunction with synchrotron motion can explain observations of slow beam loss and emittance growth, which are often accompanied by changes in the longitudinal beam profile. In this paper we review the recent progress in understanding and modeling of the underlying mechanisms, highlight the differences and similarities between space charge and electron cloud, and discuss simulation results in the light of experimental observations, e.g., at GSI, CERN, and BNL. In particular, we address the role of the pinched electrons and describe in detail the complexity of the electron pinch formation. We present simulation results within a dipole or in a field-free region of the beam pipe, which reveal the morphology and main features of this phenomenon, explain the physical origin of the complex electron structures like stripe in either field configuration, and discuss the dependence on some key parameters.

Investigation of Carbon $X$-Pinches as a Source for Point-Projection Radiography

We report on the first investigations of the use of carbon fibers in an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -pinch load using a 250-kA linear transformer driver (LTD). Multiframe laser shadowgraphy is used to examine the evolution of the pinch and shows that carbon loads demonstrate wire expansion and cross-point pinch and gap formation as observed in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -pinches constructed from high Z materials. Radiographs taken using the carbon <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> -pinch as the source demonstrate both that sufficient flux is emitted to provide a good contrast image at source-to-image distances of > 10 cm and that the cross point produces a relatively small hot spot. Radiographs of a series of fine wires (5-30 mum) using X-rays > 500 eV demonstrated that 25-mum wires can be resolved in this energy range. Time-resolved X-ray emission measurements showed that, while emission in the hnu > 500 eV range shows long (> 100 ns) timescales, emission in the hnu > 1 keV range shows a multiple-peaked structure with durations as short as 20 ns.