Plasma Compression Research Articles

Spot and diffuse mode of cathode attachments in a magnetically rotating arc plasma generator at atmospheric pressure

Adjusting the strength of an axial magnetic field is an effective method to control the cathode attachment. In this paper, a magnetically rotating arc plasma generator is constructed to study the cathode attachment modes under different magnetic fields. Two cathode attachment modes are observed: a spot mode and a diffuse mode. Images of cathode attachments, temperature distribution of the cathode surface, and arc voltage characteristics correlating to different cathode attachment modes are investigated. Results show that the spot mode is favored by the low magnetic field. With an increase in the magnetic field, the cathode attachment region expands gradually, until the spot mode evolutes to the diffuse mode. The diffuse mode is associated with a significantly increased arc voltage, indicating that the transition is an abrupt process rather than a gradual process. For the diffuse mode, the cathode end has a higher average and lower peak temperature, but there exists varying temperature distribution on the cathode end, such as the ring-shaped high temperature region. Additionally, a two-dimensional coupled model is applied to qualitatively discuss the effect of magnetic field on the cathode attachment modes. Simulation results reveal that energy flux to the cathode surface increases with the increase of the magnetic field, and the major increment is thermal conduction heating from the arc column to the cathode surface, which possibly arises from the axial compression of arc plasma. Thus, the diffuse mode tends to always operate in the large magnetic field.

Bright Gamma-Ray Flares Powered by Magnetic Reconnection in QED-strength Magnetic Fields

Abstract Strong magnetic fields in the magnetospheres of neutron stars (NSs) (especially magnetars) and other astrophysical objects may release their energy in violent, intense episodes of magnetic reconnection. While reconnection has been studied extensively, the extreme field strength near NSs introduces new effects: radiation cooling and electron–positron pair production. Using massively parallel particle-in-cell simulations that self-consistently incorporate these new radiation and quantum-electrodynamic effects, we investigate relativistic magnetic reconnection in the strong-field regime. We show that reconnection in this regime can efficiently convert magnetic energy to X-ray and gamma-ray radiation and thus power bright, high-energy astrophysical flares. Rapid radiative cooling causes strong plasma and magnetic field compression in compact plasmoids. In the most extreme cases, the field can approach the quantum limit, leading to copious pair production.

Ступенчатое сжатие плазмы многозарядных ионов в протяженном продольном малоиндуктивном сильноточном Z-разряде

The features of the multicharged ion plasma dynamics in the extended low-inductive high-current discharge with power supply system based on high-voltage generator and transmission lines was analyzed. It is shown that the power supply systems of the considered type allow one to obtain the stepwise plasma compression and heating, which creates additional programming possibilities for physical and spectroscopic characteristics, in particular, of the ionic composition of the plasma.

Solar Wind Streams of Different Types and High-Latitude Substorms

The effects of different large-scale solar wind structures on magnetospheric substorms at high geomagnetic latitudes are studied. Two types of high-latitude substorm disturbances are considered: substorms observed in quiet conditions, when the auroral oval contracts and shifts towards high latitudes (contracted oval substorms, or polar substorms), and those observed in disturbed conditions, when the auroral oval expands (expanded oval substorms, or expanded substorms). Ground-based magnetic observations from the Scandinavian IMAGE network are compared with the OMNI solar wind database and the catalog of large-scale solar wind phenomena ( ftp://ftp.iki.rssi.ru/omni/ ). The study involves the following types of solar wind streams: (1) high-speed streams from coronal holes (FAST); (2) interplanetary manifestations of coronal mass ejections, i.e., magnetic clouds (MCs) or EJECTA; and (3) plasma compression regions ahead of these streams, i.e., corotating interaction regions (CIRs) and SHEATH. The study includes 186 polar and 202 expanded substorms in 1995, 1996, 1999, and 2000. It is shown that ~75% of expanded substorms are observed in FAST streams or plasma compression regions ahead of these streams (CIRs), and only ~18% of such substorms are observed in interplanetary manifestations of coronal mass ejections EJECTA and in SHEATH compression regions. While ~67% of polar substorms are observed in SLOW streams, ~19% of such substorms have been recorded in SHEATH and EJECTA but only against the background of a slow solar wind.

Mechanisms of the energy transfer across the magnetic field by Alfvén waves in toroidal plasmas

Physics of the transverse energy transfer by Alfvén waves in toroidal plasmas is elucidated. It is found that, in contrast to the classical Alfvén waves in infinite plasmas, the Alfvén waves in toroidal systems produce plasma compression due to coupling with fast magnetoacoustic waves, which provides the energy transfer. The radial group velocities of the traveling waves constituting the Global Alfvén Eigenmodes and Toroidicity-induced Alfvén Eigenmodes are calculated. It is shown that equations for Alfvén eigenmodes derived in the approximation of vanishing wave field along the equilibrium magnetic field reproduce the longitudinal magnetic field of the wave and lead to correct transverse energy flux. The obtained results explain how Alfvén eigenmodes can provide the spatial energy channeling—the transfer of the energy by these modes from the unstable plasma region to the region where the mode damping dominates.

Kinetic Equilibrium of Dipolarization Fronts

The unprecedented high-resolution data from the Magnetospheric Multi-Scale (MMS) satellites is revealing the physics of dipolarization fronts created in the aftermath of magnetic reconnection in extraordinary detail. The data shows that the fronts contain structures on small spatial scales beyond the scope of fluid framework. A new kinetic analysis, applied to MMS data here, predicts that global plasma compression produces a unique particle distribution in a narrow boundary layer with separation of electron and ion scale physics. Layer widths on the order of an ion gyro-diameter lead to an ambipolar potential across the magnetic field resulting in strongly sheared flows. Gradients along the magnetic field lines create a potential difference, which can accelerate ions and electrons into beams. These small-scale kinetic effects determine the plasma dynamics in dipolarization fronts, including the origin of the distinctive broadband emissions.

Experimental investigation of the compression and heating of an MHD-driven jet impacting a target cloud

Adiabatic compression has been investigated by having an MHD-driven plasma jet impact a gas target cloud. Compression and heating of the jet upon impact were observed and compared to theoretical predictions. Diagnostics for comprehensive measurements included a Thomson scattering system, a fast movie camera, a translatable fiber-coupled interferometer, a monochromator, a visible-light photodiode, and a magnetic probe array. Measurements using these diagnostics provided the time-dependent electron density, electron temperature, continuum emission, line emission, and magnetic field profile. Increases in density and magnetic field and a decrease in jet velocity were observed during the compression. The electron temperature had a complicated time dependence, increasing at first, but then rapidly declining in less than 1 μs which is less than the total compression time. Analysis indicates that this sudden temperature drop is a consequence of radiative loss from hydrogen atoms spontaneously generated via three-body recombination in the high-density compressed plasma. A criterion for how fast compression must be to outrun radiative loss is discussed not only for the Caltech experiment but also for fusion-grade regimes. In addition, the results are analyzed in the context of shocks the effects of which are compared to adiabatic compression.

Ballooning instability of azimuthally small scale coupled Alfvén and slow magnetoacoustic modes in two-dimensionally inhomogeneous magnetospheric plasma

This paper investigates the conditions of the ballooning instability of the coupled Alfvén and slow magnetoacoustic modes in the dipole model of Earth's magnetosphere taking into account plasma and magnetic field inhomogeneity in the direction along the magnetic field lines. The diamagnetic condition (meaning vanishing perturbation of the total pressure) is satisfied. It was shown that the instability develops on the slow magnetoacoustic oscillation branch, but the instability threshold is determined by the coupling with the Alfvén mode. The symmetric (with respect to the magnetic equator) modes were found to be more unstable than antisymmetric ones. In this case, the instability threshold depends on plasma compressibility: the finite sound velocity raises the instability threshold. For all other equal conditions, the instability threshold decreases with the decrease in the field line curvature radius on the equator.

Energy Distribution of Pickup Ions at the Solar Wind Termination Shock

Abstract In-situ measurements taken by the Voyager 2 spacecraft suggest that the solar wind termination shock is significantly affected by the presence of pickup ions that are produced in the inner heliosphere due to charge exchange between interstellar neutrals and the solar wind ions. We use a fully kinetic particle-in-cell method to self-consistently simulate the shock with all physical properties available from Voyager 2. We have performed a set of simulations with varying velocity distribution functions for the pickup ions, since it was not determined by Voyager’s measurements. We show that the measurements suggest that the pickup ions upstream of the shock are more energetic than generally believed. If their velocity distribution function assumes a filled-shell shape in the wind frame, the maximum cutoff speed for the pickup ions should be ≳650 km s−1 in order to reproduce the measurements, which is almost twice the local wind speed. We suggest that pickup ions upstream of the shock are energized by adiabatic compression of the solar wind plasma as well as due to an enhanced level of turbulence in a broad foreshock region.

Propagation of a global coronal wave and its interaction with large-scale coronal magnetic structures

Context. Global coronal waves associated with solar eruptions (the so-called EIT waves) often encounter coronal holes and solar active regions and interact with these magnetic structures. This interaction leads to a number of observed effects such as wave reflection and transmission. Aims. We consider the propagation of a large-scale coronal shock wave and its interaction with large-scale non-uniformities of the background magnetic field and plasma parameters. Methods. Using the Lare2d code, we performed 2.5-dimensional simulations of the interaction of a large-scale single-pulse fast-mode magnetohydrodynamic shock wave of weak-to-moderate intensity with the region of enhanced Alfvén speed as well as with that of reduced Alfvén speed. We analysed simple models of non-uniformity and the surrounding plasma to understand the basic effects in wave propagation. Results. We found the reflected waves of plasma compression and rarefaction, transmitted waves that propagate behind or ahead of the main part of the wave, depending on properties of the plasma non-uniformity, and secondary wave fronts. The obtained results are important to the correct interpretation of the global coronal wave propagation in the solar corona, understanding of theoretical aspects of the interaction of large-scale coronal shock waves with large-scale coronal magnetic structures, and diagnostics of coronal plasma parameters.

A Power Module Designed for Compressed Plasma in EAST

Magnetic compression (MC) technology was recommended for tokamak to study compressed plasma and a power module have been designed here for developing its server power supplier. The minor-radius compression is one of the most effective methods to improve the performance parameters of existing tokamaks, enabling the fusion plasmas operated at high density, high temperature, and high beta. In this paper, a high-frequency and high-power supply system is proposed, used for powering coils of MC within the vacuum chamber of the experimental advanced superconducting tokamak. The basic of the power module is an ac/pulse converter, implemented by a series-resonant full-bridge circuit and controlled by pulsewidth modulated. Also, control method adopts current closed-loop proportional–integral control, which has less than 3-ms current response time in real time. The converter is analyzed and the design procedure is discussed. Experimental results obtained from a 3-kW converter prototype are presented to validate the converter’s performance with the redesigned control board.

Micro-scale fusion in dense relativistic nanowire array plasmas

Nuclear fusion is regularly created in spherical plasma compressions driven by multi-kilojoule pulses from the world’s largest lasers. Here we demonstrate a dense fusion environment created by irradiating arrays of deuterated nanostructures with joule-level pulses from a compact ultrafast laser. The irradiation of ordered deuterated polyethylene nanowires arrays with femtosecond pulses of relativistic intensity creates ultra-high energy density plasmas in which deuterons (D) are accelerated up to MeV energies, efficiently driving D–D fusion reactions and ultrafast neutron bursts. We measure up to 2 × 106 fusion neutrons per joule, an increase of about 500 times with respect to flat solid targets, a record yield for joule-level lasers. Moreover, in accordance with simulation predictions, we observe a rapid increase in neutron yield with laser pulse energy. The results will impact nuclear science and high energy density research and can lead to bright ultrafast quasi-monoenergetic neutron point sources for imaging and materials studies.

Features of the Application of the Magnetic-probe Method for Diagnostics of High-temperature Plasma

Some aspects of the applicability of the magnetic-probe technique in high-power pulsed discharges are analyzed. The influence of an electron beam and an intense X-ray yield of (~1 TW/cm2), which result from the compression of high-current plasma in the interelectrode gap of a Z-pinch discharge, on the correctness of measurements using magnetic-field probes was studied. We considered the use of multilayer shells as a method for protecting the sensing element of a magnetic probe. The results of experimental testing of probes of a new design in experiments with wire assemblies on the Angara-5-1 facility at discharge currents of up to 4 MA are presented. Experiments on the Angara-5-1, PF-3, and PF-1000 high-power electrophysical facilities show the effect of the shape and material of the probe shell on the perturbation of plasma that flows around a probe and, as a consequence, on the accuracy of the magnetic-field measurements.

Interaction of laser beams with magnetized substance in a strong magnetic field

Laser-driven magneto-inertial fusion assumed plasma and magnetic flux compression by quasisymmetric laser-driven implosion of magnetized target. We develop a 2D radiation magnetohydrodynamic code and a formulation for the one-fluid two-temperature equations for simulating compressible non-equilibrium magnetized target plasma. Laser system with pulse radiation with 10 ns duration is considered for numerical experiments. A numerical study of a scheme of magnetized laser-driven implosion in the external magnetic field is carried out.

MAGO-IX Experiment

The results of the last (in the thermonuclear program “Magnetic compression”) MAGO-IX experiment with a plasma chamber including a third compartment designed to compress plasma with a converging liner are presented. An X-ray pulse consisting of an intense peak of 1-μs duration, followed by a low-intensity tail with a duration of more than 10 μs, was recorded. In the MAGO-IX experiment, the neutrons were generated mainly in the third compartment. A neutron yield of 2 × 1012 was obtained. The results demonstrate that the expected compression of preheated plasma in chambers similar to MAGO-IX is promising for achieving thermonuclear ignition.

Magnetothermodynamics: measurements of the thermodynamic properties in a relaxed magnetohydrodynamic plasma

We have explored the thermodynamics of compressed magnetized plasmas in laboratory experiments and we call these studies ‘magnetothermodynamics’. The experiments are carried out in the Swarthmore Spheromak eXperiment device. In this device, a magnetized plasma source is located at one end and at the other end, a closed conducting can is installed. We generate parcels of magnetized plasma and observe their compression against the end wall of the conducting cylinder. The plasma parameters such as plasma density, temperature and magnetic field are measured during compression using HeNe laser interferometry, ion Doppler spectroscopy and a linear ${\dot{B}}$ probe array, respectively. To identify the instances of ion heating during compression, a PV diagram is constructed using measured density, temperature and a proxy for the volume of the magnetized plasma. Different equations of state are analysed to evaluate the adiabatic nature of the compressed plasma. A three-dimensional resistive magnetohydrodynamic code (NIMROD) is employed to simulate the twisted Taylor states and shows stagnation against the end wall of the closed conducting can. The simulation results are consistent to what we observe in our experiments.

Compression and heating of a laser-produced plasma using single and double induction coils

The results of an experiment on magnetohydrodynamic compression and heating of a laser-produced plasma in vacuum are described. The plasma was produced by laser ablation of copper at 2 J cm−2. A pulsed magnetic field, with an amplitude of 0.3 T and a period of 2.2 µs, was produced by a three-turn spiral induction coil placed 10 mm above the ablation spot. Time-resolved imaging revealed that the magnetic field had a strong influence on both the plasma between the coil and the target, and on the plasma which flows through the aperture in the coil. The plasma flow through the coil aperture is strongly pinched due to the Lorentz interaction of the induced current and the coil magnetic field. Heating of the plasma is evidenced by strong enhancement of the overall visible light emission and the appearance of Cu+ line emission. Magnetic compression and plasma heating were also observed in a setup using two induction coils separated by 10 mm. This technique could be used to enhance the sensitivity of laser-induced breakdown spectroscopy, increase the ion yield in laser plasma ion sources, or control the ablation plume expansion in pulsed laser deposition.

Improving Machining Efficiency Methods of Micro EDM in Cold Plasma Jet

This paper investigates the characteristics of micro electrical discharge machining (EDM) in cold plasma jet using transistor pulse generator. In order to improve the processing efficiency and quality in stable process conditions, oxygen-assisted nitrogen plasma jet, nitrogen-oxygen mixed plasma jet and external compressed air assisted nitrogen plasma jet are used in micro-EDM experiments. It was found that the material removal rate and surface roughness increases with the increase of oxygen flow rate in oxygen-assisted nitrogen plasma jet experiment. In addition, it was found that micro-EDM with a compressed air-assisted plasma jet is superior to oxygen-assisted nitrogen plasma jet in surface quality and edge quality. Nitrogen element is not found on the machined surface in micro-EDM based in NPJ or NJ. The content of oxygen element has an increasing trend when compressed air is used as an auxiliary gas.

Influence of the Doppler effect on radiative transfer in a spherical plasma under macroscopic motion of substance

The non-LTE radiative transfer in spherical plasma containing resonantly absorbing light ions has been studied numerically under conditions of macroscopic motion of substance. Two types of macroscopic motion were simulated: radial expansion and compression (pulsation) of spherical plasma; rotation of plasma relative to an axis of symmetry. The calculations of absorption line profile of transmitted broadband radiation and the emission line profile were performed for the optically dense plasma of calcium ions on the resonance transition with wavelength 397 nm. Numerical results predict frequency shifts in the emission line profile to red wing of the spectrum for radial expansion of the plasma and to blue wing of the spectrum for the plasma compression at an average velocity of ions along the ray of sight equal to zero. The width of the emission line profile of a rotating plasma considerably exceeds the width of the profile of the static plasma, and the shift of the central frequency of resonance transition from the resonance frequency of the static plasma gives a linear velocity of ion motion along a given ray trajectory in units of thermal velocity. Knowledge of the linear radial velocity of ions can be useful for diagnostic purposes in determining the frequency and period of rotation of optically dense plasmas.

Study of Implosion of Combined Nested Arrays

New experimental data on the implosion of plasma of nested kapron−tungsten arrays are obtained at the Angara-5-1 facility. The mode of plasma implosion is implemented in which a shock wave region forms in the space between the inner and outer arrays where a transition from the super-Alfvenic (V r > V A ) to sub-Alfvenic (V r < V A ) plasma flow takes place. Specific features of the formation and decay of the shock region are studied using laser shadow imaging and X-ray frame photography. The plasma density in the transition region is estimated. By comparing the experimental data with the results of simulations of quasi-steady implosion of a nested array with allowance for extended plasma production, the physical conditions are determined at which the implosion mode with the formation the shock region takes place. Stable compression of the plasma of the inner array was observed during the implosion of combined nested arrays with a fiber outer array and tungsten inner array. Suppression of magnetic Rayleigh-Taylor instability during the compression of the inner array plasma results in the formation of a compact radiating Z-pinch and generation of a soft X-ray pulse with a peak power of 4 TW and duration of about 5 ns.