Current-carrying Plasma Research Articles

МОДЕЛИРОВАНИЕ ДИНАМИКИ ПЛАЗМЫ В ПЛАЗМЕННОМ ФОКУСЕ ТИПА ФИЛИППОВА

We have simulated motion of the current-carrying plasma sheath (CPS) in the PF-3 facility — from discharge start to appearance of plasma focus. For this purpose, we modified one-component MHD model to take into account the Hall effect near electrodes. The simulation reaffirmed the key role of the Hall effect in the phenomenon of the CPS slip along the electrode, which was assumed earlier. The extended simulation results are in agreement with the CPS «runaway» mode, obtained in the PF-3 facility. Modeling of CPS movement taking into account the Hall effect is in good agreement with the experimental results, which were obtained at the facility PF-3 previously. It was shown that the parameters of the axial plasma stream are is significantly affected by the degree of the pinch compression.

Two-fluid and magnetohydrodynamic modelling of magnetic reconnection in the MAST spherical tokamak and the solar corona

Twisted magnetic flux ropes are ubiquitous in laboratory and astrophysical plasmas, and the merging of such flux ropes through magnetic reconnection is an important mechanism for restructuring magnetic fields and releasing free magnetic energy. The merging-compression scenario is one possible start-up scheme for spherical tokamaks, which has been used on the Mega Amp Spherical Tokamak (MAST). Two current-carrying plasma rings or flux ropes approach each due to mutual attraction, forming a current sheet and subsequently merge through magnetic reconnection into a single plasma torus, with substantial plasma heating. Two-dimensional resistive and Hall-magnetohydrodynamic simulations of this process are reported, including a strong guide field. A model of the merging based on helicity-conserving relaxation to a minimum energy state is also presented, extending previous work to tight-aspect-ratio toroidal geometry. This model leads to a prediction of the final state of the merging, in good agreement with simulations and experiment, as well as the average temperature rise. A relaxation model of reconnection between two or more flux ropes in the solar corona is also described, allowing for different senses of twist, and the implications for heating of the solar corona are discussed.

Current sheets with inhomogeneous plasma temperature: Effects of polarization electric field and 2D solutions

We develop current sheet models which allow to regulate the level of plasma temperature and density inhomogeneities across the sheet. These models generalize the classical Harris model via including two current-carrying plasma populations with different temperature and the background plasma not contributing to the current density. The parameters of these plasma populations allow regulating contributions of plasma density and temperature to the pressure balance. A brief comparison with spacecraft observations demonstrates the model applicability for describing the Earth magnetotail current sheet. We also develop a two dimensional (2D) generalization of the proposed model. The interesting effect found for 2D models is the nonmonotonous profile (along the current sheet) of the magnetic field component perpendicular to the current sheet. Possible applications of the model are discussed.

Study of magnetic fields and current in the Z pinch at stagnation

The structure of magnetic fields in wire-array Z pinches at stagnation was studied using a Faraday rotation diagnostic at the wavelength of 266 nm. The electron plasma density and the Faraday rotation angle in plasma were calculated from images of the three-channel polarimeter. The magnetic field was reconstructed with Abel transform, and the current was estimated using a simple model. Several shots with wire-array Z pinches at 0.5–1.5 MA were analyzed. The strength of the magnetic field measured in plasma of the stagnated pinch was in the range of 1–2 MG. The magnetic field and current profile in plasma near the neck on the pinch were reconstructed, and the size of the current-carrying plasma was estimated. It was found that current flowed in the large-size trailing plasma near the dense neck. Measurements of the magnetic field near the bulge on the pinch also showed current in trailing plasma. A distribution of current in the large-size trailing plasma can prevent the formation of multi-MG fields in the Z pinch.

Kinetic theory of the filamentation instability in a collisional current-driven plasma with nonextensive distribution

The evolution of filamentation instability in a weakly ionized current-carrying plasma with nonextensive distribution was studied in the diffusion frequency region, taking into account the effects of electron-neutral collisions. Using the kinetic theory, Lorentz transformation formulas, and Bhatnagar-Gross-Krook collision model, the generalized dielectric permittivity functions of this plasma system were achieved. By obtaining the dispersion relation of low-frequency waves, the possibility of filamentation instability and its growth rate were investigated. It was shown that collisions can increase the maximum growth rate of instability. The analysis of temporal evolution of filamentation instability revealed that the growth rate of instability increased by increasing the q-parameter and electron drift velocity. Finally, the results of Maxwellian and q-nonextensive velocity distributions were compared and discussed.

The microscopic Z-pinch process of current-carrying rarefied deuterium plasma shell

For insight into the microscopic mechanism of Z-pinch dynamic processes, a code of two-dimensional particle-in-cell (PIC) simulation has been developed in cylindrical coordinates. In principle, the Z-pinch of current-carrying rarefied deuterium plasma shell has been simulated by means of this code. Many results related to the microscopic processes of the Z-pinch are obtained. They include the spatio-temporal distributions of electromagnetic field, current density, forces experienced by the ions and electrons, positions and energy distributions of particles, and trailing mass and current. In radial direction, the electric and magnetic forces exerted on the electrons are comparable in magnitude, while the forces exerted on the ions are mainly the electric forces. So in the Z-pinch process, the electrons are first accelerated in Z direction and get higher velocities; then, they are driven inwards to the axis at the same time by the radial magnetic forces (i.e., Lorentz forces) of them. That causes the separations between the electrons and ions because the ion mass is much larger than the electron's, and in turn a strong electrostatic field is produced. The produced electrostatic field attracts the ions to move towards the electrons. When the electrons are driven along the radial direction to arrive at the axis, they shortly move inversely due to the static repellency among them and their tiny mass, while the ions continue to move inertially inwards, and later get into stagnation, and finally scatter outwards. Near the stagnation, the energies of the deuterium ions mostly range from 0.3 to 6 keV, while the electron energies are mostly from 5 to 35 keV. The radial components, which can contribute to the pinched plasma temperature, of the most probable energies of electron and ion at the stagnation are comparable to the Bennett equilibrium temperature (about 1 keV), and also to the highest temperatures of electron and ion obtained in one dimensional radiation magnetohydrodynamic simulation of the plasma shell Z-pinch. The trailing mass is about 20% of the total mass of the shell, and the maximum trailing current is about 7% of the driven current under our trailing definition. Our PIC simulation also demonstrates that the plasma shell first experiences a snow-plow like implosion process, which is relatively stable.

Characteristics of the double layer associated with terrestrial bow shock by THEMIS observation

This study presents observation and detailed analysis on the double layers (DLs) in the ramp and the foreshock contacting with the foot of the terrestrial bow shock by THEMIS on September 14, 2008 under enhanced dynamic pressure in the solar wind. The results reveal that: (1) The time duration of the double layers is mainly 3-8 ms, and their max parallel electric field is minus 10-40 mV/m. (2) On assuming a propagation speed at the ion acoustic speed ( u s), their spatial scale is estimated to be 0.3-1.15 km (about 75-200 l D ). (3) The net potential drop of DLs is estimated to be 5-32 V. (4) The DLs in the ramp and the foreshock contacting to the foot of the bow shock is current-carrying as a result of development and evolution of nonlinear phase of instability in the self-consistent current-carrying plasma. The DLs may play an important role in strong turbulence in the foreshock contacting with the foot of the bow shock.

Small-scale instability and nonlinear structures in current-carrying atmospheric plasma in the D region of the ionosphere

It is shown that the electric field may cause instability of low-frequency potential oscillations with a period of about 1 min in the D region of the ionosphere. This is related to the fact that the attachment rate of electrons to molecules is a nonmonotonic function of the temperature: it increases at low temperatures and decreases at high temperatures. The temperature corresponding to the maximum attachment rate is about 1000 K. The development of instability can result in the formation of quasi-steady layers of the electron density with a characteristic spatial period of several tens of meters. Such inhomogeneities can affect the propagation of radio waves with wavelengths on the order of or shorter than 10 m.

Suppression of vertical instability in elongated current-carrying plasmas by applying stellarator rotational transform

The passive stability of vertically elongated current-carrying toroidal plasmas has been investigated in the Compact Toroidal Hybrid, a stellarator/tokamak hybrid device. In this experiment, the fractional transform f, defined as the ratio of the imposed external rotational transform from stellarator coils to the total rotational transform, was varied from 0.04 to 0.50, and the elongation κ was varied from 1.4 to 2.2. Plasmas that were vertically unstable were evidenced by motion of the plasma in the vertical direction. Vertical drifts are measured with a set of poloidal field pickup coils. A three chord horizontally viewing interferometer and a soft X-ray diode array confirmed the drifts. Plasmas with low fractional transform and high elongation are the most susceptible to vertical instability, consistent with analytic predictions that the vertical mode in elongated plasmas can be stabilized by the poloidal field of a relatively weak stellarator equilibrium.

The impact of Hall physics on magnetized high energy density plasma jets

Hall physics is often neglected in high energy density plasma jets due to the relatively high electron density of such jets (ne ∼ 1019 cm−3). However, the vacuum region surrounding the jet has much lower densities and is dominated by Hall electric field. This electric field redirects plasma flows towards or away from the axis, depending on the radial current direction. A resulting change in the jet density has been observed experimentally. Furthermore, if an axial field is applied on the jet, the Hall effect is enhanced and ignoring it leads to serious discrepancies between experimental results and numerical simulations. By combining high currents (∼1 MA) and magnetic field helicity (15° angle) in a pulsed power generator such as COBRA, plasma jets can be magnetized with a 10 T axial field. The resulting field enhances the impact of the Hall effect by altering the density profile of current-free plasma jets and the stability of current-carrying plasma jets (e.g., Z-pinches).

Relating reconnection rate, exhaust structure and effective resistivity

The magnetic reconnection structure consists of a central diffusion region (CDR) and a cone or wedge shaped reconnection exhaust containing accelerated plasma flows and electromagnetic fluctuations. We predict here the relationship among the exhaust half-cone angle (θe), the half width (w) of the CDR, the outflow velocity Vo, and the effective resistivity (ηeff), which includes the effects of all the nonideal terms in the generalized Ohm's law. The effective resistivity is defined as the ratio of reconnection electric field Erec to the current density Jy at the X point and it essentially represents the loss of momentum from the current-carrying plasma particles due to scattering by waves, their inertia or outflux from the CDR. The relation is checked against relevant results previously reported from laboratory experiments, space observations, and simulations, showing excellent agreement. The relation can be used for estimating the ad-hoc effective resistivity often used in magnetohydrodynamic modeling of reconnection.

Particle in cell simulation of relativistic Buneman instability in a current-carrying plasma

Non-linear evolution of the relativistic Buneman instability in a current-carrying plasma is investigated by a particle in cell simulation. These simulations show that as the time progresses, some electrons are trapped in phase space holes and thus counter-streaming and plateau can be formed. Moreover, the electron and ion density profiles indicate a periodic pattern of the density steepening. This density distribution is similar to the generation of the grating-like patterns which strongly depends on the initial electron velocity and saturation time. It is also shown that the electric field profile has the sawtooth form; charged particles can be accelerated by this field. Finally, it is found that increasing the electron velocity increases the saturation time and consequently the growth rate decreases which is in good agreement with the result obtained by the fluid model.

Formation of current filaments and magnetic field generation in a quantum current-carrying plasma

The nonlinear dynamics of filamentation instability and magnetic field in a current-carrying plasma is investigated in the presence of quantum effects using the quantum hydrodynamic model. A new nonlinear partial differential equation is obtained for the spatiotemporal evolution of the magnetic field in the diffusion regime. This equation is solved by applying the Adomian decomposition method, and then the profiles of magnetic field and electron density are plotted. It is shown that the saturation time of filamentation instability increases and, consequently, the instability growth rate and the magnetic field amplitude decrease in the presence of quantum effects.

Action of an electromagnetic pulse on a plasma with a high level of ion-acoustic turbulence. Field diffusion and subdiffusion

Specific features of the interaction of a relatively weak electromagnetic pulse with a nonisothermal current-carrying plasma in which the electron drift velocity is much higher than the ion-acoustic velocity, but lower than the electron thermal velocity, are studied. If the state of the plasma with ion-acoustic turbulence does not change during the pulse action, the field penetrates into the plasma in the ordinary diffusion regime, but the diffusion coefficient in this case is inversely proportional to the anomalous conductivity. If, during the pulse action, the particle temperatures and the current-driving field change due to turbulent heating, the field penetrates into the plasma in the subdiffusion regime. It is shown how the presence of subdiffusion can be detected by measuring the reflected field.

Numerical magnetohydrodynamic simulations of expanding flux ropes: Influence of boundary driving

The expansion dynamics of a magnetized, current-carrying plasma arch is studied by means of time-dependent ideal MHD simulations. Initial conditions model the setup used in recent laboratory experiments that in turn simulate coronal loops [J. Tenfelde et al., Phys. Plasmas 19, 072513 (2012); E. V. Stenson and P. M. Bellan, Plasma Phys. Controlled Fusion 54, 124017 (2012)]. Boundary conditions of the electric field at the “lower” boundary, intersected by the arch, are chosen such that poloidal magnetic flux is injected into the domain, either localized at the arch footpoints themselves or halfway between them. These conditions are motivated by the tangential electric field expected to exist in the laboratory experiments due to the external circuit that drives the plasma current. The boundary driving is found to systematically enhance the expansion velocity of the plasma arch. While perturbations at the arch footpoints also deform its legs and create characteristic elongated segments, a perturbation between the footpoints tends to push the entire structure upwards, retaining an ellipsoidal shape.

Simulation of filamentation instability of a current-carrying plasma by particle in cell method

The nonlinear dynamics of filamentation instability in a weakly ionized current-carrying plasma in the diffusion frequency region is studied using particle in cell simulation. The effects of electron thermal motion and ion-neutral collision on the evolution of this instability in the nonlinear stage of the filaments coalescence are discussed. It is found that the coalescence of the current filaments is enhanced by increasing the temperature and is delayed by increasing the collision frequency.

MICROWAVE QUASI-PERIODIC PULSATION WITH MILLISECOND BURSTS IN A SOLAR FLARE ON 2011 AUGUST 9

An peculiar microwave quasi-periodic pulsation (QPP) accompanying with a hard X-ray (HXR) QPP of about 20 s duration occurred just before the maximum of an X6.9 solar flare on 2011 August 9. The most interesting is that the microwave QPP is consisting of millisecond timescale superfine structures. Each microwave QPP pulse is made up of clusters of millisecond spike bursts or narrow band type III bursts. There are three different frequency drift rates: global frequency drift rate of microwave QPP pulse group, frequency drift rate of microwave QPP pulse, and frequency drift rate of individual millisecond spikes or type III bursts. The physical analysis indicates that the energetic electrons accelerating from a large-scale highly dynamic magnetic reconnecting current sheet above the flaring loop propagate downwards, impact on the flaring plasma loop, and produce HXR bursts. The tearing-mode (TM) oscillations in the current sheet modulate HXR emission and generate HXR QPP; the energetic electrons propagating downwards produce Langmuir turbulence and plasma waves, result in plasma emission. The modulation of TM oscillation on the plasma emission in the current-carrying plasma loop may generate microwave QPP. The TM instability produces magnetic islands in the loop. Each X-point will be a small reconnection site and accelerate the ambient electrons. These accelerated electrons impact on the ambient plasma and trigger the millisecond spike clusters or the group of type III bursts. Possibly each millisecond spike burst or type III burst is one of the elementary burst (EB). Large numbers of such EB clusters form an intense flaring microwave burst.

Toroidal Alfvén Eigenmodes and Fast Particles in Tokamaks

A general introduction is given to ideal magnetohydrodynamic (MHD) waves, using linear perturbations of ideal MHD equations. Subsequently the shear Alfvén waves are shown to form a discrete spectrum of Global Alfvén Eigenmodes in current-carrying cylindrical plasmas and Toroidal Alfvén Eigenmodes in toroidal plasmas. A comparison between theory and experiment is presented for the observed discrete spectra of Alfvén waves driven by energetic ions in Joint European Torus. The mechanism of excitation of Alfvén instabilities is described by considering particle-to-wave power transfer, and two main mechanisms of TAE-induced re-distribution and losses of energetic ions are discussed.

Study of Nonlinear Dynamics of the Filamentation Instability in a Current-Carrying Plasma Using Adomian Decomposition Method

Using Adomian decomposition method (ADM), we have theoretically investigated the nonlinear dynamics of the magnetic field diffusion and filamentation instability in a current-carrying plasma. Based on the Adomian solutions, the spatiotemporal behavior of the magnetic field is obtained, and it is shown that its profile changes from a sinusoidal shape to a saw-tooth one. Furthermore, the electron density distribution becomes highly peaked. In contrast to the numerical methods, the ADM predicts the microcurrents. In addition, the observation time of other events, like the filamentation instability as well as steepening of electron density, is tremendously increased in comparison with the time obtained by the traditional methods.

Coupled azimuthal modes propagating in current-carrying plasma waveguides

AbstractThe paper is devoted to the theory of electromagnetic surface waves propagating along the azimuthal direction in cylindrical metal waveguides, which are filled with current-carrying plasmas. The problem is solved by the method of successive approximation. Adequacy of this method application is proved here. To study the coupling of ordinary (O-) and extraordinary (X-) azimuthal modes, the linear theory of the eigenazimuthal X- and O-modes is applied as zero approximation. Plasma particles are described in the framework of magneto-hydrodynamics, electromagnetic fields of the coupled azimuthal modes are determined from Maxwell equations. Spatial distribution of electromagnetic field of these coupled modes and their damping caused for different reasons are studied. Possibility to observe experimentally the phenomena, which accompany propagation of these coupled modes, is estimated numerically. Branches of their possible utilization are discussed as well.