Closed Field Line Configuration Research Articles

Basic investigations of electrostatic turbulence and its interaction with plasma and suprathermal ions in a simple magnetized toroidal plasma

Progress in basic understanding of turbulence and its influence on the transport both of the plasma bulk and of suprathermal components is achieved in the TORPEX simple magnetized torus. This configuration combines a microwave plasma production scheme with a quasi-equilibrium generated by a toroidal magnetic field, onto which a small vertical component is superimposed, simulating a simplified form of tokamak scrape-off layers. After having clarified the formation of blobs in ideal interchange turbulence, TORPEX experiments elucidated the mechanisms behind the blob motion, with a general scaling law relating their size and speed. The parallel currents associated with the blobs, responsible for the damping of the charge separation that develops inside them, hence determining their cross-field velocity, have been measured. The blob dynamics is influenced by creating convective cells with biased electrodes, arranged in an array on a metal limiter. Depending on the biasing scheme, radial and vertical blob velocities can be varied. Suprathermal ion transport in small-scale turbulence is also investigated on TORPEX. Suprathermal ions are generated by a miniaturized lithium source, and are detected using a movable double-gridded energy analyser. We characterize vertical and radial spreading of the ion beam, associated with the ideal interchange-dominated plasma turbulence, as a function of the suprathermal ion energy and the plasma temperature. Experimental results are in good agreement with global fluid simulations, including in cases of non-diffusive behaviour. To investigate the interaction of plasma and suprathermal particles with instabilities and turbulence in magnetic configurations of increasing complexity, a closed field line configuration has recently been implemented on TORPEX, based on a current-carrying wire suspended in the vacuum chamber. First measurements indicate the creation of circular symmetric profiles centred on the magnetic axis, and instabilities driven in the region of strong gradients, with a strong ballooning character.

Consequences of violation of frozen-in-flux: Evidence from OpenGGCM simulations

[1] It is widely believed that during a substorm, plasma instabilities occur before the onset of magnetic reconnection, signaling the end of the growth phase. Despite many years of effort, however, the details of how the instability and the onset of reconnection develop from closed field line configuration with finite normal magnetic field are not well understood. In this paper, we study an idealized simulation of a substorm that occurred on 23 March 2007, based on the Open Geospace General Circulation Model (OpenGGCM). Our analysis emphasizes the time development of the distribution of the entropy parameter and its convective time derivative, which should be zero in ideal MHD. In the late growth phase, the simulation exhibits, over a range of local times, a systematic violation of conservation of entropy that corresponds to what is called “antidiffusion.” Out of this background, a more localized disturbance develops in a region of high magnetic stretching, resulting in formation of a strong reduction of entropy (bubble) earthward of a local enhancement (blob). The process is accelerated when the current density exceeds a threshold for triggering an explicit resistivity in the code. The bubble moves earthward and the blob tailward, which leads to a reduction of the normal magnetic field and a thinning of the current sheet between them, making the magnetospheric configuration more conducive to tearing and other instabilities (we do not address specifically which instability has occurred). This positive feedback gives rise to increased violation of the perfect conductivity relation and eventually reconnection.

Quasilinear theory of interchange modes in a closed field line configuration

Two important issues for any magnetic fusion configuration are the maximum achievable values of β and energy confinement time when ideal magnetohydrodynamic (MHD) modes are excited. It is well known that the excitation of the MHD unstable modes typically can lead to violent restructuring of the plasma profiles. The particle and energy transport associated with these modes normally dominates all other transport mechanisms and can lead to plasma disruptions and a rapid loss of energy. This paper analytically investigates the transport of particle density, energy, and magnetic field due to the ideal MHD interchange mode in a closed-line system using the quasilinear approximation. The transport equations are derived for a static plasma in a hardcore Z-pinch configuration and generalized to an arbitrary axisymmetric toroidal closed poloidal field line configuration. It is shown that violation of the marginal stability criterion leads to rapid quasilinear transport that drives the pressure profile back to its marginal profile and forces the particle density to be inversely proportional to ∮dl∕B. The applicability of the quasilinear approximation is numerically tested for the hardcore Z-pinch magnetic configuration using a full nonlinear code.

The effect of sheared axial flow on the interchange mode in a hard-core Z pinch

It is well known that a static (i.e., v=0) closed field line configuration, such as a levitated dipole, or a hard-core Z pinch, can be stabilized against ideal magnetohydrodynamic (MHD) interchange modes when the edge pressure gradient is sufficiently weak. The stabilizing effect is provided by plasma compressibility. However, many laboratory plasmas exhibit a sheared velocity flow (i.e., n∙∇v≠0), and this flow may affect the marginal stability boundary. The present work addresses this issue by an analysis of the effect of axially sheared flow on interchange stability in a hard-core Z pinch, a cylindrical model for the levitated dipole configuration. Specifically, the goal is to learn whether sheared flow is favorable, unfavorable, or neutral with respect to MHD stability. Analytic calculations of marginal stability for several idealistic velocity profiles show that all three options are possible depending on the shape of the shear profile. This variability reflects the competition between the destabilizing Kelvin-Helmholtz effect and the fact that shear makes it more difficult for interchange perturbations to form. Numerical calculation are also presented for more realistic experimental profiles and compared with the results for the idealized analytic profiles.

Plasma gun supersonic vortex magnetohydrodynamic dynamo for magnetized target fusion

Supersonic plasma jets injected with a large azimuthal velocity component into a magnetic cusp field entrap the radial field component in between the coalescing jets, with the cushioning effect of the magnetic field diminishing the formation of shocks which otherwise would occur by the collision of the jets. With the bending of the radial magnetic lines of force into an azimuthal direction by the vortex flow of the coalescing jets, a magnetohydrodynamic dynamo with an axial current is established. With the radial velocity component of the jets compressing the magnetic field and the plasma formed by the coalescing jets, a closed field line configuration is established. If dense denterium–tritium (DT) gas is injected into the vortex core, the axial current passes through the DT resulting in a dense pinch discharge stabilized by the rapidly rotating vortex flow.

Electrostatic drift modes in a closed field line configuration

The stability of electrostatic drift waves in a closed field line configuration in collisionality regimes ranging from collisional to collisionless is compared. The maximum sustainable pressure gradient is found to be dependent on the ratio of the temperature and density gradients (η≡d ln T/d ln n). The eigenmodes are seen to be flute-like. The stability boundary was found to be similar when both ions and electrons are collisional, when they are collisionless, and for collisional electrons and collisionless ions. The largest stable pressure gradients are obtained for η⩾2/3. As the collisionality is reduced one observes some reduction of the region of stability.

Magnetohydrodynamic Origin of Jets from Accretion Disksa

Abstract: A review is made of recent magnetohydrodynamic simulations of outflows from accretion disks for different distributions of magnetic field threading the disk. The problem of the opening of a closed field line configuration linking a magnetized star and an accretion disk is also discussed.

Submegajoule Liner Implosion of a Closed Field Line Configuration

The adiabatic compression of a preformed closed field line configuration by an imploding liner is considered. Three configurations are discussed: the field-reversed configuration, the spheromak, and the Z-pinch. It is shown that by employing a two-dimensional compression, one can reach a breakeven condition with an energy input into the plasma as low as 100 kJ. A brief discussion of various phenomena affecting the plasma wall confinement is presented. It is shown that heat losses to the walls are modest and do not limit the plasma enhancement factor Q. The derived scaling law for Q versus the input parameters of the system shows a relatively weak dependence of Q on the input energy. 34 refs., 9 figs., 2 tabs.

Simulation of the ejection of plasma from the polar ionosphere

A numerical model calculates densities, fluxes and temperatures of H+ and O+ ions and the temperature of electrons on a convecting magnetic field line. The one‐dimensional fluid equations are solved along a field line path extending from 100 km altitude to many tens of Earth radii out in the magnetotail. Effects of photochemical reactions and charge exchange are included in the low altitude regions while dynamical effects due to field line convection, such as centrifugal acceleration and flux tube compression/dilation, are important at the higher altitudes. The model employs a variable spatial step to accommodate the disparate size scales. The evolution of O+ and H+ densities is calculated on field lines convecting over the polar cap. The simulation starts on a closed field line configuration on the dayside of the polar cap, the field line opens out into the magnetotail over the polar cap and eventually closes again on the nightside aurora zone. The results show that (1) centrifugal force is effective in accelerating ionospheric H+ plasma outward into the magnetotail, (2) the smothering effect of the H+ plasma on top of the O+ plasma prevents any ejection of O+ plasma, so long as the scale height of the O+ plasma is smaller than that of the H+ plasma (this indicates that heating processes specific to O+ are necessary for the appearance of O+ in the magnetotail) and (3) the upward component of the E×B drift on the dayside of the polar cap lifts the F region ionosphere to altitudes where charge exchange with neutrals and recombination is very slow so that ionization created on the dayside is transported across the polar cap with little loss. An enhanced F region peak in ionization density is created on the nightside of the polar cap because of compression of the O+ plasma by the subsequent downward component of the E×B drift.