Interchange Instability Research Articles

Investigation of gas puff induced fluctuation suppression in ADITYA tokamak

An experiment aimed at investigating the cause of fluctuation suppression by gas puffing in the edge and scrape-off layer plasma of the ADITYA tokamak is reported. The fluctuation suppression is accompanied by a flattening of the radial profiles of the floating potential, plasma density and electrostatic Reynolds stress. Both the fluctuation suppression and the simultaneous reduction in particle flux occur throughout the observed region of the plasma edge. The bolometer data also show an increase in the edge radiation from 20% to 40% of the input power during the gas puff. These results indicate a significant role of edge cooling in the fluctuation suppression mechanism and are shown to be well explained in terms of a concomitant reduction in the growth rate of the interchange instability associated with the electron temperature gradient.

Pressure-driven, resistive magnetohydrodynamic interchange instabilities in laser-produced high-energy-density plasmas

Recent experiments using proton backlighting of laser-foil interactions provide unique opportunities for studying magnetized plasma instabilities in laser-produced high-energy-density plasmas. Time-gated proton radiograph images indicate that the outer structure of a magnetic field entrained in a hemispherical plasma bubble becomes distinctly asymmetric after the laser turns off. It is shown that this asymmetry is a consequence of pressure-driven, resistive magnetohydrodynamic (MHD) interchange instabilities. In contrast to the predictions made by ideal MHD theory, the increasing plasma resistivity after laser turn-off allows for greater low-mode destabilization (m>1) from reduced stabilization by field-line bending. For laser-generated plasmas presented herein, a mode-number cutoff for stabilization of perturbations with m> approximately [8pibeta(1+D_{m}k_{ perpendicular};{2}gamma_{max};{-1})];{1/2} is found in the linear growth regime. The growth is measured and is found to be in reasonable agreement with model predictions.

Transport scaling in interchange-driven toroidal plasmas

Two-dimensional fluid simulations of a simple magnetized torus are presented, in which the vertical and toroidal components of the magnetic field create helicoidal field lines that terminate on the upper and lower walls of the plasma chamber. The simulations self-consistently evolve the full radial profiles of the electric potential, density, and electron temperature in the presence of three competing effects: the cross-field turbulent transport driven by the interchange instability, parallel losses to the upper and lower walls, and the input of particles and heat by external plasma sources. Considering parameter regimes in which equilibrium E×B shear flow effects are weak, we study the dependence of the plasma profiles—in particular the pressure profile scale length—on the parameters of the system. Analytical scalings are obtained that show remarkable agreement with the simulations.

Observations and Simulations of Magnetic Fluctuations in MCX

Recently magnetic fluctuations in the Maryland centrifugal experiment (MCX) have been measured by an azimuthal array of 16 probes in the edge region of the plasma. A detailed analysis indicates that there is primarily a convection of m = 2 fluctuations by the azimuthally rotating plasma. However, the frequency spectrum of this mode is broad and is almost of the same order as the frequency. Furthermore, bicoherence analysis indicates dominant nonlinear interaction between m = 2 and a low frequency m = 0 mode. We utilize a 2D MHD code to investigate the dynamics of the primary interchange instability. For very low sheared rotation there is a broad spectrum (in m) of unstable modes. However, as the sheared rotation is increased the high mode numbers become stabilized. We will present detailed comparisons of spatio-temporal dynamics of our simulations with the data from the magnetic probes.

A robust method for measurement of fluctuation parallel wavenumber in laboratory plasmas

Measuring the parallel wavenumber is fundamental for the experimental characterization of electrostatic instabilities. It becomes particularly important in toroidal geometry, where spatial inhomogeneities and curvature can excite both drift instabilities, whose wavenumber parallel to the magnetic field is finite, and interchange instabilities, which typically have vanishing parallel wavenumber. We demonstrate that multipoint measurements can provide a robust method for the discrimination between the two cases.

Global modes of flute instability of a rotating cylindrical plasma

The influence of rotation on the flute instability of a cylindrical gravitating plasma in a straight inhomogeneous magnetic field is studied in the framework of one-fluid magnetohydrodynamics. The dispersion relation and integral expression for the instability growth rate of eigenmodes are derived. It is shown that, in the framework of the given problem, rotation is a destabilizing factor, and the corresponding theorem is proved for the general case. For a linear radial profile of the rotation frequency, the structure of eigenmodes is calculated. The growth rate of these modes is shown to increase with increasing rotation velocity and azimuthal mode number. It is found that plasma rotation in the eigenmode localization region leads to the displacement of perturbation from the rotation region, which results in a decrease in the instability growth rate. The absence of eigenmodes (i.e., exponential instability of the system) for certain profiles of the density and rotation frequency is demonstrated.

Modification of Titan's ion tail and the Kronian magnetosphere: Coupled magnetospheric simulations

Three‐dimensional multifluid/multiscale modeling is used to study the coupled interaction between the Kronian magnetosphere and Titan's induced magnetosphere. The multifluid aspect of the model allows ions from the solar wind, Saturn's ionosphere, Enceladus torus, and Titan's ionosphere to be tracked separately. The multiscale aspect of the model allows simultaneous resolution of global and local processes down to 100 km near Titan. In the simulation, Titan is placed in the premidnight sector at 2100 Saturn local time. Two orientations of interplanetary magnetic field (IMF) are considered: parallel and antiparallel to Saturn's planetary field. During parallel IMF, plasma conditions near Titan are relatively stable and Titan's ion tail extends to 10 Saturn radii. The ion tail produces mass loading of the Kronian plasma disk and leads to a modification of the position of the plasma sheet. During antiparallel IMF, fingers of cold dense plasma from the inner magnetosphere propagate outward, driven by the centrifugal interchange instability. Titan's interaction with the fingers causes enhanced outflow as well as large changes in the orientation of Titan's ion tail. The plasma fingers are also disrupted by the interaction with Titan. In both IMF configurations, ions lost from Titan do not produce a complete ion torus. However, Titan's ion tail has global importance because it influences the inner boundary of the plasma sheet and has the potential to modify Kronian auroral processes when it is in this position in the Kronian magnetosphere.

Studies of blob formation, propagation and transport mechanisms in basic experimental plasmas (TORPEX and CSDX)**Paper from the 35th EPS Conference on Plasma Physics (Hersonissos, Crete, Greece, 9–13 June 2008).

The findings of previous blob studies in the interchange-dominated regime of TORPEX helium (Müller et al 2007 Phys. Plasmas 14 110704) and hydrogen plasmas (Furno et al 2008 Phys. Rev. Lett. 100 055004) are summarized and compared. The onset of blobs is studied as a function of the vertical magnetic field Bz, proving the existence of blobs also in the drift-interchange-dominated regime characterized by Bz < 1 mT. These blobs, despite being inherently three-dimensional and sheath-disconnected, exhibit statistical properties similar to the blobs in the interchange regime. Using conditionally averaged density and potential measurements, the entire time line of an interchange instability leading to the formation of wave-like structure patterns and blobs could be experimentally observed. These results show that a background E × B shear flow is not essential for the generation of blobs and that the phase shift between density and potential during the generation of blobs is π/2 in all studied cases, demonstrating the interchange nature of blobs in TORPEX. Fast-camera and Langmuir probe measurements of blobs in the linear device CSDX (Tynan et al 2004 Phys. Plasmas 11 5195) support the hypothesis that an interchange instability is also responsible for the generation of blobs in the linear geometry, where the necessary effective gravity is provided by centrifugal forces.

Correction to “Pressure-driven and ionosphere-driven modes of magnetospheric interchange instability”

[1] In the paper ‘‘Pressure-driven and ionosphere-driven modes of magnetospheric interchange instability’’ by A. Miura (Journal of Geophysical Research, 114, A02224, doi:10.1029/2008JA013663, 2009), there were several printing errors in in-line and numbered equations. [2] In the third sentence in paragraph 18, the correct sentence should read ‘‘Therefore, r J? can change sign at the ionosphere according to the change of rB 1 and therefore r (Jkb) can also change sign at different places in the ionosphere.’’ [3] In the second term of the integrand of equation (21), dot products were mistakenly printed as vector products. The correct equation (21) should read

Investigation of flute instability in application to laboratory astrophysics and Z-pinch experiments

Interaction of plasma flows with magnetic fields can lead to excitation of flute-like oscillations. These oscillations can also be excited in Z-pinch plasmas. We have developed understanding of nonlinear dynamics of compressible flute mode turbulence with spatial scales comparable to the ion Larmor radius in a high beta plasma, when these modes become electromagnetic and in the presence of a non-uniform magnetic field. It is shown that the flute waves are responsible for generation of large scale structures of streamer and zonal flow types as well as Kolmogorov type spectra in the short scale region. The relevance of the obtained results to laboratory astrophysics and Z-pinch experiments is discussed.

Reduction of the turbulent blob transport in the scrape-off layer by a resonant magnetic perturbation in TEXTOR

During the static 6/2 Dynamic Ergodic Divertor experiments in TEXTOR, a significant influence of the edge resonant magnetic perturbation (RMP) on the turbulent blob transport in the scrape-off layer (SOL) has been observed. In ohmic discharges without the RMP, the blobs extend 4–5 cm deep into the SOL with a radially outward moving speed of about 1 km s−1 and hence constitute a strong outflow of mass. With the application of the RMP, the blob amplitudes and their radially moving velocity are both reduced, resulting in a significant reduction of the blob transport in the SOL. The reduction effect of the RMP on blobs is found to be robust to changes in the operational regime and to phasing variations of the RMP as well. The blob dynamics appears to be consistent with the paradigm of the radial motions of the blob structures driven by the interchange instability.

Pressure‐driven and ionosphere‐driven modes of magnetospheric interchange instability

A general stability criterion for magnetospheric interchange instability, which includes an ionospheric destabilizing contribution, is derived for an arbitrary finite‐β magnetospheric model satisfying the magnetohydrostatic force balance. The derivation is based on the magnetospheric energy principle. Unperturbed field‐aligned currents in finite‐β nonaxisymmetric magnetospheric models are assumed to close via diamagnetic currents in the magnetosphere or in the ionosphere. By exploiting the limit of a very large perpendicular wave number and the eikonal representation for the perpendicular plasma displacement, the magnetospheric interchange mode is shown to be compressible. In this limit the kink mode makes no contribution to the change in the magnetospheric potential energy. By using magnetospheric flux coordinates, the explicit form of the magnetospheric potential energy change is calculated for interchange perturbations, which do not bend magnetospheric magnetic fields. For a nonaxisymmetric finite‐β magnetospheric model, a combined effect of the pressure gradient and field line curvature, not only in the meridional plane but also in the plane parallel to the longitudinal direction, is responsible for pressure‐driven interchange instability. For an axisymmetric, north‐south symmetric and low‐β magnetospheric model, in which the magnetic field is approximated by a dipole field, the m = 1 or m = 2 ionosphere‐driven mode, where m is the azimuthal mode number, has an upper critical equatorial β value for instability in the order of 1. Thus a substantial region of the inner magnetosphere or the near‐Earth magnetosphere may be unstable against ionosphere‐driven interchange instability caused by a horizontal plasma displacement on the spherical ionospheric surface.

Self-Generated Toroidal Flow in a High-Beta Compact Toroid with Mirror Configuration

In the field-reversed configuration (FRC) plasma, rotational instability with toroidal mode number n = 2 is known as a sole destructive MHD instability. Though an ideal FRC is unstable for higher n interchange instabilities, those have never been observed in past experiments. As a possible stabilized mechanism, self-generated toroidal rotation and its shear have been investigated experimentally.

Regulation of the centrifugal interchange cycle in Saturn's inner magnetosphere

Multifluid modeling of Saturn's magnetosphere produces the first numerical simulation showing the development of hot, tenuous plasma from the plasma sheet interchanging with cold, denser plasma from the inner magnetosphere. Individual injection events are seen regularly by Cassini, but with a single observation it is impossible to determine the global distribution. Multifluid simulations enable us to characterize the growth and development of not merely one injection event but show that it is a global process dependent on both the plasma distribution of ions from Enceladus and forcing by solar wind conditions. Development of the interchange arises in a fashion similar to a Rayleigh‐Taylor instability, except that the heavy ions are being driven outward not by gravity but by centrifugal forces. Interplanetary magnetic field (IMF) parallel to the planetary magnetic field reduces centrifugal forcing, whereas antiparallel IMF increases the forcing, by altering the bowl‐like shape of the plasma sheet. However, the interchange instability also develops under normally quiet parallel IMF conditions when the mass loading of the Enceladus torus is increased. The total number of interchange events is 1–2 higher for the antiparallel case versus the increased mass case. Interchange develops in the vicinity of 7 RS, and once the fingers of cold plasma reach ∼12–14 RS (close to the inner edge of the plasma sheet) they spread in the azimuthal direction, because of the fact that the magnetic field is too weak to keep the fingers solidly locked in rotation. The derived energy characteristics of the interchanging plasma are shown to be consistent with Cassini data.

Edge turbulence and SOL transport in tokamaks

Some key topics in tokamak edge plasma transport and turbulence are reviewed. Multi-device results reveal a new paradigm of scrape-off layer (SOL) transport. Radial transport is driven by intermittency throughout the SOL, in between edge localized modes (ELMs) in H-mode, and comprised of plasma filaments that are generated near the last closed flux surface likely by interchange instability. The filaments travel radially at speeds of ∼1 km/s into the SOL and have a poloidal size of 1–3 cm in most devices. The radial transport in the SOL is poloidally asymmetric, by factors of 2–5, causing a pressure peak in the low field side. This asymmetry and other neo-classical terms, such as Pfirsch–Schlüter currents, are found to drive strong SOL flows. The intermittent particle flux, is 20% of the total, including ELMs, at low collisionality, becoming 70% of total at high collisionality. Numerical and analytical models can reproduce the scaling of intermittency with collisionality as well as many details of the filament dynamics in the SOL.

Plasma behaviour at high β and high density in the Madison Symmetric Torus RFP

Pellet fuelling of improved confinement Madison Symmetric Torus (MST) plasmas has resulted in high density and high plasma beta. The density in improved confinement discharges has been increased fourfold, and a record plasma beta (βtot = 26%) for the improved confinement reversed-field pinch (RFP) has been achieved. At higher β, a new regime for instabilities is accessed in which local interchange and global tearing instabilities are calculated to be linearly unstable, but experimentally, no severe effect, e.g., a disruption, is observed. The tearing instability, normally driven by the current gradient, is driven by the pressure gradient in this case, and there are indications of increased energy transport (as compared with low-density improved confinement). Pellet fuelling is also compared with enhanced edge fuelling of standard confinement RFP discharges for the purpose of searching for a density limit in MST. In standard-confinement discharges, pellet fuelling peaks the density profile where edge fuelling cannot, but transport appears unchanged. For a limited range of plasma current, MST discharges with edge fuelling are constrained to a maximum density corresponding to the Greenwald limit. This limit is surpassed in pellet-fuelled improved confinement discharges.

MHD study of the reactor-relevant high-beta regime in the Large Helical Device

In the Large Helical Device, the volume averaged beta value ⟨βdia⟩ of 5%, which is the highest value in all heliotron/stellarators and relevant to the reactor requirement, was achieved by optimizing the magnetic configuration from the viewpoint of magneto-hydrodynamic (MHD) characteristics, transport and heating efficiency of the neutral beam. This beta value was instantaneously obtained by pellet injection and maintained for more than 10τE, whereas the steady-state plasma with a maximum ⟨βdia⟩ of 4.8% was sustained for 85τE by gas-puff fueling. While it is theoretically predicted that stochastization of the peripheral magnetic field structure develops with an increment of ⟨βdia⟩, no serious degradation of the global confinement has been observed in the present ⟨βdia⟩ range. The several low-order MHD activities located in the periphery were enhanced with the beta value and sometimes affect the local profiles. The amplitude of the mode in the periphery strongly depends on the magnetic Reynolds number, which is close to that of the growth rate and/or the radial mode width of the resistive interchange instability.

Rice Convection Model simulation of the 18 April 2002 sawtooth event and evidence for interchange instability

We present the results of a Rice Convection Model (RCM) simulation of the 18 April 2002 sawtooth event. This event occurred as a series of quasi‐periodic substorms during fairly stable solar wind conditions. It is modeled by (1) prescribing a solar‐wind‐driven magnetic field model (T01_s) augmented by additional current loops representing the magnetic effects of the substorm current wedge and (2) by carefully specifying a substorm‐phase‐dependent plasma distribution at the RCM outer boundary at 8 Re such that a hot and attenuated plasma distribution is used after every substorm onset. The set of input parameters was adjusted to make the simulation results agree with the primary signatures of the sawtooth event, specifically the sequence of magnetic field stretching and dipolarization observed by the GOES spacecraft and the associated sharp increases and gradual decreases in the flux of energetic protons measured by the LANL/Synchronous Orbit Plasma Analyzer (SOPA) instruments on other geosynchronous spacecrafts. The results suggest the important role that higher temperature and lower density plasma‐sheet plasma plays in producing flux enhancements at geosynchronous orbit. The results also confirm that induction electric fields associated with magnetic field collapse after substorm onsets can serve as a likely mechanism for the energization of particles up to 25 keV. Synthetic high‐energy neutral atom images are compared with IMAGE/HENA measurements for 10–60 keV hydrogen atoms. Magnetic field dipolarization over a large range of local time resulted in a dramatic reduction in the plasma entropy parameter PV5/3 on the boundary. The simulation indicates that the ring current intensified 10–20 minutes after every onset, associated with the injection of low PV5/3 flux tubes through the boundary. The low PV5/3 plasma also produced an interchange convection in the inner magnetosphere, which drives Birkeland currents in a quasi‐periodic upward‐downward pattern with a lifetime of 40–60 minutes and spatial extent of 1.5–2.0 hours. The results suggest that the spatial quasi‐periodic and nearly north–south‐aligned auroral arcs observed by an IMAGE/FUV WIC detector might be caused by interchange instability.

Study of plasma pressure distribution in the inner magnetosphere using the low altitude satellite data and its relevance for magnetospheric dynamics

Plasma pressure distribution in the inner magnetosphere is one of the key parameters for understanding the main magnetospheric processes including geomagnetic storms and substorms. However, the pressure profiles obtained from in-situ particle measurements by the high-altitude satellites inside the plasma sheet do not allow tracking the pressure variations related to the magnetospheric dynamics, because time interval needed to do this generally exceeds the characteristic times of the main magnetospheric processes. On the contrary, fast movement of low-altitude satellites makes it possible to restore quasi-instantaneous profiles of plasma pressure along the satellite trajectory, using the precipitating particle flux data in the regions of isotropic plasma pressure. It was found that during quiet geomagnetic conditions profiles obtained from low-altitude and high-altitude satellites coincide. Nevertheless, the plasma pressure profiles change significantly during the development of storms and substorms, that indicates possibility of the interchange (storm) or modified interchange (substorm) instability development.

Analysis and modeling of edge fluctuations and transport mechanism in the Maryland Centrifugal Experiment

The Maryland Centrifugal Experiment [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] is a mirror machine designed to have a plasma axially confined by supersonic rotation and dominantly interchange stable by the radial shear in the azimuthal velocity. Nevertheless, residual fluctuations still persist. To investigate the presence of such fluctuations, an azimuthal array of 16 magnetic pickup coils at the edge region of the plasma has been employed. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m=0 and m=2 modes are dominant; yet, the observed frequency spectrum is broadband. Using higher order spectral analysis, clear evidence of nonlinear mode coupling is detected. It is also observed that the amplification of magnetic fluctuations leads to enhanced transport consistent with the drop of the plasma density and voltage. As a result, the magnetic fluctuations start to decrease in amplitude as the central plasma pressure drops. In return, the anomalous radial particle and momentum transport are reduced; thus, the plasma confinement improves. As the plasma pressure starts to build up, the plasma voltage increases, destabilizing the m=2 interchange mode. The cycle of enhanced transport and intermittent fluctuations repeats itself. A two-dimensional magnetohydrodynamics code in slab geometry is employed to investigate the dynamics of the primary interchange instability and to assess the level of transport. For very low sheared rotation, a broad spatial spectrum of unstable modes is obtained. As the sheared rotation is increased, the high mode numbers become stabilized and low mode numbers dominate the spectrum. Both the experimental data obtained from the azimuthal array probes and the simulations in case of parabolic shear flow show clear evidence of nonlinear mode coupling, explaining the broadband frequency spectrum for low mode numbers. A detailed comparison of spatiotemporal dynamics of simulations with the experimental data is presented.