Current Convective Instability Research Articles

Effects of magnetic geometry on dynamics of current-convective turbulence in tokamak divertor plasma

The current-convective instability is one of the physical mechanisms that can drive fluctuating transport of plasma in the tokamak divertor. The impact of the tokamak magnetic geometry on spatial and temporal properties of saturated turbulence, driven by the instability, is investigated. The analysis is performed by employing the basic model of the instability [A. A. Stepanenko and S. I. Krasheninnikov, Phys. Plasmas 25, 012305 (2018)] extended to include contributions from the interchange drive. Turbulence simulations are carried out in BOUT++ under DIII-D-like conditions. The dependencies of amplitudes, spatial and frequency spectra of saturated plasma fluctuations on the curvature radius, and connection and shear lengths of magnetic field lines are found. It is demonstrated that taking the finite curvature of the magnetic field into account leads to a large increase in the turbulence amplitude, whereas the shear and connection lengths mainly affect the temporal dynamics of fluctuations and their spatial localization. In all cases shown, the turbulence frequency and wave-number spectra demonstrate better agreement with the experimentally observed ones, compared to the results of previous simulations based on the basic model of the current-convective instability.

Impact of divertor plasma parameters on characteristics of current-convective turbulence under DIII-D-like detached conditions

Recent observations of turbulence in plasma of a DIII-D inner divertor revealed the formation of poloidally localized fluctuations with frequencies in the low-kilohertz range, when the machine was operating in the fluctuating state of detachment. This phenomenon can be related to the onset of the current-convective instability (CCI). In this contribution, we use the basic physical model of the instability to simulate the saturated current-convective turbulence under the DIII-D-like detached conditions. Parameter scans of turbulence characteristics are performed. The spatial and temporal spectra of fluctuations are obtained and compared with the available experimental data. It is demonstrated that simulations are able to qualitatively reproduce most of the distinct features of turbulence found in the experiments, indicating that the CCI can be a plausible mechanism responsible for plasma activity recently observed in DIII-D.

Spatial–Temporal Properties of Current-Convective Turbulence in Divertor Plasma under DIII-D-Like Detached Conditions

Recent experimental observations of divertor plasma turbulence at the ASDEX Upgrade tokamak have demonstrated the onset of strong fluctuations of plasma radiation intensity in the near X-point region within the inner divertor leg, when the machine was operating with the pronounced asymmetry in detachment of plasma between the inner and outer divertors. As been recently shown, one of the possible mechanisms responsible for these fluctuations can be related to the current-convective instability (CCI). In this contribution, we assess whether the CCI can drive turbulence with spatial and temporal parameters similar to those observed lately in DIII-D experiments. It is demonstrated that the CCI drives saturated plasma fluctuations, dominated by low-frequency modes with toroidal wavelengths on the order of the machine circumference in the inner divertor region—features that are similar to turbulence characteristics found in experiments.

Numerical study of the current-convective instability driven by asymmetry of detachment in inner and outer divertors

One of the possible mechanisms responsible for strong radiation fluctuations observed in recent experiments with detached plasmas at ASDEX Upgrade [Potzel et al., Nucl. Fusion 54, 013001 (2014)] can be related to the onset of the current-convective instability (CCI) driven by strong asymmetry of detachment in the inner and outer divertors of the tokamak [S. Krasheninnikov and A. Smolyakov, Phys. Plasmas 23, 092505 (2016)]. In this study, we present the physical model, used to simulate the CCI, and the first numerical results of modeling of the CCI dynamics in ASDEX Upgrade-like conditions. The simulation results provide frequency spectra of turbulent divertor plasma oscillations showing reasonably good agreement with the available experimental data.

Magnetic Shear Damped Polar Convective Fluid Instabilities

AbstractThe influence of the magnetic field shear is studied on the E × B (and/or gravitational) and the Current Convective Instabilities (CCI) occurring in the high‐latitude F layer ionosphere. It is shown that magnetic shear reduces the growth rate of these instabilities. The magnetic shear‐induced stabilization is more effective at the larger‐scale sizes (≥ tens of kilometers) while at the scintillation causing intermediate scale sizes (∼ a few kilometers), the growth rate remains largely unaffected. The eigenmode structure gets localized about a rational surface due to finite magnetic shear and has broken reflectional symmetry due to centroid shift of the mode by equilibrium parallel flow or current.

Stability of divertor detachment

The 2D simulations of edge plasma transport show that unlike some earlier publications, the impurity radiation loss per se does not cause the bifurcation-like transition to detached divertor regime. However, for the case where anomalous plasma transport is increasing with advancement to detachment, like it was recently observed experimentally, the transition to detachment exhibit the bifurcation-like character. Some other plausible reasons for similar bifurcation-like evolution to detachment are discussed. It is demonstrated that the current convective instability can be triggered in detached inner divertor plasma for the condition when outer divertor is still attached. This can explain the fluctuations of radiation loss observed recently experimentally for similar conditions. The self-sustained oscillations observed recently in numerical simulations and related to the interplay of the thermal force effects in impurity transport and impurity radiation loss are further investigated. It is shown that for some conditions these oscillations are ubiquitous, since no stable solutions possible.

Current convective instability in detached divertor plasma

The asymmetry of inner and outer divertors, which cause the inner divertor to detach first, while the outer one is still attached, results in the large temperature difference between the vicinities of inner and outer targets and the onset of large electric potential drop through detached plasma of the inner divertor. A large potential drop along with the inhomogeneity of the resistivity of detached plasma across the divertor leg drives the current convective instability in the inner divertor and subsequent fluctuations of radiation loss similar to that observed in experiments. The estimates of the frequency of plasma parameter fluctuations due to the current convective instability are in a reasonable agreement with experimental data. Once the outer divertor also detaches, the temperature difference between the vicinities of inner and outer targets disappears, and the driving force for the current convective instability, and resulting oscillations of radiation loss, vanishes. This feature is indeed observed in experiments.

Polar cap plasma patch primary linear instability growth rates compared

AbstractFour primary plasma instability processes have been proposed in the literature to explain the generation of phase scintillation associated with polar cap plasma patches. These are the gradient drift, current convective, and Kelvin‐Helmholtz instabilities and a small‐scale “turbulence” process. In this paper the range of possible values of the linear growth rates for each of these processes is explored using Dynamics Explorer 2 satellite observations. It is found that the inertial turbulence instability is the dominant process, followed by inertial gradient drift, collisional turbulence, and collisional shortwave current convective instabilities. The other processes, such as Kelvin‐Helmholtz, collisional gradient drift, and inertial shortwave current convective instabilities, very rarely (<1% of the time) give rise to a growth rate exceeding 1/60, that is deemed to be significant (in publications) to give rise to GPS scintillation.

Instabilities, waves, and nonequilibrium structures in a dense low-frequency plasma (a look at the problem after 50 years publications in the journal High Temperature)

This review is devoted to one of the fundamental problems of plasma physics, namely, waves and instabilities in a dense low-temperature plasma. Instabilities are characteristic of a large number of natural phenomena. They affect the existence of nonequilibrium systems and cause a transition from one state to another. Ensuring stability of a process is the central problem in many technologies in which a low-temperature plasma is used as an working medium. This review is aimed at revealing the key mechanisms which lead to instabilities and at presenting the instability criteria in a clear physical form. A method that allows considering the modern state of the problems of instabilities and propagation of waves with a specified wave number in a dense low-temperature plasma of gases and metal vapors from a unified standpoint is discussed. Acoustic, overheating, current-convective, ionization, and ionization-field instabilities are considered in detail. This study is performed in the framework of continuum models both for a dense plasma in which the temperatures of electrons and heavy particles (atoms and ions) do not almost differ (Ta ≈ Te) and a plasma in which the electron temperature differs from the temperature of heavy particles (Ta ≠ Te). Particular attention is given to the effects of the Ampere force, Joule heating, and the processes related to changes in the plasma composition (ionization and recombination) and the energy exchange between the plasma components in the absence of temperature equilibrium. The contribution of the half-century publications in High Temperature to investigation of this problem is traced.

On the Gradient-Current Mechanism of Formation of the Irregular Structure of the High-Latitude Upper Ionosphere

We consider the gradient-current instability of an inhomogeneous magnetoactive plasma in the approximation of double-fluid magnetohydrodynamics. Unlike the known gradient-drift and current-convective instabilities, the gradient-current instability is related to generation of nonpotential quasistatic electric fields polarized orthogonal to the external magnetic field B0 and excited by eddy currents whose density vector lies in the plane passing through the vectors of the magnetic field B0 and large-scale electron-density gradient. It is shown that in the high-latitude upper ionosphere, in the regions containing large-scale currents flowing in and out of the ionosphere along the magnetic field, the gradient-current instability can lead to the appearance of sheet-like irregularities extended predominantly in the plane passing through the geomagnetic-field and regular plasma-drift velocity vectors.

Theory of Flute and Current-Convective Instability in a ThinMagnetically Confined Plasma Layer

The flute and current-convective instabilities of a magnetizedplasma layer are investigated when the inhomogeneity size of thedensity near the layer surface is much smaller than the fieldinhomogeneity (a sharp boundary model) and as a result the geometricaloptic approximation is not applicable. It is studied both cases ofthick (in comparison with a field inhomogeneity) and thin plasmalayers in the limits of collisionless and collisional plasmas. Theobtained growth rates of instabilities are compared with those foundin the geometrical optic frame.

Magnetopause crossings without a boundary layer

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence by Drake et al. [1994], who propose the current convective instability as the dominant process for current transport at the magnetopause.

Turbulence and transport in the magnetopause current layer

The current convective instability is identified as the dominant mechanism for producing turbulence and transport in the magnetopause current layer. A linear stability analysis demonstrates that the instability should be robustly unstable for the parameters of the magnetopause current layer. Three‐dimensional nonlinear simulations indicate that the current layer evolves to a strongly turbulent state. The current convective instability is driven unstable by the cross‐field gradient of the parallel current in collisionless plasma. A simple physical picture of the instability is presented. For narrow current layers with scale length L less than an ion Larmor radius ρi, the growth rates exceed the ion gyrofrequency even when Ti ≫ Te. A set of nonlinear fluid equations are derived in the unmagnetized ion limit ρi > L. In three‐dimensional simulations based on these equations the current layer evolves to a turbulent state with layers of extended but irregular clifflike structures with transverse scale length of the order of ρes, the electron Larmor radius based on Ti. The anomalous cross‐field transport rate D⊥ of current resulting from this instability is given by D⊥ ∼ ρes υy with υy the electron parallel streaming velocity. The present linear theory and simulations are limited to electrostatic disturbances. The implications of these results for understanding the structure of the magnetopause current layer are discussed.

Two-fluid theory of thermal transport in current-carrying edge plasma

A nonlinear nalysis of the current-convective instability is carried out on the basis of a classical, two-fluid model that includes the effects of parallel electron current and thermal conductivity, electron compressibility and thermal force. Restriction is made to time rates of change slow compared with the resistive diffusion rate across a perpendicular wavelength. Assuming the background gradient of electron temperature to be much greater than that of the density, a set of mode-coupling equations between electron temperature and electrostatic potential Fourier components is analysed numerically. The heat flux is computed explicitly. The influence of parallel current and electron conductivity on plasma edge transport due to thermal excitations is determined and discussed.

The effect of finite “blob” size on the current convective instability in the auroral ionosphere

It has been suggested that the current convective instability may be responsible for the structuring, i.e., generation of density irregularities, of density enhancements (known as “blobs”) in the auroral ionosphere. However, previous theories have neglected the finite extent of the “blob” along the geomagnetic field. In this paper we develop a nonlocal theory of the current convective instability which considers the finite extent of an ionospheric “blob” parallel to the geomagnetic field. We find that the growth rate of the instability can be substantially reduced in the finite‐sized “blob” case from the value obtained in the local approximation for an infinitely long blob. For auroral ionosphere parameters, the reduction in the growth rate for medium‐scale irregularities (1–10 km) can be 1 to 2 orders of magnitude for the typical observed values of “blob” sizes (approximately a few hundred kilometers). Thus it appears that the current convective instability is not a viable mechanism to generate scintillation causing irregularities, i.e., 1‐ to 10‐km irregularities.

A case study of plasma processes in the dayside cleft

On November 28, 1983, between 1638 UT and 1642 UT, the HILAT satellite moving approximately along the noon meridian passed over northeastern Canada and encountered the dayside high latitude cleft. During the passage an upward current sheet bounded to the north and south by weaker downward current sheets was observed. The current sheets, particularly the upward current sheet, were immersed in a region of fairly intense low energy electron precipitation of the type commonly observed in the polar cleft. Simultaneous radar measurements were made in a region slightly to the east of the satellite track and in which beacon measurements between HILAT and a ground station at Sondre Stromfjord, Greenland, were being conducted. The radar indicated an extended, magnetic east‐west‐aligned region of small‐scale irregularities at the same magnetic latitude as the cleft‐related current sheets and precipitation. The HILAT beacon experiment showed the strongest scintillations within the same region. On the basis of the available data, it appears as if the convective flows from the dawn and dusk sectors were converging and rotating poleward within the observational volume. The irregularities producing the scintillations and radar backscatter may have been produced by the combined influence of the gradient drift and current convective plasma instabilities, the former process being excited by the poleward convection of plasma parallel to the precipitation‐induced density gradient and the latter process by the upward field‐aligned current. The apparent absence of irregularities further into the polar cap may be accounted for by the stabilizing effect of plasma drift antiparallel to the density gradient on the poleward edge of the cleft precipitation zone.

Dissipative instabilities at the edge of reversed-field pinches driven by a combination of plasma gradients and a parallel current

Stability in the edge region of reversed-field pinches is analyzed within the context of a two-fluid model. Two major sources of instability are identified: in combination with a parallel electric field, either an electron temperature gradient and/or a density gradient leads to rapid growth (of several to many Ohmic heating rates) over a region of several millimeters around the mode-rational surfaces in the edge region. The basic signature of both instabilities is electrostatic. In the case of the density gradient mode, it relies on the effects of electron compressibility, whereas the temperature gradient mode can be identified as the current-convective instability by taking the limit of zero density gradient, thermal force, and electron compressibility. The possibility of enhanced particle loss and thermal convection as a nonlinear consequence of the instability is indicated. A criterion for the overlap of the linearly unstable regions in the edge of reversed-field pinches is obtained.

Chaotic behavior of thermal excitations in edge plasmas

It is found that an electromagnetic analog of the current convective instability in the nonlinear regime may develop into a chaotic system. Comparison is made with the corresponding electrostatic strongly nonlinear case which may show chaotic behavior of the Lorenz type, and the nonlinear cascade properties are discussed.

Effects of finite current channel width on the current convective instability

The effects of the finite current channel width on the current convective instability are studied both analytically and numerically. First, using a sharp boundary field‐aligned current distribution which has a finite width along the plasma density gradient, the dispersion relation is obtained analytically. It is found that for the long wavelength modes (kyd ≪ 1) where the nonlocal effects are most prominent, the growth rate γ is proportional to in the collisional limit (νin ≫ |ω|), where d is the half width of the current channel, L is the plasma density gradient scale length, is the field‐aligned current velocity, νin is the ion‐neutral collision frequency, and ω is the perturbation frequency. For the long wavelength modes in the inertial limit (νin ≪ |ω|) the growth rate γ scales as , where α = 1/2 (2/3) for kz²/ky² Ωe/νei much less than (greater than) |ω|/Ωi, kz(ky) is the wave number parallel (perpendicular) to the magnetic field, Ωe(Ωi) is the electron (ion) gyrofrequency, and νei is the electron‐ion collision frequency. Numerical results are also presented for a diffuse boundary current velocity distribution. Applications to the high‐latitude ionosphere are discussed.

Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity

An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range‐type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E‐region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh‐Taylor instability and irregularity growth enhancement by the current convective and (wind‐driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity‐influencing processes. The role of field‐aligned currents, associated with the longitudinal gradient in integrated E‐region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.