Sheared Plasma Flow Research Articles

Dust Sheared Flow Driven Instability of Dust Drift Waves in a Nonuniform Magnetoplasma

We investigate instability of dust drift waves in a nonuniform dusty magnetoplasma containing transverse sheared plasma flow that is produced by a nonuniform electric field. By using Boltzmann distributed electrons and ions, Poisson’s equation, as well as the dust continuity equation with perpendicular guiding center dust drift speed, we derive an eigenvalue equation, which strongly depends on the profiles of dust sheared flow and dust density gradient. The eigenvalue equation is analytically solved to obtain expressions for the growth rate and threshold of a convective instability arising from resonant interactions between the dust drift waves and sheared flows. The result may be relevant to the understanding of short wavelength (in comparison with the ion gyroradius) electrostatic fluctuations in magnetized plasmas of Saturn rings and in cometary tails.

Shear-driven magnetic reconnection in viscous resistive incompressible plasmas

Planar magnetic reconnection, driven by shear flows in viscous resistive incompressible plasmas, is analyzed. Time-dependent profiles of the flow velocity and the reconnecting magnetic field are computed by solving the magnetohydrodynamic equations for disturbances of a magnetostatic configuration. The resulting asymptotic profiles for large times are characterized by boundary layers, formed at the boundaries of the region, rather than by a localized reconnecting current sheet within the region. Analysis of steady viscoresistive solutions confirms that a smooth solution, reached by a dynamically evolving system, must be nonlocalized. The results imply that at least two-dimensional shear flows are required for driving magnetic reconnection in viscous resistive plasmas.

Rocket and radar investigation of background electrodynamics and bottom-type scattering layers at the onset of equatorial spread <i>F</i>

Abstract. Sounding rocket experiments were conducted during the NASA EQUIS II campaign on Kwajalein Atoll designed to elucidate the electrodynamics and layer structure of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each comprised of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets deployed trails of trimethyl aluminum (TMA) which yielded wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. Electron density profiles were also measured with rocket beacons and an ionosonde. Strong plasma shear flow was evident in both experiments. Bottom-type scattering layers were observed mainly in the valley region, below the shear nodes, in westward-drifting plasma strata. The layers were likely produced by wind-driven interchange instabilities as proposed by Kudeki and Bhattacharyya (1999). In both experiments, the layers were patchy and distributed periodically in space. Their horizontal structure was similar to that of the large-scale plasma depletions that formed later at higher altitude during ESF conditions. We argue that the bottom-type layers were modulated by the same large-scale waves that seeded the ESF. A scenario where the large-scale waves were themselves produced by collisional shear instabilities is described.

Towards turbulence control in magnetised plasmas

A reduction of turbulent transport is an important task on the way of an economic fusion power production. The knowledge gathered on the driving forces and the dynamics of plasma turbulence helps to identify different techniques, which have been successfully applied in fusion experiments to improve confinement. The techniques can be divided into two classes, those related to sheared plasma flows and the others related to an optimisation of the magnetic configuration. The paper gives an overview of the different approaches and aims at an intuitive understanding of the mechanisms leading to reduced transport. To cite this article: U. Stroth, M. Ramisch, C. R. Physique 7 (2006).

Onset conditions for equatorial spread F determined during EQUIS II

An investigation into the dynamics and layer structure of the postsunset ionosphere prior to the onset of equatorial spread F (ESF) took place during the NASA EQUIS II campaign on Kwajalein Atoll on August 7 and 15, 2004. On both nights, an instrumented rocket measured plasma number density and vector electric fields to an apogee of about 450 km. Two chemical release rockets were launched both nights to measure lower thermospheric wind profiles. The Altair UHF/VHF radar was used to monitor coherent and incoherent scatter. In both experiments, strong plasma shear flow was detected. Periodic, patchy bottom‐type scattering layers were observed in the westward‐drifting plasma below the shear nodes. The large‐scale plasma depletions that formed later during ESF reproduced the periodic structure of the original, precursor layers. The layers were therefore predictive of the ESF that followed. We surmise that collisional shear instabilities may have given rise to large‐scale plasma waves that were highlighted by the bottom‐ type layer structure and that preconditioned the postsunset ionosphere for ESF.

Shear Flow Instabilities in Low-Beta Space Plasmas

We study instabilities driven by a sheared plasma flow in the low-frequency domain. Two unstable branches are found: the ion-sound mode and the kinetic Alfven mode. Both instabilities are aperiodic. The ion-sound instability does not depend on the plasma β (gas/magnetic pressure ratio) and has a maximum growth rate of about 0.1 of the velocity gradient dV 0/dx. On the other hand, the kinetic Alfven instability is stronger for larger β and dominates the ion-sound instability for β > 0.05. Possible applications for space plasmas are shortly discussed.

On the field‐aligned currents in the vicinity of prenoon auroral arcs

The EISCAT Svalbard radar (at 75° MLAT) was used to investigate the ionospheric plasma flows along auroral arcs in the 7–11 MLT sector with the radar beam directed along the L‐shell. In nine cases auroral arcs appeared in the immediate vicinity of the radar beam allowing for observations of plasma flow simultaneously at different distances across the arc. In eight cases the arcs were associated with a plasma flow shear indicating sheets of upward field‐aligned current (FAC) over the arcs. However, in one case the data indicate a downward net current in the region of the arc, with a flow shear and, especially, a downward FAC sheet adjoining the arc on the equatorial side. We suggest that in the downward FAC sheet, the Kelvin‐Helmholtz instability can generate a series of auroral spots forming an arc, if the current sheet is co‐located with a plasma boundary inside the magnetosphere.

The stability of ballooning modes in tokamaks with internal transport barriers

Modern tokamaks can produce transport barriers (TBs)—localized regions with an increased energy confinement. Previous studies have been unable to examine the stability of internal TBs to radially extended short-wavelength magnetohydrodynamic instabilities (“ballooning modes”), for the usual case with a sheared plasma flow and a magnetic shear that passes through zero near the TB. An established technique is adapted to study this situation, finding instability if (1) there is a low-pressure gradient, and if (2) the nearest “resonant surface” at which a Fourier mode is resonant, is sufficiently close. Surprisingly, flow shear is no more stabilizing than for magnetic shears of order one. This is explained. Without a strongly stabilizing mechanism, ballooning modes will fundamentally limit a TB's radial extent, preventing them from extending across the entire plasma radius.

Formation of a sheared flow Z pinch

The ZaP Flow Z-Pinch project is experimentally studying the effect of sheared flows on Z-pinch stability. It has been shown theoretically that when dVz∕dr exceeds 0.1kVA the kink (m=1) mode is stabilized. [U. Shumlak and C. W. Hartman, Phys. Rev. Lett. 75, 3285 (1995).] Z pinches with an embedded axial flow are formed in ZaP with a coaxial accelerator coupled with a 1m assembly region. Long-lived, quiescent Z pinches are generated throughout the first half cycle of the current. During the initial plasma acceleration phase, the axial motion of the current sheet is consistent with snowplow models. Magnetic probes in the assembly region measure the azimuthal modes of the magnetic field. The amplitude of the m=1 mode is proportional to the radial displacement of the Z-pinch plasma current. The magnetic mode levels show a quiescent period which is over 2000 times the growth time of a static Z pinch. The axial velocity is measured along 20 chords through the plasma and deconvolved to provide a radial profile. Using data from multiple pulses, the time evolution of the velocity profile is measured during formation, throughout the quiescent period, and into the transition to instability. The evolution shows that a sheared plasma flow develops as the Z pinch forms. Throughout the quiescent period, the flow shear is greater than the theoretically required threshold for stability. As the flow shear decreases, the magnetic mode fluctuations increase. The coaxial accelerator provides plasma throughout the quiescent period and may explain the evolution of the velocity profile and the sustainment of the flow Z pinch.

The ZaP Flow Z-Pinch: Plasma Flow Shear and Stability

The ZaP Flow Z-pinch plasma device at the University of Washington produces a small diameter (20-30 mm) dense Z-pinch plasma with typical electron density 1022-1023 m-3 and ion plus electron temperature 100-200 eV. The plasma is stable, with relatively low magnetic mode activity, for tens of microseconds. This is orders of magnitude longer than predicted by a simple ideal magnetohydrodynamic calculation. The probable stabilizing mechanism is radial shear in the axial plasma flow. The axially flowing Z-pinch is generated with a coaxial accelerator coupled to a pinch assembly chamber. After the pinch assembles a quiescent period occurs, during which the mode activity is significantly reduced. Multichord Doppler shift measurements of impurity lines show a large, sheared flow during the quiescent period and low, uniform flow profiles during periods of high mode activity. The plasma has a sheared axial flow that exceeds the theoretical threshold for stability during the quiescent period and is lower than the threshold during periods of high mode activity. The Z-pinch plasmas are globally stable for 700-2000 times the theoretically predicted kink growth time of a static Z-pinch. The end of the quiescent period corresponds to a decrease in acceleration of plasma and possibly suggests a means to extend the experiment to quasi-steady-state operation.

Role of flow shear in the ballooning stability of tokamak transport barriers.

A tokamak's confinement time is greatly increased by a transport barrier (TB), a region having a high pressure gradient and usually also a strongly sheared plasma flow. The pressure gradient in a TB can be limited by ideal magnetohydrodynamic instabilities with a high toroidal mode number n ("ballooning modes"). Previous studies in the limit n--> infinity showed that arbitrarily small (but nonzero) flow shears have a large stabilizing influence. In contrast, the more realistic finite n ballooning modes studied here are found to be insensitive to sub-Alfvénic flow shears, provided the magnetic shear s approximately 1 (typical for TBs near the plasma's edge). However, for the lower magnetic shears that are associated with internal transport barriers, significantly lower flow shears will influence ballooning mode stability, and flow shear should be retained in the analysis of their stability.

The stability of ideal magnetohydrodynamic ballooning modes in plasmas with internal transport barriers

The stability and spatial structures of short wavelength ideal magnetohydrodynamic ballooning modes (i.e., those modes with moderate to large values of the toroidal mode number, n) that can exist in regions of zero or small magnetic shear are investigated. This is a situation relevant to discharges with internal transport barriers (ITBs). The generic properties of such instabilities are discussed by considering the s–α equilibrium. In regions of low s the ballooning formalism fails for large but finite, values of n. In this limit a complementary approach is developed, based on solving the recurrence relation describing the toroidal coupling of radially localized “modelets” on adjacent mode rational surfaces. This technique extends the stability analysis to lower s and finite n, capturing effects arising from the discreteness of mode rational surfaces. Consideration of equilibrium trajectories in the s–α stability diagram corresponding to profiles of ITB discharges allows one to determine the global stability of such discharges to these modes. It is shown that the stability of ITBs can be parameterized in terms of αmax, the value of the peak α, and the steepness of the barrier pressure profile relative to the shape of the q profile. Inclusion of the stabilizing effects of favorable average curvature at finite aspect ratio, ε, leads to stable high-pressure ITB configurations. The stabilizing influence of low-order rational values of qmin also emerges from the theory. The influences of the bootstrap current and plasma flow shear at ITBs are briefly discussed.

Effect and Production Mechanism of Flow Velocity Shear in Toroidal Plasmas

Production mechanism of spontaneous plasma flow and flow velocity shear in the toroidal plasma is described. The plasma flow parallel to the magnetic field is produced by the radial electric field through the viscosities in the plasma. The observation of plasma flow driven by the thermodynamic force such as pressure gradient are presented. The effect of flow velocity shear on the transport, which determines the pressure profiles, is also discussed.

Sheared Flow Excitation and Suppression of Electrostatic Instabilities in Laboratory Collisionless Magnetoplasmas

The ion-cyclotron instabilities in a collisionless magnetoplasma are observed to be stabilized by a perpendicular plasma flow shear independently of the sign of the shear. This suppression of the ion-cyclotron instabilities is caused only when the perpendicular flow shear exists outside the disk electrode which excites the instabilities by applying a positive bias voltage to it. This phenomenon can be explained by the excitation mechanism of ion-cyclotron instabilities such as potential driven model.

FRC Formation Process of Two Colliding Spheromaks with Counterhelicity

A merging formation of field-reversed configuration (FRC) has been studied for the first time without central conductor, using two merging spheromaks with opposing toroidal magnetic field. Without the n=1 stabilization effect of the central conductor, the merging process was maintained stably probably due to (1) plasma flow shear generated by the counterhelicity reconnection and (2) line-tying effect of the magnetic field lines. This relaxation from the force-free (≈0.05-0.1) spheromaks to the high-beta (≈0.7-1) FRC is attributed to efficient (≈80%) conversion of the toroidal magnetic energy of the former to the ion thermal energy of the latter. This energy conversion was caused by a direct ion heating and acceleration effect of reconnection under the toroidally symmetric condition. This ion heating energy was found to increase inversely with the field amplitude parallel to the X-point line. Further plasma acceleration by high-field compression was observed to increase the reconnection speed, revealing the new fast reconnection mechanism called the current sheet ejection.

Ion cyclotron waves in the high altitude cusp: CLUSTER observations at varying spacecraft separations

We have analysed high‐resolution Cluster magnetic field data during three high‐altitude cusp crossings in 2001 and 2002. The Cluster separations for these crossings varied between 100 and 600 km and therefore provided an unique opportunity to study wave properties at different length scales. In the cusp Cluster sees frequent intervals of magnetic field fluctuations with clear peaks in power close to the local ion cyclotron frequency, and both left‐ and right‐handed polarisations. At large separations the power seen at different spacecraft can differ by orders of magnitude. For smaller separations, the power seen at the four spacecraft agrees better but still shows some differences. For all separations there was no significant correlation between the signals seen at different spacecraft, indicative of very local structure. The origin of the waves appears to lie in highly filamented sheared plasma flows present in the cusp.

Structure of the flank magnetopause for horizontal IMF: INTERBALL 1 observations

A case study is presented of a flank duskside magnetopause crossing by the INTERBALL 1 spacecraft during an interval of duskward horizontal interplanetary magnetic field. There is strong evidence that reconnection takes place in the flank magnetopause, where the draped sheath field has a large shear relative to the magnetospheric field. In our case the satellite was on the duskside above the equator. The assumed reconnection site is on the duskside of the northern flank slightly sunward at higher latitudes with respect to the satellite. The observations show energetic particles streaming along the field in the magnetosheath from the reconnection site, as expected. The deHoffman‐Teller frame analysis shows that propagation of the magnetopause perturbations originates from the derived reconnection site. Observations are, in general, consistent with reconnection models that are based on the one‐dimensional assumption. However, there is some significant deviation from the model prediction. We attribute the deviation to the presence of sheared plasma flow. The magnetopause in this case could not be described as a simple one‐dimensional rotational discontinuity.

Stabilization of magnetic islands due to the sheared plasma flow and viscosity

It is shown that the asymmetric deformation of the magnetic island caused bythe finite plasma viscosity gives a stabilizing effect on the magneticislands in the Rutherford regime. Such stabilization typically overcomes thedestabilizing effect of the ion polarization current.

Local Buoyant Instability of Magnetized Shear Flows

The effect of magnetic shear and shear flow on local buoyant instabilities is investigated. A simple model is constructed allowing for an arbitrary entropy gradient and a shear plasma flow in the Boussinesq approximation. A transformation to shearing magnetic coordinates achieves a model with plasma flow along the magnetic field lines where the coordinate lines are coincident with the field lines. The solution for the normal modes of the system depends on two parameters: the Alfven Mach number of the plasma flow and the entropy gradient. The behavior of the unstable normal modes of this system is summarized by a stability diagram. Important characteristics of this stability diagram are the following: magnetic shear is stabilizing, and the entropy gradient must exceed a threshold value for unstable mode growth to occur; flow acts to suppress mode growth in a substantially unstable regime as expected, yet near marginal stability it can lessen the stabilizing effect of magnetic shear and enhance the growth rates of the instability; and, as the Alfven Mach number approaches 1, the instability is completely stabilized. Analytical work is presented supporting the characteristics of the stability diagram and illuminating the physical mechanisms controlling the behavior of the model. A derivation of the stability criterion for the case without shear flow, asymptotic solutions in the limit that the Alfven Mach number approaches 1 and in the limit of zero growth rate, a complete WKB solution for large growth rates, an exactly soluble bounded straight field case, and energy conservation relations are all presented. The implications of this work for astrophysical and fusion applications and the potential for future research extending the results to include compressibility are discussed.

Reynolds stress and shear flow generation

The so-called Reynolds stress may give a measure of the self-consistent flow generation in turbulent fluids and plasmas by the small-scale turbulent fluctuations. A measurement of the Reynolds stress can thus help to predict flows, e.g. shear flows in plasmas. This may assist the understanding of improved confinement scenarios such as H-mode confinement regimes. However, the determination of the Reynolds stress requires measurements of the plasma potential, a task that is difficult in general and nearly impossible in hot plasmas in large devices. In this work we investigate an alternative method, based on density measurements, to estimate the Reynolds stress, and demonstrate the validity range of this quantity, which we term the pseudo-Reynolds stress. The advantage of such a quantity is that accurate measurements of density fluctuations are much easier to obtain experimentally. Prior to the treatment of the pseudo-Reynolds stress, we present analytical and numerical results which demonstrate that the Reynolds stress in a plasma, indeed, generates a poloidal shear flow. The numerical simulations are performed both in a drift wave turbulence regime and a resistive interchange turbulence regime. Finally, the implications of misaligned probe arrays on the determination of Reynolds stresses are investigated, and alignment is found to be important but not severe.