Electric Drift Research Articles

Magnetic-curvature-driven interchange modes in dusty plasmas

The magnetic-curvature-driven interchange mode instability of a weakly inhomogeneous dusty plasma is rigorously investigated. It is shown that the electric drift convection of the equilibrium dust charge density is a stabilizing factor for long wavelength interchange modes. In a fully nonlinear regime, the finite amplitude interchange modes may self-organize in the form of a dipolar vortex. The present results should be useful in the understanding of the properties of the interchange mode turbulence in nonuniform magnetized plasmas containing charged dust particles.

Low-frequency instabilities in a laboratory magnetized plasma column

A detailed analysis of the phase velocity of unstable low-frequency waves is performed in a laboratory magnetized plasma column. The measurement of the radial profiles of the density, electron temperature and plasma potential allows to determine the radial profile of the electric drift velocity and electron diamagnetic drift for increasing values of the magnetic field. In the case of a large diameter plasma column, only diamagnetic drift waves without E×B Doppler shift occur. On the other hand, in the case of a restricted diameter plasma column, the radial electric field induces a rotation of the plasma column. At low magnetic field the recorded unstable waves are in that case flute modes propagating azimuthally at the E×B drift velocity.

Reaction constants for main cationic native defects in narrow-gap Hg1−xCdxTe crystals

The equilibrium defect structure of Hg1−xCdxTe crystals and the diffusion of mercury in this material have been investigated. The results are presented. The results were obtained by using an optical method for free carrier concentration measurements at T≈300K. A new model for mercury diffusion was used, taking into account the effect of charged defect electric drift. As result, a consistent system of reaction constants for mercury vacancies and the mercury interstitials in Hg1−xCdxTe crystals has been obtained.

Discharge plasma instabilities in the presence of an external constant electric field

The instabilities of a weakly ionized quasineutral plasma embedded in an external constant electric field are investigated. Special attention is paid to the dispersion equation with taking into account the interparticle collisions, the ionization and the recombinations. Dependency of the electron-neutral collision frequency, the electron-ion recombination coefficient and the ionization frequency on the electron temperature are considered. Most of the instabilities investigated are stipulated by the sign change of derivative of kinetic coefficients with respect to temperature. A new instability in which the plasma is assumed to be near the equilibrium state is found although an electric drift and heating of light particles (electrons) exists.

Unified quantum approach for the electric field-dependent drift velocity and the diffusion coefficient

Based on the Wannier–Stark picture, a unified microscopic approach for the field-dependent drift velocity and diffusion coefficient is derived. This single-particle theory, which is exact for all finite electric field strengths, is applied to study electro-phonon resonances in superlattices.

Three-dimensional flow and turbulence structure in electrostatic precipitator by stereo PIV

Stereo PIV was employed to study the three-dimensional velocity and turbulence fields in a laboratory model of a negative corona, barbed-wire, smooth-plate electrostatic precipitator. The calculated electric drift velocity of charged seeding particles is subtracted from the measured particle velocity to obtain the gas velocity. Results show how the strength of the secondary flow (in the form of longitudinal rolls) and the level of turbulence (as well as its degree of anisotropy) increase with increasing axial position, increasing current density, and decreasing bulk velocity. Both of these features of the gas flow are crucial to the performance of precipitators.

Modeling the inner plasma sheet protons and magnetic field under enhanced convection

In order to understand the evolution of the protons and magnetic field in the inner plasma sheet from quiet to disturbed conditions, we incorporate a modified version of the Magnetospheric Specification Model (MSM) with a modified version of the Tsyganenko 96 (T96) magnetic field model to simulate the protons and magnetic field under an increasing convection electric field with two‐dimensional (2‐D) force balance maintained along the midnight meridian. The local time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer (LLBL). We previously used this model to simulate the inner plasma sheet under weak convection corresponding to a cross polar cap potential drop (ΔΦPC) equal to 26 kV and obtained 2‐D quiet time equilibrium for proton and magnetic field that agrees well with observations. We start our simulation for enhanced convection with this quiet time equilibrium and time‐independent boundary particle sources and increase ΔΦPC steadily from 26 to 146 kV in 5 hours. Simulations are also run separately to steady states by keeping ΔΦPC constant after it is increased to 98 and 146 kV. The magnitudes of proton pressure, number density, and temperature and their increase from quiet to moderate activity (ΔΦPC = 98 kV) are consistent with most observations. Our simulation at high activity (ΔΦPC = 146 kV) underestimates the observed pressure and temperature. This disagreement indicates possible dependence of the boundary particle sources on activity and possible effects of solar wind dynamic pressure enhancements that have not yet been included in our simulation. The simulated equatorial pressures and temperatures show stronger enhancement on the dusk side than on the dawn side as convection is increased, while density profiles show an increase on the dawn side and a decrease on the dusk side. The simulated proton flow speed at the equatorial plane increases with enhancing convection while the overall flow direction does not change significantly, a result of enhancement in both the earthward electric drift and the azimuthal diamagnetic drift. The equatorial magnetic field strength decreases more in the near‐Earth plasma sheet than at larger radial distances as ΔΦPC increases, resulting in an increasing flat radial profile with enhancing convection. The feedbacks from diamagnetic drift and magnetic fields to increasing convection are found to restrain the pressure increase. Based on the good agreement between our results and observations at moderate activity, our magnetic field indicates that the plasma and magnetic field in the plasma sheet can be in a state far from possible force balance inconsistency during periods of moderately enhanced convection. A scale analysis of our results shows that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet for moderate activity.

Structural and Electrical Characterization of Porous Silicon Carbide Formed in n-6H-SiC Substrates

The wide bandgap of silicon carbide ~SiC! semiconductor gives it the edge over other materials for making high power, high temperature, and high frequency devices. High thermal conductivity, saturation electric drift velocity, and breakdown electric field adds to its better thermal and electronic properties. In the last few years it has been recognized that nanostructured porous semiconductor networks show interesting optoelectrical properties different from those of bulk semiconductors. These properties are related to the presence of a three-dimensional ~3-D! interfacial structure with a huge internal surface area and huge volume density of surface-localized electrons. At present, extensive research is devoted to nanostructured semiconductor networks. It is believed that such networks will play an important role in future ~opto-! electronic devices ~solar cells, light emitting diodes, chemical sensors, electrochromic devices, single electron transistors !. In recent years, porous silicon carbide has been of interest due to its more efficient luminescence compared to bulk SiC. 1 Also, electroluminescent and gas sensor devices based on porous SiC have been demonstrated. 2,3 In order to use porous SiC in device application, the correlation between electrical characteristics and structural morphology of the porous layer must be understood. The goal of this work was to investigate the surface and pore morphology of 6H-SiC with respect to the effect of varying current density used during electrochemical anodization. The characterization technique used to study the surface and pore morphology has never been reported before.

Two‐dimensional quiet time equilibrium for the inner plasma sheet protons and magnetic field

In order to study the quiet time proton flow and magnetic field in the inner plasma sheet resulting from electric and magnetic drifts, we incorporate a modified version of the Magnetospheric Specification Model with a modified Tsyganenko 96 magnetic field model to self‐consistently simulate plasma sheet protons and magnetic fields with two‐dimensional force balance maintained along the midnight meridian. The resulting equatorial proton flows and pressures agree very well with observations quantitatively. Because of this agreement, the simulated force‐balanced magnetic field configuration should give more realistic plasma sheet magnetic field variations for quiet times than does the original T96 model. This 2‐D self‐consistent simulation reproduces the observed flows and pressures better than our previous 1‐D self‐consistent simulation. The improvement results from a more accurate modeling of the coupling between plasma and magnetic field in the 2‐D self‐consistent simulation.

Observations of plasma injection into the ring current during substorm expansive phase

The injection of plasmas during the expansive phase of four substorms was investigated with global auroral images and simultaneous measurements of plasmas and magnetic fields in the plasma sheet near the magnetic equator. The high‐resolution images were gained at far‐ultraviolet wavelengths with the Earth Camera on board the Polar spacecraft and the observations of plasmas and magnetic fields were acquired with the Geotail spacecraft at radial distances in the range of 11 to 15 RE. For each of the substorms an ion flow burst was detected with maximum speeds in the range of 200 to 400 km/s and durations of several minutes. The ion bulk flow was directed earthward. For the first time auroral images recorded the position of the flow bursts as the leading edge of the westward or eastward traveling surge during the expansive phase of the substorm. The time intervals from the substorm onset as recorded by the camera until the detection of the flow pulse at the Geotail spacecraft ranged from 3 to 11 min. With the exception of one event which was located near the duskside magnetopause, the ion velocity distributions were characterized by the same densities and temperatures during and after the flow pulse for each of the individual events. For three of the events the electron temperature increased by a factor of about 2 after passage of the flow pulse. For the fourth event the spacecraft was not in the plasma sheet prior to the flow pulse, and hence this temperature increase could not be determined. Substantial fluctuations of the magnetic fields accompany the flow pulses. The electron and ion spectra were smoothly varying during the flow pulses without evidence of the highly structured spectral features due to combined gradient and electric field drifts which are well documented for particle injections near the 6.6‐RE synchronous orbit. The measurements at the Geotail spacecraft position support the existence of the previously proposed “injection front” for plasmas which is due to transient earthward convection during the expansive phase of substorms. This convection occurs during the diversion of the equatorial current sheet into the ionosphere at locations inside the Geotail position.

Progress in ergodic divertor operation on Tore Supra

Upgrade of the Tore Supra ergodic divertor (ED) has led to significant progress in ED physics. Pulse durations of 30 s with LHCD have been achieved demonstrating the heat exhaust capability of both the actively cooled technology at hand and of this specific divertor concept. The disruptive limit governed by the stochastization of the outer magnetic surfaces is found to occur for a value of the Chirikov parameter reaching two on the magnetic surface q = 2+(3/12). This experimentally observed robustness allows one to operate at very low safety factor on the separatrix (q∼2). Numerical analysis of ballooning turbulence in a stochastic layer indicates that the decay of the density fluctuations is associated with an increase of the fluctuating electric drift velocity. This results in an enhanced cross-field transport in the vicinity of the target plates. This lowering of confinement appears to be compensated by an intrinsic transport barrier on the electron temperature. The three-dimensional response of the temperature field is computed with a fluid code. The code can recover the intrinsic transport barrier at the separatrix, reported experimentally, together with small amplitude temperature modulations in the divertor volume. Experimental evidence for the three density regimes (linear, high recycling and detachment) is reported. The low critical density values for transitions between these regimes indicate that similar parallel physics governs the axisymmetric and ED, despite the open configuration of the latter. Measurement and understanding of these density regimes provide a means for feedback control of plasma density and an improvement in ion cyclotron radiofrequency heating coupling scenarios. Experimental data also indicated that particle control with the vented target plates is effective. Increase of both impurity control and radiation efficiency are reviewed. Global power balance has been analysed in order to account for non-axisymmetric radiation. These results, taken together, confirm the large radiation capability of the ED.

End-shorting and electric field in edge plasmas with application to field-reversed configurations

The shorting of open field lines where they intersect external boundaries strongly modifies the transverse electric field all along the field lines. The modified electric field is found by an extension of the familiar Boltzmann relation for the electric potential. This leads to a prediction of the electric drift. Flow generation by electrical shorting is applied here to three aspects of elongated field-reversed configurations: plasma rotation rate; the particle-loss spin-up mechanism; and the sustainability of the rotating magnetic field current drive method.

Numerical modelling of pump limiter biasing on TEXTOR-94 and Tore Supra

The two-dimensional multifluid code TECXY has been used to model the biasing (with respect to the first wall) of the toroidal belt limiter ALT-II on the tokamak TEXTOR-94 and of the new toroidal pump limiter being installed on Tore Supra tokamak in the framework of the CIEL project. It is well known that the edge flow pattern can be influenced by the poloidal electric drifts from imposing radial electric fields. The modelling with TECXY introduces imprinted bias currents in the scrape-off layer (SOL) for the case of negative (limiter) biasing, and imprinted bias potentials for the case of positive biasing. This allowed us to simulate sufficiently well the experimental I-V characteristics for either biasing of ALT-II and also reproduced the essential features and trends of the observed plasma profiles in the SOL of TEXTOR-94. For negative biasing a moderate improvement of the pumping exhaust efficiency can be achieved in the case of TEXTOR. For Tore Supra, however, only a negligible improvement of the limiter performance with biasing can be predicted, which is explained by the relatively weak drift flows in Tore Supra.

Limiting equivalent electric field response in Ar+SF/sub 6/ subjected to orthogonal electric and magnetic fields

The limiting equivalent electric fields in Ar+SF/sub 6/ binary gas mixtures due to Townsend discharges are evaluated directly from a Monte-Carlo simulation when the mixture is subjected to orthogonal electric and magnetic fields. Along with the limiting equivalent electric fields, transverse and perpendicular drift velocities, electron mean energies and collision frequencies are also determined within the scope of the Monte-Carlo simulation. The equivalent reduced electric field (EREF) concept for the corresponding limiting electric fields is also investigated for the calculated mean energy levels and collision frequencies. The EREF values are found to be in good agreement with the previously published limiting electric field data.

Formation of steep gradients with plasma detachment at grazing B-field incidence at a plasma-wall transition

We use an Eulerian Vlasov code to study the problem of the formation of a plasma sheath with plasma detachment (i.e. no plasma transport to the wall) at a plasma-wall transition, in front of an absorbing wall. The ions are described by a kinetic equation in the full velocity space which integrates exactly the ion orbits. We consider a one-dimensional slab geometry, where the inhomogeneous direction is perpendicular to the wall. The electrons, assumed to only move along the magnetic field lines, are described by a parallel-B kinetic equation. The magnetized electrons are restricted to move along the magnetic field because the surface-parallel diamagnetic and electric drifts are equal and opposite. The present study shows how an ambipolar flow in the direction normal to the wall is established. For very small values of the angleα between the magnetic field and the plane of the wall, steep density and potential gradients are established in front of the plate surface with almost no plasma transport towards the wall.

Test-Particle Trajectories in a ``Sheared'' Stationary Field: Newton-Lorentz and First Order Drift Numerical Simulations1

We study a magnetic field distribution that is nonuniform and sheared like in tangential discontinuities. This distribution is an input parameter for the numerical integration of the equations of motion of the test-particle and of its guiding center. Two different electric field distributions are alternatively tested. In the first case, the electric field is uniform and constant like the electric field prescribed in the large-scale, steady-state reconnection models. The numerical solution shows that in this case the test-particle is trapped within the discontinuity into a region where (i) B goes to zero or (ii) the magnetic vector becomes exactly parallel to the electric field. In the second case, we consider an electric field, which is nonuniform. Its components are computed such that the zero order (or electric) drift is everywhere perpendicular to the discontinuity surface and its value is conserved throughout the simulation. In this case the numerically integrated trajectory of the test-particle penetrates the discontinuity for any angle of shear of B. Direct comparison between exact (Newton–Lorentz) and approximated (first order drift) numerical solutions shows that the mathematical singularities of the latter do not correspond to any physical singularity of the exact equation of motion of the particle.

Laboratory simulation of plasma jet propagation across geomagnetic field at presence of plasma background

In the experiments with laser-produced plasma jets the main processes of their interaction with transverse magnetic field were studied to simulate various plasma injections in space and the effect of influence of dilute background plasma onto electric drift motion of jet was reproduced and investigated experimentally for the first time

Understanding Io's space environment interaction: Recent energetic electron measurements from Galileo

We present energetic (>20 keV) electron measurements from the Energetic Particle Detector on the Galileo spacecraft during recent (orbits I24 and I27) near encounters with Jupiter's satellite Io in the upstream (with respect to the plasma flow) and anti‐Jupiter flank regions. No magnetic field‐aligned electron beams were observed comparable to those observed during Galileo's J0 encounter within Io's plasma wake; only a hint of a weak field‐aligned intensity enhancement was observed during the I27 flank encounter. Motivated by these new findings, the J0 wake observations are reexamined by combining high‐energy (>20 keV) and recently published low‐energy (<10 keV) electron beam components. Comparisons with observations of Earth auroral processes suggest that the Io electron beams are created at low Jovian altitudes in a downward (with respect to Jupiter) electric current region. We thus suggest that the observed electron beams are not responsible for the spot‐like auroral emissions from Jupiter at the magnetic foot point of Io, but rather constitute part of the return current of the electric circuit that connects Io to Jupiter. During I27 Galileo entered a region depleted of energetic electrons that well matches modeled expectations of Io shadowing arising from combined electric and magnetic drifts. Also, within the shadow region Galileo moved onto field lines that are connected directly to Io or its atmosphere. Detailed angle distributions reveal the magnetic field strengths (BIo) where the particles were absorbed at Io along Io's flanks. BIo values are greater than the values at Galileo's location.

Particle acceleration at an X‐type reconnection site with a parallel magnetic field

The acceleration of charged particles at a two‐dimensional magnetic reconnection site is investigated. The magnetic field has an X‐type neutral point, while reconnection is driven by a uniform transverse electric field; the effect of including a uniform magnetic field component parallel to the driving electric field and transverse to the plane of the X point is studied. We focus on the adiabatic motion of strongly magnetized particles, a valid assumption everywhere for sufficiently strong parallel magnetic fields but one which excludes a region around the neutral point for weaker fields. The regime of interest is fast driven reconnection, in which the electric drift is strong. The trajectories of particles and their dependence on the magnitude of the parallel magnetic field component are investigated. Particles can be accelerated along the magnetic field lines both because of the coupling of the perpendicular electric drift with the parallel motion, which occurs in an inhomogeneous magnetic field, and the direct acceleration by the electric field. The energy spectra of particles leaving the reconnection site are also calculated.

The convection electric field in auroral substorms

Dynamics Explorer 2 (DE 2) electric field and ion drift data are used in a statistical study of the ionospheric convection electric field in bulge‐type auroral substorms. Thirty‐one individual DE 2 substorm crossings were carefully selected and organized by the use of global auroral images obtained by DE 1. The selected passes, which occurred during substorm expansion phase, maximum, or early recovery phase, cover the entire nighttime substorm. The organization of the data used the method developed by Fujii et al. [1994], which divided the data into six local time sectors covering the nighttime substorm region. Following the procedures employed in the paper by Gjerloev and Hoffman [2000b], the latitudinal width and location of each auroral oval crossing was then adjusted to fit the sector average. In addition to the detailed study of the characteristics of the field within each sector this database enabled us to compile a model of the ionospheric convection electric field. The characteristics of the premidnight convection reversal show a pronounced local time dependency. Far west of the surge it is a fairly well defined point reversal or convection shear. Approaching the surge and within the surge it is a region of weak electric fields increasing in width toward midnight that separates regions of equatorward and poleward electric fields. Therefore we adopt the term Harang region rather than the Harang discontinuity for the premidnight convection reversal. A relatively narrow convection channel is coincident with the highest conductances located just poleward of the Harang region. This channel drives the substorm current wedge component of the westward electrojet in the surge and middle surge sectors. It is present in all premidnight passes and consequently is an integral part of the three‐dimensional substorm current wedge system.