Radial Currents Research Articles

Study of The Potential Structure from the Plug-Barrier Cell to the End Plate in Gamma 10

For understanding of the plug potential generation in a tandem mirror, the potential structure of the whole plasma should be investigated. In particular, the potential structure from the plug-barrier cell to the end plate installed on the end wall of the vacuum vessel has physical import. The fundamental ECRH at the plug region generates an electromotive force by driving the axial flow of electrons. This electromotive force is divided into the positive plug potential and the negative end plate potential. This paper shows the variations of these potential with currents flowing through each region. The end plate potential increases with this current. Analysis of a current carrying sheath is applied to the end plate potential. The plug potential decreases with a radial ion current in the peripheral region of the plug-barrier cell. To illustrate this point, a plasma shot with NBI is examined in which a trapped plasma is generated in the cell.

Self-consistent model for the radial current generation during fishbone activity

Line broadened quasilinear burst model, originally developed for the bump-on-tail instability [H. L. Berk et al., Nucl. Fusion 35, 1661 (1995)], is extended to the problem of sheared flow generation by the fishbone burst. It is supposed that the radial current of the resonant fast ions can be sufficient to create the transport barrier.

New expressions for calculating the magnetic field created by radial current in massive disks

In this paper, new expressions are presented for three-dimensional magnetic-field calculation for a massive disk currying radial currents. These expressions have been obtained in analytical form as functions of incomplete elliptical integrals (of the first, second, and third kind) and an integral to be solved numerically. This approach enables one to easily calculate the magnetic field everywhere in space at reduced computational cost. In addition, it can be employed to calculate the magnetic field in conductor disk configurations involving complicated geometry as well as to evaluate their self and mutual inductance.

Transient dynamics of vulcanian explosions and column collapse.

Several analytical and numerical eruption models have provided insight into volcanic eruption behaviour, but most address plinian-type eruptions where vent conditions are quasi-steady. Only a few studies have explored the physics of short-duration vulcanian explosions with unsteady vent conditions and blast events. Here we present a technique that links unsteady vent flux of vulcanian explosions to the resulting dispersal of volcanic ejecta, using a numerical, axisymmetric model with multiple particle sizes. We use observational data from well documented explosions in 1997 at the Soufrière Hills volcano in Montserrat, West Indies, to constrain pre-eruptive subsurface initial conditions and to compare with our simulation results. The resulting simulations duplicate many features of the observed explosions, showing transitional behaviour where mass is divided between a buoyant plume and hazardous radial pyroclastic currents fed by a collapsing fountain. We find that leakage of volcanic gas from the conduit through surrounding rocks over a short period (of the order of 10 hours) or retarded exsolution can dictate the style of explosion. Our simulations also reveal the internal plume dynamics and particle-size segregation mechanisms that may occur in such eruptions.

Noise Reduction of the DC Motor for the Electrical Power Steering by Short Circuit Wires for Symmetrical Position

The market of the electrical power steering is expanding rapidly because of its fuel efficiency. The DC motor is generally used in this system, whose noise reduction is demanded. The DC motor has some dispersion in contact number of segments or contact resistance between the brush and the commutator. When these conditions are different with plural brushes, currents in armature coils are in asymmetrical distribution and the unbalanced magnetic pull occurs in radial direction.We measured the radial force and currents in armature coils with the developed experimental apparatus that can also set up the eccentricity of rotor to stator. In this paper, we investigated the factor and the process of the radial force generated in the DC motor. Then we propose short circuit wires for symmetrical position that makes winding current symmetrical and realizes the noise reduction.

Fast wave current drive at high ion cyclotron harmonics on DIII-D

Current driven by the fast Alfvén wave is measured at the fourth and eighth harmonics of the deuterium ion cyclotron frequency in identical plasmas on the DIII-D tokamak. In non-sawtoothingdischarges with neutral beam injection heating, the radial profile ofthe fast wave current drive (FWCD) is determined by the response of theloop voltage profile to co- and counter-antenna phasings. The dimensionless current drive efficiency is a factor of two greater for the eighth harmonic case compared to the fourth harmonic case. Modelling of the fast wave absorption using a ray tracing code shows that the decrease in FWCD efficiency for the latter situation can be explained by high harmonic damping of the fast waves on energetic ions.

MHD simulation of the three-dimensional structure of the heliospheric current sheet

The existence of the radial component of the electric current flowing toward the Sun is revealed in numerical simulation. The total strength of the radial current is ~ A. The only way to fulfil the electric current continuity is to close the radial electric current by means of field- aligned currents at the polar region of the Sun. Thus, the surface density of the closure current flowing along the solar surface can be estimated as ~4 A/m, and the magnetic field produced by this current is T, i.e. several percent of the intrinsic magnetic field of the Sun. This seems to mean that any treatment of the solar magnetic field generation should take into account the heliospheric current circuit as well as the currents flowing inside the Sun.

DC column plasma kinetics in a longitudinal magnetic field

The cylindrical column plasma of a neon dc glow discharge under the influence of a weak longitudinal magnetic field is studied. An extended, fully self-consistent model of the column plasma has been used to determine the kinetic quantities of electrons, ions and excited atoms, the radial space charge field, and the axial electric field for given discharge conditions. The model includes a nonlocal kinetic treatment of the electrons by solving their spatially inhomogeneous kinetic equation, taking into account the radial space charge field and the axial magnetic field. The treatment is based on the two-term expansion of the velocity distribution and comprises the determination of its isotropic and anisotropic components in the axial, radial, and azimuthal direction. A transition from a distinctly nonlocal kinetic behavior of the electrons in the magnetic-field-free case to an almost local kinetic behavior has been found by increasing the magnetic field. The establishment of the electron cyclotron motion around the column axis increasingly restricts the radial electron energy transport and reduces the radial ambipolar current. The complex interaction of these transport phenomena with the alterations in the charge carrier production leads finally to a specific variation of the electric field components. The axial field increases by applying weak magnetic fields, however, decreases with increasingly higher magnetic fields. At higher magnetic fields, the radial space-charge field is considerably reduced.

Contribution of the ion diamagnetic flow to poloidal rotation and pressure asymmetries in the SOL

Under typical conditions of high recycling divertor plasmas, the ion diamagnetic flow makes the largest contribution to the ion poloidal rotation in the SOL. Owing to toroidal effects, the net poloidal rotation in the SOL gives rise to ion parallel Pfirsch-Schlüter flows, which are sustained by an up-down pressure asymmetry which reverses with toroidal field (Bϕ) reversal. The up-down pressure asymmetry in turn gives rise to net, surface averaged, radial currents j̄r arising from the surface averaging of local diamagnetic currents jr = (1/rB)(∂p/∂θ) due to toroidal effects. The radial divergence of radial currents j̄r can exist in the SOL because of electrical contact with the target. The resulting net toroidal j̄r Bθ force in the direction of the main plasma current creates an in-out pressure asymmetry in favour of the ion drift side (which is the inner side in normal Bϕ and the outer side in reversed Bϕ plasmas). This pressure asymmetry should result in higher density and low temperature plasma in the inner divertor leg in normal Bϕ discharges and should also make the distribution of density and temperature between the targets more equal in reversed Bϕ discharges, consistent with the main trends observed in experiments with toroidal field reversal. The proposed mechanism can therefore provide an alternative explanation (to the effect of the E × B drifts analysed earlier) for the observed changes in target asymmetries associated with Bϕ reversal.

Calculation of 2D profiles for the plasma and electric field in the boundary layer of the TEXTOR-94 Tokamak

An improved version of the 2D multifluid code TECXY is used to study the plasma parameters and the electric field in the boundary layer of the TEXTOR-94 tokamak. An anomalous perpendicular shear viscosity η ⊥ is incorporated into the ion momentum balance which allows to fulfil the global (magnetic surface averaged) ambipolarity constraint for the radial electric current, and to determine the electric field in the transition layer by a second-order ordinary differential equation for E r( r). The code modelling results suggest the existence of a narrow global circulation layer for the poloidal plasma velocity just inside the separatrix which is connected with a negative electric field spike. A proper choice of η ⊥ gives better agreement with experimental velocity profiles.

Observation of a Radial Current at a Plug/Barrier Cell in GAMMA10

This paper presents a picture of current flow relevant to the plug/barrier cell and an end plate installed on the end wall of the GAMMA 10 tandem mirror. Detailed analyses show that, during electron cyclotron resonance heating for plug potential formation, a net electron current flows through the end plate. In the plug/barrier cell, on the other hand, ion currents are detected during ECRH on ring electrodes newly installed near the side wall of the vacuum vessel. A part of the radial ion currents is connected to the axial net electron current flowing through the end plate. This indicates that a residual non-ambipolar diffusion still remains. The radial ion current in the plug/barrier cell includes another component that originates form the axial ion flux emanating from the central region. This suggests radial diffusion of ions during potential plugging.

Magnetodynamic nonlinearity of the electrical properties of uncompensated metals

The magnetodynamic nonlinearity of the electrical properties of uncompensated metals, wherein the drift of charge carriers at a high density in crossed electric and magnetic fields leads to generation of a self-field of the current, is investigated experimentally and analytically. The experiment is done on cylindrical conductors made from high-purity polycrystallinealuminum, in an arrangement in which the radial current is acted on by an external magnetic field coaxial to the sample. The electric potential and the nonlinear correction to it are determined over a wide range of values of the energy dissipation, all the way up to levels corresponding to the boiling crisis of liquid helium. In the approximation that the contributions to the resistive effect of the external field and self-field are additive, the results of a calculation based on the macroscopic field equations are found to be in good agreement with experiment. Questions pertaining to the concentration of magnetic field energy of the current for a cylindrical geometry of the conductor are discussed in the approximations of long and short solenoids.

Numerical modeling of L-H transitions in the presence of a radial current at the edge of a tokamak plasma

Hydrodynamic equations describing wall plasma turbulence are analyzed numerically using a two-dimensional four-field model. Turbulent transport coefficients are calculated with consideration of the radial current. Numerical analysis revealed a possible scenario for L-H transitions that is associated with the radial current driven by nonambipolar processes. It is shown that the transition of a plasma to an improved confinement mode can also be triggered by other mechanisms.

The Role of Radial Electric Fields in the Tokamaks TEXTOR-94, CASTOR, and T-10

Radial electric fields (E r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma. The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime. The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call "separatrix biasing" is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E×B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E×B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates (≈ 5×106 s-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements. During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter. The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.

Comment on “Generation of plasma rotation by ion cyclotron resonance heating in tokamaks” [Phys. Plasmas 6, 1969 (1999)

It is shown by considering the general properties of solution of the kinetic equation that rf wave heated tokamak plasmas cannot be modeled correctly by a radial current.

Kinetic aspects of the Jovian current sheet

In this work, self‐consistent two‐dimensional particle simulations are used to examine the structure and dynamics of the current sheet in the middle and outer regions of the Jovian magnetotail. A plasma source, representing plasma transported outward from Io, is injected into the simulation system of a corotating magnetodisc configuration that is initially at hydromagnetic equilibrium. Simulations show that the corotating magnetodisc equilibrium evolves into a new configuration in which plasma outflow is present and the ion and electron outflows are different. The initial hydromagnetic equilibrium also develops a cross‐sheet charge separation, an electrostatic potential, and a polarization electric field normal to the sheet. We observe a relatively stable current sheet and a steady, moderate outflow inside the Alfvén radius. Outside the Alfvén radius, a corotation lag spiral magnetic field, radial outward currents carried by the ions, and electron diamagnetic drifts in the polarization field are found. In this region the centrifugally slung plasma may break the magnetic field topology through driven reconnection. During strong reconnection the magnetic field configuration in the plasmoid region is observed to change from corotation lag to corotation lead; the magnetodisc plasma escapes into a super‐Alfvénic outflow, and fast‐shock‐like layers form. Ion heating and non‐Maxwellian velocity distributions are found in these regions.

Stability of the Base Flow to Axisymmetric and Plane-Polar Disturbances in an Electrically Driven Flow Between Infinitely-Long, Concentric Cylinders

The method of normal modes is used to examine the stability of an azimuthal base flow to both axisymmetric and plane-polar disturbances for an electrically conducting fluid confined between stationary, concentric, infinitely-long cylinders. An electric potential difference exists between the two cylinder walls and drives a radial electric current. Without a magnetic field, this flow remains stationary. However, if an axial magnetic field is applied, then the interaction between the radial electric current and the magnetic field gives rise to an azimuthal electromagnetic body force which drives an azimuthal velocity. Infinitesimal axisymmetric disturbances lead to an instability in the base flow. Infinitesimal plane-polar disturbances do not appear to destabilize the base flow until shear-flow transition to turbulence.

Enhanced current collection by a positively charged spacecraft

The collection of electrons by a conducting spherical body at a high voltage Φo from a magnetized plasma is studied by means of a fully three‐dimensional particle‐in‐cell code. A relative motion between the plasma and the body is included to simulate the orbital motion of a spacecraft. The current‐voltage (I‐V) characteristic of the body as seen from the simulations is compared with that predicted by the theory giving the upper‐bound current Ipm [Parker and Murphy, 1967]. In agreement with measurements in the tethered satellite experiments TSS‐1 and TSS‐1(R), the simulations give currents I(Φo), which are higher than the corresponding currents Ipm(Φo). For sufficiently large values of Φo, Ipm∝Φo0.5, while from the simulations we find that Ipm∝Φo0.62, giving Isim/Ipm∝Φoδ with δ∼0.12. Thus our simulations predict a voltage‐dependent enhancement of the current although the dependence is weak. Despite this enhancement simulations show that the dependence of Isim on the body radius rs is the same as that given by Ipm(rs). In order to better understand these behaviors of the current collection, the distributions of potential and current in the plasma around the body are examined. When eΦo > Wo, the ram ion energy, the reflection of ram ions forms a virtual anode in front of the body in the ram region. The potential structure extends from the virtual anode along the magnetic field B0 and not along the magnetic shadow of the body. The current pattern in the plasma contributing to the collection of electrons reveal that when eΦo > Wo, the current flow is approximately magnetic‐field aligned at parallel distances |z|>rs in a cylindrical volume aligned with the magnetic field lines passing through the virtual anode. When |z|<rs, the parallel currents give way to azimuthal and radial currents in the ram volume in front of the body. The latter currents predominately contribute to the electron collection, which maximizes near the equatorial (z∼0) ram surface of the body. The current patterns are further studied by tracing electron trajectories in the potential distributions in the sheath of the body, showing that the E×B drifts of the electrons in the self‐consistently determined poteial distribution do indeed enable the collection of the electrons near the equatorial ram surface.

X-Point Heating, Potentials, and Temperature Asymmetries in Edge Plasmas

Three-dimensional simulations of turbulence in the boundary plasma of a diverted tokamak indicate strongly localized heating and radial currents on closed flux surfaces in the vicinity of the X point, and over bllrstiness. We apply a long-mean-free path theory of particle and energy exchange between passing and potential-trapped electrons to show that these turbulent phenomena combined with natural fueling.asymmetries may lead to electron temperature, density and potential maxima in this region. Such maxima have been observed on the DIII-D tokamak.

X-Point Heating, Potentials, and Temperature Asymmetries in Edge Plasmas

Three-dimensional simulations of turbulence in the boundary plasma of a diverted tokamak indicate strongly localized heating and radial currents on closed flux surfaces in the vicinity of he X point, and overall burstiness. We apply a long-mean-free path theory of par icle and energy exchange between passing and potential-trapped electrons to show that these turbulent phenomena combined with natural fueling asymmetries may lead to electron temperature, density and potential maxima in this region. Such maxima have been observed on the DIII-D tokamak.