Cross-field Motion Research Articles

Neutral gas and plasma shielding (NGPS) model and cross-field motion of ablated material for hydrogen–neon mixed pellet injection

A neutral-gas and plasma shielding (NGPS) model is applied for cryogenic hydrogen–neon mixed pellet injection used for the mitigation of tokamak disruptions. The NGPS model is useful to evaluate the characteristics of ionized plasmoid (size, density, temperature, and radiation), while reproducing the scaling of the neutral gas shielding model. It is found that even if one takes into account the optical thickness for radiation, the energy loss due to line radiation associated with neon is strong enough to limit the temperature and the pressure of the ionized plasmoid for pure neon or hydrogen–neon mixed pellets in the early phase of the material homogenization. Consequently, the ionized plasmoid of the neon mixed pellets is expected to homogenize along the magnetic field line where the ablated material is released—without significantly making the cross-field drift motion—as compared to pure hydrogen pellets.

Localized spiraling plasma ejection contributing the ion-flux broadening in the detached linear plasma

We have performed multipoint measurements with segmented electrodes and a microwave interferometer in the linear plasma device NAGDIS-II, in order to reveal cross-field motion and axial localization of the enhanced radial transport in the detached plasma. By changing the neutral pressure successively and applying several statistical analysis techniques, it was clarified that there is axially localized ion flux broadening accompanying an enhanced plasma ejection from the center with radially elongated spiraling structure. The spiraling plasma ejection accompanies the m = 0 mode drop near the center with the similar time scale. Further, such behavior composed of f > 1 kHz fluctuations is modulated by several-hundred-hertz fluctuation with m = 0. This cross-field transport causes non-negligible effect for the reduction of the ion flux peak in the detached plasma.

Impact of cross-field motion on ablation of high-Z dust in fusion edge plasmas

The impact of cross-field motion of high-Z dust grains on their shielding by ablation cloud in edge plasmas of tokamaks is analyzed. The modification of the existing high-Z dust shielding theory is developed, which takes the dust motion effects into account. It is shown that the cross-field motion can lead to a large factor increase of the dust ablation rate, as compared to the previous model. It is also shown that the motion effects take place when the dust cross-field velocity exceeds a threshold value. The dependencies of the dust ablation flux on the dust velocity and of the threshold velocity on the dust size and the ambient plasma temperature are obtained.

Drift resonance effect on stochastic runaway electron orbit in the presence of low-order magnetic perturbations

During major disruptions, an induced loop voltage accelerates runaway electrons (REs) towards high energy, being in the order of 1–100 MeV in present tokamaks and ITER. The stochastization mechanisms of such high-energy RE drift orbits are investigated by three-dimensional (3D) orbit following in tokamak plasmas. Drift resonance is shown to play an important role in determining the onset of stochastic drift orbits for different electron energies, particularly in cases with low-order perturbations that have radially global eigenfunctions of the scale of the plasma minor radius. The drift resonance due to the coupling between the cross-field drift motion with radially global modes yields a secondary island structure in the RE drift orbit, where the width of the secondary drift islands shows a square-root dependence on the relativistic gamma factor γ. Only for highly relativistic REs (γ ≫ 1), the widths of secondary drift islands are comparable with those of magnetic islands due to the primary resonance, thus the stochastic threshold becoming sensitive to the RE energy. Because of poloidal asymmetry due to toroidicity, the threshold becomes sensitive not only to the relative amplitude but also to the phase difference between the modes. In this paper, some examples of 3D orbit-following calculations are presented for analytic models of magnetic perturbations with multiple toroidal mode numbers, for both possibilities that the drift resonance enhances and suppresses the stochastization being illustrated.

Cross-field motion of plasma blob-filaments and related particle flux in an open magnetic field line configuration on QUEST

Blob-filaments have been observed by combined measurement with a fast camera and a movable Langmuir probe in an open magnetic field line configuration of electron cyclotron resonance (ECR) heating plasma in QUEST. Blob-filaments extended along field lines do correspond to over-dense plasma structures and propagated across the field lines to the outer wall. The radial velocity of the blob structure, Vb, was obtained by three methods and was dominantly driven by the E×B force. The radial velocity, size of the blob showed good agreements with the results obtained by sheath-connected interchange theoretical model. Vb corresponds to roughly 0.02–0.07 of the local sound speed (Cs) in QUEST. The higher moments (skewness S and kurtosis K) representing the shape of PDF of density fluctuation are studied. Their least squares fitting with quadratic polynomial is K=(1.60±0.27)S2−(0.46±0.20). The larger blob structures, occurring only 10% of the time, can carry more than 60% loss of the entire radial particle flux.

Formation of Forward and Reverse Shock Waves in a Magnetized Plasma: Two-Dimensional Particle Simulations

Interactions of exploding and surrounding plasmas are studied with two-dimensional (2D) electromagnetic particle simulations, for a case in which the initial velocity of the exploding plasma is perpendicular to an external magnetic field. It is confirmed that essentially the same phenomena (the penetration of exploding ions, the formation of a strong magnetic-field pulse, and the generation of forward and reverse shock waves) as in the previous one-dimensional (1D) simulations occurred in the 2D simulation. Further, modified two-stream instabilities excited by relative cross-field motion between ions and electrons are investigated with attention to their effects on the 2D structure of the strong magnetic-field pulse and on ion reflection by the pulse.

Exploitation of a general-coordinate guiding centre code for the redistribution of fast ions in deformed hybrid tokamak equilibria

Self-consistent fast ion distributions are usually obtained using a code that solves the guiding-centre equations, with an appropriate fast ion source (e.g. NBI pinis) and sink (e.g. collision operators). Straight field-line coordinate systems, such as Boozer coordinates, are ordinarily convenient due to the simple separation of longitudinal and cross-field motion, and the simple expression of magnetic differential operators. However, these coordinates are found to be near-singular at the boundary of the internal helical region associated with an n = m = 1 infernal mode. These important configurations are associated with many tokamak phenomena, including snakes and long-lived modes [1] in spherical or more conventional devices. Such internal helical states occur when there is a radially extended region where the safety factor is close to unity. Recent calculations predict the possibility of helical equilibria in ITER hybrid scenarios [2]. The ANIMEC code [3] conveniently produces an equilibrium helical state despite choosing for example an axisymmetric fixed boundary. The corresponding magnetic field in these coordinates can now be fed to the newly devised Particle-In-Cell (PIC) code VENUS-LEVIS, which has been upgraded with phase-space Lagrangian guiding-centre orbit equations [4], embodying full 3D anisotropic electromagnetic fields and a formulation that is independent of coordinate choice, despite retaining intrinsic Hamiltonian properties. The simulations are applied to MAST experiments where the presence of a long-lived mode can effect confinement of neutral beam ions, potentially affecting NBI heating and current drive [1]. Neighbouring equilibria from ANIMEC, one helical in the core and the other axisymmetic, permits a precise means of identifying the effect of 3D geometry on the simulated confinement properties of MAST's neutral beam fast ion population. In agreement with the compared experimental data from MAST neutron camera, a significant fraction of particles are pushed out of the helical core region affecting both the measured radial neutron distribution and the heating and current drive properties of the neutral beam population.

Modeling of Drift Displacement of the Pellet Ablated Material for Outboard Side Injection in Large Helical Device

The outward drift displacement of the pellet ablated material is studied for low-field side injection in the Large Helical Device (LHD). Stopping of the drift acceleration is shown to be mainly due to the formation of an internal current circuit owing to helical variation of the magnetic field gradient. This process is the most efficient for stopping the cross-field motion of the ablatant in the LHD because, in helical configurations, the parallel scale length of the gradient variation is shorter than in tokamaks. Simulated ablation and deposition profiles are shown to compare well with the Hα emission and post-injection density and temperature profiles.

Magnetosheath excursion and the relevant transport process at the magnetopause

Abstract. A large-amplitude excursion of the magnetosheath (MS) in quiet solar wind conditions on 17 March 2004 was recorded simultaneously by the Cluster and TC-1 spacecraft. During this period, the IMF Bz was entirely northward. The coherence between the bow shock motion and magnetopause (MP) motion is revealed and the excursion velocities of the bow shock motion are analyzed. In addition, the relevant plasma transport phenomenon in the form of flux fluctuations below the ion gyrofrequency at the MP is exposed and is interpreted as manifestation of the drift instability. Correlated observations on charge accumulation and electrostatic potential perturbation are recorded by electron measurements in high energy regime, and also the eventual cross-field vortex motion in the nonlinear stage and the consequential mass exchange are exhibited. The present investigation gives some new insight into the MS plasma transport mechanism across the subsolar MP region in quiet solar wind conditions during a period of northward IMF.

Cross-Field Motion of Plasma Blobs in an Open Magnetic Field Line Configuration

The radial propagation of blobs generated from plasma instabilities is investigated in an open magnetic field line configuration. Blob cross-field velocities and sizes are obtained from internal probe measurements using pattern recognition. By varying the ion mass, the normalized vertical blob scale a[over] is scanned from a[over] < 1 to a[over] > 1. An analytical expression for the blob velocity including cross-field ion polarization currents, parallel currents to the sheath, and ion-neutral collisions is derived and shows good quantitative agreement with the experimental data. In agreement with previous theoretical studies, this scaling shows that, for a[over] < 1, the blob velocity is limited by cross-field ion polarization currents, while for a[over] > 1 it is limited by parallel currents to the sheath.

Chromospheric heating by the Farley-Buneman instability

Context. Chromospheric heating produces UV emissions that can only occur in an enhanced electron temperature medium. In the quiet Sun the radiative losses are orders of magnitude larger than those in the much hotter corona. Chromospheric heating mechanisms considered previously (e.g. shock waves and nanoflares) have failed to account for the observed persistency and uniformity of UV lines and continua. Also, resistive magnetic free-energy dissipation is not efficient enough in the highly electrically conductive solar atmosphere. Aims. In this paper we consider plasma effects in the low chromosphere and propose that the Farley-Buneman (hereafter FB) plasma-instability mechanism provides the mechanism for dissipating the energy of convectively driven motions of neutral atoms into chromospheric heating in the Sun and other cool stars that have a partially ionized chromosphere. Methods. Analysis of the ion acoustic sound speed and consideration of recent measurements of magnetic field in the quiet, internetwork, solar low chromosphere are carried out in the context of understanding the characteristics and onset of chromospheric heating. The FB instability is triggered by the cross-field motion of the partially ionized gas at velocities in excess of the ion acoustic velocity. The ions acquire their cross-field velocities through collisions with the much denser chromospheric neutral atoms. Estimates of cross-field velocities are obtained from consideration of both spectral line widths and convection numerical simulations that indicate values from a few to several km s -1 at the top of the practically radiative-equilibrium low chromosphere. Results. The FB instability is triggered by the cross-field motion of the neutral component of the partially ionized gas at velocities in excess of the ion acoustic velocity. This instability occurs in the solar chromosphere because electrons become strongly magnetized just above the photosphere, while heavy ions and protons remain unmagnetized, and only at the very top of the chromosphere do they become magnetized. Conclusions. We find that convective overshoot motions are drivers of the FB instability and provide enough energy to account for the upper chromospheric radiative losses in the quiet-Sun internetwork and network lanes.

Shock formation processes in colliding two collisionless plasmas in a magnetic field

Interactions of exploding and surrounding plasmas are analyzed with theory and particle simulations for the case in which the initial velocity v0 of the exploding plasma is perpendicular to the weak external magnetic field B0; because of the fast v0 and weak B0, the gyroradius v0∕Ωi is much longer than shock widths. After the penetration of the exploding ions, the electric field induced by the cross-field plasma motion quickly accelerates the surrounding ions in the direction perpendicular to v0 and B0. Owing to the magnetic force, however, the surrounding ions that were once overtaken by the exploding ions can pass them later. Because the electron guiding centers move with the E×B drift, the two kinds of electrons are not mixed. Further, because of the deceleration of the exploding ions in the front layer, the plasma density and magnetic-field strength go up there. This field reflects the surrounding ions forward and exploding ions backward. This generates two large pulses, which then develop into forward and reverse shock waves. Up to the generation of the two pulses, the time and length scales are basically determined by v0 and B0.

Transport across complex magnetic environment

It is well known that the rich, highly complicated field configurations of the Heliospheric Magnetic Field has an essential effect on the cross-field propagation of charged particles of relatively low rigidity. In this process the field line separation is the most significant factor of the magnetic structure. The aim of this paper is to develop a framework, in which the particle transport can be investigated for various types of field-line separation. It turns out that the presence of any diffusive cross-field motion of the particles, even if it is arbitrarily small, can wash out most of the details belonging to a specific field line separation model. In that case the overall cross-field motion is forced to become diffusive, irrespectively of the type of the field line spreading, whether this spreading is diffusive, or even non-diffusive in nature. For the various separation models the expressions derived are very similar.

High‐Altitude Particle Acceleration and Radiation in Pulsar Slot Gaps

We explore the pulsar slot gap (SG) electrodynamics up to very high altitudes, where for most relatively rapidly rotating pulsars both the standard small-angle approximation and the assumption that the magnetic field lines are ideal stream lines break down. We address the importance of the electrodynamic conditions at the SG boundaries and the occurrence of a steady-state drift of charged particles across the SG field lines at very high altitudes. These boundary conditions and the cross-field particle motion determine the asymptotic behavior of the scalar potential at all radii from the polar cap (PC) to near the light cylinder. As a result, we demonstrate that the steady-state accelerating electric field, E(sub ll), must approach a small and constant value at high altitude above the PC. This E(sub ll) is capable of maintaining electrons moving with high Lorentz factors (approx. a few x 10(exp 7)) and emitting curvature gamma-ray photons up to nearly the light cylinder. By numerical simulations, we show that primary electrons accelerating from the PC surface to high altitude in the SG along the outer edge of the open field region will form caustic emission patterns on the trailing dipole field lines. Acceleration and emission in such an extended SG may form the physical basis of a model that can successfully reproduce some pulsar high-energy light curves.

Preliminary oscillating fluxes current drive experiment in DIII-D tokamak

Abstract A preliminary oscillating flux helicity injection experiment was done on DIII-D tokamak. The toroidal flux was modulated by programming the plasma elongation. Instead of programming the surface voltage directly, the plasma current was programmed with a periodic modulation at some phase shift. The theoretical basis of this modulation is discussed in terms of the helicity injection and also introduced by cross-field motion of the modulated plasma. Because the primary winding is well coupled with the plasma current and the power supply is strong, the plasma current behaves as programmed. However, as the plasma shape is not coupled strongly with the shaping and equilibrium coils, the elongation amplitude and phase are affected by the change of plasma current and do not behave as programmed. Because of this, the voltage induced by the helicity injection is low, and the experiment did not test the principle of helicity injection. The injection powers of helicity and energy, and the electric field intensity of the helicity injection model and the cross-field motion of plasma are compared with each other experimentally. The improvement necessary to do the experiment is also proposed.

Stability theorem for a single species plasma in a toroidal magnetic configuration

A stability theorem is developed for a single species plasma that is confined by a purely toroidal magnetic field. A toroidal conductor is assumed to bound the confinement region, and frequencies are ordered so that the cyclotron action and the toroidal action for each particle are good adiabatic invariants. The cross-field motion is described by toroidal-average drift dynamics. In this situation, it is possible to find plasma equilibria for which the energy is a maximum, as compared to all neighboring states that are accessible under general constraints on the dynamics. Since the energy is conserved, such states must be stable to small-amplitude perturbations. This theorem is developed formally using Arnold’s method, and examples of stable equilibria are obtained.

A self-consistent MHD ablation model: pellet penetration depth prediction for a reactor-temperature plasma

A time dependent quasi-two-dimensional ablation model has been developed. The ablation of a pellet injected into a magnetically confined plasma and the temporal variation of the properties of the cloud surrounding it are calculated in a self-consistent manner. The energy deposition in the shielding cloud is determined by stopping length calculations applied along the magnetic field lines with allowance for electrostatic shielding effects. The stopping length calculations are supplemented by thermal diffusion calculations, thus redistributing the energy deposited in the discrete energy group approximation. The neutral cloud is allowed to expand also in the direction perpendicular to the magnetic field. Finite rate equations are used for determining the time history of the ionization state of the ablated substance. The deceleration and full stopping of the cross-field motion is calculated by means of an MHD model (iteratively with the axial expansion dynamics), thus determining the transient variation of the lateral cloud dimension and the corresponding modifications of the field aligned density and temperature distributions used in the stopping length calculations. The expulsion of the magnetic field lines by the expanding and ionized ablatant and their rediffusion into the partial magnetic cavity thus formed are also calculated. The variation of the ablation rate along the pellet path (i.e. that of its value averaged over the residence time of a pellet of given injection velocity in a flux tube defined by the local ionization radius of the ablatant) are calculated at a sequence of flux surfaces, thus determining the particle deposition profile. Results of penetration depth calculations are compared with values stemming from experiment and from other ablation models. Good correspondence is found at moderate plasma temperatures. Predictive calculations performed for a reactor-grade plasma yield, owing to magnetic shielding effects, substantially larger penetration depths than those obtained with the standard NGS a

Cross Field Ion Motion at Sawtooth Crash

Turbulent ion motion caused by sawtooth crash is analyzed in the framework of the magnetic stochasticity model of the sawtooth crash. The average current profile changes much more slowly than the pressure profile after the onset of the crash, so that inhomogeneity of the parallel current along the field line is generated. This causes a perpendicular current with a short scale length, leading to the ion cross field motion.

Evolutions of Plasma Potential Controlledby Biased Segmented Endplates

Temporal and spatial evolutions of plasma potential formation are experimentally investigated in an ECR plasma terminated by biased segmented endplates under a mirror configuration of magnetic field. The time scale of the potential formation is determined by ion transport perpendicular to the magnetic field. Nonaxisymmetric potential profiles formed by the biased segmented endplates are also observed to depend on the cross-field ion motion.

A two-dimensional model of plasma expansion in the ionosphere

The motion of artificial plasma clouds injected perpendicular to the geomagnetic field ( B) in the ionosphere was studied using a time-dependent, two-dimensional, electrostatic model. Various geophysical and gas release conditions were considered, including different solar cycle, seasonal, and geomagnetic activity levels and different cloud sizes, injection velocities, release altitudes, and species. The effects of both magnetospheric electric fields and thermospheric winds on the cross-field motion were also considered. This systematic study led to the following conclusions: (1) initially, the newly created plasma clouds (Ba + and Li +) move across B with the injected neutral clouds (for t ≲ 1 s), but the ions are decelerated much faster than the neutrals and eventually become tied to the magnetic field; (2) at high altitudes (~400 km), the shape of a plasma cloud becomes long and narrow because it is formed via photoionization from the more rapidly moving neutral cloud. However, the precise cloud configuration and deceleration rate are determined by the ionospheric and release conditions; (3) the deceleration of plasma clouds is faster in summer than in winter, but the cloud Pedersen conductivities have similar magnitudes; (4) solar cycle and geomagnetic activity variations have a similar effect on the cross-field motion of plasma clouds. At solar maximum and high geomagnetic activity, the plasma clouds are decelerated faster and the cloud Pedersen conductivities are greater than at solar minimum and low geomagnetic activity; (5) at high altitudes, structure forms on the rear side of Ba + clouds in winter at solar minimum and low geomagnetic activity, but not for other geophysical conditions or for lithium; (6) the electrodynamic drift associated with the magnetospheric electric field acts to distort the shape of plasma clouds, change their direction of motion, and change their orientation relative to the neutral clouds; (7) at low altitudes (~200 km), the cross-field motion of both the plasma and neutral clouds is much slower, the cloud Pedersen conductivity is much greater (by two orders of magnitude), and the plasma cloud configuration is more isotropic than at high altitudes; (8) a thermospheric wind has an important effect on the cross-field motion of plasma clouds at 200 km, but its effect is small at 400 km. At low altitudes, the neutral wind acts to polarize the plasma cloud and eventually it moves in the direction of the thermospheric wind (a well-known result); (9) for high injection velocities and high cloud densities, the plasma cloud moves a greater distance across B and the cloud tends to become longer and narrower, particularly at high altitudes; (10) increasing the initial cloud size has an effect that is similar to lowering the cloud injection velocity; (11) if a cloud is partially ionized at the time of release, the motion of the initial ionization is affected by the electric field associated with the subsequent photoionization of the more rapidly moving neutral cloud; and (12) comparing comparable barium and lithium releases, the lithium ion cloud has a lower Pedersen conductivity and is decelerated faster because of the lower photoionization rate and smaller mass for lithium.