Poloidal Plasma Rotation Research Articles

Structure of parallel-velocity-shear-driven mode in toroidal plasmas

It is shown that the Fourier-ballooning representation is appropriate for the study of short-wavelength drift-like perturbation in toroidal plasmas with a parallel velocity shear (PVS). The radial structure of the mode driven by a PVS is investigated in a torus. The Reynolds stress created by PVS turbulence, and proposed as one of the sources for a sheared poloidal plasma rotation, is analyzed. It is demonstrated that a finite ion temperature may strongly enhance the Reynolds stress creation ability from PVS-driven turbulence. The correlation of this observation with the requirement that ion heating power be higher than a threshold value for the formation of an internal transport barrier is discussed.

Driven plasma rotation in the Compact Auburn Torsatron

Poloidal plasma rotation has been driven in the Compact Auburn Torsatron (CAT) [R. F. Gandy et al., Fusion Technol. 18, 281 (1990)]. In these studies, a biased electrode is used to establish the rotation. Currents of up to 1 A are injected into hydrogen, helium, and argon plasmas from a biased electrode placed inside of the last closed magnetic flux surface of the CAT device. The response of the plasma to the injected current is measured with Langmuir, Mach, and emissive probes. Poloidal plasma velocities of up to 10 km/s in hydrogen plasmas are measured by the Mach probe. The plasma density and electron temperature increased during the driven rotation, and there are indications of improved particle confinement. The experimental measurements show generally good agreement with a model of plasma rotation by Coronado and Talmadge. Experiments also show that the sign of the injected current (i.e., the direction of the radial electric field) has a significant impact on the effect of the driven rotation. A reduction in the particle transport is observed only for radially inward electric fields.

Destabilization of poloidal plasma rotation during electron cyclotron resonance heating in tokamaks

The poloidal plasma rotation destabilized by electron cyclotron resonance heating (ECRH) is proposed in both collision and banana regions of tokamak plasmas. The poloidal electric field, which results in poloidally inhomogeneous ion accumulation, can be produced by ECRH due to either the resonance localization or the asymmetric rf power deposition. When the ion accumulation rate overwhelms the damping rate of the ion–ion collision on the poloidal plasma rotation, the poloidal plasma rotation will be destabilized. Based on the linearized drift kinetic equation and the fluid equations, including a model term of the density perturbation driven by rf waves, the criterion of the destabilization of the poloidal plasma rotation is analytically derived. It is shown that the poloidal plasma rotation in tokamaks can be generated by ECRH in the practical rf power level by this manner.

Toroidal and poloidal plasma rotation in the reversed field pinch RFX

In this paper, the toroidal and poloidal rotation velocities measured in the reversed field pinch device RFX from the Doppler shift of impurity ion lines are presented and their relation with the plasma velocity is discussed. It is found that the toroidal velocity at the edge has the opposite direction with respect to the core toroidal velocity, indicating a shear of the plasma rotation velocity of about . Starting from the rotation measurements and the momentum equation, a one-dimensional impurity diffusion model for the core plasma and a Monte Carlo code for the edge region are used to estimate the radial electric field, that is found to be around outward directed in the plasma core, and about inward directed at the edge, consistent with that measured at the edge by Langmuir probes.

Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through time-resolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

Sheared rotation effects on kinetic stability in enhanced confinement tokamak plasmas, and nonlinear dynamics of fluctuations and flows in axisymmetric plasmas

Sheared rotation dynamics are widely believed to have significant influence on experimentally-observed confinement transitions in advanced operating modes in major tokamak experiments, such as the Tokamak Fusion Test Reactor (TFTR) [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)], with reversed magnetic shear regions in the plasma interior. The high-n toroidal drift modes destabilized by the combined effects of ion temperature gradients and trapped particles in toroidal geometry can be strongly affected by radially-sheared toroidal and poloidal plasma rotation. In previous work with the FULL linear microinstability code, a simplified rotation model including only toroidal rotation was employed, and results were obtained. Here, a more complete rotation model, which includes contributions from toroidal and poloidal rotation and the ion pressure gradient to the total radial electric field, is used for a proper self-consistent treatment of this key problem. Relevant advanced operating mode cases for TFTR are presented. In addition, the complementary problem of the dynamics of fluctuation-driven E×B flow is investigated by an integrated program of gyrokinetic simulation in annulus geometry and gyrofluid simulation in flux tube geometry.

Equations for the evolution of the radial electric field and poloidal rotation in toroidally symmetric geometry

A set of three coupled ordinary differential equations that give the evolution of the density, the radial electric field, and poloidal plasma rotation are derived from the continuity equation and momentum balance. They include the effect of those processes considered to be of importance for the L- (low) to H- (high) mode transition: neutral friction, neoclassical viscosity, the radial pressure gradient, orbit losses, a radial current through a probe, anomalous stresses, and Stringer spin up. The equations are valid for arbitrary toroidally symmetric geometry and include effects of non-uniformity (of for instance the neutral friction) in the magnetic surface. As an example, non-uniform neutral friction in an elongated geometry is discussed.

Destabilization of the poloidal rotation in a multiple ion species tokamak plasma

The destabilization of the poloidal rotation for the species of ions with the poloidally inhomogeneous sources or sinks is studied in multiple ion species tokamak plasmas. Based on the fluid equations, the evolution equation of the poloidal ion rotation speed is derived analytically. The magnitude of the inhomogeneous sources or sinks needed to destabilize the rotation is shown to be proportional to the ion density. It is suggested that the poloidal plasma rotation can be generated by destabilizing the rotation of the minority ions.

Poloidal Flow Destabilized by Electron Cyclotron Resonant Heating in Collision Tokamak Plasmas

The production of the poloidally sheared flow during electron cyclotron resonant heating (ECRH) in the collisional tokamak plasmas, which refers to the edge region, is studied. Based on the linearized kinetic drift equation and the fluid equations including the density perturbation term driven by rf waves, the criterion of the destabilization of the poloidal plasma rotation is analytically derived. It is shown that the edge poloidal plasma rotation can be destabilized during ECRH in the present rf power level.

Effect of subsonic toroidal flows on plasma poloidal rotation and ion heat conductivity. In collisional plasmas of elongated tokamaks

The ion poloidal rotation and heat conductivity in collisional plasmas of axially-symmetric tokamaks with elongated cross-sections and with subsonic toroidal plasma flows are considered. It is shown that subsonic toroidal plasma flows, induced by neutral beam injection or radio frequency waves, can strongly affect the poloidal plasma velocity and ion heat conductivity in collisional plasmas of tokamaks. The transport coefficients also depend on the tokamak ellipticity parameter which, in combination with the Mach number, allows to operate transport processes at smaller values of the toroidal Mach number. The importance of taking into account the ion-electron heat exchange and electron temperature toroidal perturbations to find ion temperature toroidal perturbations is demonstrated.

POLOIDALLY SPIN UP GENERATED BY ELECTRON CYCLOTRON RESONANCE HEATING IN CORE TOKAMAK PLASMAS

A new approach is presented to produce core plasma poloidal rotation during electron cyclotron resonance heating (ECRH) in Tokamaks. The physical mechanism employed here is as follows:in a Tokamak plasma with high power ECRH, the resonance particle localization causes poloidal electric field, which consequently induce a poloidally asymmetric ion profile. This kind of poloidal ion accumulation would overwhelm the damping of poloidal rotation from the magnetic pumping, and make the plasma spin up poloidally. Based on the fluid equations and the drift kinetic equation, we have derived the criterion of plasma spin up in the collisionless region. It is shown that the core plasma poloidal rotation can be generated by ECRH in the practical rf power level.

Analysis of electrostatic fluctuations in the presence of plasma poloidal rotation

An experimental characterisation of electrostatic fluctuations in the presence of a radial electric field has been performed in the steady-state toroidal Thorello device. Both kinematic and dynamic effects on the fluctuations are analysed along the radius of the torus.

Plasma biasing by radial electric fields induced in front of lower hybrid grills via poloidal ponderomotive forces

This article treats the stationary radial electric field that the lower hybrid wave induces in front of the grill of the exciting antenna structure. The source of the induced electric field is the poloidal ponderomotive force of lower hybrid waves. It is shown that electric fields of the order of tens of volts per cm are possible in tokamak discharges. The required poloidal ponderomotive force would be possible if the mutual phasing of the horizontal waveguide rows of the grill were of a suitable value. It has been found that the induced radial electric field and the associated poloidal and toroidal plasma rotation will undergo sign changes if the poloidal phasing of the grill is altered appropriately.

Measurement of Plasma Rotation Velocities with Limiter Biasing on the CT-6B Tokamak

H-mode-like phenomenon is observed with positive limiter biasing on the CT-6B Tokamak. Poloidal rotation of plasma in the boundary region is slowed and even changed its direction with positive bias, effect of positive bias on the toroidal rotation is also presented.

SPECTROSCOPIC MEASUREMENT OF PLASMA POLOIDAL ROTATION ON THE CT-6B TOKAMAK

By means of a high-resolution spectroscopic detection system, the radial profiles of plasma poloidal rotation velocity have been measured from the Doppler shifts of impurity ion lines OⅡ464.2nm, CⅢ464.7nm and the hydrogen Hα line on the CT-6B Tokamak. The results show that inside the plasma, the poloidal rotation velocity of impurity ions is in the electron diamagnetic drift direction and reaches its maximum value of～3.5km/s at the minor radius around 9cm. It reverses to the ion diamagnetic drift direction with its maximum value at r=10cm near the limiter. The internal electric field deduced from these data is directed towards the plasma center, and its maximum value is 18V/cm. There is also a poloidal rotation velocity component for neutral hydrogen but it is only in the electron diamagnetic drift direction with a value notably less than that of the ions. A preliminary discussion of the results is also given.

Poloidal plasma rotation and its effect on fluctuations in a toroidal plasma

An experimental study on poloidal plasma rotation induced by electrode biasing and its effect on fluctuations is carried out in a purely toroidal plasma having no poloidal magnetic field. Poloidal plasma rotation and, concomitantly, significant reduction in the fluctuation level are observed. The radial profiles of poloidal flows and fluctuations are measured and compared. The dependence of the poloidal plasma rotation on the bias voltage and the toroidal magnetic field is presented. It is found that the large poloidal plasma rotation gives rise to a transition of the plasma state from a turbulent to a coherent mode

Theoretical estimates of radial electric fields and plasma rotation induced in H-1NF by RF waves

The purpose of this study is to estimate, for planned experiments at the H-1NF heliac, nonlinear variations of the stationary radial electric field, and poloidal and toroidal plasma rotation, that result from ponderomotive forces exerted by large amplitude RF waves in the H-1NF. Similarly as in the previous studies of nonlinear transport effects induced by RF waves, the nonlinear ponderomotive force effects on the radial electric field, toroidal and poloidal plasma rotation become important for dissipated powers of the order of 1 MWm−3 for RF waves with frequency of about 10 MHz. At these high RF powers, the nonlinear ponderomotive force effects might therefore result in important changes in plasma confinement and RF wave coupling in H-1NF.

Toroidal ηi-mode global vortices in a rotating plasma

Model equations describing dynamics of a toroidal ηi mode in a rotating plasma are derived. The stationary solution of the model equations is investigated analytically and the condition for global vortex formation is found. The form of the solution is verified numerically and it is shown that the shape of global vortices is affected strongly by the shear of the plasma poloidal rotation. In weak shear, the stationary solution looks like a dipole vortex, while in strong shear, the main part of the plasma cross section is occupied by vortex flow.

Analysis of Poloidal Rotation and Suppression of Plasma Turbulence Using a Dynamo Model

The effect of plasma rotation on suppression of magnetohydrodynamic (MHD) turbulence is studied using a turbulent dynamo model. This dynamo model consists of the equations for the turbulent MHD energy, its dissipation rate, and the cross helicity, which are solved numerically. The temporal evolutions of the turbulent MHD energy and the turbulent cross helicity are compared between states with and without poloidal plasma rotation. It is shown that the turbulent MHD energy is greatly suppressed through the turbulent cross-helicity effect in the presence of poloidal plasma rotation. The cross-helicity production mechanism is also discussed.

Self-consistent description of L-H mode transition dynamics in tokamaks

A theoretical model of L-H transition in a tokamak, based on the self-consistent analysis of plasma transport and taking the suppression of anomalous transport by sheared poloidal plasma rotation and the establishment of a radial electric field due to non-ambipolar particle transport into account, is presented. The transition to an improved confinement regime for a plasma with or without suprathermal electron component is studied. A numerical simulation shows that this model adequately describes the main, experimentally observed features of L-H transition dynamics during electron cyclotron or lower hybrid heating. It is also shown that in certain conditions such a transition can be 'auto-oscillatory', which may be considered as an ELMs feature. The dependence of the localization region of the ELMs and their characteristics (frequency and magnitude) on the plasma parameters are investigated