Effect Of Toroidal Plasma Rotation Research Articles

Effect of toroidal plasma rotation on double tearing modes in cylindrical geometry

The effect of toroidal plasma rotation on q = 3 double tearing modes (DTMs) was studied numerically in cylindrical geometry using the method of reduced magnetohydrodynamic simulation. The results indicate that toroidal plasma rotation can reduce the growth rate of DTMs, but the magnitude of toroidal velocity has weak effect, especially without shear. When the shear of toroidal velocity exists, the suppression effect becomes better. Whether the velocity flow has shear or not, the growth rate of DTMs decreases as the magnitude of toroidal velocity increases. With the increase of velocity shear, the DTMs grow slowly. And the suppression effect of toroidal plasma rotation in early growth and transition stage is better, which means that the toroidal plasma rotation can suppress the linear growth of islands. Furthermore, the toroidal plasma rotation can suppress the evolution of poloidal stream. And the toroidal velocity shear on the q = 3 rational surface is more dominant than the magnitude of toroidal velocity in determining the DTM characteristics.

Local measurements of screening currents driven by applied RMPs on TEXTOR

In order to understand the mechanism by which resonant magnetic perturbations (RMPs) mitigate or suppress edge-localized modes, it is necessary to understand the plasma response to the application of RMPs. TEXTOR's fast movable Mirnov probe can provide direct measurements of the plasma response to RMPs applied using the dynamic ergodic divertor. The effect of toroidal plasma rotation is investigated, and a change in the phase of the plasma response at certain values of rotation is found. Jumps in the phase of the magnetic field are found to occur on resonant surfaces, indicating the formation of screening currents on these surfaces. The first observations of screening currents on multiple surfaces are presented, and the transition from screening to field penetration with increasing strength of the applied RMP field is observed.

The effect of toroidal plasma rotation on low-frequency reversed shear Alfvén eigenmodes in tokamaks

The influence of toroidal plasma rotation on the existence of reversed shear Alfvén eigenmodes (RSAEs) near their minimum frequency is investigated analytically. An existence condition is derived showing that a radially decreasing kinetic energy density is unfavourable for the existence of RSAEs. The Coriolis effect is typically unfavourable for modes moving slower than the plasma or moving in the opposite direction. The generality of the analysis allows for the description of downwards sweeping RSAEs and also rotation-induced modes in regular shear plasmas.

The effect of plasma profile variation on the stability of the n = 1 internal kink mode in rotating tokamak plasmas

The sensitivity of the stability of the ideal n = 1 internal kink mode to variations in the plasma profiles is analysed both analytically and numerically in rotating tokamak plasmas. These stability analyses have been carried out including the centrifugal effects of toroidal plasma rotation upon the equilibrium, and also inconsistently when the equilibrium is treated as static. The change in plasma stability due to rotation is partially (consistent equilibrium) or wholly (inconsistent treatment) determined by the radial profiles of the plasma density and rotation velocity. It is found that the internal kink mode stability is strongly influenced by small variations in these plasma profiles. The implications of this extreme sensitivity are discussed, with particular reference to experimental data from MAST.

Importance of centrifugal effects for the internal kink mode stability in toroidally rotating tokamak plasmas

Analytical theory and two different magnetohydrodynamical stability codes are used in a study of the effects of toroidal plasma rotation on the stability of the ideal, internal kink mode in tokamak ...

Low-frequency magnetohydrodynamics and geodesic acoustic modes in toroidally rotating tokamak plasmas

This paper analyses low-frequency magnetohydrodynamic (MHD) modes, especially the geodesic acoustic modes (GAMs), in toroidal plasmas with large aspect ratio and circular cross section, including the effects of toroidal plasma rotation. A system of equations describing MHD modes with frequency of the order of the sound frequency in such plasmas is derived from the Frieman–Rotenberg equation, using a technique where the plasma perturbation \U0001d6cf and the perturbed magnetic field Q are expanded separately in the inverse aspect ratio ε = r/R, where r and R denote the minor and major radii of the plasma torus, respectively. The large-scale, ideal MHD properties of the GAM induced by toroidal rotation (Wahlberg 2008 Phys. Rev. Lett. 101 115003) are thereafter analysed in more detail employing this system of equations. It is shown that both the axisymmetric GAMs existing in rotating plasmas are localized on a specific magnetic surface only to leading order in ε, and that a ‘halo’ consisting of finite components of both \U0001d6cf and Q with dominant poloidal mode numbers m = ±2 appears outside this magnetic surface to higher orders in ε.

The effect of toroidal plasma rotation on sawteeth in MAST

Results from MAST concerning the effect of toroidal rotation driven by neutral beam injection (NBI) on sawteeth are presented. The sawtooth period is shown to increase as the co-NBI power, and thus the toroidal plasma rotation, is increased. As the counter-NBI power is increased, the sawtooth period decreases to a minimum that is shorter than in Ohmically heated plasmas, before lengthening at high toroidal flows. The sawtooth period reaches a minimum when the sawtooth precursor changes the direction of rotation. This can be interpreted as the counter-NBI inducing a toroidal flow which balances the intrinsic ion diamagnetic rotation of the plasma, at which point the mode precursor changes direction. Magnetohydrodynamic stability analyses of the ideal n = 1 internal kink mode with respect to toroidal rotation at finite ion diamagnetic frequency show good accordance with the experimental results. When MAST discharges are modelled at the experimental ion diamagnetic frequency, , the ideal n = 1 internal kink mode is stabilized completely by co-rotation approaching the ion sound speed. When the rotation is oriented in the counter- direction, the mode is initially driven more unstable, before being stabilized by high toroidal flows. Experimentally, the radial extent of the sawtooth crash, and hence the q = 1 surface, varies with respect to the sawtooth period. Stability analyses show that the marginally stable radial location of the q = 1 surface reaches a minimum at approximately the same counter-toroidal rotation as that which minimizes the sawtooth period experimentally. The magnitude of the toroidal rotation at the q = 1 surface determines the stabilizing effect of the toroidal flow upon the ideal n = 1 internal kink mode.

Modeling the effect of toroidal plasma rotation on drift-magnetohydrodynamic modes in tokamaks

A new code, MISHKA-F (Flow), has been developed as an extension of the ideal magneto-hydrodynamic (MHD) code MISHKA-1 [Mikhailovskii et al., Plasma Phys. Rep. 23, 844 (1997)] in order to investigate the linear MHD stability of ideal and resistive eigenmodes with respect to the effects of toroidal rotation in tokamaks in general toroidal geometry with the ion diamagnetic drift effect taken into account. Benchmark test results of the MISHKA-F code show good agreement with analytic theory [A. B. Mikhailovskii and S. E. Sharapov, Plasma Phys. Controlled Fusion 42, 57 (2000)] for the stability limits of the ideal n∕m=1∕1 internal kink mode. The combined stabilizing effects of the ion diamagnetic drift frequency, ω*i, and the toroidal flow shear are also studied. The ω*i stabilization of the internal kink mode is found to be more effective at finite flow shear. Finite-n ballooning modes are studied in plasmas with the toroidal flow shear effect included. The stabilization of the ballooning modes by toroidal rotation is found to agree well with earlier predictions [Webster et al., Phys. Plasmas 11, 2135 (2004)]. The effect of high flow shear is analyzed for a sawtoothing discharge typical in the Mega Ampère Spherical Tokamak (MAST) [Sykes et al., Nucl. Fusion 41, 1423 (2001)]. It is found that the ideal n=1 internal kink mode can be stabilized by toroidal rotation at values observed experimentally.

Stability of resistive wall modes in reversed field pinches with longitudinal flow and dissipative effects

The stability of the nonresonant external kink modes in a reversed field pinch (RFP) under resistive wall boundary conditions is investigated. The effects of toroidal plasma rotation and parallel viscosity, as well as the finite β are included in the analysis. The influence of the equilibrium parameters on the mode instability is carefully studied by employing the conventional α–Θ0 equilibrium models. It is found that the behavior of the resistive wall mode (RWM) is rather sensitive to the reversal parameter F of RFPs. The effects of plasma rotation together with the viscosity and/or ion sound wave continuous spectrum damping can stabilize the RWM for certain positions of the wall relative to the plasma for both the low β (or zero β) and high β plasmas. For typical RFP operating parameters, the rotation velocity required to stabilize the modes is in the range between the ion sound wave and Alfvén wave velocities. This is larger than the natural rotation velocity in current RFPs, hence based on the above linear theory active feedback is needed for a long pulse RFP. A further nonlinear study is necessary.

Resistive wall mode stabilization by plasma rotation in advanced tokamaks

By combining previous results of Betti and Freidberg [Phys. Rev. Lett. 74, 2949 (1995)] and Eriksson [Phys. Plasmas 2, 3095 (1995)], a fully analytical description is obtained for the stabilizing effect of toroidal plasma rotation in a large aspect ratio tokamak surrounded by a resistive wall. As in advanced tokamak configurations with a large fraction of bootstrap current, it is assumed that the current gradient near the plasma edge is large. This assumption enables an analytical analysis of external kink modes with low poloidal mode numbers. An expression is obtained, showing explicitly how the window of stable wall distances depends on the current profile.