Rameswar Singh

Physics of non-diffusive turbulent transport of momentum and the origins of spontaneous rotation in tokamaks

Recent results in the theory of turbulent momentum transport and the origins of intrinsic rotation are summarized. Special attention is focused on aspects of momentum transport critical to intrinsic rotation, namely the residual stress and the edge toroidal flow velocity pinch. Novel results include a systematic decomposition of the physical processes which drive intrinsic rotation, a calculation of the critical external torque necessary to hold the plasma stationary against the intrinsic residual stress, a simple model of net velocity scaling which recovers the salient features of the experimental trends and the elucidation of the impact of the particle flux on the net toroidal velocity pinch. Specific suggestions for future experiments are offered.

Intrinsic rotation and electric field shear

A novel mechanism for the generation and amplification of intrinsic rotation at the low-mode to high-mode transition is presented. The mechanism is one where the net parallel flow is accelerated by turbulence. A preferential direction of acceleration results from the breaking of k‖→−k‖ symmetry by sheared E×B flow. It is shown that the equilibrium pressure gradient contributes a piece of the parallel Reynolds stress, which is nonzero for vanishing parallel flow, and so can accelerate the plasma, driving net intrinsic rotation. Rotation drive, transport, and fluctuation dynamics are treated self-consistently.

Influence of the radio frequency ponderomotive force on anomalous impurity transport in tokamaks

Trace impurity transport in tokamaks is studied using an electrostatic, collisionless fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence in the presence of radio frequency (rf) fields, and the results are compared with neoclassical predictions. It is shown that the inward impurity convective velocity (pinch) that is usually obtained can be reduced by the rf fields, in particular close to the wave resonance location where the rf ponderomotive force may be significant. However, the impurity diffusivity and convective velocity are usually similarly affected by the ponderomotive force, and hence the steady-state impurity density peaking factor −∇nz∕nz is only moderately affected by the rf fields.Trace impurity transport in tokamaks is studied using an electrostatic, collisionless fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence in the presence of radio frequency (rf) fields, and the results are compared with neoclassical predictions. It is shown that the inward impurity convective velocity (pinch) that is usually obtained can be reduced by the rf fields, in particular close to the wave resonance location where the rf ponderomotive force may be significant. However, the impurity diffusivity and convective velocity are usually similarly affected by the ponderomotive force, and hence the steady-state impurity density peaking factor −∇nz∕nz is only moderately affected by the rf fields.

Zonal flow generation in collisionless trapped electron mode turbulence

In the present work the generation of zonal flows in collisionless trapped electron mode (TEM) turbulence is studied analytically. A reduced model for TEM turbulence is utilized based on an advanced fluid model for reactive drift waves. An analytical expression for the zonal flow growth rate is derived and compared with the linear TEM growth, and its scaling with plasma parameters is examined for typical tokamak parameter values.

New glance at resistive ballooning modes at the edge of tokamak plasmas

To understand the L?H transition, one has to identify the modes to be stabilized at the edge of L-mode plasmas, roughly from ??=?0.7 to the last closed flux surface. To address this issue, realistic edge tokamak parameters inspired by three different L-modes from DIII-D and Tore Supra have been investigated with a gyrokinetic code GENE (Jenko et al 2000 Phys. Plasmas 7 1904). Former fluid theories for such parameters predict resistive ballooning modes (RBMs) to be unstable (Rogers et al 1998 Phys. Rev. Lett. 81 4396). In this paper, linear gyrokinetic simulations demonstrate that, under realistic L-mode conditions, RBMs are linearly unstable at every edge, i.e. ????0.93. These modes predominantly drift in the electron diamagnetic direction at low wave numbers and are destabilized by higher collisionality. They are further destabilized by higher normalized temperature gradient and higher q. The magnetic shear and the density gradient length have a weaker impact.

Study of electromagnetic fluctuations in high beta plasma of a large linear device

Observation of electromagnetic fluctuations in lower hybrid range of frequencies is reported in a large volume linear plasma device. The instability is observed in the plasma core when a narrow multifilamentary source is used and it is absent when a broad source is used. This instability is observed in high beta plasma and it is characterized by broadband turbulent spectra with central frequency ω=5×104 s−1 and wave number k⊥=0.2 cm−1 and satisfies the condition k⊥ρe≤1, where ρe is the electron Larmor radius. When increasing the axial magnetic field reduces plasma beta, the instability weakens in magnitude and magnetic component is totally suppressed at plasma beta less than 0.5. Several possible explanations are considered and it is indicated that either the pressure gradient modified by energetic electrons or the electron temperature gradient may be responsible for the instability.

Intrinsic toroidal and poloidal flow generation in the background of ion temperature gradient turbulence

The generation of intrinsic toroidal and poloidal flows in the background of ion temperature gradient (ITG) driven microturbulence has been studied. It is shown that the dynamics of mean toroidal and poloidal flows is coupled. The radial fluxes of toroidal and poloidal momentum have been derived. It is shown that the polarization drift driven toroidal momentum flux is independent of mean flow shear and hence complements the shear driven k∥ symmetry breaking mechanism (Gurcan et al 2007 Phys. Plasmas 14 042306) of intrinsic rotation. The radial flux of poloidal momentum due to polarization drift is found to vanish at the steady state. Comparison of residual toroidal and poloidal momentum fluxes, in the absence of seed flows, shows that toroidal flow dominates over poloidal flow.

Feedback control of tearing modes via modulated neutral beams

Tearing modes are suspected to be a principal cause of both minor and major disruptions in a tokamak. Suppression of these modes via a neutral beam which supplies momentum, energy and particles of appropriate phase and amplitude is considered. Model theoretical calculations are presented for the stabilization of drift tearing modes and it is found that for ADITYA [S.B. Bhatt et al., Ind. J. Pure Appl. Phys. 27, 710 (1989)] and other present-day tokamak parameters, stabilization is achievable using modest levels of beam power, current and density.

Comprehensive comparisons of geodesic acoustic mode characteristics and dynamics between Tore Supra experiments and gyrokinetic simulations

In a dedicated collisionality scan in Tore Supra, the geodesic acoustic mode (GAM) is detected and identified with the Doppler backscattering technique. Observations are compared to the results of a simulation with the gyrokinetic code GYSELA. We found that the GAM frequency in experiments is lower than predicted by simulation and theory. Moreover, the disagreement is higher in the low collisionality scenario. Bursts of non harmonic GAM oscillations have been characterized with filtering techniques, such as the Hilbert-Huang transform. When comparing this dynamical behaviour between experiments and simulation, the probability density function of GAM amplitude and the burst autocorrelation time are found to be remarkably similar. In the simulation, where the radial profile of GAM frequency is continuous, we observed a phenomenon of radial phase mixing of the GAM oscillations, which could influence the burst autocorrelation time.

Finite ballooning angle effects on ion temperature gradient driven mode in gyrokinetic flux tube simulations

This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch (k⊥ρi > 1) when growth rates maximized over all ballooning angles are considered. However, the SW branch disappears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremely high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle.