Search

In ASDEX Upgrade hybrid discharges, it is found that an externally applied n = 1 field preferentially distorts the plasma in the core, leading to significant flow damping there and elsewhere across the plasma radius. MARS-F/Q modeling of a neoclassical toroidal viscous NTV) torque that results from an amplified internal kink-type displacement in the plasma core is qualitatively consistent with the measured internal displacements, beta dependence, and rotation damping. Sensitivity studies indicate that the internal kink response and the resulting core flow damping critically depend on the plasma equilibrium pressure, the initial flow speed, the coil phasing and the proximity of q0 to 1. No appreciable flow damping is found for a βN plasma. A relatively slower initial toroidal flow results in a stronger core flow damping, due to the enhanced NTV torque. Weaker flow damping is achieved as q0 is assumed to be farther away from 1. Finally, a systematic coil phasing scan finds the strongest (weakest) flow damping occurring at the coil phasing of approximately 20 (200) degrees, quantitatively agreeing with experiments. This study points to the important role played by the internal kink response in plasma core flow damping in high-beta hybrid scenario plasmas such as that foreseen for ITER.

The influence of energetic particles (EPs) on the ideal internal kink mode, in rotating tokamak plasmas, is numerically investigated by simultaneously solving MHD-kinetic hybrid equations together with a toroidal momentum balance equation utilizing the MARS-Q code (Liu et al 2013 Phys. Plasmas 20 042503). The neoclassical toroidal viscous (NTV) torque, induced by precessional drift resonances of trapped energetic particles, acts as the momentum sink term to damp the plasma flow. Quasi-linear initial value simulations show local reduction of the flow amplitude and enhancement of the flow shear near the q = 1 rational surface (q is the safety factor) due to EP induced NTV. Both effects in turn destabilize the internal kink mode. These numerical findings are robust against the initial linear stability of internal kink, the initial plasma flow profile, as well as the equilibrium distribution model for EPs.

A new damping model on the resistive wall mode (RWM) instability is studied, via the turbulence induced plasma viscosity (in short, turbulent viscosity). \bl{In a cylindrical plasma}, the synergistic effect on suppressing the RWM is investigated between this new damping mechanism and the plasma flow. An eigenmode formulation is derived based on magneto-hydrodynamic (MHD) theory, where the momentum equation is extended by including the turbulent viscosity term with a proportionality coefficient $\chi$. Numerical results show that, in the absence of plasma flow, increasing $\chi$ decreases the RWM growth rate but does not fully stabilize the mode. However, in the presence of sufficiently fast plasma flow, turbulent viscosity can lead to full suppression of the RWM, when $\chi$ exceeds a critical value. Similarly, at a given $\chi$ value, the plasma flow can fully suppress the mode when the flow velocity exceeds a threshold value. In particular, turbulent viscosity significantly reduces the threshold value of the flow velocity required for full stabilization of the RWM.

Non-linear interaction between the edge localized infernal mode (ELIM) and the plasma toroidal flow is numerically investigated, by solving the initial value problem for the n = 1 ELIM, where n is the toroidal mode number. The linear results show that the ELIM instability is strongly affected by a close-fitting resistive wall. The presence of a resistive wall can fully stabilize an otherwise flow-shear destabilized ELIM. The computed toroidal torques based on linear ELIM eigenfunctions show different radial distributions: neoclassical toroidal viscous torque has a global distribution, whilst the torques associated with the Maxwell and Reynolds stresses largely peak near rational surfaces. The non-linear interplay between the unstable ELIM and the plasma flow tends to push the mode towards a more stable domain, no matter where the instability starts. This may help to understand some of the underlying physics for the experimentally observed edge harmonic oscillations in tokamak H-mode plasmas.

Effects of toroidal plasma flow, magnetic drift kinetic damping as well as feedback control, on the resistive wall mode instability in HL-2M tokamak are numerically investigated, using the linear stability codes MARS-F/K (Liu et al 2000 Phys. Plasmas 7 3681, Liu et al 2008 Phys. Plasmas 15 112503). It is found that the precession drift resonance damping due to trapped thermal particles ensures a robust passive stabilization of the n = 1 (n is the toroidal mode number) RWM in the 2 MA double-null advanced plasma scenario designed for HL-2M, provided that the toroidal flow speed is not too fast: . With two rows of magnetic control coils designed for HL-2M, the optimal poloidal location for the RWM stabilization is found to be . Toroidal modeling also shows that the plasma flow damping, drift kinetic damping and magnetic feedback can be arranged to synergistically stabilize the RWM in HL-2M, by tuning the feedback gain phase and/or including derivative actions in the control loop. The numerical results obtained by MARS-F/K are qualitatively well re-produced by an analytic single-pole model.