Nonlinear stabilization of tokamak microturbulence by fast ions.

Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al. [Phys. Rev. Lett. 107, 135004 (2011)], this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.

Global plasma turbulence simulations of q = 3 sawtoothlike events in the RTP tokamak.

A two-fluid computer model of electromagnetic tokamak turbulence, CUTIE, is used to study the dynamic structure and turbulent transport in the Rijnhuizen Tokamak Project tokamak. A discharge with dominant, off-axis electron cyclotron heating is the main focus of the simulations which were extended over several resistive diffusion times. CUTIE reproduces the turbulent transport and MHD phenomena of the experiment. The noninductive components of the current density profile, viz., the dynamo current and the bootstrap current, are identified as key players in the turbulent transport and its suppression and in off-axis MHD events.

Heat pinches in electron-heated Tokamak plasmas: theoretical turbulence models versus experiments.

Two fluid turbulence models, the drift wave based quasilinear 1.5D Weiland model and the electromagnetic global 3D nonlinear model CUTIE, have been used to account for heat pinch evidence in off-axis modulated electron cyclotron heating experiments in the Rijnhuizen Tokamak Project. Both models reproduce the main features indicating inward heat convection in mildly off-axis cases. In far-off-axis cases with hollow electron temperature profiles, the existence of outward convection was reproduced only by CUTIE. Turbulence mechanisms driving heat convection in the two models are discussed.

Toroidal plasma rotation induced by the dynamic ergodic divertor in the TEXTOR tokamak.

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.