Gyrokinetic simulations of microinstabilities in stellarator geometry

A computational study of microinstabilities in general geometry is presented. The ion gyrokinetic is solved as an initial value problem. The advantage of this approach is the accurate treatment of some important kinetic effects. The magnetohydrodynamic equilibrium is obtained from a three-dimensional local equilibrium model. The use of a local magnetohydrodynamic equilibrium model allows for a computationally-efficient systematic study of the impact of the magnetic structure on microinstabilities

Metamorphosis of plasma turbulence-shear flow dynamics through a transcritical bifurcation

The structural properties of an economical model for a confined plasma turbulence governor are investigated through bifurcation and stability analyses. A close relationship is demonstrated between the underlying bifurcation framework of the model and typical behavior associated with low- to high-confinement transitions such as shear flow stabilization of turbulence and oscillatory collective action. In particular, the analysis evinces two types of discontinuous transition that are qualitatively distinct. One involves classical hysteresis, governed by viscous dissipation. The other is intrinsically oscillatory and non-hysteretic, and thus provides a model for the so-called dithering transitions that are frequently observed. This metamorphosis, or transformation, of the system dynamics is an important late side-effect of symmetry-breaking, which manifests as an unusual non-symmetric transcritical bifurcation induced by a significant shear flow drive.Comment: 17 pages, revtex text, 9 figures comprised of 16 postscript files. Submitted to Phys. Rev.

Performance Near Operational Boundaries , The JET Team and the ASDEX Upgrade Team

ABSTRACT The performance of ELMy H-mode operation in ASDEX Upgrade and JET is compared. Special attention is paid to variations (usually reductions) in this performance near the operational limits which will need to be approached in a next step device. In JET it is found that input powers substantially above the H-mode threshold power are required to obtain discharges with energy confinement enhancement factors at or above the usual ELMy H-mode scalings. Such a margin (as much as a factor of two in JET) is not observed in ASDEX Upgrade. It is proposed that this difference may be due to the higher edge collisionality in ASDEX and the results are compared to a recent theory based on interchange instabilities and magnetic flutter. In ASDEX Upgrade, the confinement in Type I ELMy discharges degrades as the density is raised due to a stiffness of the temperature profiles which leads to a degradation of the core confinement. This type of stiffness is observed in JET only at relatively high edge densities. In JET, the edge confinement degrades as the density is increased by external gas fuelling, consistent with a constant edge pressure gradient and an edge barrier width which reduces in proportion to the edge ion poloidal Larmor radius. In both machines, H-mode performance is limited at high density by a transition first to the Type III ELM regime and then to L-mode. The confinement penalty, relative to good Type I ELM discharges, of operating with Type III ELMs is about 25-30%. The maximum densities for operation with Type I or Type III ELMs can be substantially increased by increasing the plasma triangularity in both machines

Study of Type III ELMs in JET

This paper presents the results of JET experiments aimed at studying the operational space of plasmas with a Type III ELMy edge, in terms of both local and global plasma parameters. In JET, the Type III ELMy regime has a wide operational space in the pedestal n(e)-T-e diagram, and Type III ELMs are observed in standard ELMy H-modes as well as in plasmas with an internal transport barrier (ITB). The transition from an H-mode with Type III ELMs to a steady state Type I ELMy H-mode requires a minimum loss power, P-TypeI-P-TypeI decreases with increasing plasma triangularity. In the pedestal n(e)-T-e diagram, the critical pedestal temperature for the transition to Type I ELMs is found to be inversely proportional to the pedestal density (T-crit proportional to 1/n) at a low density. In contrast, at a high density, T-crit, does not depend strongly on density. In-the density range where T-crit proportional to 1/n, the critical power required for the transition to Type I ELMs decreases with increasing density. Experimental results are presented suggesting a common mechanism for Type III ELMs at low and high collisionality. A single model for the critical temperature for the transition from Type III to Type I ELMs, based on the resistive interchange instability with magnetic flutter, fits well the density and toroidal field dependence of the JET experimental data. On the other hand, this model fails to describe the variation of the Type III n(e)-T-e operational space with isotopic mass and q(95). Other results are instead suggestive of a different physics for Type III ELMs. At low collisionality, plasma current ramp experiments indicate a role of the edge current in determining the transition from Type III to Type I ELMs, while at high collisionality, a model based on resistive ballooning instability well reproduces, in term of a critical density, the experimentally observed q(95) dependence of the transition from Type I to Type III ELMs. Experimental evidence common to Type III ELMs in standard ELMy H-modes and in plasmas with ITBs indicates that they are driven by the same instability