ELM triggering conditions for the integrated modeling of H-mode plasmas

Recent advances in the integrated modeling of ELMy H-mode plasmas are presented. A model for the H-mode pedestal and for the triggering of ELMs predicts the height, width, and shape of the H-mode pedestal and the frequency and width of ELMs. Formation of the pedestal and the L-H transition is the direct result of ExB flow shear suppression of anomalous transport. The periodic ELM crashes are triggered by either the ballooning or peeling MHD instabilities. The BALOO, DCON, and ELITE ideal MHD stability codes are used to derive a new parametric expression for the peeling-ballooning threshold. The new dependence for the peeling-ballooning threshold is implemented in the ASTRA transport code. Results of integrated modeling of DIII-D like discharges are presented and compared with experimental observations. The results from the ideal MHD stability codes are compared with results from the resistive MHD stability code NIMROD.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Microtearing modes in tokamak discharges

Microtearing modes (MTMs) have been identified as a source of significant electron thermaltransport in tokamak discharges. In order to describe the evolution of these discharges, it isnecessary to improve the prediction of electron thermal transport. This can be accomplished byutilizing a model for transport driven by MTMs in whole device predictive modeling codes. Theobjective of this paper is to develop the dispersion relation that governs the MTM driven transport.A unified fluid/kinetic approach is used in the development of a nonlinear dispersion relation forMTMs. The derivation includes the effects of electrostatic and magnetic fluctuations, arbitraryelectron-ion collisionality, electron temperature and density gradients, magnetic curvature, and theeffects associated with the parallel propagation vector. An iterative nonlinear approach is used tocalculate the distribution function employed in obtaining the nonlinear parallel current and the nonlineardispersion relation. The third order nonlinear effects in magnetic fluctuations are included,and the influence of third order effects on a multi-wave system is considered. An envelope equationfor the nonlinear microtearing modes in the collision dominant limit is introduced in order to obtainthe saturation level. In the limit that the mode amplitude does not vary along the field line, slab geometry,and strong collisionality, the fluid dispersion relation for nonlinear microtearing modes isfound to agree with the kinetic dispersion relation. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4953609]I. INTRODUCTIONMicro-instabilities can result in turbulence that influencesenergy confinement in tokamak discharges. One suchmicro-instability is the microtearing mode (MTM), atearing-parity mode centered on high-order rational surfaces.Microtearing instability can provide a significant contributionto the electron thermal transport in low-aspect ratiotokamaks.1–5 The MTMs lead to a tearing and subsequentreconnection of the magnetic field. MTMs are shortwavelengthion scale (low kh) electromagnetic instabilitiesthat are driven by electron temperature gradients.6–8 It wasproposed that when the magnetic field has a component inthe same direction as the electron temperature gradient, acurrent is driven in the direction of the magnetic field line,which can destabilize MTMs. These modes propagate in theelectron diamagnetic drift direction and depend on the electronion collisionality.9,10 Consequently, transport driven byMTM instabilities depends on both the electron ion collisionfrequency and the electron temperature gradient. Theresearch carried out in this paper indicates that when theelectrostatic effects are included, MTMs also depend on thedensity gradient

Experimental tests of paleoclassical transport

Abstract Predictions of the recently developed paleoclassical transport model are compared with data from many toroidal plasma experiments: electron heat diffusivity in DIII-D, C-Mod and NSTX ohmic and near-ohmic plasmas; transport modelling of DIII-D ohmic-level discharges and of the RTP ECH 'stair-step' experiments with electron internal transport barriers (eITBs) at low order rational surfaces; investigation of a strong eITB in JT-60U; H-mode T e edge pedestal properties in DIII-D; and electron heat diffusivities in non-tokamak experiments (NSTX/ST, MST/RFP, SSPX/spheromak). The radial electron heat transport predicted by the paleoclassical model is found to be in reasonable agreement with a wide variety of ohmic-level experimental results and to set the lower limit (within a factor 2 in tokamaks) on the radial electron heat transport in most resistive, current-carrying toroidal plasmas-for T e T crit e B 2/3ā1/2 keV where it is expected to be dominant over fluctuation-induced anomalous transport that scales with a gyro-Bohm diffusion coefficient

Estimation of the radial size and density fluctuation amplitude of edge localized modes using microwave interferometer array

A novel technique to estimate the range of radial size and density fluctuation amplitude of edge localized modes (ELMs) in the KSTAR tokamak plasma is presented. A microwave imaging reflectometry (MIR) system is reconfigured as a multi-channel microwave interferometer array (MIA) to measure the density fluctuations associated with ELMs, while electron cyclotron emission imaging (ECEI) system is used as a reference diagnostics to confirm the MIA observation. Two dimensional full-wave (FWR2D) simulations integrated with optics simulation are performed to investigate the Gaussian beam propagation and reflection through the plasma as well as the MIA optical components and obtain the interferometric phase undulations of individual channels at the detector plane due to ELM perturbation. The simulation results show that the amplitude of the phase undulation depends linearly on both radial size and density perturbation amplitude of ELM. For a typical discharge with ELMs, it is estimated that the ELM structure observed by the MIA system has density perturbation amplitude in the range similar to 7 % to 14 % while radial size in the range similar to 1 to 3 cmclos