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Small edge resonant magnetic perturbations are used to control the pedestal transport and stability in low electron collisionality ( e * ), ITER relevant, poloidally diverted plasmas. The applied perturbations reduce the height of the density pedestal and increase its width while increasing the height of the electron pedestal temperature and its gradient.The effect of the perturbations on the pedestal gradients is controlled by the current in the perturbation coil, the poloidal mode spectrum of the coil, the neutral beam heating power and the divertor deuterium fueling rate. Large pedestal instabilities, referred to as edge as an array of higher frequency electromagnetic waves and turbulence [7]. Outside the plasma, sources such as field-errors from asymmetries in toroidal and poloidal magnetic field coils, magnetic materials, vacuum vessel image and return currents [8], external control coils used to stabilize plasma modes, and correction coils [9] used to minimize perturbations from known field-errors on low integer rational surfaces all contribute to the structure of the magnetic field in which the plasma resides. At the edge of the plasma where the safety factor [ q ( ) defined as the rate of change in toroidal magnetic flux with poloidal magnetic flux ] increases rapidly, all of these perturbations contribute to the creation of closely spaced resonant magnetic islands which may result in the formation of edge stochastic layers. In a poloidally diverted tokamak, the high magnetic shear q across the edge of the plasma results in an increased density of island states and a significantly higher probability of forming open stochastic layers that connect magnetic field lines to plasma facing material surfaces [10]. Thus, the edge plasma is immersed in a dynamically complex magnetic topology over the same region where substantial radial flows of mass and energy, driven by large gradients, compete with strong turbulent transport in highly sheared toroidal and poloidal plasma flow fields. Large radial gradients in the plasma pressure are often established by a strong reduction in turbulent 4 transport due to sheared plasma flow. The large edge gradients are a key factor in the establishment of good confinement levels that make the tokamak the leading candidate for fusion reactors. However, they also lead to the instabilities known as ELMs which drive impulsive energy losses that can be detrimental to plasma facing surfaces. It is easy to understand why the development of tools to control the pedestal transport and stability is a compelling issue for improving the performance and operational safety of high energy density tokamak based fusion confinement systems.A strong motivation for understanding the physics of edge stochastic layers is to enable the development of predictable and reliable tools for controlling key fusion plasma pedestal processes such as: the plasma temperature and pressure at the top of the pedestal, the size and frequency of ELMs, the energy and particle exhaust rate in steady-state cond...

Edge localized mode (ELM) triggering by pellet injection in the DIII-D tokamak has been simulated with the non-linear MHD code JOREK with a view to validating its physics models. JOREK has been subsequently applied to evaluate the requirements for ELM control by pellet injection in ITER. JOREK modelling results for DIII-D show that the key parameter for the triggering of ELMs by pellets is the value of the localized pressure perturbation caused by pellet injection which leads to a threshold minimum pellet size for a given injection velocity, injection geometry and H-mode plasma characteristics. The minimum pellet size for ELM triggering is found to depend on injection geometry with the largest value being required for injection at the outer midplane, intermediate for injection near the X-point and the smallest one for injection at the high-field side. The first results of studies for ELM triggering by pellet injection in ITER 15 MA Q = 10 plasmas with the foreseen injection geometry in ITER are presented.

Discharges which can satisfy the high gain goals of burning plasma experiments have been demonstrated in the DIII-D tokamak under stationary conditions at relatively low plasma current (495 > 4). A figure of merit for fusion gain (P~&9/4&) has been maintained at values corresponding to Q = 10 operation in a burning plasma for >6 s or 36 ZE and 22R. The key element is the relaxation of the current profile to a stationary state with qrnin > 1. In the absence of sawteeth and fishbones, stable operation has been T. C. Luce, et al. STATIONARY HIGH-PERFORMANCE DISCHARGES IN THE DIII-D TOKAMAKissues which need to be addressed for projection to a burning plasma experiment will be discussed, followed by summary and conclusions. 2 GENERAL ATOMICS REPORT GA-A24 146 STATIONARY HIGH-PERFORMANCE DISCHARGES IN THE DIII-D TOKAMAK T. C. Luce, et al.