PhD and MPhil Thesis Classes - Université Libre de Bruxelles

1.7 Plasma Operational Limits1.7.2 L-modeIt was recognized early that the ohmic regime was inefficient for achieving the necessarytemperatures. As temperature increased, the resistive heating decreased, and,therefore, auxiliary heating would help to increase the plasma temperature to the levelrequired for significant fusion power production. Auxiliary heating was performed bya variety of techniques including neutral beams and Radio Frequency heating (RF).While the temperature and stored energy increased with this auxiliary heating, the incrementalincrease in stored energy was less than that expected from the ohmic scaling,resulting in a degradation of global confinement. This mode of operation with degradedenergy confinement time, is called L-, or low confinement, mode. Characteristic featuresof L-mode plasma are the low temperatures and temperature gradients near theplasma periphery.1.7.3 H-modeThe high confinement mode (H-mode), associated with a spontaneous formation of anedge transport barrier, was first discovered in ASDEX (8) and has now been seen ona wide variety of magnetic confinement devices under a wide range of conditions. TheH- mode exhibits global energy confinement values about a factor of two better thanL-mode. Part of this is due to formation of the edge transport barrier. Another part ofthis improvement is due to a reduction in local transport throughout the plasma afterthe L-H transition.The H-mode is reached above a certain threshold of power, which depends on plasmaconditions and machine size, see reference (3).1.7.4 Global InstabilitiesA number of large scale MHD phenomena can have an impact on global confinement.Two of these are the periodic sawtooth instability, which can have a significant effecton the profiles of temperature, density and impurities in the central core region, andthe ELMs, which periodically affect plasma edge region. In the following sections wewill briefly review these two instabilities.11