Effect of Ripple-Induced Ion Thermal Transport on H-mode Performance

The effects of ripple-induced losses on the performance of ELMy H-mode plasmas have been found in a number of experiments. A noticeable difference in plasma performance, plasma rotation, ELM frequency and amplitude between JET (with ripple amplitude d~0.1%) and JT-60U (with d~1%) was found in otherwise identical discharges. JT-60U plasma generally has somewhat lower edge pressure with more frequent and smaller ELMs. It was previously shown in JET experiments with enhanced ripple that a gradual increase in the ripple amplitude first leads to a modest improvement in plasma confinement, which is followed by the degradation of edge pedestal and further transition to the L-mode regime, if d increases above a certain critical level. Marginally better confinement has been recently reported by the DIII-D Team [3] in experiments where edge transport was enhanced by an externally applied magnetic perturbation. The relative high impact of ripple or stochastisity induced transport on H-mode plasma is attributed to its reliance on the edge transport barrier as the main cause of improved confinement. Since transport in a narrow layer at the edge of H-mode plasma is suppressed to a very low, neo-classical level, the effects of any additional edge transport can be very discernible. This paper presents the results of the comprehensive numerical modelling of the ripple losses of thermal ions in JET and JT-60U magnetic configuration as well as dynamics of ELMy H-mode JET plasma relevant to a JET/JT-60U similarity experiment.
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