(TH/P8-5) Theoretical analysis and predictive modelling of ELMs mitigation by enhanced toroidal ripple and ergodic magnetic field

V. Parail1), T. Evans2), T. Johnson3), J. Lonnroth4), N. Oyama5), G. Saibene6), R. Sartori6), A. Salmi4), P. de Vries1), M. Becoulet7), G. Corrigan1), J. Hastie1), C. Gimblett1), C.M. Greenfield2), T. Hatae5), D. Howell1), V. Hynonen4), Y. Kamada5), T. Kiviniemi4), T. Kurki-Suonio4), A. Loarte6), E. Nordon7), K. Shinohara5)
1) EURATOM/UKAEA Fusion Association, Culham, United Kingdom of Great Britain and Northern Ireland
2) General Atomics, San Diego, California USA
3) Association EURATOM-VR, Alfven Laboratory, Stockholm, Sweden
4) Association EURATOM-TEKES, Helsinki University of Technology, Finland
5) Japan Atomic Energy Egency, Naka, Japan
6) EFDA CSU-Garching, Germany
7) Association EURATOM-CEA, Cadarache, France

Abstract.  Ripple-induced transport and externally driven stochastic magnetic perturbations near the separatrix are both considered as prospective methods of ELM mitigation in present day tokamaks and ITER. Although these methods rely on completely different physics to generate extra transport, the influence of this transport on plasma dynamics and ELM mitigation is either similar or supplementary. ELM mitigation by ripple-induced transport relies on extra transport of thermal ions provided by the vertical drift of toroidally trapped particles. The parametric dependence of ion losses and their radial distribution as function of main dimensionless plasma parameters and magnetic field geometry is studied using Monte Carlo orbit following code ASCOT. This code also allows the study of the role of ripple losses in the formation of the edge radial electric field. The latter parameter influences L-H transition thresholds and transition from type-III to type-I ELMs. The results of ASCOT analyses were used in the 1.5D core transport code JETTO to predict how ripple-induced transport influence ELM dynamics. These results are compared with recent experimental observations from JET and JT-60U. Unlike ripple, stochastic magnetic field near the plasma edge induces mainly electron transport. This method was successfully used in DIII-D tokamak, where it was shown that externally driven Resonant Magnetic Perturbation (RMP) fully suppresses ELMs in low collisionality plasma. Two unconventional results of recent experiments are discussed in this report. The first relates to experimentally observed trend that the RMP increases particle transport more that electron thermal conductivity. The opposite result is expected from theoretical considerations. Secondly, experiment reveals that an observed level of stochastic transport is two orders of magnitude smaller than that given by the field tracing code. Possible explanations to both observations are presented. Relatively stronger manifestation of enhanced particle diffusion might be explained as a compensation of the opposite trend, observed in a conventional ELMy H-mode (where suppressed electron transport within ETB leads to uncontrolled density rise). A much lower level of stochastic transport might be a result of magnetic field screening by rotating plasma. Work conducted under EFDA and partly supported by Euratom and the UK EPSRC.

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