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Research on EAST Reveals How RMP Suppresses ELMs

Sep 26, 2016

Edge localized mode (ELM) is one of the most frequent edge magnetohydrodynamic (MHD) instabilities in high confinement operation modes in tokamaks. It brings periodic transient heat flow to both the divertor and other first wall units, for which it is a great challenge for International Thermonuclear Experimental Reactor (ITER) operation and future fusion reactors’. In recent decades, resonant magnetic perturbation (RMP) was found effective to mitigate or suppress (ELMs). However, its physical mechanics still remains unclear.

In an attempt to achieve further understanding, a study team led by Prof. SUN Youwen from Institute of Plasma Physics, Hefei Institutes of Physical Science of the Chinese Academy of Sciences recently conducted a series of investigations on Experimental Advanced Superconducting Tokamak (EAST) to answer how RMP suppresses ELMs. The results bring better physical understanding on this control method, which makes it promising to expand this control method to the future fusion reactors.

The team, for the first time, suppresses ELMs on EAST in the ITER-like radio frequency (rf) wave dominated heating, low torque injection condition via the recently installed RMP coils.

The linear MHD modeling reveals the total RMPs field strength, including the plasma contribution rather than the vacuum one, and determines the optimized coil configuration for full ELM suppression. The phase of response field gradually deviates the linear MHD result and approaches the vacuum field one during the transition from mitigation to suppression.

This suggests that different harmonics penetrate in turns and the degree of edge topological changes enhances gradually during this stage. It explains also the observed stairlike changes of electron density and ELM frequency during the ramping up of the RMPs current before the final ELM suppression.

The footprint splitting and the sudden increase of particle fluxing to the divertor also support the edge topological change during ELM suppression. An additional sudden increase of edge perpendicular rotation triggers the transition from ELM mitigation to full suppression and it suggests the existence of a threshold in edge topological change for full suppression.

Therefore, not only the formation of a magnetic island near the edge, but also a critical level of magnetic topological change taking into account plasma response, plays a key role in accessing final ELM suppression.

“The research is the fruit of Institute of Plasma Physics’ long-term international and domestic collaboration. It is also a result of group contributions and supports from the whole EAST team.”, said Prof. SUN, “The scientists from Forschungszentrum Juelich GmbH (FZJ) in Germany, the Culham Center for Fusion Energy (CCFE) in UK, Daliang University of Technology (DLUT) and University of Science and Technology of China (USTC) have participated and supported the research.”

The study was published in journal Physical Review Letters with title Nonlinear Transition from Mitigation to Suppression of the Edge Localized Mode with Resonant Magnetic Perturbations in the EAST Tokamak. 

 

With gradually increasing RMP coil current, ELMs in rf wave heating condition were fully suppressed above a certain RMP coil current threshold. (Image by SUN Youwen) 

 

The ELM nonlinear mitigation-to-suppression transition process due to magnetic perturbation reveals the critical role of the change in edge magnetic field topology in fully suppressing ELMs. (Image by SUN Youwen) 

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