Multi-Resonance Effect in Type-I ELM Control with Low n Fields on JET

Multiple resonances in the Edge Localized Mode (ELM) frequency (fELM) as a function of the edge safety factor q95 have been observed for the first time with an applied low n (=1,2) field on the JET tokamak. Without an n = 1 field applied, fELM increases slightly from 20 to 30Hz by varying the q95 from 4 to 5 in a type-I ELMy H-mode plasma. However, with an n = 1 field applied, a strong increase in fELM by a factor of 4-5 has been observed with resonant q95 values, while the fELM increased only by a factor of 2 for non-resonant values. A model, which assumes that the ELM width is determined by a localised relaxation triggered by an unstable ideal external peeling mode, can qualitatively predict the observed resonances when low n fields are applied.
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