Multiresonance effect in type-I edge-localized mode 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 30 Hz 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 nonresonant values. A model, which assumes that the ELM width is determined by a localized relaxation triggered by an unstable ideal external peeling mode, can qualitatively predict the observed resonances when low n fields are applied.