Observations of Multi-Resonance Effect in ELM Control with Magnetic Perturbation Fields on the JET Tokamak

Multiple resonances of the ELM frequency caused by application of low n (= 1 or 2) magnetic perturbations for Edge Localized Mode (ELM) control has been observed on JET. With a low n field applied, a strong increase in ELM frequency, fELM , by a factor of ∼ 4.5 was found in many separated narrow windows of q95 (resonant q95), while the fELM increased only by a factor of ∼ 2 for non-resonant q95 values. Both the global effect (no q95 dependence) and the multi-resonance effect (strong q95 dependence) depend on the amplitude of the perturbation field. The fractions of increase in fELM with different resonant q95 values are not the same. An analysis of ideal external peeling modes shows that both the dominant unstable peeling mode number and fELM depend on the amplitude of the normalized edge current density as well as the edge safety factor, qa.