Active Control of Type-I Edge Localized Modes with n = 1 and n = 2 fields on JET
Recent experiments on JET have shown that type-I Edge Localized Modes (ELMs) can be controlled by applying static low n = 1 External Magnetic Perturbation Fields (EMPFs) produced by four external Error Field Correction Coils (EFCC) mounted far away from the plasma between the transformer limbs. When an n = 1 EMPF with an amplitude of a few Gauss at the plasma edge (r > 0.95) is applied during the stationary phase of a type-I ELMy H-mode plasma, the ELM frequency rises from ~30Hz up to ~120Hz and follows the applied perturbation field strength. The energy loss per ELM normalised to the total stored energy, DWELM/Wp, decreased from 7 % to below the resolution limit of the diamagnetic measurement (~ 2%). Transport analysis using the TRANSP code shows no or a modest reduction of the thermal energy confinement time because of the density pump-out, but when normalised to the IPB98(y,2) scaling the confinement shows almost no reduction. Stability analysis of mitigated ELMs shows that the operational point moves from intermediate n peeling-ballooning (wide mode) boundary to low-n peeling (narrow mode) boundary with n = 1 perturbation fields. The first results of ELM mitigation with the n = 2 EMPFs on JET demonstrate that the frequency of ELM can be increased by a factor of 3.5, only limited by the available EFCC coil current. During the application of the n = 1, 2 EMPFs, a reduction in the ELM size (DWELM) and ELM peak heat fluxes on the divertor target by roughly the same factor as the increase of the ELM frequency has been observed. The reduction in heat flux is mainly due to the drop of particle flux rather than the change of the electron temperature. Similar plasma braking effect has been observed with n = 1 and n = 2 external fields when a same EFCC coil current was applied. Compensation of the density pump-out effect has been achieved by means of gas fuelling in low triangularity plasmas. An optimised fuelling rate to compensate the density pump-out effect has been identified. Active ELM control by externally applied fields offers an attractive method for next-generation tokamaks, e.g. ITER.