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 the application of static low n = 1 external magnetic perturbation fields produced by four external error field correction coils (EFCC) mounted far away from the plasma between the transformer limbs. When an n = 1 field with an amplitude of a few mT at the plasma edge (the normalized poloidal flux, Ψ, is larger than 0.95) is applied during the stationary phase of a type-I ELMy H-mode plasma, the ELM frequency rises from ∼30 Hz up to ∼120 Hz. The energy loss per ELM normalized to the total stored energy, ΔWELM/W, decreased from 7% to values below the resolution limit of the diamagnetic measurement (<2%). Transport analysis using the TRANSP code shows up to 20% reduction in the thermal energy confinement time because of density pump-out, but when normalized to the IPB98(y, 2) scaling the confinement time shows almost no reduction. Stability analysis of controlled ELMs suggests that the operational point with n = 1 perturbation field moves from the intermediate-n peeling–ballooning boundary to the low-n peeling boundary, and the radial width of the most unstable mode reduced from ∼3% down to ∼1% of the normalized minor radius. The first results of ELM control with n = 2 fields on JET demonstrate that the frequency of ELMs can be increased by a factor of 3.5 with the present capability of the EFCC power supply. During the application of the n = 1, 2 fields, a reduction in the absolute ELM size (ΔWELM) and ELM peak heat fluxes on the divertor target by roughly the same factor as the increase in the ELM frequency has been observed. The reduction in heat flux is mainly due to the drop in particle flux rather than a change in the electron temperature. Similar plasma braking effects have been observed with n = 1 and n = 2 external fields when the 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 optimized fuelling rate to compensate the density pump-out effect has been identified. When the n = 1 field is applied in plasmas with reduced toroidal rotation and density due to increased TF ripple of 8%, both the magnitude of the toroidal braking and density pump-out are found to become smaller; however, the increase in the ELM frequency with the n = 1 field is still observed. Active ELM control by externally applied fields may offer an attractive method for next-generation tokamaks, e.g. ITER.