Characterization of Toroidal Mode Number of Edge Localized Modes in JET plasmas. Comparison between Spontaneous and Controlled ELMs

The foreseen operating scenario for ITER is the type-I ELMy H-mode. Nevertheless, the energy load onto plasma facing components, caused by type-I ELMs is too high, calling for methods for controlling them, while maintaining adequate confinement. Several control mechanisms have been proposed and recently explored on JET, including ELM mitigation by resonant magnetic perturbations using the Error Field Correction Coils (EFCCs), ELM pacing with shallow pellet injection [3], ELM magnetic pacing by fast movements of the plasma column with imposed radial electric fields ('vertical kicks'). In this work, the magnetic perturbation spectra of ELMs during EFCCs control experiments on JET are compared with those of spontaneous type-I ELMs. The toroidal mode numbers of ELMs is extracted from the phase of the magnetic perturbations measured by edge Mirnov coils, on the low field side, toroidally separated by 10 degrees (maximum n = 11). The method, based on the combination of wavelet functions and a two-point correlation analysis, allows the reconstruction of the power spectral density P(n,f) over short time windows before and during the ELM crash. For each ELM, P(n,f) has been computed over time steps of 100ms during a 10ms window centred at the time of the crash. The n grid is constructed over unitary steps, Dn = 1, in such a way that all wavenumber values (phase shift divided by coil separation) that fall between n ± 0.5 are associated to n.
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