First Observation of Edge Localized Modes Mitigation with Resonant and Nonresonant Magnetic Perturbations in ASDEX Upgrade

First experiments with nonaxisymmetric magnetic perturbations, toroidal mode number $n=2$, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I edge localized modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density, and lower tungsten impurity concentration.

Figure 1

Poloidal cross section of ASDEX Upgrade showing the locations of the Bu and Bl coils, the plasma cross section for the present experiments, interferometer chords, the location of the pedestal top ECE measurement, and divertor thermography views.

Figure 2

Toroidal arrangement of the $B$ coils in operation (black) and those to be yet installed (gray); normal field direction in even and odd parity configurations.

Figure 3

Time traces of discharge 26 081 with $B$ coils operating in odd parity (resonant configuration). Time intervals marked “1” and “2” show a reduction of type-I ELM frequency and full suppression of type-I ELMs, respectively.

Figure 4

Time traces for an unmitigated ELMy phase ($B$ coils off).

Figure 5

Time traces for a phase with mitigated ELMs ($B$ coils operating with odd parity). The inset shows small transport events as seen in pedestal top electron temperature and outer divertor power.

Figure 6

Measurement of coil current threshold for ELM mitigation with odd (left) and even (right) parity.

Figure 7

Poloidal mode number ($m$) spectra (taken for $n=2$ component at the $q=5$ surface) for discharges 26 196 (odd parity) and 26 201 (even parity).