Pedestal Bifurcation and Resonant Field Penetration at the Threshold of Edge-Localized Mode Suppression in the DIII-D Tokamak

Rapid bifurcations in the plasma response to slowly varying $n=2$ magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of $n=2$ fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied $n=2$ field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.

Figure 1

Pedestal bifurcations with slowly varying resonant fields. (a) Upper and lower row I-coil relative phase. (b) ${\mathrm{D}}_{\alpha }$ light near the outer strike point (red) and pedestal density $n_{e,\text{ped}}$ (black). (c) Pedestal electron temperature $T_{e,\text{ped}}$. (d) Edge impurity velocity in the $\mathrm{co}\text{−}{I}_P$ direction.

Figure 2

(a) Contours of $T_e$ vs ${\psi }_n$ and ${\mathrm{D}}_{\alpha }$ signal (black); contours of $B_{\text{pol}}$ (gauss) vs time and toroidal angle for (b) $n=1$ inner wall, (c) $n=1$ outer wall, (d) $n=2$ inner wall, (e) $n=2$ outer wall.

Figure 3

(a) $T_e$, (b) $n_e$ vs ${\psi }_n$ averaged over 50 ms before ELM suppression (red) and during ELM suppression (blue), (c) the $E\times B$ frequency, (d) the perpendicular electron flow frequency, (e) the $n=2$ resonant magnetic field strength on edge rational surfaces as calculated by M3D-C1. The circles in (a) and (b) denote the location at the top of the pedestal.

Figure 4

Spontaneous bifurcations into and out of ELM suppression with static $n=2$ fields. (a) ${\mathrm{D}}_{\alpha }$ at the outer strike point, (b) density fluctuation power spectra measured using BES at ${\psi }_n\approx 0.94$, (c) impurity toroidal rotation at the plasma edge, (d) $n=2$ poloidal field on inner wall. Blue vertical lines indicate intervals of ELM suppression, and red vertical lines indicate the ELMing phase.

Figure 5

During the back transition to conditions favorable to ELMs. (a) Edge impurity rotation (red) and ${\mathrm{D}}_{\alpha }$ at outer strike point (black), (b) spectrogram of DBS signal, (c) contours of $B_{\text{pol}}$ (G) vs time and toroidal angle on inner wall, (d) zoom-in view of contours of the AC coupled $B_{\text{pol}}$ (G) on the inner wall.