Resonant Pedestal Pressure Reduction Induced by a Thermal Transport Enhancement due to Stochastic Magnetic Boundary Layers in High Temperature Plasmas

Good alignment of the magnetic field line pitch angle with the mode structure of an external resonant magnetic perturbation (RMP) field is shown to induce modulation of the pedestal electron pressure $p_e$ in high confinement high rotation plasmas at the DIII-D tokamak with a shape similar to ITER, the next step tokamak experiment. This is caused by an edge safety factor $q_{95}$ resonant enhancement of the thermal transport, while in contrast, the RMP induced particle pump out does not show a significant resonance. The measured $p_e$ reduction correlates to an increase in the modeled stochastic layer width during pitch angle variations matching results from resistive low rotation plasmas at the TEXTOR tokamak. These findings suggest a field line pitch angle resonant formation of a stochastic magnetic edge layer as an explanation for the $q_{95}$ resonant character of type-I edge localized mode suppression by RMPs.

Figure 1

Pedestal time traces in DIII-D discharge #132741 with $q_{95}$ ramp-down (from top to bottom): $q_{95}(t)$, $T_e(t)$, and $n_e(t)$ and the resulting $p_e(t)$ at ${\Psi }_N=0.8$ including the $D_{\alpha }$-recycling emission light at the inner divertor strike line. ELM suppressed periods are marked with grey shaded boxes and the RMP period is indicated on top. The square markers show $n_e$, $T_e$, and $p_e$ values for a second discharge at fixed $q_{95}=3.5$. The error bars shown are the fitting and statistical measurement uncertainties.

Figure 2

Comparison of $T_e(t)$ at ${\Psi }_N=0.7$ obtained from electron cyclotron (red dots) and Thomson scattering measurement (blue dots) during $q_{95}$ ramp-down. The square markers show the $T_e$ values with and without RMP for stationary $q_{95}$.

Figure 3

Modulation of electron pressure $p_e$ during ramp-down of the safety factor (a) at DIII-D and (b) in TEXTOR.