Formation of a High Pressure Staircase Pedestal with Suppressed Edge Localized Modes in the DIII-D Tokamak

We observe the formation of a high-pressure staircase pedestal ($\approx 16–20\mathrm{kPa}$) in the DIII-D tokamak when large amplitude edge localized modes are suppressed using resonant magnetic perturbations. The staircase pedestal is characterized by a flattening of the density and temperature profiles in midpedestal creating a two-step staircase pedestal structure correlated with the appearance of midpedestal broadband fluctuations. The pedestal oscillates between the staircase and single-step structure every 40–60 ms, correlated with oscillations in the heat and particle flux to the divertor. Gyrokinetic analysis using the cgyro code shows that when the heat and particle flux to the divertor decreases, the pedestal broadens and the $E\times B$ shear at the midpedestal decreases, triggering a transport bifurcation from the kinetic ballooning mode (KBM) to trapped electron mode (TEM) limited transport that flattens the density and temperature profiles at midpedestal and results in the formation of the staircase pedestal. As the heat flux to the divertor increases, the pedestal narrows and the $E\times B$ shear at the midpedestal increases, triggering a back transition from TEM to KBM limited transport. The pedestal pressure increases during the staircase phase, indicating that enhanced midpedestal turbulence can be beneficial for confinement.

Figure 1

(a) Balmer signal $D_{\alpha }$ from inner strike point and I-coil current ($I_C$), (b) BES density fluctuations at $\rho \approx 0.85$, (c) inner strike point heat flux ${\tilde{P}}_{\mathrm{ISP}}(\text{red})$ measured by IR camera and 2 ms average of heat flux in blue (d) pedestal top temperature from ECE (e) ion saturation current density at the inner strike point. Plasma parameters are $q_{95}\approx 5.3$, $R\approx 1.68\mathrm{m}$, $a\approx 0.6\mathrm{m}$, $B_T\approx -1.90\mathrm{T}$, $Ip\approx 1.15\mathrm{MA}$, $P_{\mathrm{NBI}}\approx 10\mathrm{MW}$, $P_{\mathrm{EC}}\approx 3.4\mathrm{MW}$, and ITER shape $\delta =0.55$.

Figure 2

Power spectrum of density fluctuation measured by BES at (a) pedestal top and (b) midpedestal. (c) Heat flux ${\tilde{P}}_{\mathrm{ISP}}$ at the inner strike point. (d)–(g), respectively, electron temperature, electron density, ion temperature, and $E\times B$ toroidal rotation frequency (${\omega }_E$).

Figure 3

Single-step and intermediate pedestal profiles and nonlinear flux calculations. (a) Electron density for single step (red) staircase blue and intermediate profile (dashed) (b) $E\times B$ shearing rate for the single-step (red), staircase (blue), the intermediate (dashed) (c) Nonlinear fluxes from cgyro of particle, electron energy and ion energy (${\mathrm{\Gamma }}_e$, $Q_e$, $Q_i$) in their respective gyro-Bohm units (gB), for the single-step pedestal (red) and the intermediate pedestal (black).

Figure 4

Local particle flux vs $a/L_n$, squares, and circle are the nonlinear fluxes, and dashed lines are from quasilinear calculation.