Empirical impact of near-separatrix plasma and neutral transport on the pedestal in the transition between EDA and ELMy H-modes on Alcator C-Mod

Abstract

The transition between the ELMy H-mode and the EDA H-mode is studied on Alcator C-Mod using an experimental database and predictive pedestal models. High-resolution Thomson scattering measurements are used to compare the pedestal density, $n_{e}^\mathrm{ped}$, and the separatrix density, $n_{e}^\mathrm{sep}$ with main chamber neutral measurements. $n_{e}^\mathrm{ped}$ is sensitive to neutral sources only in the ELMy H-mode regime and not in the EDA H-mode regime. Density fluctuation spectra reveal that quasi-coherent structures become stronger at higher densities and more coherent in the EDA relative to the inter-ELM phases of ELMy H-modes, before weakening again at the highest values of $n_{e}^\mathrm{ped}$. The Saarelma-Connor pedestal density prediction model is validated for ELMy H-modes up to $n_{e}^\mathrm{ped} = 2.0 \times 10^{20}$ m$^{-3}$. An additional transport channel driven by resistive ballooning modes (RBM), $D_\mathrm{RBM}$, scaling directly with $α_{t}$ and inversely with $k_\mathrm{RBM}^{2}\hat{q}_\mathrm{cyl}$ is shown to improve the prediction for EDA H-modes, finding good model agreement up to $n_{e}^\mathrm{ped} = 3.0 \times 10^{20}$ m$^{-3}$. EPED scans in $n_{e}^\mathrm{ped}$ are then performed at three values of $n_{e}^\mathrm{sep}/n_{e}^\mathrm{ped}$. Increasing this ratio moves the peeling-ballooning branch transition to lower $n_{e}^\mathrm{ped}$, increasing $p^\mathrm{ped}$ in the peeling branch and decreasing it in the ballooning branch. Agreement is found for large ELM H-modes. SPARC pedestal density predictions for an ELMy and an EDA/QCE-like H-mode are performed and found consistent with assumptions used in previous EPED modeling. Inclusion of $D_\mathrm{RBM}$ significantly weakens the density gradient near the separatrix, lowering $n_{e}^\mathrm{ped}$ by approximately 20%.

Figures (19)

• Figure 1: Operational space in terms of $T_{e}$ and $n_{e}$ at the separatrix (left) and pedestal top (right), showing discharges with small ELMs (light red squares), large ELMs (dark red squares), mixed EDA and ELMy (purple circles), and only EDA (blue diamonds). At left, dash-dotted black and purple lines show $\alpha_{t}$ and $\beta_{e}$ contours at the separatrix from miller_determination_2025. At right, the dashed black line shows the contour of constant collisionality, $\nu^{*} = 1.5$ ($\alpha_{t} = 0.05$), and the dashed purple line shows the contour of constant electron pressure, $p_{e} = 12$ kPa ($\beta_{e} = 10^{-3}$).
• Figure 2: Profiles of $n_{e}$ (top) and $T_{e}$ (bottom) against $\psi_{n}$ showing a high $n_{e}$ EDA H-mode in dark blue, a medium $n_{e}$ EDA H-mode in light blue, a medium $n_{e}$ large ELMy discharge as dark red, and a low $n_{e}$ small ELMy discharge in light red.
• Figure 3: D$_{\alpha}$ emission as a function of time from beginning of time window, $t_\mathrm{min}$, for selected discharges from Fig \ref{['fig:elmy_eda_profiles']}.
• Figure 4: $n_{e}^\mathrm{sep}$ (top, left) and $n_{e}^\mathrm{ped}$ (bottom, left) plotted against $p_{0}^\mathrm{OMP}$ and $p_{0}^\mathrm{OMP}$ plotted against $-\nabla n_{e}^\mathrm{sep}$ (right) for discharges with small ELMs (light red squares), large ELMs (dark red squares), mixed EDA and ELMy (purple circles), and only EDA (blue diamonds).
• Figure 5: Fourier-transformed time series of line-averaged density fluctuations as measured by the PCI for channel #15 of four different discharges. Blue discharges are EDA H-modes and red discharges are ELMy H-modes. Solid gray and dashed black curves are used to show fits to the fluctuation spectra using Equations \ref{['eq:hf']} -- \ref{['eq:gf']}.