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 ${\alpha }_t$ 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%.