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%.
Categories: physics.plasm-ph