AI Insight
This study examines the transition between two types of high-confinement plasma modes (ELMy H-mode and EDA H-mode) in the Alcator C-Mod tokamak fusion reactor. Researchers found that pedestal density responds differently to neutral particle sources in each regime, and that including an additional transport mechanism driven by resistive ballooning modes significantly improves predictions for EDA H-mode behavior. The models were validated against experimental measurements and applied to predict plasma pedestal characteristics for the SPARC fusion reactor, showing that the additional transport reduces predicted pedestal density by approximately 20%.
Why it matters
Accurate prediction of plasma pedestal properties is critical for designing and optimizing future fusion reactors, including SPARC. Understanding the transport mechanisms that govern different operational regimes enables better control strategies and more reliable performance predictions for next-generation fusion energy systems.
Understand the Science
arXiv:2603.16515v2 Announce Type: replace
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%.