Description
Understanding the physics of high n_e regimes free of Type-I ELMs will be critical for reactor operation. This study focuses on one such regime, the EDA H-mode on Alcator C-Mod, using experimental profile and interpretive stability analysis. Recent work focused on analysis of dimensionless parameters at the separatrix [1], following from a similar study of the QCE on AUG [2]. The dataset includes a variation of the programmed L-mode density at fixed $I_P$ = 0.9 MA, $B_t$ = 5.6 T, and shape, with input ICRF power between 2 – 3 MW. As programmed density decreases, plasmas transition from the EDA to the ELMy regime in the H-mode transition. In the ELMy H-mode, $n_e^\mathrm{ped}$ and $n_e^\mathrm{sep}$ both decrease with the programmed density decrease, but in the EDA H-mode, only $n_e^\mathrm{sep}$ decreases, whereas $n_e^\mathrm{ped}$ remains constant across all EDA points. At the highest fueling rate, EDA H-modes feature a pronounced density shoulder, qualitatively like that of the QCE. This motivates characterization of pedestal stability for C-Mod EDAs at different fueling rates. Predictive pedestal scans are performed with EPED1.0 at three values of $n_e^\mathrm{sep}$/$n_e^\mathrm{ped}$, using an empirically-chosen coefficient for the width-height KBM scaling of C=0.076. The coefficient well describes the pedestal structure of many EDA H-modes except for those at high $n_e$ with broad density shoulders, as well as wide pedestals. The EPED-predicted pedestal height matches the experimental value for H-modes with large ELMs and overpredicts that of many EDA H-modes, as expected. For EDAs at high $n_e$, however, it matches and even underpredicts p^ped. These results motivate interpretive stability analysis using the MISHKA-HELENA stability chain. Local ballooning stability analysis is performed for two EDA H-modes, one at high and one at moderate fueling, with $n_\mathrm{sep}=1.2\times10^{20}$ m$^{-3}$ and $1.6\times10^{20}$ $m$^{-3}$, respectively. Results are compared with the hypothesis from AUG that ballooning modes limit moderately-fueled EDAs at the middle of the pedestal and highly-fueled QCEs at the bottom of the pedestal [3].
[1] M.A. Miller et al 2025 Nucl. Fusion 65 052002
[2] M. Faitsch et al 2023 Nucl. Fusion 63 076013
[3] L. Radovanovic et al 2022 Nucl. Fusion 62 086004