Determination of confinement regime boundaries via separatrix parameters on Alcator C-Mod based on a model for interchange-drift-Alfvén turbulence

Abstract

The separatrix operational space (SepOS) model [Eich \& Manz, \emph{Nuclear Fusion} (2021)] is shown to predict the L-H transition, the L-mode density limit, and the ideal MHD ballooning limit in terms of separatrix parameters for a wide range of Alcator C-Mod plasmas. The model is tested using Thomson scattering measurements across a wide range of operating conditions on C-Mod, spanning ${\overline{n}}_e=0.3-5.5\times 10^{20}$m${}^{-3}$, $B_t=2.5-8.0$ T, and $B_p=0.1-1.2$ T. An empirical regression for the electron pressure gradient scale length, $\hat{I}{»}_{p_e}$, against a turbulence control parameter, $\hat{I}{\pm }_t$, and the poloidal fluid gyroradius, $\ddot{I}{}_{s,p}$, for H-modes is constructed and found to require positive exponents for both regression parameters, indicating turbulence widening of near-SOL widths at high $\hat{I}{\pm }_t$ and an inverse scaling with $B_p$, consistent with results on AUG. The SepOS model is also tested in the unfavorable drift direction and found to apply well to all three boundaries, including the L-H transition as long as a correction to the Reynolds energy transfer term, $\hat{I}{\pm }_{\mathrm{RS}}<1$ is applied. I-modes typically exist in the unfavorable drift direction for values of $\hat{I}{\pm }_t\lesssim 0.35$. Finally, an experiment studying the transition between the type-I ELMy and EDA H-mode is analyzed using the same framework. It is found that a recently identified boundary at $\hat{I}{\pm }_t=0.55$ excludes most EDA H-modes but that the balance of wavenumbers responsible for the L-mode density limit, namely $k_{\mathrm{EM}}=k_{\mathrm{RBM}}$, may better describe the transition on C-Mod. The ensemble of boundaries validated and explored is then applied to project regime access and limit avoidance for the SPARC primary reference discharge parameters.