The role of the separatrix in establishing boundaries for operational regimes on Alcator C-Mod and prospects for projection to SPARC*

A model describing operational boundaries based on separatrix plasma parameters (SepOS) [1] is validated with Alcator C-Mod data. The model defines boundaries in terms of separatrix parameters, both dimensionless and mapped to dimensional space, for the L-H transition, the L-mode density limit, and the ideal MHD limit. Edge Thomson Scattering (ETS) measurements of electron density, n_e, and electron temperature, T_e, as well as their gradient scale lengths are used for model validation. Measurements from ETS allow for consistent identification of the separatrix position by estimating T_e at the separatrix, assuming Spitzer-Härm conductivity. For the typical toroidal magnetic field and plasma current, B_t = 5.4 T, I_P = 0.8 MA, the C-Mod separatrix conditions correlate well with the SepOS boundaries. This work extends validation of the theory of SepOS boundaries to higher B_t, lending credibility to projections to SPARC. Generation of these boundaries for SPARC informs prediction of access to and avoidance of operational regimes, like I-mode and EDA H-mode, as opposed to Type-I ELMy H-mode, and limits, like the ideal MHD limit and the L-mode density limit. Estimates are made for values for the minimum n_e at the separatrix, n_e^sep, required for the L-H transition, as well as maximum values of n_e^sep before an H-L back-transition or an L-mode density limit. Estimates for typical values of n_e^sep, in H-mode enable higher fidelity predictions of achievable n_e^ped, important for core performance modeling.

[1] T. Eich et al 2021 Nucl. Fusion 61 086017