Dependence of the Heat Flux Width on Average Pressure in Alcator C-Mod

The Alcator C-Mod heat flux width (λ_q) database suggests a strong correlation with the inverse square root of the volume-averaged plasma pressure, unified across L-, I-, and H-mode plasmas [1]. This implies that a high performance core at high pressure will lead to challenging heat and particle exhaust at small λ_q. This concern motivates our work, as does research from ASDEX Upgrade, where λ_q was found to be more strongly correlated with the edge electron pressure and its gradient length than with the average pressure [2]. This study adds density, temperature, and pressure profile data from the Thomson scattering and electron cyclotron emission diagnostics to the C-Mod database that was used to develop the p_ave^-0.5 scaling. The profiles are fit using Gaussian process regression. Within the extended database, the average pressure still shows a stronger correlation with λ_q than any localized value of the profiles or gradient lengths. Pressure values in the region of ψ_norm < 0.8 are found to be better correlated with λ_q than pressure values further out. Finally, scalings of λ_q with localized profile values in C-Mod are found to differ across confinement regimes.

[1] D. Brunner et al., Nuclear Fusion 58, (2018).

[2] D. Silvagni et al., Plasma Phys. Control. Fusion 62, 045015 (2020).