High-resolution heat flux width measurements at reactor-level magnetic fields and observation of a unified width scaling across confinement regimes in the Alcator C-Mod tokamak

Abstract

New data from Alcator C-Mod have extended the range of heat flux measurements and scalings to poloidal magnetic fields above ITER-level. Knowledge of how the scrape-off layer heat flux width (λq) scales with machine parameters is crucial for designing fusion reactors and developing a power exhaust solution. An international database indicated that λq scaled approximately inversely with the poloidal magnetic field (Bp) and had no other significant dependencies. However, reactor-class tokamaks are expected to have at least 50% higher Bp than the maximum of that database (0.8 T). Alcator C-Mod has been the only diverted tokamak capable of operating at reactor-level Bp, up to ~1.3 T. A major focus of the final experimental campaign on Alcator C-Mod was to characterize λq over a wide range of conditions, utilizing a unique array of heat flux sensors with unprecedented spatial resolution and heat flux dynamic range. The heat flux width scaling is found to extend up to Bp ~ 1.3 T in H-mode. Looking across confinement regimes we find the remarkable result that λq exhibits a unified dependence on volume-averaged core plasma pressure (). Within a standard deviation of about 20%, the heat flux width in any of the C-Mod plasmas studied (L-, I-, and H-mode) is proportional to the inverse square root of . It is also found that the standard prescription of representing the target plate heat flux profile as a convolution of exponential and Gaussian functions does not capture the heat flux profile measured in the private zone; a purely exponential decay fits the data better in this region to >3 orders of magnitude in heat flux dynamic range.