Heat flux width scaling and detachment in high heat flux experiments on DIII-D

A multi-tokamak scaling predicts narrowing heat flux widths and increasing heat flux density in reactors well beyond the limits which existing armor materials can sustain. However, more recent experiments suggest that broadening effects observed at higher scrape-off-layer (SOL) pressure might change how the divertor heat flux profiles scale, easing this heat flux challenge. Experiments at the DIII-D tokamak use a range of plasma current (I_p =1.0-1.9 MA) and neutral beam heating power (P_inj=8-16 MW, for a power-into-SOL of P_SOL≈5-10 MW) H-mode discharges to investigate the physical processes that may limit the maximum parallel heat flux into the divertor. Analysis of infrared camera measurements in the divertor shows heat flux broadening between ELMs at the outer target in the P_SOL=10 MW case compared to the 5 MW case at I_p=1.9 MA. Density scans provide data both attached and detached conditions, with detachment onset in the high power case at about 59% the Greenwald limit. Detached cases at I_p=1.9 and P_SOL=10 MW give some of the highest normalized divertor parameters achieved on DIII-D, with neutral ionization length, heat flux width, and Lyman mean free path length within factors of ≈2.5-3.2 compared to that expected in a reactor.