Large Target Heat Fluxes from Thermoelectric Current

Experimental measurements from DIII-D show that SOL currents can carry enough energy to dominate the peak target heat flux in agreement with UEDGE modeling. Parallel electric currents in the scrape-off layer (SOL) are mainly driven by the asymmetry in electron temperature at the two sheath entrances for each flux tube. The electrons always convect towards the hotter target, acting to enhance the already existing temperature asymmetry. The current flux flowing into the grounded target (j₀) can exceed the ion saturation current flux density (j_sat) by several fold in agreement with [Brida PPCF 2020], and j₀ can drive large heat fluxes (q_ej0). In QH-mode [Ernst PRL 2024] q_ej0 is the dominant component of the outer target peak heat-flux, and accounting for it more than doubles the approximation commonly used with Langmuir probes. The importance of SOL currents was predicted by UEDGE modeling which shows that electron convection can carry a dominant fraction of the total heat, exceeding the contributions from Spitzer heat conduction and ambipolar convection combined. q_ej0 is related to the ‘detachment cliff’ since it drops abruptly at the onset of detachment. The q_ej0 profile is very narrow and acts to reduce the heat flux width λ_q.