Parallel Energy Transport in Detached DIII-D Divertor Plasmas

A comparison of experiment and modeling of detached divertor plasmas in DIII-D is examined in the context of parallel energy transport due to electron conduction and plasma convection in order to validate and improve models used for divertor design. Power balance analysis is carried out to determine parallel heat flux and energy dissipation as a function of distance from the divertor target. The relative fractions of conductive and convective heat flux are determined from Thomson scattering measurements of the divertor parallel T_e gradient. Modeling with the fluid code SOLPS is found to underestimate experimental measurements of divertor heat flux radiative dissipation due to steeper T_e gradients in the region of T ≤ 20 eV resulting in smaller radiating volume. VUV spectroscopy measurements of CIII, CIV and Ly-a reveal a much more distributed radiation pattern from the X-point to the target than produced by modeling. Possible causes of this discrepancy that are examined include increased parallel energy transport through plasma convection and plasma turbulence.