Assessment of the role of molecules in divertor plasma detachment for tokamak devices using the multi-fluid code UEDGE

The importance of molecules for onset and degree of plasma detachment is demonstrated for a one-dimensional and a two-dimensional slab geometry. Plasma detachment, experimentally observed for $T<3$~eV [1], decreases the particle and heat fluxes incident on the divertor due to volumetric recombination. Molecular processes become relevant at temperatures relevant to detachment [2], and may result in sub-eV plasma temperatures, when electron-ion recombination become large. Equipartition of hydrogen ions and atoms with molecules cools the ions and atoms, and the decreased ion temperature diminishes the electron heating due to electron-ion thermal coupling. When the collisionality becomes sufficiently high, the ion temperature decreases below the electron temperature due to ion-molecule equipartition and ion-electron equipartition becomes an electron energy sink. It is shown that this electron energy sink results in sub-eV electron temperatures, yielding high electron-ion recombination rates and, subsequently, high degrees of plasma detachment not observed in simulations without molecules.\\\\\noindent[1] D. Eldon, et al., {\it Nucl.~Fusion} {\bf57}, 066039 (2017).\\\noindent[2] U. Fantz, et al., {\it J.~Nucl.~Mat.} {\bf290}, 367 (2001).