A novel hydrogenic spectroscopic technique for inferring the role of plasma-molecule interaction on power and particle balance during detached conditions
Abstract
Detachment, an important mechanism for reducing target heat deposition, is achieved through reductions in power, particle and momentum; which are induced through plasma-atom and plasma-molecule interactions. Experimental research in how those reactions precisely contribute to detachment is limited. In this work, we investigate a new spectroscopic technique to utilise Hydrogen Balmer line measurements to 1) disentangle the Balmer line emission from the various plasma-atom and plasma-molecule interactions; and 2) quantify their contributions to ionisation, recombination and radiative power losses. During detachment, the observed Hα emission often strongly increases, which could be an indicator for plasma-molecule interactions involving H2+ and/or H-. Our analysis technique quantifies the Hα emission due to plasma-molecule interactions and uses this to 1) quantify the Balmer line emission contribution due to H2+ and/or H-; 2) subsequently estimate its resulting particle sinks/sources and radiative power losses. Its performance is verified using synthetic diagnostic techniques of both detached TCV and MAST-U SOLPS-ITER simulations. Experimental results of this technique on TCV data show a bifurcation occurs between the measured total Hα and the atomic estimate of Hα emission, indicative of the presence of additional Hα due to plasma-molecule interactions with H2+ (and/or H-). An example analysis shows that the hydrogenic line series, even Lyα as well as the medium-n Balmer lines can be significantly influenced by plasma-molecule interactions by tens of percent during which significant Molecular Activated Recombination (MAR) is expected.