Hydrogen release from plasma-facing components into fusion plasmas - recent results from a spectroscopic approach

Hydrogen isotopes are released from the walls, divertor plates and limiters of fusion devices in different forms, as atoms or molecules. The density and velocity distributions of these particles upon release can heavily influence the boundary plasma, especially through their penetration depth, and, indirectly, the plasma and its confinement properties as a whole. Recent experiments on different tokamaks have brought to light the deep interdependence between atomic and molecular species in this respect. Investigating this complexity calls for sophisticated spectroscopic diagnostics and new data on molecules like D2 and HD. Hydrogen and deuterium atoms with extremely low energies, definitely below 1 eV, are observed and give indications on the release processes, i.e. on those where molecules are involved. It turns out that corrections to the estimated hydrogen flux may be required - a procedure is proposed in the present work to obtain values for an effective S/XB coefficient for atomic hydrogen (≠15), which denotes the ratio of the collisional ionization (S) to the excitation (X) rate coefficients, divided by the branching ratio B. Moreover, attention is drawn to a possible heating mechanism of these very cold atoms by the proton/deuteron background (from 0.2 up to 10 eV). The derived information on the detailed release mechanisms should contribute towards improving the various numerical codes in which neutrals play a role. Eventually, the strong influence of the surface temperature on the ratio of atoms to molecules has to be considered in the choice of materials for plasma-facing components.