The detailed physical mechanisms of hydrogen recycling are not yet completely clear. But, their understanding is required for the correct interpretation of spectroscopic measurements that are intended to provide us routinely with the total particle fluxes as well as with sound extrapolations to fusion devices of the next generation. Thanks to its large observation ports, TEXTOR provides ideal conditions for the combination of optical diagnostics based on completely different techniques, which can be applied simultaneously, with high resolving powers (/ = 2 × 104 to 2 × 105).

It is shown how Zeeman spectroscopy on the Balmer-alpha transition (656.1 nm) and laser-induced fluorescence at Lyman-alpha (121.5 nm) both point to the presence of a substantial amount of cold hydrogen atoms (with kinetic energy <1 eV) in front of plasma-facing components, which is a phenomenon that, surprisingly, is largely independent of the local plasma parameters. This has led to a strong development of the spectroscopy of hydrogen molecules (Fulcher band), which may be a dominant source of atomic hydrogen in the plasma edge, and, as a final result, to an explanation for the phenomenological correction applied to the inverse photon efficiencies S/XB that are commonly used in the conversion of the photon fluxes into particle fluxes.