Spectroscopic investigation of ion sinks/sources during TCV detachment

Power exhaust is one of the major challenges for ITER and beyond. It is envisioned that operating in, at least, a partially detached divertor is necessary for ITER and fusion reactors. This investigation aims at providing a better physical understanding of which processes drive target ion current loss during detachment on the TCV tokamak. A novel spectroscopic technique is employed to separate ionisation and recombination contributions from Balmer line emission, enabling the determination of both the source (ionisation) and sink (recombination) of ions in the divertor plasma. One of the key characteristics of detachment is the roll-over of divertor target ion flux during a core density ramp. Previous research on TCV has indicated that three body recombination, as a sink for ions, is too small to explain the observed drop in divertor target ion flux [1]. Bolometric measurements suggests that radiated power losses reduce the power available for divertor ionization. Therefore, radiation losses could limit the ionisation source and facilitate the ion current roll-over. New measurement of divertor ion sources due to ionization, based on several Balmer series lines, appear to support this hypothesis. We present results for a range of divertor geometries with different levels of poloidal flux expansion on which the above analysis was applied. Preliminary evidence indicates the magnitude of both recombination and ionisation sinks/sources can be influenced separately by the magnetic divertor topology. This leads to variations in trends of the observed particle flux [2]. Inferences of both the recombination and ionisation sink/sources can hence provide more physical insight in the role of magnetic topology on detachment.