Hydrogen recycling and puffing at a poloidal limiter of TJ-II

Abstract Hydrogen recycling and puffing through a hole in a poloidal limiter of TJ-II is studied by spatially resolved H α -emission spectroscopy with a CCD camera. Very different emission profiles are obtained depending on whether the hydrogen enters the plasma when recycling at the carbon limiter or whether it is puffed through a hole in the limiter directly inside the last closed flux surface of the plasma. From the emission profiles, the neutral density profile of the H α -emission precursor is deduced and compared to that calculated with the help of a simple one-dimensional model. The analysis shows, that in front of a limiter with T e  > 30 eV, the H α -emission has two main sources: dissociative excitation of thermally desorbed molecular hydrogen and excitation of atomic hydrogen with kinetic energy of typically 0.3 eV. The relative importance of this last term decreases in front of a strong gas-puffing source. If the puffing rate is very strong, the plasma is measured to be locally perturbed and this can be the origin of the different emission profiles measured with and without puffing.