Radial dependence of decay times of emissions from injected impurities in alcator A tokamak

The characteristics of EUV emissions from the ionization states of injected impurities which exist outside the core of the Alcator A plasma (r/a ≥ 0.5) are presented. These results are compared with the emissions from the core ionization states to investigate their radial dependence. In deuterium discharges, the ionization states of silicon which exist outside the core (e.g. Si XII) have shorter decay times than those of the core states (e.g. Si XIII), but the scalings of the decay times with plasma current and toroidal field are similar for the two ionization states in the two regions. The difference between the decay times of the ionization states in the two regions increases as the mass of the background ion is increased (3He and 4He discharges). The behaviour of other injected impurities (Al, Fe, and Mo) is presented; it is also observed that the decay times of the ionization states of these impurities which exist outside of the plasma core are shorter than the corresponding core states. The difference in the decay times is modelled with a computer code and is found to be qualitatively consistent with a radially constant spreading diffusion coefficient with no (inward) convection.