The influence of the limiter on EUV emissions from light impurities in the Alcator A tokamak

EUV emissions from light impurities were observed simultaneously with two spectrometers — one at the limiter, the other 90° away toroidally – on the Alcator A tokamak. A difference in the time history of the emission from the limiter was noted during the low-density phase of the discharge. In addition, differences in the spatial profiles at the limiter are discussed. A simple model is used to explain the difference in the observed time histories. The model indicates that, if either re-cycling or a local source is present at the limiter, the emission will exhibit a toroidal asymmetry in agreement with the experimental data.