Radial profiles of impurity lines from a scanning mirror VUV system in a compact helical system

A scanning mirror VUV system with an octagonal rotating mirror was constructed and routinely operated in a compact helical system to observe the impurity radial profiles after installation of carbon plates to shut out the wall reflection of VUV light. The system covers a practical range of 40–160 nm and exhibits a good time resolution of 2.5 ms. It is observed that the plasma always begins to shrink from the edge when the radiation collapse happens. The data obtained are analyzed with magnetic surfaces and are well described, considering the distortion of the magnetic surface by the β effect.