INTEGRATED SPECTROSCOPIC EDGE DIAGNOSTICS ON COMPASS-D

摘要

The investigation of edge physics in tokamaks often requires high spatial resolution, for example: transport barrier scale lengths in COMPASS-D H-modes are ～lcm. With this comes a requirement for accurate inter-diagnostic spatial mapping, since alignment uncertainties can make it very difficult to combine data from different sources. This is especially so when dealing with poloidally separated regions, but when working at the highest resolution, even toroidal symmetry may not be adequately preserved. On COMPASS-D we have addressed this problem by combining the visible light collection for two separate spectroscopic diagnostics into a single assembly. "HELIOS" (measuring HeI line ratios and Balmer D_α light) and "CELESTE" (HeII Doppler measurements) share a collection lens with a poloidal view through a thermal helium jet, and have optical fibres located in the same ferrule (see Figure 1). The extreme proximity of the collection fibres and their respective collection volumes in the plasma ensures a near-perfect mapping, permitting the combination of the data to yield the radial electric field E(r). This greatly enhances studies of H-mode dynamics. A further benefit of the combined collection system is that only one viewing port is required on the tokamak. The diagnostics are used for H-mode transition studies, for which good time resolution is essential. The use of a localised source ensures excellent signal strength, allowing high-speed operation, and also means that no spatial inversion is required.