The Motional Stark Effect diagnostic at TEXTOR-94

Knowledge of the current distribution in a tokamak is indispensable for an understanding of the energy transport in the plasma and controlling instabilities. A diagnostic exploiting the Motional Stark Effect (MSE) has the potential to obtain this quantity. This is based on a measurement of the polarisation of the Balmer-a light emitted by neutral particles injected into tokamaks. On the tokamak TEXTOR-94 a new MSE system is under development which exploits the full spectral information, along with its polarisation. This system consists of 30 radial channels, measuring the spectrum at two orthogonal angles. This is accomplished by inserting a Glan-Laser prism in the optical path. All the optics is housed in five camera modules and located inside the vessel to improve the radial resolution and minimise changes in polarisation due to optical elements. Fibres transfer the light by a vacuum feedthrough towards a spectrometer. The simple construction of the fibre feedthrough and its reliable operation opens the possibility to observe plasma light at all conceivable angles and positions, irrespective of suitable locations of windows. (C) 2001 Elsevier Science B.V. All rights reserved.