Spectra polarimetry of the motional Stark effect at TEXTOR-94

The Balmer-alpha light emitted by neutral particles injected into tokamaks is polarized with respect to the Lorentz electric field experienced by these atoms: E-l = nu xB, known as the motional Stark effect (MSE). On TEXTOR-94 a new MSE system is under development which exploits the full spectral information, along with its polarization. The advantages of exploiting the full spectral information are obvious: beam velocity, observation volume, absolute value of B, the plasma radial electric field and the beam density can in principle all be extracted from the measurement, whereas the polarization can be detected as several lines simultaneously. The TEXTOR-94 MSE system consists of 30 radial channel, of which the full Balmer-alpha spectrum is measured at two orthogonal angles. This is accomplished by inserting a Glan-Laser prism in the optical path. All the optics is located inside the vessel to improve the radial resolution and minimize changes in polarization due to optical elements. Fibers transfer the light by a vacuum feedthrough towards a spectrometer. It is shown that the measured spectra can be adequately fitted. The accuracy for determining the safety factor q is estimated to be 10%-15% and for the radial electric field to be 30 kV/m. (C) 2001 American Institute of Physics.