High-resolution ruby laser Thomson scattering diagnostic for the W7-AS stellarator

Abstract

After reconstruction, the ruby Thomson scattering diagnostic for the W7-AS stellarator is now in operation to again cover the complete plasma cross section (420 mm), where the former photomultiplier-based polychromator system has been replaced by two individual Littrow-type polychromator setups (focal length 50 cm), using intensified charge coupled device cameras for light detection. A single-pulse ruby laser with high pulse energy is used as a light source. The light scattered by the plasma electrons is imaged upon a set of 45 fiber bundles. 30 bundles provide a high spatial resolution of 4 mm at the inner edge of the plasma. The remaining part of the view chord is observed with a spatial resolution of 20 mm. At the output, the fiber bundles form the entrance slits of the Littrow-type spectrometers. The spectral resolution of the edge spectrometer can be modified by exchanging the diffraction grating. Depending on the installed diffraction grating (600 or 1800 lines/mm) a total wavelength range of 80 nm (suitable for plasma edge investigations) or 320 nm (suitable for gradient investigations) can be surveyed. The spectral intensity and geometrical calibration of the presented diagnostic setup can be done in situ by means of a neon spectral lamp and a calibrated tungsten strip lamp. The absolute sensitivity calibration for the system is achieved using either Raman (at 723.8 nm) or Rayleigh (at laser wavelength) scattering as hydrogen gas filled up in the vacuum vessel (up to 100 mbar). Examples of electron density and temperature profiles measured under different plasma conditions are presented.