Four-channel dispersion interferometer at TEXTOR

For magnetic fusion experiments like the TEXTOR tokamak or the Wendelstein 7-X stellarator, interferometry is a well established method for the measurement of the line-integrated plasma density. As the line averaged electron density is obtained independent of the signal power and thus of a possible degradation of windows and optical elements, interferometry is widely used for plasma density control or the position control for the plasma column. For a common interferometer, the measured phase shift is generated by the dispersive effect of the plasma (proportional to the line-integrated density) and the change in the optical path due to moving (e.g. vibrating) optical components. The latter effect needs to be compensated. One possibility is the dispersion (sometimes 'second harmonics') interferometer. Here, a part of the probing beam is converted into its second harmonics, the phase shift is generated for both wavelengths. After crossing the plasma, the fundamental wave is frequency doubled again, and both second harmonics interfere. With this method the vibration induced part of the phase shift cancels out nearly completely. In addition, due to the coherence of both wavelengths, an additional reference beam is not necessary, which makes very compact assemblies possible. At the TEXTOR tokamak in Juelich, Germany, a multi-channel dispersion interferometer was developed, using the 10.6 μm radiation of a CO2 laser for the fundamental beam. The paper will focus on the first results of the four channel system and outline the possibilities for density and plasma position control. The next stages of expansion and the possibilities to implement such a system for steady state operation at Wendelstein 7-X will be outlined and discussed. This document is composed of the slides of the presentation. (authors)