Interferometric plasma position detection for position control in Tokamak experiments

In large tokamak experiments plasma position control is an important task of the machine control system. Usually the plasma position is detected by magnetic measurement techniques. An alternative method is suggested here which is capable to detect very slow plasma displacements and could be applied to cross check a magnetic detection system. Special attention is given to on-line data acquisition for reasons of feedback control for a toroidally confined plasma. Two 155 GHz microwave interferometer channels with automatic fringe counting are used. The electron line density is measured along two trajectories through the boundary regions of the plasma on the inner and outer side of the minor plasma diameter. The difference of the integral densities is continuously evaluated. Frequency modulation of the microwave carrier is not required. Instead a logic counting of the zero-crossing signals of two detectors is applied. The sign and the amount of the difference counting is a measure of plasma asymmetry with regard to the vessel axis. The difference account can be used to control a feedback system for plasma position adjustment. The sensitivity of the dual-channel microwave system was calculated for various plasma parameters as they are expected in the TEXTOR tokamak. The influence of an error source, the refraction of the electromagnetic wave in the density gradient of the plasma, is evaluated and discussed. (orig.)