Real-time fringe correction algorithm for the JET interferometer

On the JET facility, the line-integrated electron density is measured by a multichannel far-infrared (FIR) interferometer. The basic source of radiation is a deuterium cyanide laser (? = 195 ?m) measuring the density along four vertical and four nearly horizontal (lateral) channels. During high-density discharges the system suffers from fringe jumps that are manually partially corrected by an off-line program. This intermittent phenomenon prevents reliable usage of the measured line-integrated density in a real-time feedback controller. To solve this problem we have developed a method to correct the fringe jumps on the lateral channels in real time. The method uses the additional methyl alcohol laser (? = 118.8 ?m) interferometer present on the lateral channels to compensate the vibrations. The method has been successfully tested with off-line test data, before implementation on a VERSA Module Eurocard system for the real-time control of the JET discharges.