Real-time recovery of the electron density from interferometric measurements affected by fringe jumps

Abstract

Optical interferometers are normally used in magnetically confined plasmas to measure the refractive index of the plasma by comparing the phase shift variation between a reference and a probe laser beam, from which the line-integrated electron density can be derived. Unfortunately, interferometric measurements are affected by fringe jumps, which are basically the erroneous phase difference determination due to the loss of signals or a phase difference bigger than 2π. The multiple causes include the refraction, wavelength of the laser radiation used, sensitivity, and time resolution of the measurements. On the other hand, the plasma density has become an essential piece of information for many real-time control schemes, which can therefore be completely jeopardized by fringe jumps. To overcome this problem at JET two main approaches can be adopted. The first approach consists of performing a real-time correction of the affected chords, eliminating the spurious effect of the fringe jumps, and providing a corrected line integral of acceptable quality. This is done at JET by complex algorithms that have inputs of various interferometry and polarimetry measurements. A second approach can be adopted based on the observation that, for many real-time experiments, an approximate estimate of the density profile is sufficient. In JET, it was demonstrated that the density profile of the vast majority of configurations could be determined with sufficient accuracy by using only the line-integrated density profile provided by two chords; one external and one internal. The various solutions were tested and results compared in order to verify the most suitable one for the various plasma configurations and operational scenarios. A “general purpose” version of the correction algorithm was implemented and is now normally running during JET operation.