Analysis of the JET FIR interferometer beam phase changes during plasmas and application for fast fringe jump corrections by electronics

Abstract

The interferometer diagnostics using Far Infrared (FIR) laser beams provide, by phase measurement techniques, precise line-electron density measurements of magnetic fusion plasmas. But the FIR beams still suffer the effect of refraction when traversing the plasma. This can cause, when the density gradients are strong, temporary losses of signal that induce the so called fringe jumps on the estimated phase. On the CEA Tore Supra tokamak, a Field Programmable Get Array (FPGA) electronics has been developed using a time delay method to calculate the phase and correct the fringe jumps at the frequency rate of the probe sinusoidal signal (100 KHz). To test the efficiency of the algorithm on various plasma scenarios, a prototype of the CEA electronics has been installed on the JET tokamak and the data have been compared with those issued from the present JET electronics, which calculates the phase by a different method. Statistical comparisons between the two methods on more than 1500 JET shots are reported in this article and show that the two methods give similar results but that none of them is 100% reliable as still some fringe jumps remain, in particular when Edge Localised Modes (ELMs), pellet injections or disruptions occur. To understand this phenomenon, an analysis of the fast changes of the 100 KHz raw input signals during ELMs and pellet injections has been done with a 1MHz numerical acquisition. The typical durations of signal losses have been found to be few hundreds of micro-seconds. Meanwhile, the line density can increase and then return to its original value. Simulations show that an algorithm that would block the phase calculation during a longer time (i.e. 500 μs) than the disturbed period would help to avoid fringe jumps.