Advanced electronics for tokamak far infrared interferometers

Tore Supra uses a ten channel FIR interferometer to measure the plasma density. The plasma density is deduced from the phase between a shifted reference beam and a probing beam that crosses the plasma, however the counting of the fringes is affected by several kinds of plasma events that make the signal too low to be reliably measured. The last upgrade of the interferometer electronic instrumentation has been carried out by the responsible physicist and the Tore Supra electronics team in order to reduce the number of so-called fringe jumps in real time. Efficient correction algorithms were developed for both Tore Supra and JET tokamaks. The use of the FPGA (Field Programmable Gate Array) technology with the VHDL language makes possible to refine more and more specific fringe jump correction algorithms. The article will focus on the technology used to implement such an instrument and will present the next generation that is planned to equip the new JET interferometer channels.