Development of efficient FPGA-based phase meters for IR-interferometers. optimizations for multi-channel interferometers

Infrared (IR) interferometry is a method for measuring integrated electronic density in fusion plasmas. The great performance achieved by FPGAs in resolving digital signal processing tasks has suggested to use this type of technology in the two-color IR interferometers of the modern stellarators, such as TJ-II and the future interferometer of W7-X. TJ-II is a medium scale stellarator that employs a two-color heterodyne IR interferometer (CO₂, λ₁ = 10.591 μm and NdYAG, λ₂ = 1.064 μm) for measuring line average electronic density. W7-X is a stellarator in which, due to technical restrictions, an IR interferometer of (CO₂, λ₁ = 10.591 μm and CO, λ₂ = 5.295 μm) needs to be developed. The electronic density computation in this type of diagnostics basically involves three steps: (i) detection of the interference measuring signals, (ii) computation of the phase differences between the measuring and reference signals, and (iii) calculation of the line-integrated electronic density from the optical path-length differences. The possibility of using the measurements of these diagnostics as real-time feedback signals for control purposes is opened. Current phase-meters based on general purpose processors do not permit real-time calculus directly over the signals in these type of interferometers. In this contribution a solution to this problem based on a specific processor implemented in an FPGA is addressed. Several signal processing techniques as well as a phase measuring algorithm have been defined and finally the specific processor has been implemented in an FPGA. This FPGA is integrated in a system that includes high speed analog-to-digital converters and a computer that controls the FPGA. The implementation of this processor in the FPGA with several optimizations for multi-channel systems is detailed. Finally results from TJ-II and from W7-X pro totype are presented.