High precission fpga based phase meters for infrared interferometer fusion diagnostics

Resumen

Interferometers are devices used to measure optical path-length variations. If the target of the interferometer is a plasma, these variations are proportional to the line-electron density of the plasma. The wavelength used in the interferometer depends on several limitations. First, the maximum expected electron density must be lower than the cut-off density for that wavelength, which ensures that the waves can still propagate through the plasma when the maximum density is reached. Another limitation is given by refraction, which causes a deflection of the beams and eventually the loss of the interference signals. This effect is more pronounced the closer the maximum expected density is to the cut-off. However, it can be moderated by reducing the wavelengths, for instance with the use of interferometers operating in the middle infrared region. With such low wavelengths mechanical vibrations are significant and a complementary interferometer is typically used to cancel these vibrations. These arrangements are called two-color interferometers. Two beams are used as references and other two beams cross the plasma. When these beams reach a detector, two interference signals are generated. By comparing the phases of these signals with two sinusoidal references extracted directly from an oscillator, the optical path-length variations are obtained. The TJ-II is a medium size flexible Heliac type stellarator installed in the Laboratorio Nacional de Fusión of the CIEMAT, Spain, for the study of helical axis plasmas over a wide range of parameters. The plasma is firstly generated using an Electron Cyclotron Resonance Heating (ECRH) system based on two gyrotrons working at 53.2 GHz, each one delivering a power of up to 200 kW to the plasma. With this method, plasmas up to average densities of ne = 1.7*10(19) m-3 can be generated. Beyond this value, the cut-off density for the 53.2 GHz waves is reached and the waves are reflected back. To further increase the density of the plasmas, a Neutral Beam Injection (NBI) heating system is used. The TJ-II NBI heating system consists of two NBI injectors (hydrogen neutrals) that can deliver a power to the plasma up to 550 kW each. With this system, plasmas up to a density near to (ne) = 8*10(19) m-3 can be generated. Now, in order to measure in this density range an IR heterodyne interferometer is used. This interferometer has a single channel and uses two wavelengths, CO2-Nd:YAG 10.591 µm-1.064 µm. For several experimental campaigns an off-line phase measuring system has been routinely working. Currently, an expanded beam multichannel interferometer is under development for electron density spatial profile measurements in the TJ-II stellarator . In its initial version, this setup will be capable of measuring three chord integrals. For this purpose, the phase measurement must be carried out for eight signals: three CO2 interference signals, three Nd:YAG interference signals and two reference signals. To perform this procedure online, very fast acquisition and processing systems are required as well as efficient signal processing algorithms. For multi-channel infrared interferometers the acquisition systems must have multiple inputs, and the signals must be de-noised and processed simultaneously at very high speeds. The current digital technology allows this to be done using FPGAs, in a more flexible, accurate and faster way. In this Thesis, several novel digital signal processing algorithms to compute the line integrated electron density in fusion IR-interferometers are presented. These algorithms have been implemented in an FPGA device, where after a high speed digitization of the signals the line integrated electron density is computed in real time. Several sources of error have been considered during the design of the processing system. In particular, the implementation in the FPGAs is performed in fixed-point format which causes the apparition of quantization effects such as the round-off noise. Since the implemented algorithms are non-linear a new technique to evaluate the quantization effects based on modified affine arithmetic and Legendre polynomial chaos expansion has been developed. Finally, experimental results from the TJ-II infrared interferometer under plasma operation, and from the W7-X infrared interferometer test-bench under static conditions are presented.