Infrared (IR) interferometry is a method for measuring the line-electron density of fusion plasmas. The significant performance achieved by FPGAs in solving digital signal processing tasks advocates the use of this type of technology in two-color IR interferometers of modern stellarators, such as the TJ-II (Madrid, Spain) and the future W7-X (Greifswald, Germany). In this work the implementation of a line-average electron density measuring system in an FPGA device is described. Several optimizations for multichannel systems are detailed and test results from the TJ-II as well as from a W7-X prototype are presented.

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Development of Efficient FPGA-Based Multi-Channel Phase Meters for IR-Interferometers

Infrared interferometers are used in magnetic fusion devices for measuring the line-integrated electron density of the plasmas. An FPGA-based processing system is currently being used in the TJ-II infrared interferometer to compute the line-integrated electron density. In high performance Digital Signal Processing (DSP) applications, the computations carried out in the FPGAs are usually performed in fixed point. The floating-point values of the algorithm description must be quantized to their fixed-point counterparts, introducing some deviations with respect to the un-quantized case. These deviations are modeled as Round-Off Noise (RON) sources, whose effects are propagated through the different parts of the system. Thus, these quantization operations significantly affect the maximum attainable system performance. In the TJ-II system, the amount of RON is a limiting factor. Therefore, its analysis and reduction is essential to perform the control operations in real time.

Quantization Analysis of the Infrared Interferometer of the TJ-II Stellarator for its Optimized FPGA-Based Implementation

Microwave interferometry has been used extensively for plasma diagnostics in tokamak for real-time electron density measurements. Real-time density measurement is an indispensable part of advanced tokamaks, as it can be used as feedback to control the plasma operation. In this article, we report the development of the phase extraction method in a heterodyne interferometer for real-time density estimation using a field-programmable gate array (FPGA). FPGA provides parallel processing, minimum control delay, more efficient processing architecture, and jitter-free synchronization between different control layers. Digital CORDIC algorithm is used for the estimation of the phase between two arms of the interferometer. A LabVIEW program is developed for the FPGA target and host controller to acquire the data and estimate electron density in real time, which can be used for feedback purpose by regulating the amount of gas injection using piezoelectric valve that requires a high voltage (~100 V) for about 1 ms. The delay in generating the feedback signal is proportional to the sampling speed (100 kHz) of the analog signal. In this work, line-averaged electron density measurement using digital signal processing (DSP) and LabVIEW-FPGA technology is presented. The developed interferometer and data acquisition (DAQ) system installed on ADITYA-U tokamak can measure the electron density in the range of $1 \times 10^{18} - 1 \times 10^{20}\,\,\text{m}^{-3}$ . The algorithm used for the estimation of phase difference is validated using a conventional arctan method. The developed reconfigurable FPGA-based DAQ system uses low power, has a reconfigurable hardware structure for fast real-time signal processing, and can be easily upgraded.

LabVIEW-FPGA-Based Real-Time Data Acquisition System for ADITYA-U Heterodyne Interferometry

In GAMMA 10/PDX, divertor simulation studies have been started as a new research plan by using end-loss plasma flux at the end-mirror region. A divertor simulation experimental module (D-module) was installed in the west end-cell. Here we introduce a novel microwave interferometer system with a phase imaging method, and all the crucial components are designed and tested. Finally, we tested the interferometer system using a Teflon plate in substitution for plasma. In order to improve the system accuracy, the phase detection circuit was modified and the phase difference of each channel was verified.

A Novel Frequency-Multiplied Interferometer System with 1-D Horn-Antenna Mixer Array in the GAMMA 10/PDX End Divertor Module

A fringe jump compensation algorithm has been developed for a phase measurement that measures the phase within a single fringe. The algorithm is extremely useful in the case of the time-averaging zero-crossing phase detector on noisy environments. When the noise level on the measurements is not sufficiently suppressed, the signals near the fringe jump show a negative slope instead of a sharp drop. The slope brings an ambiguity over the compensation process. An algorithm with an additional channel that measures the phase of a half fringe shift has been applied in the millimeter-wave interferometer on the Korea Superconducting Tokamak Advanced Research device. These techniques removed the ambiguity in most cases. The algorithm can provide a most simple, robust, and cost-effective solution for the phase measurement system in various fields.

Fringe jump compensation techniques for the time-averaging zero-crossing phase measurement in the KSTAR millimeter-wave interferometer.

Interferometry is used for measuring line average electronic densities in fusion plasmas. The W7-X stellarator will employ a two-color CO2 (10.591 m) and CO (5.295 m) heterodyne-infrared interferometer as an electronic density measurement diagnostic. The frequency displacement is 40 MHz for the CO2 wavelength and 25 MHz for the CO, so these values will fix the heterodyne frequencies. Because the frequency gap between the two carriers is wide enough and the detector sensitivity is similar for both wavelengths, it is possible to use a single detector for the two signals; nevertheless, they should be split with filters. Traditionally, the intermediate-frequency signals should be filtered, downconverted to a lower frequency by the use of analog circuitry, and then processed. A new approach is proposed. The intermediate-frequency signals are directly sampled by means of high-speed analog-to-digital converters followed by a digital diplexer and a specific phase-meter processor implemented in a field-programmable gate array. Preliminary results from the W7-X infrared interferometer prototype, without plasma, are presented.

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M.E. Sánchez

Papers

Continuous Phase Measurement in the W7-X Infrared Interferometer by Means of a FPGA and High-Speed ADCs