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

Abstract

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.