Synthetic aperture microwave imaging with active probing for fusion plasma diagnostics

TL;DR

This paper presents a novel synthetic aperture microwave imaging system for fusion plasma diagnostics, capable of passive and active imaging, achieving 2-D imaging and turbulence velocity mapping in real experiments.

Contribution

The development of a SAMI system that combines passive and active imaging modes with high-speed frequency switching for detailed fusion plasma analysis.

Findings

First 2-D images of BXO mode conversion windows.

First turbulence velocity maps obtained with active probing.

System demonstrated effective performance on MAST tokamak.

Abstract

A Synthetic Aperture Microwave Imaging (SAMI) system has been designed and built to obtain 2-D images at several frequencies from fusion plasmas. SAMI uses a phased array of linearly polarised antennas. The array configuration has been optimised to achieve maximum synthetic aperture beam efficiency. The signals received by antennas are down-converted to the intermediate frequency range and then recorded in a full vector form. Full vector signals allow beam focusing and image reconstruction in both real time and a post processing mode. SAMI can scan over 16 preprogrammed frequencies in the range of 10-35GHz with a switching time of 300ns. The system operates in 2 different modes simultaneously: both a passive imaging of plasma emission and also an active imaging of the back-scattered signal of the radiation launched by one of the antennas from the same array. This second mode is similar to so-called Doppler backscattering (DBS) reflectometry with 2-D resolution of the propagation velocity of turbulent structures. Both modes of operation show good performance in a real fusion plasma experiments on Mega Amp Spherical Tokamak (MAST). We have obtained the first ever 2-D images of BXO mode conversion windows. With active probing, the first ever turbulence velocity maps have been obtained. In this article we present an overview of the diagnostic and discuss recent results.