Microwave diagnostics

Abstract

Microwave-based diagnostics have found broad application in magnetic fusion plasma diagnostics and are expected to be widely employed in future burning plasma experiments (BPXs). Most of these techniques are based directly on the dispersive properties of the plasma medium that, as shown in the body of the paper, results in the microwave/millimeter wave portion of the electromagnetic spectrum being particularly well suited for a variety of measurements of both magnetic fusion plasma equilibrium parameters and their fluctuations. Electron cyclotron emission provides a measurement of electron temperature and its fluctuations while electron cyclotron absorption potentially can provide a measurement of electron pressure (the product of electron density and temperature) as well as information on the suprathermal electron distribution. Electron Bernstein wave emission is also employed for electron temperature radiometric measurements in devices including reversed field pinches, spherical tori, and higher-aspect-ratio tokamaks and stellarators that operate at high beta. The radar-based microwave reflectometry technique measures the electron density profile and its fluctuations by means of the reflection of electromagnetic waves at the plasma cutoff layer. Coherent Thomson scattering in the microwave region yields information on the fast ion population. Wave number resolved microwave collective scattering is also widely employed for measuring nonthermal (turbulent) density fluctuations or coherent electrostatic waves. The approach taken in this review is to address each technique separately beginning with the physical principles followed by representative implementations on magnetic fusion devices. In each case, the applicability to future BPXs is discussed. It is impossible in a short review to capture fully the numerous significant accomplishments of the many clever scientists and engineers who have advanced microwave plasma diagnostics technology over many decades. Therefore, in this paper, we can reveal only the basic principles together with some of the most exciting highlights while outlining the major trends, and we hope it will serve as an exciting introduction to this rich field of plasma diagnostics.