Fusion plasma diagnostics with microwaves

An introductory overview is given on the established methods of probing magnetically confined hot fusion plasmas with microwaves in the mm and sub-mm wavelength regions. Since the plasma dielectric properties are mainly determined by the electron component of the plasma, primarily information on this component is being achieved. The methods can be divided into active and passive diagnostics. Applying the active ones, like interferometry, reflectometry, polarimetry, and wave scattering the changes in phase, time delay, polarization state, and spectrum of a launched electromagnetic wave are measured to yield information on the local electron density and temperature as well as the magnetic field and plasma currents respectively. The passive radiometry of the cyclotron emission of the electrons gives local information on the electron temperature, in general the electron energy distribution function. The physics basis of the various methods is well known since many years. However, the development of reliable microwave components, both sources and heterodyne detection systems, and recently also two dimensional detector arrays and the application of signal correlation techniques, makes multi-view phase and amplitude imaging applicable, delivering new insights into turbulent plasma structures most important in understanding plasma heat and particle transport properties. The paper gives in an elementary way the physics basis and describes the principles of the various active and passive techniques.