Sailing on Far Infrared and Submillimeter Waves Plasma Diagnostics, towards THz-TDS and beyond

In the preface of their iconic book [1] M.A. Heald and C.B. Wharton summarize the concept of plasma diagnostics, as the instrumentation "to make unperturbed measurements of plasma parameters". The term diagnostics has medical origin, having been acquired from the studies of the effect of nuclear radiation by the Atomic Energy Commission (AEC) when the Sherwood Program for controlled nuclear fusion research was launched. This paper will give an overview of plasma diagnostics and plasma heating systems in the Far Infrared Radiation (FIR) and submillimeter waves range (100-2000 GHz) for magnetically confined plasmas in tokamaks [2]. Then the novel addition to the diagnostics landscape coming from Terahertz Time Domain Spectroscopy (THz-TDS) applications will be discussed.