Progress on THz Applications for Plasma Diagnostics

We discuss an extensive set of experimental results about the spectroscopic properties of different materials in THz spectral range, as part of the development of a THz-based Plasma Diagnostic for Nuclear Fusion Applications. The Terahertz (THz) band of the electromagnetic spectrum is defined as the frequency range between microwaves and midinfrared light [1,2]. Since 2010 a successful collaboration between ENEA Frascati and the Photonics Group at Clarendon Laboratory, Oxford University, has been in place to extend the use of THz Time Domain Spectroscopy (TDS) techniques to harsh environment applications, namely Tokamak Plasma diagnostics for Fusion research [3]. The simultaneous use of large portions of the electromagnetic spectrum in the form of THz pulses produced with femtosecond mode-locked lasers provides an appealing tool to diagnose plasma phenomena spanning above and below the plasma frequency [4]. THz pulses can be used as very sensitive and versatile probes of widely varying plasma parameters especially for diagnostic applications in Tokamaks where plasma characteristics are non-uniform and evolve during the discharge. We designed and assembled a table-top free-air THz-TDS setup based on a femtosecond infrared laser pulse (790 nm) and photoconductive GaAs plates [2]. The system has been designed with a great deal of flexibility, to experiment different solutions for coupling optical systems, path difference scan and Group Velocity Delay (GVD) compensation. Recently we used a commercial Advantest TAS7500TS Terahertz Analysis System for a preliminary test of the long range optics and to measure the spectroscopic properties of materials and components relevant for Plasma Physics diagnostics, over an unprecedentedly large spectral range.