FTU Diagnostic System Based on THz Time-domain Spectroscopy

We present the state of development of a THz-based Time-Domain Spectroscopy (TDS) system for FTU Tokamak. TDS is ideal to measure plasma characteristics that are non-uniform and/or evolve during the discharge: THz pulses (0.1-2 THz) produced with femtosecond mode-locked lasers (790 nm) conveniently span the spectrum above and below the plasma frequency and, thus, can be used as very sensitive and versatile probes of widely varying plasma parameters. This work is based on an existing collaboration between ENEA Frascati and the Photonics Group at Clarendon Laboratory, Oxford University, where a THz-TDS experimental apparatus has been assembled and characterised. The spectral response of diagnostic-relevant components and materials has been tested. The instrument is now being equipped with fiber-optics to guide the laser beam to the THz Emitter and Receiver to ease access through the Tokamak ports. Broadband THz pulse propagation in plasma has been simulated using a simple, yet effective Fourier Transform-based model. In the Tokamak environment, THz TDS can be used in conjunction with other diagnostic techniques, such as, interferometry and ECE, for the simultaneous measurement of dispersion and absorption to launch a bridge between major areas of research in the science landscape.