A Broadband 693 GHz Traveling Wave Tube for Burning Plasma Diagnostics in Fusion Reactors

As the fusion science research advances towards the demonstration of practical reactors for commercial adoption there is a growing need to develop diagnostic systems for monitoring various plasma parameters in real time for control and shaping of the plasma density and profile. Recent, experiments have demonstrated the potential of a high-k Scattering system for studying turbulence physics studies by providing a measurement of the kθ-spectrum of both electron temperature gradient (ETG) and ion temperature gradient (ITG) modes. Currently, a large and bulky Far Infrared (FIR) laser system is in use at the National Spherical Torus Experiment - Upgrade (NSTX-U) for such experiments. Also, the receiver systems use multiple solid-state sources to work as local oscillators for the receiver bank. The mixers are operated at higher harmonics of the local oscillator due to limited power available at 693 GHz. We propose to develop a compact broadband TWT at 693 GHz providing >100 mW of power to work as a local oscillator for a bank of receivers. The mixers can operate at fundamental mode providing higher conversion efficiency. Furthermore, this TWT can also be used on the transmitter side of the diagnostic to eliminate the need for a large far infrared laser system in the high-k scattering diagnostic. The TWT can also enable the creation of the much-needed reflectometer diagnostic to monitor divertor performance in a burning plasma machine. This technology can be extended to any frequency in the 1 THz range using currently available fabrication techniques and hence have wide applicability in different fusion reactors operating over a range of plasma density and magnetic field. The development of such a source will be essential for real time plasma monitoring and reflectometry in the divertor region in a commercial reactor. In the short term, such receivers will advance further development of this technique and methods at experiments such as the National Spherical Torus Experiment - Upgrade (NSTX-U). The proposed TWT technology can also be used for Dynamic Nuclear Polarization (DNP) enhanced Nuclear Magnetic Resonance (NMR) spectroscopy for structure determination of proteins for drug development.