New developments of density measurement by fast wave interferometer/reflectometer

A fast-wave interferometer and a reflectometer can measure the mass density of charged particles and the hydrogen isotope density ratio in plasma, respectively. The frequency band of probe wave is about 10-100 MHz, and the transmission system installed in a vacuum vessel is not an optical system but an antenna. These instruments have the advantages of being less susceptible to plasma wear, impurity deposition, high radiation, or temperature changes. Therefore, it is considered possible to use them in the future nuclear fusion reactor, which will make a harsh environment for many measuring instruments. The fast-wave interferometer/reflectometer have received principle verification at DIII-D (Tokamak), and further improvements are being made to upgrade their performances, and a new attempt such as fluctuation measurement are currently underway. This paper introduces the measurement principles of fast-wave interferometer/reflectometer as well as the measurement results obtained to date, and explains the prospects for their applications to fusion reactors and the challenges to be addressed. (A.O.)