Fast wave interferometer and reflectometer for density measurements on DIII-D and prospects for magnetic confinement fusion reactors

Abstract The fast wave interferometer and reflectometer (FWIR) provide the ion mass density and isotope ratio of a magnetically confined fusion plasma, respectively. The ion mass density can be a proxy for the electron density for fueling control while the isotope ratio is an essential parameter to control plasma burning in a future fusion reactor. In-vessel components of the FWIR such as a waveguide, a coaxial cable, and an antenna are less deteriorated by radiation, erosion, and impurity deposition than in-vessel optics, electronics, and detectors used for other diagnostics. Hence frequent maintenance and replacement even under burning plasma conditions will not be necessary. From these advantages, the FWIR are promising candidates for a reactor diagnostics. The FWIR have been tested and its feasibility is demonstrated on DIII-D. In order to overcome fringe jump errors and ensure measurement failure independence, a fast wave pulse delay technique was tested on DIII-D. The density evaluated from the measured pulse delay is consistent with that measured with the CO_2 laser interferometer. The frequency ranges of the FWIR in future reactors are similar to those on DIII-D, which increases the relevance of this work for future machines.