High Field Side Scrape-off-Layer Reflectometry

Microwave reflectometry is commonly used in plasmas to measure density profiles and fluctuations. Using reflectometry to measure scrape-off-layer density profiles and fluctuations on the high field side of a tokamak is desired to address important boundary physics issues as well as possible heating and current drive applications. This application of reflectometry, however, has not been done before due to the challenging spatial constraints, large frequency bandwidth ratio, and low frequencies necessary. While the frequency sources and transmission line are challenging, the most challenging aspect is the design of the antenna in a highly spatially constrained and high magnetic field region. Broadband antennas with coaxial inputs are available commercially but will need to be modified for the nuclear fusion environment. This poster outlines preliminary results for microwave modeling and bench-top testing of a commercial 5-27 GHz antenna for this reflectometry application. Plans for possible modifications of the antenna to measure scrape-off-layer densities on the high field side of a tokamak are discussed.