Tuesday, September 22, 2020
Abstract: Lower hybrid current drive (LHCD) is a common method of driving non-inductive current in tokamaks, and high efficiency of this current drive is required if it is to be used in future power-generating fusion devices. The coupling of these lower hybrid waves to the plasma for current drive in tokamaks has been shown to be highly dependent on the scrape off layer (SOL) density profile in front of the lower hybrid launcher. The measurement of this density profile is critical for understanding the coupling mechanism and for simulation. Reflectometry allows for high spatial and temporal resolution measurements of the scrape off layer density profile and is planned to support the upcoming high field side lower hybrid current drive experiments on
DIII-D. The reflectometry system will utilize the O-mode cutoff frequency in the range of 6-27 GHz using frequency sources from an existing, unused reflectometer. However, due to limited space on the high field side, a new compact antenna will be required for the reflectometer to fit in the available space. This work outlines the design of a compact, ultrawide bandwidth antenna for the reflectometry measurement. The details of the optimization process using COMSOL and the final antenna design will be presented.