Fabry-Pérot Cavity Measurement Method of Ohmic Loss in ECH Transmission Lines

Electron cyclotron heating (ECH) is a widely used heating method in magnetic confinement fusion. Transmission efficiency is an important factor for upcoming projects with long (~100m) ECH transmission lines, such as ITER. However, obtaining an accurate measurement of the ohmic loss in millimeter wave over-moded waveguide is difficult because the loss can be extremely low (~0.01 dB/m). Accurate measurements require high power or large test assemblies. We aim to demonstrate a new method of measuring ohmic loss through a waveguide section in a Fabry-Pérot cavity setup. The setup consists of two photonic mirrors with high reflectivity to trap the wave inside the waveguide and create a high Q resonator. Using a vector network analyzer, we measure the transmission through the cavity, and determine the wall loss from the bandwidth of the measured resonances. The method provides a measurement of the ohmic loss from short waveguide sections at low power, which will allow more efficient characterization and prototyping of ECH transmission lines, contributing to higher efficiency waveguide design.