Use of near-infrared detector to sense RF antenna heating

The three antennas used for ion cyclotron heating (ICH) experiments on DIII-D have experienced localized heating of the Faraday shield rods during plasma operations which has resulted in some melting. This melting is of great concern not only because of the damage it does to the rf system´s ability to deliver RF to the plasma, but because of its potential to contaminate the plasma during a shot and cast the experimental results from the shot into question. A real-time sensor to detect the temperature of the antennae during plasma operations is described. The sensor uses an avalanche photodiode (APD) with sensitivity from 0.4 to 1.0 μm to monitor the temperature of the antennae. Calculations for the detector sensitivity based on Planck´s law are compared with experimental results and detector data taken during plasma operations are presented