Graphite millimeter-wave waveguide and mirror for high temperature environments

A graphite helix corrugated waveguide with a miter mirror has been fabricated and used for 135 GHz pyrometer measurements on a high temperature plasma arc furnace. The guide has an internal diameter of 3.81 cm, a length of 123 cm, and a corrugation of 32 grooves/inch. One end of the guide was sealed with a Teflon window having moth eye surfaces to reduce reflections. The room temperature insertion loss of this guide assembly for HE₁₁ mode propagation and launch was measured to be 0.5±0.1 dB. It was used successfully in a high temperature environment where the miter mirror end reached incandescent temperatures in access of 1200°C. High temperature graphite surface corrosion typically increased the insertion loss to 1.2±0.2 dB but did not significantly affect the beam divergence