Quasi-optical resonant ring for high power millimeter-wave testing

摘要

Gyrotrons of > 1-MW cw power in the 110- to 160-GHz frequency range with HE(sub 11) output beams are being developed for electron cyclotron heating (ECH) of plasmas. Windows are required for gyrotrons and for waveguide transmission systems at the plasma device to provide vacuum isolation and containment. Windows ar difficult to build for these systems because the window dielectric losses increase with frequency and the centrally peaked output power beam concentrates the power deposition near the center. Development and testing of a window independent of gyrotron development is desirable since window failure on a cw gyrotron usually means an expensive reprocessing of the entire tube or possibly even total loss. A quasi-optical resonant ring is being developed for testing of millimeter wave components, windows and low-loss materials at very high power levels using medium power level sources. The resonant ring generates a traveling wave resonance of uniform amplitude along the waveguide that is ideal for testing components and materials. Both smooth-wall TE(sub 01) mode and a corrugated-wall HE(sub 11) mode versions have been constructed. These units use highly oversized waveguide and four miter bends to form a quasi-optical resonant ring. A perforated plate miter bend serves as the input directional coupler. A water-cooled tube array is being designed for a coupler capable high-power cw operation. A theoretical power gain of > 10 is possible using the 63.5 mm HE(sub 11) version at 53 GHz. Low power measurements have been performed to confirm the operation and > 1.5 MW high power tests using a 200 kW gyrotron are expected in the near future.