Low-Power Testing of Losses in Millimeter-Wave Transmission Lines for High-Power Applications

Abstract

We report the measurement of small losses in transmission line (TL) components intended for high-power millimeter-wave applications. Measurements were made using two different low-power techniques: a coherent technique using a vector network analyzer (VNA) and an incoherent technique using a radiometer. The measured loss in a 140 GHz 12.7 mm diameter TL system, consisting of 1.7 m of circular corrugated waveguide and three miter bends, is dominated by the miter bend loss. The measured loss was 0.3±0.1 dB per miter bend using a VNA; and 0.22±0.1 dB per miter bend using a radiometer. Good agreement between the two measurement techniques implies that both are useful for measuring small losses. To verify the methodology, the VNA technique was employed to measure the extremely small transmission loss in a 170 GHz ITER prototype TL system consisting of three lengths of 1 m, 63.5 mm diameter, circular corrugated waveguide and two miter bends. The measured loss of 0.05±0.02 dB per miter bend may be compared with the theoretical loss of 0.027 dB per miter bend. These results suggest that low-power testing of TL losses, utilizing a small, simple TL system and a VNA, is a reliable method for evaluating performance of low-loss millimeter-wave TL components intended for use in high-power applications.

Loss Estimate for ITER ECH Transmission Line Including Multimode Propagation

“…However, their results were obtained exclusively for the HE 11 and TE 01 modes. We have extended that theory to estimate the loss for higher order LP mn modes.…”

“…The HE 11 mode of a corrugated waveguide is the operating mode of the TL. The TL provides extremely low Ohmic loss of the HE 11 mode in the straight waveguide sections.…”

“…However, research shows that gyrotron beams coupled onto the TL often excite high order modes (HOMs) in addition to the fundamental HE 11 mode. The excitation of HOMs is caused by imperfections in the Gaussian-like beam from the gyrotron (phase errors, incorrect waist size, etc.)…”

“…High output mode purity therefore can only be achieved with significantly higher input mode purity. Combining both Ohmic and mode conversion loss, for 1 MW of power generated by the gyrotron, the output power in the HE 11 mode at the end of the ITER TL can be roughly estimated in theory as 920 kW times the fraction of input power in the HE 11 mode.…”

Millimeter-Wave Heating, Radiometry, and Calorimetry of Granite Rock to Vaporization

“…Longer MMWs would remove this limitation because small particle Rayleigh scattering losses scale as 1/λ 4 . Furthermore, MMW sources are more efficient and have higher average power relative to lasers [11,12] and can be efficiently guided as a beam over long distances [13] compatible with borehole dimensions. This suggests the economic possibility of completely vaporizing rock to facilitate extraction as a nanoparticle particle smoke.…”