Miter bend loss and higher order mode content measurements in overmoded millimeter-wave transmission lines

Abstract

High power applications require an accurate calculation of the losses on overmoded corrugated cylindrical transmission lines. Previous assessments of power loss on these lines have not considered beam polarization or higher order mode effects. This thesis will develop a theory of transmission that includes the effect of linearly polarized higher order modes on power loss in overmoded corrugated transmission line systems. This thesis derives the linearly polarized basis set of modes for corrugated cylindrical waveguides. These modes are used to quantify the loss in overmoded transmission line components, such as a gap in waveguide or a 900 miter bend. The dependence of the loss in the fundamental mode on the phase of higher order modes (HOMs) was investigated. In addition, the propagation of a multi-mode beam after the waveguide was quantified, and it was shown that if two modes with azimuthal (m) indices that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are related through a constant of the motion. These theoretical calculations are useful for high-power applications, such as the electron cyclotron heating in plasma fusion reactors. In addition, this thesis develops a low-power S-Parameter Response (SPR) technique to accurately measure the loss in ultra-low loss overmoded waveguide components. This technique is used to measure the loss of components manufactured to ITER (an experimental fusion reactor) specifications, operated at 170 GHz with a diameter of 63.5 mm and quarter-wavelength corrugations. The loss in a miter bend was found to be 0.022+0.08 dB. This measurement is in good agreement with theory, which predicts 0.027 dB loss per miter bend, and past measurements [18]. The SPR was used to measure the loss in a gap of waveguide and the results were in good agreement with the well-established theoretical loss due to gap, which demonstrates the accuracy of the SPR technique. For both of these measurements, a baseline analysis determined the effects of a small percentage (1-2%) of higher order modes in the system.