Transmission Line Design for Broadband Microwave Diagnostic Systems

Abstract

Up to now mostly smooth-wall waveguides are used for low power applications in fusion plasma diagnostic systems. However ohmic losses and mode conversion in mitre bends are a serious problem for precise amplitude and phase measurements e.g. for ECE and reflectometry. For microwave diagnostic systems on future fusion experiments like ITER with very long waveguide runs (typically several tens of meters), oversized corrugated waveguides propagating the HE11 mode which exhibits extremely low losses are a good candidate. Such type of transmission line is widely used for high-power applications, especially for electron cyclotron heating of plasmas, however only at single frequencies.1 The theory is presented in several publications, e.g. Ref.2,3 and measurements at single frequencies have confirmed the low attenuation of the HE11 mode.4,5 This paper assesses the possibilities and problems which arise if these transmission lines are used in broadband (low-power) diagnostic systems. Besides the low ohmic attenuation such waveguides offer several additional advantages in comparison to smooth-wall oversized waveguides like the possibility to build compact quasi-optical mitre bends. Radiated from the open-ended waveguide, the HE11 mode couples almost perfectly to the Gaussian free-space mode and therefore simplifies the antenna design. In such waveguides high order spurious modes can be suppressed by including gaps into the transmission line which cause only very low losses for the main HE11 mode. At the same time these gaps act as DC breaks and movement compensators. Due to the almost linear polarization of the HE11 mode, broadband Brewster angle windows can be applied. Two combined Brewster angle windows can be used as an effective beam splitter.