Abstract
A six-pole Q-band waveguide filter with a notch frequency above the Q-band has been developed for plasma diagnostics. The previous paper [Nishiura et al., J. Instrum. 10, C12014 (2015)] reported that the notch frequency exists within the standard band. In this study, the newly required notch filter extends the function, which prevents a thorny wave from being mixed into an instrument beyond the standard bandwidth of the waveguide. The mode control technique for cavities realizes a deep and sharp filter shape for Q-band notch filters with 56 and 77GHz notches, respectively. The former filter has an attenuation more than 50 dB at 56.05GHz and a bandwidth of 1.1GHz at -3 dB. The latter filter has an attenuation more than 55 dB at 76.95GHz and a bandwidth of 1.6GHz at -3 dB. The electron cyclotron emission imaging and the electron cyclotron emission (ECE) diagnostics for the Q-band implemented a pair of the fabricated filters and demonstrated the ECE measurement successfully in the intense stray radiation from a 56GHz gyrotron.
Highlights
In fusion plasma experiments, intense millimeter waves for plasma production and heating are injected into a vacuum vessel from a mega-watt class-gyrotron
The electron cyclotron emission imaging (ECEI) and the ECE diagnostics for the Q-band implemented a pair of the fabricated filters, and demonstrated the ECE
The stray radiation of the intense beams interferes with signals of measurement systems, for example, electron cyclotron emission (ECE) [1], reflectometer and collective Thomson scattering (CTS)
Summary
Intense millimeter waves for plasma production and heating are injected into a vacuum vessel from a mega-watt class-gyrotron. Intense millimeter wave radiation can damage a mixer in receivers. To avoid such situations, we have started developing a notch filter with a narrow stop band and the wide pass band in the frequency range from 74 to 80 GHz [4, 5]. The stray radiation of GHz gyrotron above the Q-band range reaches the mixers, and is converted down to the intermediate frequency (IF). We have designed a notch filter composed of an E-band waveguide with mode-controlled cavities. Those designs are reported in the references [4, 5]. A notch filter for the unwanted stray radiation, which exists above the standard band of a waveguide, requires a new concept.
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