Design of a 170GHz quasi-optical notch filter for microwave-based diagnostics protection on the burning plasma device

Abstract

170 GHz gyrotrons will be used for Electron Cyclotron Heating (ECH) on the under-development burning plasma device, with a maximum power output of 16 MW with center frequency of 170 GHz. The reliable 170 GHz quasi-optical notch filters protect microwave-based diagnostic instruments from unabsorbed ECH power. This design utilizes a metal circular periodic structure, which is straightforward to manufacture, to achieve reflection and absorption of 170 GHz electromagnetic waves transmitted through space, thereby achieving the notch filtering effect (attenuation > 60 dB). Unlike the rectangular ring design used in low-frequency quasi-optical notch filters, the circular design caters to the shorter wavelength of 170 GHz, which necessitates reduced fabrication error tolerance. To achieve both high quality filter and higher processing yields, we compensate for the performance degradation of the notch filter caused by processing errors by optimizing the controllable metal circle radius and incident angle. This paper discusses the impact of material size selection and machining accuracy deviations on the performance of the notch filter.