Notch Filter in 70 GHz Range for Microwave Plasma Diagnostics ∗ )

A notch filter for the rejection of stray light from gigahertz range heating sources was developed to protect a vulnerable microwave plasma diagnostic system. As one of the applications, we consider the installation of the notch filter into the receiver of a collective Thomson scattering diagnostic in the Large Helical Device. Experimental observations indicate that two types of notch filters are required for main and spurious mode rejection; they have very narrow, steep shapes to avoid disturbing the diagnostic signal. On the basis of numerically simulated results, notch filters were fabricated, and their performance was evaluated. An attenuation level of 35 dB at 74.746GHz with a 3 dB bandwidth of 0.49GHz is achieved by two pairs of resonator cavities. This attenuation is acceptable in our study.