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Description
The Electron Cyclotron Emission diagnostic for ITER will provide essential information for plasma operation and for establishing performance characteristics. It will measure Core and Edge Electron Temperature Profile, ELM (Edge Localized Mode) temperature transient, NTM (Neo-classical Tearing Mode) and TAE (Toroidal Alfvén Eigenmode) δT/Te, as well as contribute to the measurement of βp and runaway electrons [1].
In ITER, the ECE measurement frequency range of interest is 70 to 1000 GHz. The ECE radiation is transmitted through oversized smooth-walled circular waveguides from tokamak to the diagnostics building where the Radiometer and Fourier transform Spectrometer are placed. A powerful 24 MW Electron Cyclotron (EC) system operating at 170 GHz shall be installed on the ITER tokamak for central heating and current drive (H&CD) applications. These RF sources give high levels of RF-stray radiation, which are potentially harmful to the low power mm-wave components in the ECE diagnostic. Therefore, we need to devise mechanisms for protecting the ECE diagnostic from these stray radiations. Employing a quasi-optical notch filter at 170 GHz within the transmission line in front of sensitive instruments appear to offer a viable solution to this problem. This quasi-optical notch filter will filter out the 170 GHz radiation before it enters the Radiometer and the Fourier Transform Spectrometer.
The quasi-optical notch filter is basically a Frequency Selective Surface with an array of square loops on a dielectric surface [2]. The filter is designed to operate at 170 GHz with a low insertion loss in the frequency range of 70 GHz to 1000 GHz. This filter is designed analytically and simulated using MATLAB and Computer Simulation Technology (CST) Microwave Studio. The FSS notch filter designed has a low insertion loss of less than 2 dB over the pass band of 70 GHz to 1000 GHz. It offers a suppression of ~ 22 dB at 170 GHz with 3 dB band width of ~ 8 GHz. Considering the complexity and the numerous challenges involved in the design of the notch filter, we have attained a fairly satisfactory performance following extensive optimization efforts aimed at mitigating the harmonic effects.
This paper presents the design of an FSS notch filter for the ITER ECE application.The methods employed for optimizing and suppressing the multiple harmonics shall also be discussed briefly.
