Abstract
In ITER, an Electron Cyclotron Emission (ECE) diagnostic is planned to measure the electron temperature by measuring the cyclotron radiation in the frequency range of 70-1000 GHz. The cyclotron radiation is usually of low power and needs to be transported with low attenuation over a long distance of ~ 43 m, through a suitable transmission system. Pertaining to long distance, the transmission system will consist of straight waveguide sections, miter bends and waveguide joints. Low power, low loss transmission in a broadband frequency range over long distance makes the design of the transmission system challenging. To arrive at a suitable transmission system, attenuation measurements of three types of transmission lines (TLs) have been performed i.e. circular smooth walled, corrugated and dielectric coated waveguide. A polarizing Michelson interferometer based on Martin-Puplett design has been used to measure the spectrum from waveguide set ups and liquid nitrogen has been used as the black body radiation source. The measured spectrum shows atmospheric water vapour absorption lines in all types of TLs. The preliminary measurement shows that the attenuation of smooth walled waveguide is found to be comparable to corrugated waveguide up to ~ 600GHz and better than corrugated waveguide above 600 GHz for the chosen set of experimental conditions. Further, to avoid water absorption lines, a smooth walled TL is evacuated up to rough vacuum (~10-2mbar) and it was observed that the attenuation is decreased and overall transmission is improved.
Highlights
In ITER, the electron cyclotron emission (ECE) diagnostic [1] will be used to determine the plasma electron temperature, plasma energy, Electron Cyclotron Emission (ECE) radiated power and runaway electrons in the frequency range 70 GHz - 1 THz by measuring the cyclotron radiation
A polarizing Michelson interferometer based on Martin-Puplett design has been used to measure the spectrum from waveguide set ups and liquid nitrogen has been used as the black body radiation source
The preliminary measurement shows that the attenuation of smooth walled waveguide is found to be comparable to corrugated waveguide up to ~ 600GHz and better than corrugated waveguide above 600 GHz for the chosen set of experimental conditions
Summary
In ITER, the electron cyclotron emission (ECE) diagnostic [1] will be used to determine the plasma electron temperature, plasma energy, ECE radiated power and runaway electrons in the frequency range 70 GHz - 1 THz by measuring the cyclotron radiation. The typical ECE system consists of front end optics including in-situ calibration sources, a set of transmission lines (TLs), and ECE radiation measurement instruments like radiometers and Michelson interferometers. One of the challenging requirements for the ITER ECE diagnostic is achieving low attenuation in the long TL (~43m length) for the broad frequency range. This is challenging because of the low power (~ few nW) thermal radiation emitted from the in-situ calibration sources located in the port plug which needs to be measured by the ECE instruments that are located nearly ~ 43 m away in the diagnostic building. The details of TL components, experimental set up and the results of the comparative studies of attenuation in three types of TLs will be discussed in this paper
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