Millimetre wave attenuation of prototype diagnostic components for the ITER bolometers

Abstract

Bolometers in current and future fusion devices, in particular those in ITER, are vulnerable to stray radiation from electron cyclotron resonance heating (ECRH) which results in measurement errors for plasma radiation detection. To protect the detectors from this stray radiation in the millimetre wavelength range, dedicated diagnostic components have been designed and tested. One option is to place a top plate which contains a microwave-reflecting grid onto the collimators. Another option investigated is the coating of the collimator channels using a microwave absorbing ceramic. Measurements of the mm-wave attenuation of the collimator in front of the bolometer detectors with and without top plate or coated collimator channels have been performed in the frequency range of 125–420GHz. The attenuation factor of the collimator channels at 170GHz (the ECRH frequency for ITER) with neither microwave grid nor coating is typically 10dB. The coating enhances this to 40dB and including the microwave grid yields at least an attenuation factor of 70dB, which is sufficient to reduce the residual ECRH induced signal significantly below the one due to plasma radiation. Placing a bolometer camera (collimator connected to detector housing) inside the isotropic microwave field of the test facility MISTRAL, the attenuation factor of the full diagnostic set-up using a top plate was determined to be in the order of 45dB. This degraded attenuation implies that particular attention has to be paid to design and quality control of the joints of diagnostic components to prevent microwave leakage.