Microwave stray radiation measurement techniques

Abstract

In magnetic-confinement fusion experiments, such as tokamaks and stellarators, microwave stray radiation loads occur due to non-absorbed electron cyclotron heating power, or – at very high electron temperatures – due to cyclotron emission from the fusion plasma itself. The metallic first wall of the plasma vessel reflects most of the non-absorbed power and generates a more or less homogeneous stray radiation field, which causes ohmic and dielectric heating in lossy components, while in microwave receivers outside the vessel it may cause overload or even destruction of mixers and detectors. These issues are understood and mitigation measures are developed or already in place. However, accurate and fast diagnoses of stray radiation provides a challenge: stray radiation may not be a truly scrambled mixture of k-vectors and polarisation. In such case, the signal of a single-mode microwave detector will show strong fluctuations in time. Bolometers, with a size such that many modes are simultaneously measured, provide a much more stable signal due to the integration, but the time response is of the order of seconds. And an inherent aspect of bolometers is that at elevated temperatures they become non-linear due to radiative heat losses. A caloric measurement, using for instance a water-cooled microwave load, gives a calibrated and reproducible measurement but is very slow — typically several minutes. Due to size and water coolant, calorimetry is challenging to integrate into a fusion experiment. However, it is useful for cross-calibration of bolometers. In this paper microwave stray radiation is briefly introduced, then followed by an overview of measurement techniques in use at the Wendelstein 7-X stellarator (W7-X) as well as development work together with ITER.