Abstract
Due to imperfection of the single path absorption, ECRH at ASDEX Upgrade (AUG) is always accompanied by stray radiation in the vacuum vessel. New ECRH scenarios with O2 and X3 heating schemes extend the operational space, but they have also the potential to increase the level of stray radiation. There are hazards for invessel components. Damage on electric cables has already been encountered. It is therefore necessary to monitor and control the ECRH with respect to the stray radiation level. At AUG a system of Sniffer antennas equipped with microwave detection diodes is installed. The system is part of the ECRH interlock circuit. We notice, however, that during plasma operation the variations of the Sniffer antenna signal are very large. In laboratory measurements we see variations of up to 20 dB in the directional sensitivity and we conclude that an interference pattern is formed inside the copper sphere of the antenna. When ECRH is in plasma operation at AUG, the plasma is acting as a phase and mode mixer for the millimeter waves and thus the interference pattern inside the sphere changes with the characteristic time of the plasma dynamics. In order to overcome the difficulty of a calibrated measurement of the average stray radiation level, we installed bolometer and pyroelectric detectors, which intrinsically average over interference structures due to their large active area. The bolometer provides a robust calibration but with moderate temporal resolution. The pyroelectric detector provides high sensitivity and a good temporal resolution, but it raises issues of possible signal drifts in long pulses.
Highlights
When ECRH is in plasma operation at ASDEX Upgrade (AUG), the plasma is acting as a phase and mode mixer for the millimeter waves and the interference pattern inside the sphere changes with the characteristic time of the plasma dynamics
The electron cyclotron heating system at ASDEX Upgrade (AUG) consists of seven units, which routinely operate at f=140 GHz (λ0 = 2.1 mm), three units can operate at 105 GHz
The Sniffer antenna is connected to the AUG vacuum vessel by an oversized waveguide
Summary
The electron cyclotron heating system at ASDEX Upgrade (AUG) consists of seven units, which routinely operate at f=140 GHz (λ0 = 2.1 mm), three units can operate at 105 GHz. Up to 4 MW of microwave power are launched at four different toroidal positions (Figure 1). A small fraction of this power can be found elsewhere in the vacuum vessel, whenever the absorption in the plasma is incomplete. This can happen due to a variety of geometrical effects, where for example the variation of the refractive index in the plasma gives rise to a reflection. We refer to the non-absorbed ECRH power as millimeter wave stray radiation. If its intensity is too high, it may damage in-vessel components
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