Detection of stray radiation power
Monitoring the actual stray radiation power density can
- provide a safety mechanism in case of excessive power levels,
- be used to tune the ECH launch parameters for optimal absorption, and
- provide valuable physics data on plasma absorption and emission.
To this purpose sensors are being developed that have a wide dynamic range and a stable response to the incident power. It is proving very difficult to construct such sensors with microwave (semiconductor) detectors as these respond to one particular polarisation and are monitoring at one fixed position. In a stray radiation field, in contrast, the polarization and power fluctuate over the detector aperture and over time. Therefore we resort to bolometers with the penalty that they are slower but give a stable signal and have a large dynamic range. Examples are a fluid bolometer in use at the MISTRAL facility at the Max Planck Institute in Greifswald and the development of a microwave bolometers for ITER. Figure 4 shows a bolometer that is presently developed to measure stray radiation levels in ITER.
Fig. 4. Microwave bolometer under development. The device is using two bolometer bodies, one plain copper and one coated with a microwave absorber. A thermocouple is looped through over both bodies thus providing a differential measurement of the two bodies. This way incident microwave power is measured, but nuisance signals (heat sources incident on both bodies) are rejected. Work carried out by Nick Maassen and Karsten Arts.
While the ITER microwave bolometers are primarily designed for protection purposes (e.g. ECW interlock) they may also be used as a sensor in the Sensorics program of our group.