Bolometry provides an absolutely calibrated measurement of the radiation emitted by the plasma, which enters the total energy balance.
The principle of bolometry is the absorption of the radiation by a suitable absorber. The absorbed power increases the absorber’s temperature. Hence, the absorbed power can be determined by measuring the temperature.
The bolometers developed at IPP are miniaturized metal resistor bolometers. Thin film absorbers deposited with a thickness of up to 12 µm on only 3 µm thick membranes capture the light emitted from the plasma. A meander-shaped metal conductor on the back side measures the temperature change through the change in its electrical resistance. In order to increase accuracy and to compensate for changes of the ambient temperature two absorbers are operated jointly in a bridge circuit, and one of the absorbers is shielded from the plasma radiation.
ITER presents special challenges for the development due to the extreme environment close to the plasma. The detectors have to withstand high neutron fluxes as well as ambient temperatures exceeding 200 °C while delivering reliable measurements.