In order to analyze and control the processes within nuclear fusion reactors, experts need sensitive measuring instruments. As the Fraunhofer Institute for Microtechnology and Microsystems (IMM) reports, determining the power emitted from the fusion plasma is crucial here.
Strong alternating loads between vacuum and ventilation
Radiation detectors called bolometers are used as detectors, and they have to provide valid data even under extreme conditions: "We have high-energy neutrons in a very high density, high exposure to particularly hard X-rays, extreme temperatures, alternating loads in terms of vacuum and ventilation - all aspects that require considerable caution in the choice of materials," explains Stefan Schmitt, Group Leader Special Sensor Technology at the IMM.
In order to meet these requirements, the experts have developed a silicon chip measuring around 20 × 23 mm², on which four individual sensors are located. These each have two absorber surfaces measuring 1.5 × 4 mm². One of these absorbers catches the light coming from the plasma along a narrow line of sight, causing its temperature to rise. This increase is measured by resistance meander made of platinum on the side facing away from the absorber: the resistance increases equivalently. In this way, the sensor records the radiation power in the plasma from infrared to the hard X-ray range.
The experts can assign this power to spatial points in the plasma using the measurement data from the numerous lines of sight in the reaction vessel, which are aligned to complement each other, and thus calculate a cross-sectional profile of the fusion plasma. The silicon chips manufactured at the IMM are installed in cameras for this purpose, consisting of a head in which the chip is inserted and an aperture system. The cameras make it possible to use the various measurement signals to assess how well the plasma control is running in the reactor on the one hand and to determine the overall energy balance on the other.
Bolometer for specific diagnostics
One challenge was the high energies that prevail in a fusion reactor. This means that most materials are simply radiated through. Schmitt's research team therefore designed the gold or platinum absorbers to be comparatively thick: they measure around 20 μm.
The conductor or resistance meander is made of platinum, as the material does not change even when exposed to high levels of radiation. By using gold absorbers and special carbon coatings, which absorb visible light even better on the absorber, the scientists have succeeded in developing bolometers that are mechanically and electrically highly stable for every application, according to the IMM press release.
These bolometers are already being used in renowned fusion research facilities around the world, including ASDEX Upgrade in Garching, Wendelstein 7-X in Greifswald and East in China. They have also been modified for the world's largest fusion experiment, Iter, at the Cadarache nuclear research center in the south of France.