Dutch control system for magnetic islands improves fusion reactor performance
25 May 2009
Dutch researchers from the FOM Institute for Plasma Physics Rijnhuizen and the Technical University Eindhoven have successfully tested a new system to keep instabilities in future fusion reactors in check. Their 'in-sightline feedback system', installed on the German TEXTOR experiment, is capable of detecting so-called magnetic islands with great precision and immediately suppress them. The technology will be of great importance for ITER, the international fusion experiment being build in the south of France.
To harness fusion energy, the process that powers the sun, a fuel gas is heated to millions of degrees Centigrade and captured in strong magnetic fields. Chaotic fluctuations in the hot fuel can destroy the magnetic cage and dramatically decrease the efficiency of any future fusion reactor. One example is the so-called tearing mode, which creates rotating magnetic islands that tear the confining magnetic field apart.
An effective way of battling the tearing mode has been known for some time: Heat the magnetic island in exactly the right spot using powerful microwaves (comparable to heating water in a microwave oven) and it will shrink to harmless size. The islands are detected by the tiny amounts of microwaves they emit themselves. However, to avoid blinding the sensitive detectors with the strong heating beam, detection of islands typically happens far from the heating system. Assumptions have to be made about the islands behavior in order to hit the right spot.
Within the ITER-NL framework, a consortium including Dutch research institutions and high-tech industry, a new system was developed that combines detection and heating of the islands in a single beamline. No small feat, since even stray radiation of one millionth of a millionth of the heating power could blind the detector completely. To avoid this, the researchers designed a 'notch filter' that blocks the tiniest fraction of the heating beam while allowing the measuring signal to pass through. PhD-student Bart Hennen from the FOM-Institute Rijnhuizen: "It is like playing darts. Only the board is located around the orbit of the moon, moving all the time and we are looking straight into the sun. With this system we are able to hit bulls eye time and again. We’re very proud of that."
The design was put to the test at the German experimental fusion reactor TEXTOR, with great success. Measurement of the islands continues during heating, so the suppression can be observed in real time. The system automatically detects an island, destroys it and seeks on. The international consortium constructing ITER, the large experiment that will take fusion research to the next level, has expressed great interest in the technology. It is expected that tearing modes will pose an important challenge to ITER’s performance, so the system would be a welcome addition to the machine’s instrumentation.