Researchers from the FOMInstitute for Plasma Physics Rijnhuizen and the Technical University Eindhoven installed a feedback system at the TEXTOR-tokamak in Jülich. The system detects so-called magnetic islands with great precision and immediately suppresses them. The technology may be of great importance for ITER.

Magnetic confinement is essential to reach fusion conditions in a plasma. However, chaotic fluctuations in the plasma can disturb the magnetic field and considerably weaken the confinement. One example is the so-called tearing mode, which creates rotating magnetic islands that rip the confining magnetic fields apart.

An effective way of battling the tearing mode has been available for some time: heat the magnetic island at exactly the right spot using powerful microwaves and it will shrink to harmless size. The islands are detected thanks to the tiny amounts of microwaves they emit themselves. However, to avoid blinding the sensitive detectors with the strong heating beam, the detection of islands is typically carried out far from the heating system. Assumptions have to be made about the islands behaviour 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 the 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 reflection of almost the entire heating beam while allowing the measuring signal to pass through. In practice, only a very narrow frequency band is able to pass the filter.

The design was put to the test at TEXTOR in Jülich, with great success. Measurement of the islands continues during heating, so that the suppression can be observed in real time. The system automatically and continuously detects the islands and destroys them. ITER 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 instrumentarium.

Erik Min, FOM