A very strong magnet

A 150ton magnet at the heart of an international collaboration to build a huge experimental nuclear fusion reactor has passed its initial operating test.

The International Thermonuclear Experimental Reactor (ITER) will use superconducting magnet coils around a toroidal vessel to control and induce an electrical current through plasma. Fusion reactions take place when the plasma is hot and dense enough, and contained long enough for the nuclei in the plasma to start fusing together.

The magnet was recently run under full operating conditions and produced a magnetic field of 13T (about 260 thousand times more powerful than the earth’s magnetic field) with a stored energy of 640MJ at a current of 46,000A.

The purpose of the magnet (claimed to be the most powerful of its kind in the world) is to demonstrate performance parameters and manufacturing methods for the larger magnets planned for the ITER. These magnets will provide the fields needed to initiate and sustain the electrically charged gas for the fusion reaction.

Researchers hope to increase the speed at which they can reach the 13T point and come back down from it. `The magnet is only doing its job for this particular magnetic fusion application when we’re changing the magnetic field, so the faster we can do so the more effective it is,’ said Dr. Minervini, an MIT engineer. The goal is pulsed operation at rates up to 1.2T/s.

The scientists were happy to note the stability of the magnet system in charging to full current and magnetic field. This was achieved without training or quenching; time-consuming procedures common in the early operation of many superconducting magnets. This, comments Dr Minervini, `has important implications for application of this technology to other electric power systems.’

The magnet consists of over 1,000 wires twisted into six cables that twist around a hollow tube. The whole arrangement sits inside a small tube. This device (the Cable-in-Conduit-Conductor) is then wound into a coil resembling a giant spring that makes up the bulk of the magnet.

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