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Recipe for success | 18/12/2014
Take one stainless steel tokamak component, bake in the oven for seven hours at 980 degrees C till it's just right…et voilà – it's ready to go into the fusion device.
Not quite that simple, but this is the procedure to eliminate unwanted magnetism from materials that will be installed in the vessel of MAST Upgrade, the new UK fusion experiment that is being built at CCFE.
Stainless steel is used because it is strong, cheap and can be non-magnetic. For MAST Upgrade, the latter property is particularly important. In a tokamak, hot plasma fuel is confined using strong magnetic coils. So machine components with low magnetism are necessary to avoid perturbing the magnetic configurations and to achieve accurate measurement of the tokamak's magnetic field. However, stainless steel as supplied can be slightly magnetic. If left untreated this would cause stray magnetic fields when MAST Upgrade is operated and also make magnetic measurements less accurate.
Baking the steel reorders its crystal structure to reduce the extent to which it becomes magnetised without significantly affecting its other properties. Over the past nine months, various parts have been heat-treated in ovens at the Special Techniques Group workshop here on the Culham site.
“It's an effective method and although it might seem low-tech, there's a hi-tech reason behind it,” explains CCFE Work Package Manager James Foster. “There isn't one ‘recipe' that fits everything, so each component has a different formula – just like in baking at home.”