Baking the vacuum vessel

On 10 July the refrigerator turbines were started in order to cool the magnets down from 80 K to their operating temperature of 4.5 K. Following this the temperature of the vacuum vessel which will contain the JT-60SA plasma has been increased from 50°C to 200°C.

It is necessary to ‘bake’ the vacuum vessel like this in order to achieve sufficiently clean conditions for the plasma. The high temperature helps to drive out water and other impurities from the surfaces inside the vessel so that they don’t end up polluting the plasma. The quality of the vacuum will be a key factor in achieving a tokamak plasma. The vacuum pressure increases temporarily during the bake, but should ultimately be lower and the plasma-facing surfaces should be cleaner.

Meanwhile the magnets continue to get colder. They are protected from the increased thermal radiation from the vacuum vessel by the 80 K (-193°C) double-walled helium-cooled stainless steel thermal shield surrounding them. The vacuum vessel bake causes the highest thermal loads on the cryogenic system, which now consumes 8 truck loads of liquid nitrogen each day.