Coils superconducting

The central solenoid is the heart of the JT-60SA tokamak and is critical to initiate the plasma and then to drive current in it. Plasma current is a defining feature of a tokamak which substantially enhances the confinement of the superheated particles.

The central solenoid is wound from niobium tin (Nb3Sn) cable-in-conduit conductor (CICC), which in the absence of any magnetic field becomes superconducting at about 18 K (-255°C). Early on 22 July the four modules of the central solenoid reached this temperature and the resistance of the magnets dropped to almost zero, demonstrating that the central solenoid is superconducting.

The 18 toroidal field coils and 6 equilibrium field coils are made using niobium titanium (NbTi) CICC. Their superconducting transition occurs at about 9 K (-264°C), meaning that it took a couple more days to reach and could be observed on 24 July.

By allowing current to flow with no resistance (unlike a conventional electromagnet), the superconducting magnets of JT-60SA will allow plasmas to be produced for long durations without excessive power consumption. JT-60SA is the largest superconducting tokamak built so far. The largest magnet has a diameter of 12m.

Meanwhile the vacuum vessel that will contain the JT-60SA plasma continues to be ‘baked’ at an elevated temperature of 200°C in order to remove impurities attached to its surface.

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.