The vacuum vessel that will contain the JT-60SA plasma is designed to operate at 50°C. It was warmed up to this temperature over 12-13 June by circulating hot nitrogen gas through its double wall structure.

The helium coolant in the superconducting magnets and associated plant was purified by circulating it at room temperature starting on 03 June. By 13 June the dewpoint was below -70°C and less than 7 ppm nitrogen could be detected at the magnet outlet.

The cooling of the JT-60SA magnets started on 14 June and is proceeding steadily. The distribution of helium is carefully optimised to avoid excessive differences in temperature for the components while maximising heat extraction. Nevertheless, the combined mass of the toroidal field, equilibrium field and central solenoid magnets is about 640 tonnes, so it takes a long time to get down to 4 K! Today the magnets have reached about 173 K (-100°C).

Above 80 K (-193°C) refrigeration is provided using liquid nitrogen. Consumption is now around 1400 litres per hour, which means 5 deliveries by truck every day.

Following the successful leak testing of the cryostat and the helium pipes, it was confirmed that also the main vacuum vessel is leak free.

This week a number of experiments have been made in the cryostat in order to test new vacuum monitoring sensors that have been installed.

Now helium gas is being circulated at room temperature through the magnet cooling pipes and the cryoplant purification system (80 K adsorber). This will flush out any impurities such as air from the system ready for cooling down. In parallel several dry nitrogen purges have been performed in the JT-60SA cryostat to help remove water vapour adsorbed on its multi-layer insulation.

JT-60SA operations restarted on 30 May when the flanges providing access to the cryostat were sealed closed again and vacuum pumping started. This was a big moment for us after the long suspension for improvement works!

Evacuation of both the vacuum vessel, which will eventually contain the tokamak plasma, and the cryostat surrounding the machine core proceeded smoothly. Now both the cryostat and the numerous helium pipes inside it, which will supply coolant to the superconducting magnets, have been successfully leak tested.