Cooldown underway

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.