Suppression Control of Thermal Imbalance in Cooling of Cryopumps for JT-60 Neutral Beam Injector

Abstract

A cryopump system for JT-60 neutral beam injector (NBI) is composed of 14 cryopumps with the largest total pumping speed of 20, 000m³/s in the world, which are cooled by liquid helium through a long-distance liquid helium transferline of about 500m. An automatic control method has been developed and tested of the 14 cryopumps in cool-down and steady-state modes. Features of the automatic control method are as follows:
(1) Suppression control of the thermal imbalance in cooling-down of the 14 cryopumps.
(2) Stable cooling control of the cryopump due to liquid helium supply to six cryopanels by natural circulation in steady-state mode.
(3) Stable liquid helium supply control for the cryopumps from the liquid helium dewar in all operation modes of the cryopumps, considering the helium quantities held in respective components of the closed helium loop.
In the automatic operation of the cryopumps by the newly developed control method, the cool-down of 14 cryopumps was completed in 16h from the start of cool-down of the system and the cryopumps was stably cooled by natural circulation cooling in steady-state mode.