LHD Cryogenic-Control System Performance Under Various Operating Conditions

The Large Helical Device (LHD), an experimental fusion apparatus, has conducted a second experiment cycle. The infrastructure of the LHD cryogenic control system (TESS) is a combination of a distributed control system with an integrated controller linked by multiple LANs. The integrated controller shares information with four subsystems: the helium refrigerator/liquefier, the helical coil, the poloidal coil and the superconducting busline, via reflective memories. Each subsystem has a VME bus and a workstation for process control and programming. Cooldown processes of each subsystem are implemented independently by the temperature levels. Sequence programs are developed to change cooling modes for subsystems corresponding to their temperatures. Program Control Units (PCUs) proceed cooldown and warmup process in terms of gaseous-helium supply temperature and mass-flow rates. Temperature distributions in the subsystems are monitored and process will be interrupted if temperature gradients exceed 50 K. In the case of coil quench, emergency sequence programs are executed to prevent any damage to the apparatus. The paper describes the performance of LHD-TESS during cooldown, steadystate operation, and the case of emergencies.