A reliable temperature control system for the Alcator C-MOD vacuum vessel

In Alcator C-MOD, a complex magnetic field is used to hold a hydrogen plasma in a vacuum. In normal operation, the magnets and support structure are cooled to liquid nitrogen temperature (-200°C) to take advantage of the reduced resistivity of copper and the increased strength of stainless steel at cryogenic temperatures. However the vacuum vessel, which is surrounded by the cold magnets, must be kept relatively warm (20°C) to maintain the integrity of the vacuum seals. Once C-MOD has begun operation, the temperature control system must operate continuously for months at a time. The temperature control system includes 454 heaters of various types with a nominal electrical load of 250 kW. A programmable logic controller (PLC) monitors 472 K-type thermocouples to measure vacuum vessel temperatures and controls the heaters via banks of solid state relays. The relays by themselves can only permit an on-off control of the heaters. The PLC software makes it possible to implement a superior proportional-plus-integral (PI) control scheme with electrical load leveling. The PLC also provides data logging and on-line diagnostic capabilities not as readily available in a hard-wired system. The construction of C-MOD is described briefly with an emphasis on thermal considerations. The temperature limits for the various components and the overall performance requirements for the vacuum vessel temperature control system are discussed. The design of the temperature control hardware and software is presented in some detail. Problems discovered during the initial C-MOD run period, which required changes in the heaters and controls, are described. Some of the faults and failures that plague any large system with hundreds of active components, thousands of connections, and miles of wire are also discussed