DIII-D cryogenics control system status

The DIII-D cryogenics system provides liquid helium for various auxiliary systems at the DIII-D tokamak facility. The system described here executes control and supervision of the cryogenics plant and associated load systems which include four neutral beam injectors, superconducting magnets, and an in-vessel tokamak cryocondensation pump. The recent addition of this divertor cryopump represented a major increase in the scope of the control system end greater need for reliable and automatic operation. The pump must be precooled, cooled and regenerated in a relatively short period of time under automatic control. Since the pump is located in the tokamak primary vacuum, coordination with the machine vacuum control system and consideration for the requirements of physics operations and vessel wall conditioning are required. A programmable logic controller is the central element in the cryogenics control system and exercises direct or supervisory control over the liquefier, gas management and loads. In the time since the control system was built during 1989, additions and improvements have been made to simplify operation and support upgrades. Cryogenics system capacity has been upgraded by the purchase of a new 150 l/hr helium liquefier, a second 400 hp helium compressor and additional gas storage. The new liquefier incorporates a controller of its own which exchanges information with the cryogenics control system. This independent controller relieved the main system of liquefier process control tasks but necessitates much finer control of the system high and low pressures. The control system upgrades and automation will be discussed with emphasis on the divertor cryocondensation pump control.