Operating experience with ESCAR magnet cooling system

The ESCAR magnet cooling system has been successfully demonstrated. This two-phase helium cooling system includes a CTI-Sulzer gas-bearing turbine refrigerator with two-stage compression by oil-lubricated screw compressors, 120 m of 5-cm-diameter vacuum-insulated transfer line and twelve series-connected magnet cryostats with weirs for liquid level control. The refrigeration plant provides up to 1900 W of refrigeration at 4.5 K with a mass flow of 113 g/s. Heat load within the transfer line has been measured at 0.25 W/m in sub-system testing. Cool-down times to 4.5 K for the 12 warm-iron magnets with a cold mass of 2500 kg have been about 12 hours. The magnet cryostats separate liquid by gravitational extraction and fill in sequence at a rate of up to 400 ℓ/hr. A heater in the transfer line allows adjustment of the inlet coolant quality (ratio of gas to liquid) to the cryostats; flow instabilities were not present and could not be induced. Pressure levels in the cold bore, beam orbit space were below 10^-10torr. Severe pressure transients, incurred as a result of magnet transitions, have been safely handled both in terms of refrigerator response and cryostat pressure relief. Large gas loads at the compressor suction following magnet transition have not caused overloading or interruption of the refrigeration plant output. An electrical arc punctured the helium vessel and allowed liquid helium to flow into the vacuum space. This was handled by the relief system with no additional damage.