Performance and Control of a Cryogenic System Cooling 1152 VLPC Channels
A liquid helium supplied cryogenic system has been designed, built, and operated continuously since September 1996. The system cools nine cassettes (128 channels each) of visible light photon counter (VLPC) chips to 6.5 K. Thermal stability of the chips has been excellent. Liquid helium is supplied from a modified CTI 1400 helium liquifier. The liquid flows into a vacuum insulated liquid nitrogen shielded cryostat where it cools the VLPC chips. Some of the helium boil off is used to reduce the cryostat heat leak. All boil off gas is returned to the compressor suction header for the Tevatron. The exceptional control of the cassette temperatures is achieved with two levels of PID control. The coarse control (All cassettes within 0.5 K) is reached by maintaining constant cryostat liquid level, cryostat pressure, and heat intercept flow. The fine tuning (All cassettes within +/-0.010 K) is achieved with automatically controlled heaters in each cassette. The operating temperature range for the chips can be set anywhere between 5 to 15 K. The cryostat heat leak is about 2.0 W at 6.5 K.
KeywordsLiquid Helium Annular Flow Heat Leak Flow Control Valve Control System Performance
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