Experimental investigation and analysis on the pressure stabilization of subcooled liquid nitrogen circulation for the HEPS

Abstract

The High Energy Photon Source (HEPS) is a fourth generation radiation source, where lots of optical equipment are cooled down to the liquid nitrogen temperature for the better performance. The subcooled liquid nitrogen circulation is a very advantageous cooling method of monochromators and undulators, which are sensitive to fluid pressure fluctuations. Although the pressure stabilization matters a lot but it has rarely been studied. In this paper, the causes of pressure fluctuations are analyzed, and accordingly the principle of thermal balance is proposed. Based on the analysis, three pressure stabilization methods were suggested and tested. A partially liquid nitrogen filled tank was connected to the subcooled liquid nitrogen circulation serving as a pressure fluctuation buffer. The side effect is that the heat load of the tank raises the pressure gradually. Therefore, the cold source buffer and its induced heat load need good compromise. The tank heat load was cooled by saturated nitrogen, bypass liquid nitrogen and direct gas absorption: The saturated nitrogen reuses the sensible heat of cold gas; the bypass liquid nitrogen avoids the influence of circulation heat load variation and the gas absorption prevents the heat exchanger in the tank. The test results show that the peak-to-peak value of the pressure fluctuation can be controlled in the level of ± 4.0 mbar and the RMS value less than 1.0 mbar. The pressure affect factors were empirically studied.