Sudden Vacuum Loss in Long Liquid Helium Cooled Tubes

Sudden vacuum loss in straight long tubes cooled by liquid helium is investigated. The scenario resembles an accident in a superconducting particle accelerator when the beam-cavity suddenly loses its vacuum to atmosphere. Following this accident, air will propagate down the vacuum channel and freeze on the cold walls. To warrant against catastrophes of this accident, it is vital to know the propagation speed of air in the vacuum space and the heat load on the helium bath. An experimental setup has been developed to determine these parameters. Experiments are conducted wherein a large nitrogen gas tank is rapidly vented to a high vacuum tube (~10^-4 Pa) immersed in liquid helium at 4.2 K. The measurements comprise of the tube pressure to determine the propagation speed, the tank pressure to determine the gas mass flow rate into the tube, and the tube temperature to estimate the heat load. Flash solidification of the gas on the cold tube, which is apparent in the measurements, limits the propagation speed to the order of 10 m/s. Based on the mass flow measurement a heat deposition rate of 60 kW/m² on the vacuum tube is estimated, while the heat transfer rate to the helium bath is predicted to exceed 20 kW/m².