Propagation of nitrogen gas in a liquid helium cooled vacuum tube following sudden vacuum loss – Part I: Experimental investigations and analytical modeling

Abstract

This paper describes propagation of near atmospheric nitrogen gas that rushes into a liquid helium (LHe) cooled vacuum tube after the tube suddenly loses vacuum. The loss-of-vacuum scenario resembles accidental venting of atmospheric air to the beam-line of a superconducting particle accelerator and is investigated to understand how the in-flowing air will propagate in such geometry. In controlled experiments, we simulated loss of vacuum by rapidly venting a large reservoir of nitrogen gas (a substitute for air) to a vacuum tube immersed in a LHe bath at 4.2 K. The resulting rise in the tube pressure and temperature were measured by pressure probes and thermometers arranged along the tube length. The data show that the propagation of nitrogen gas in the LHe cooled vacuum tube is orders of magnitude slower than in the same tube at room temperature. Interestingly, the gas front speed in the LHe cooled tube also decreases along the tube. A gas propagation model developed by employing conservation of mass identifies mass transfer (gas condensation in the tube) as the principal cause of the slow propagation as well as of the front deceleration. Some limitations of this analytical model are discussed in the context of quantifying the propagation speed. The speed obtained from direct measurements is seen to decrease exponentially along the tube. This exponential decay form of the propagation speed well represents the data from experiments that have different mass flow rates of nitrogen gas flowing into the vacuum tube after venting.

Epoxy encapsulation of the Cernox™ SD thermometer for measuring the temperature of surfaces in liquid helium

“…The sensitivities agree very well with those documented in [2]. These encapsulates were then used in a series of experiments [6,7] (column B of Table 1), which involved several thermal cycles between 295 K and 4.2 K. After these experiments, we performed systematic tests to study the shift in calibration due to potting. Columns from C to F of Table 1 show the chronology of these thermal tests.…”

Heat and mass transfer during a sudden loss of vacuum in a liquid helium cooled tube -- Part III

“…In order to better understand the complex coupled heat and mass transfer processes involved in a beamline vacuum break event, pioneering work has been carried out in our cryogenics lab by Dhuley and Van Sciver via venting room-temperature nitrogen (N 2 ) gas from a buffer tank to a LHe cooled vacuum tube [9,10]. Their experiments with normal liquid helium (He I) showed that the gas-front propagation slowed down nearly * Corresponding: wguo@magnet.fsu.edu exponentially.…”