A superconducting point contact having the properties of a Josephson junction

Abstract

layers of thin nylon ribbon, covered with aluminum, to reduce heat influx by radiation. Measurements of the heat influx from the surroundings to the inner tube were made to compute the evaporation of liquid helium when siphoning. Cold gaseous helium at an initial temperature of 5 8 K is supplied to the siphon from a liquid helium vessel. The rise in temperature of the gas over a distance of 700 mm in the inner flexible tube was measured, with two thermometers, and depends on the mass flow and on the heat influx to the part of the siphon between the thermometers. The temperature was measured by miniature carbon resistors calibrated between 4 and 20 K. The flow of gas was measured by a measuring gas holder and a gas meter. Siphons of three forms were tested (a) of ordinary design with rigid tubes, (b) flexible of corrugated tubes, and (c) flexible with the inner corrugated tube wound with aluminized nylon ribbon (2 layers). The heat influxes from the surroundings to the inner tube were as follows (averaged values normalized to a 6 mm diameter working passage) (a) 09 to 1 W (linear m ) . (b) 30 to 34 W rn -1 , and (c) 18-21 W m -1 . These heat influxes are mainly due to radiation from the outer tube and to the heat conductivity of the spacers, since the heat transfer through the residual gas is negligibly small (the vacuum is better than 5 x 10 -5 torr). The calculated heat loss through the distance pieces, due to their heat conductivity is ~ 04 W (linear m) -~ . Winding the inner corrugated tube with aluminized ribbon almost halves the radiation heat influx. During use with liquid helium the temperature of the inner tube of the siphon is somewhat lower than in the experiments (42 K instead of 6 -12 K), but this does not have any noticeable effect on the loss of cold to the surroundings. The results obtained are therefore completely applicable to the case of siphoning liquid helium. More than a year's use of flexible siphons in these laboratories has confirmed their high performance and their greater applicability compared with rigid designs.