Abstract
A study of the means of reducing the heat flux into liquid helium in wide and short necked metal dewars has been carried out.
A set of vapour cooled baffles at approximately the 77 K temperature level almost completely absorbs room temperature radiation down the neck. Constant horizontal temperature profiles across the vapour column below the baffles have been observed. This suggests that there is good heat transfer between the walls of the neck and the vapour, and that the main function of the baffles is to absorb radiation. Little improvement in performance is obtained when the baffles are replaced by polystyrene foam plugs. Moreover, if a plug is left in an atmosphere of helium, the liquid evaporation rate increases with time.
By using a neck with a corrugated wall, the conducted heat path is lengthened, the conducted heat flux lowered, and hence the evaporation rate may be further reduced.