THE NEW CORNELL COPPER DEMAGNETIZATION STAGE

The persistent failure of a previous welded copper demagnetization sta ge to reach temperatures significantly below 100 microkelvin caused us to replace the original structure. The new stage has been machined fr om a single ingot of moderately high purity (4 9's) polycrystalline co pper, with a much more rigid cross-section. Heat treatment increased t he RRR of the copper to 4000. The cryostat is now capable of maintaini ng temperatures below 50 microkelvin for over a week. However there ap pear to be gradients between the thermometer and the copper nuclei in the main field, which may be a factor of 5 or more colder. Much of the improved performance comes from the identification and removal of an existing heat leak. The vibrational heating is dramatically smaller th an that of the old stage, inferred from the magnetic field dependence of the heat leak.