Lead Shielded Dewar Development

Results of a recent development program to increase operating time of levitated rings are presented. Fabrication, cryogenic, and mechanical testing of 20 in. and 36 in major diameter lead shielded toroidal Dewars are described. Thirty to sixty pounds of lead shielding was applied to a glass epoxy and fiberglass substructure separated by a vacuum gap from the inner chamber. A typical installation consisted of lead-copper sheet laminates applied to a tinned copper base. Cooling of the lead jacket was accomplished by a separate cooling loop or by vent gases from the inner chamber. Mechanical integrity of the lead shield assembly was evaluated by shock testing the assembly in excess of 10 g´s at 77°K. Test results indicate that the addition of 32 lbs. of lead shielding increases the isochoric operating time of a typical 36 in. ring by a factor of 2.5 or more.