Status of the relativity mission superfluid helium flight dewar

Abstract

The world's largest capacity helium flight dewar has been assembled for use on the Relativity Mission, also known as Gravity Probe-B (GP-B). Acceptance tests have been successfully performed and the dewar has been delivered to Stanford University. The science mission dewar (SMD) is the first piece of flight hardware delivered for GP-B. It will be used in testing to demonstrate payload functionality with a prototypical Science Instrument Assembly, the flight spare probe (Probe-B), and prototypical electronics. Since delivery to Stanford, the flight dewar has undergone preparations for integration of Probe-B with the dewar. This involved filling both the main tank and the well with liquid helium and installing a superconducting ultralow magnetic field shield. This shield consists of 63-μm-thick lead foil which lines the inside of the dewar well and provides an ambient magnetic field environment of <10 pT (0.1 μG) in the region of the science instrument assembly. The installation of the shield is an iterative process which will be described briefly. Once the shield is installed, the dewar and the shield must be kept cold through to the end of the science mission. The ambient field inside the shield that has just been installed has been measured to be 5 pT or less in the gyro region. After completion of testing with Probe-B, the flight probe (Probe-C) will be integrated in early 1998 in preparation for testing the flight payload. This paper reviews the key design features of the dewar, especially those driven by the ultralow magnetic field requirements. The measured mass, helium boil-off, and ground hold performance are compared to those of the previously launched superfluid helium cryostats (IRAS, COBE, and ISO).