The world's largest capacity helium flight dewar hasbeen assembled for use on the Relativity Mission, also known asGravity Probe-B (GP-B). Acceptance tests have been successfullyperformed and the dewar has been delivered to Stanford University.The science mission dewar (SMD) is the first piece of flighthardware delivered for GP-B. It will be used in testing todemonstrate payload functionality with a prototypical ScienceInstrument Assembly, the flight spare probe (Probe-B), andprototypical electronics.Since delivery to Stanford, the flight dewar has undergonepreparations for integration of Probe-B with the dewar. Thisinvolved filling both the main tank and the well with liquid heliumand installing a superconducting ultralow magnetic field shield.This shield consists of 63-μm-thick lead foil which lines the insideof the dewar well and provides an ambient magnetic fieldenvironment of < 10 pT (0.1 μG) in the region of the scienceinstrument assembly. The installation of the shield is an iterativeprocess which Will be described briefly. Once the shield isinstalled, the dewar and the shield must be kept cold through to theend of the science mission. The ambient field inside the shield thathas just been installed has been measured to be 5 pT or less in thegyro region. After completion of testing with Probe-B, the flightprobe (Probe-C) will be integrated in early 1998 in preparation fortesting the flight payload.This paper reviews the key design features of the dewar, especiallythose driven by the ultralow magnetic field requirements. Themeasured mass, helium boil-off, and ground hold performance arecompared to those of the previously launched superfluid heliumcryostats (IRAS, COBE, and ISO).
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