Abstract
The procedures for the production of a very low magnetic field of 10-11 tesla in a full scale prototype dewar have recently been completed. The GP-B Relativity Gyroscope Experiment will provide a controlled test of Einstein’s General Theory of Relativity by making observations of the precession of nearly perfect gyroscopes in Earth orbit. The gyroscopes consist of highly spherical and homogeneous fused silica rotors which are coated with a thin superconductor and operated at a temperature of 2 K. Readout of gyro precession is accomplished by measuring the orientation of the magnetic dipole moment (London moment) which is generated by the spinning superconductor and is aligned with the spin axis. The GP-B experiment requires an ambient field of ≤ 2 × 10-11 tesla (2 × 10-7 gauss) in order to minimize trapped flux in the rotors and thereby insure the proper operation of the SQUID-based gyroscope magnetic readout. The process to reach the required low field level is by iterative expansion of superconducting lead foil shields. The methods developed to effect the lead shield expansions and to accurately measure the low field are presented along with the final results.
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References
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A recent summary of key GP-B technologies and the current state of their development can be found in D. Bardas, et al., in: “Proceedings of the Sixth Marcel Grossman Meeting on General Relativity,” H. Sato and T. Nakamura, ed., World Scientific Press, Singapore (1992), p. 382.
A more detailed discussion of the GP-B gyro design and fabrication can be found in J. P. Turneaure, et al., Adv. Space Res. 9: 29 (1989) and references therein.
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The specification of 0.2% Sn in the lead foil keeps the surface of the foil largely oxide free. This allows the welds to be carried out without the use of a flux which would contaminate the shield and act as an adhesive between layers of foil.
Both the trapped field and the fringing field from the open top of the shield should be transverse. The former because the shield expansion is transverse in direction, and the latter because the external axial field has a shorter decay length than the tranverse component. See B. Cabrera, Ph.D Thesis, Stanford University (1974).
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Taber, M.A., Murray, D.O., Lockhart, J.M., Frank, D.J., Donegan, D. (1994). Production of Ultralow Magnetic Fields for Gravity Probe B (GP-B). In: Kittel, P. (eds) Advances in Cryogenic Engineering. Advances in Cryogenic Engineering, vol 39. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-2522-6_19
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DOI: https://doi.org/10.1007/978-1-4615-2522-6_19
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