Abstract
We propose and test a method for controlling the charging of the Gravity Probe B(GP-B) electrostatically suspended gyroscopes using electrons generated by field emission cathodes. The GP-B Gyroscope Experiment is designed to measure for the first time the geodetic and the frame-dragging effects predicted by Einstein’s general theory of relativity. The expected accuracy of ∼0.3 marcsec/yr (10−11 deg/h) will allow for a 0.01% measurement of the geodetic effect and a 1% measurement of the frame-dragging effect. Gyroscope charging is caused by cosmic radiation, by field emission, and by the separation of dissimilar metals. The expected charging rate for the gyroscopes is ∼1 nC/yr and consequently above the 50 pC limit dictated by disturbing torque considerations. The present charge control technique is based on ultraviolet photoemission of electrons from both the gyroscope and an auxiliary electrode. Experiments have shown this method to be effective at room temperature in ground testing, and calculations indicate that it is suitable for charge control in orbit. As an alternative we demonstrate the use of Spindt-type field emission cathodes for the control of the positive charges on the gyroscopes, by using a 10 000 tip emitter array produced by SRI International. The device requirements are (a) stable and reliable operation over two years at 2 K and 1.5×10−9 Pa, (b) average power dissipation in the device of less than 50 μW, (c) peak emission current of 1–100 pA, (d) dimensions less than 2 mm, (e) magnetization less than 10−8 G, (f) electric field at the gyroscope less than 104 V/m. The control of negative charges on the gyroscope is achievable by operating in a regime in which the secondary electron emission coefficient is greater than unity.
Published Version
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