S. Buchman et al., GRAVITY PROBE-B GYROSCOPE CHARGE CONTROL USING FIELD-EMISSION CATHODES, Journal of vacuum science & technology. B, Microelectronics and nanometer structures processing, measurement and phenomena, 11(2), 1993, pp. 407-411
We propose and test a method for controlling the charging of the Gravi
ty Probe B (GP-B) electrostatically suspended gyroscopes using electro
ns 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 approximately 0.3 marcsec/yr (10(-11) deg/h)
will allow for a 0.01% measurement of the geodetic effect and a 1% me
asurement of the frame-dragging effect. Gyroscope charging is caused b
y cosmic radiation, by field emission, and by the separation of dissim
ilar metals. The expected charging rate for the gyroscopes is approxim
ately 1 nC/yr and consequently above the 50 pC limit dictated by distu
rbing torque considerations. The present charge control technique is b
ased 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 indi
cate that it is suitable for charge control in orbit. As an alternativ
e we demonstrate the use of Spindt-type field emission cathodes for th
e control of the positive charges on the gyroscopes, by using a 10 000
tip emitter array produced by SRI International. The device requireme
nts are (a) stable and reliable operation over two years at 2 K and 1.
5 X 10(-9) Pa, (b) average power dissipation in the device of less th
an 50 muW, (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 10(4) V/m. The control of negative charges on
the gyroscope is achievable by operating in a regime in which the sec
ondary electron emission coefficient is greater than unity.