GRAVITY PROBE-B GYROSCOPE CHARGE CONTROL USING FIELD-EMISSION CATHODES

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.