VACUUM PUMPING PROPERTIES OF A HIGH VOLTAGE 60 HZ CROSSED-FIELD DISCHARGE

The principle of electron trapping in a d.c. Penning discharge is used in the design of standard vacuum pumping equipment. The feasibility of achieving vacuum pumping using high 60 Hz a.c. voltages and permanent magnets arranged to form a crossed-field configuration is investigated here. The applications are to use the a.c. voltages inherently available in high-voltage vacuum-insulated power apparatus to cause a vacuum pumping action. This would create a "self-maintained" vacuum without employing an auxiliary vacuum pump. The objective is to maintain a vacuum in the face of outgassing in systems of large surface area over long periods of time, and not necessarily to obtain a fast pump-down of the system. Many important applications exist for a device which will maintain the integrity of a vacuum, while operating with high 60 Hz a.c. voltages. Vacuum insulation of underground power cables would require pumps to maintain the vacuum while the cable remains underground for long periods of time. Other applications are in vacuum circuit breaker, lightning arrester, and surge arrester designs.; Two devices were designed, built and tested which utilize high 60 Hz a.c. voltages and permanent magnets to generate a crossed-field discharge. Experimental results using the first device demonstrated that indeed a vacuum could be maintained with 60 Hz a.c. voltages. Having demonstrated the feasibility of self-maintaining a vacuum with a.c. voltages, a second device was designed which would more realistically model an actual piece of power apparatus. Experimental results on the second device indicated better pumping performance than the first device. Also, it was discovered that there was an unexpected difference in the vacuum pumping characteristics of a.c. and d.c. voltages. It was found that the a.c. voltages provided better performance over an equivalent d.c. value equal to the rms a.c. level.; The experimental results are presented and an analysis of the electric and magnetic fields within the experimental devices is described. Mechanisms responsible for the observed superior vacuum pumping with a.c. voltages will be discussed.