EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN

Abstract

We have studied electrical conduction and electrical breakdown in a vacuum gap between molybdenum electrodes in the presence of the evaporation products of a dispenser cathode and a thoria coated tungsten filament. The field enhancement factor was determined and the work function and the emitting area were determined before and after the Mo electrodes were exposed to the vapors. The surface damage on the electrodes due to arcing was examined in a scanning electron microscope. We have studied the phenomenon of electrical conduction and breakdown in a vacuum gap between molybdenum electrodes in the presence of the evaporation products of a dispenser cathode and a thoria coated tungsten filament. The Fowler-Nordheim equation was used to determine the field enhancement factor. The work function of the molybdenum cathode and the emitting area of the field emission sites were measured before and after the Mo electrodes were exposed to the vapors. We have also examined in a scanning electron microscope the surface damage on the electrodes due to arcing. The critical voltages and local fields for breakdown have been measured. Experiments were performed in a stainless steel base bell jar systeml.Varian model Vl-221. It is equipped with a sorption pump, a 140 -g ion pump and a titanium sublimation pump. The flanged ports on the chamber were used with various feedthroughs. During experiments, the pressure in the system was =2xl09 torr. The molybdenum electrodes were cut from h inch diameter rod with a purity of 99.99%. The edges were rounded to provide a smooth profile so that a high electric field would not be produced due to sharp edge. The electrodes were screwed on supporting molybdenum rods in the vacuum chamber. The front surfaces of the electrodes were mechanically polished using finally a 0.05 um AI2O3 abrasive powder in water suspension. The high voltage electrode was mounted on a stationary high voltage ceramic insulated feedthrough and the ground potential electrode was mounted on a linear motion feedthrough. Both the electrodes could be simultaneously outgassed at =1300K by electron bombardment heating. Electrodes temperature was measured with an optical pyrometer. A 2 HI resistor was used in the voltage-current measurement circuit to protect the power supply. The high voltage was applied by a Sorensen 0-30 KV dc power supply and measured by an electrostatic voltmeter. A Keithley picoammeter was used for measuring the current. During breakdown experiments the ammeter was removed and the voltage was increased until a spark occurred between the electrodes. The electrode surfaces were examined in a JEOL model JSM-35 scanning electron microscope (SEM). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984951 C9-304 JOURNAL DE PHYSIQUE The Mo e l e c t r o d e s were outgassed s e v e r a l t imes between 1250-1350 K till s t a b l e va lues of t h e f i e l d enhancement factor ,B and t h e emi t t ing a r e a were obta ined. The d i spense r cathode used was Semicon type-S i n which a porous tungsten ma t r ix i s impregnated wi th a mixture of BaO, CaO and A1203 i n t h e mole r a t i o 4 : l : l . The d i s penser cathode was brought t o f a c e t h e s u r f a c e of t h e Mo cathode and then heated t o 10500C br igh tness . The Table 1 g ives t h e work func t ion and t h e e l e c t r o n emi t t ing a r e a of t h e molybdenum e l e c t r o d e f o r d i f f e r e n t exposure t imes t o t h e vapors which c o n s i s t of Ba, BaO and some calcium2. The work func t ion decreased a s t h e exposure time was increased. The minimum va lue obta ined was 1.OleV. From t h e known evaporat ion r a t e 2 of t h e impregnants we es t ima te t h a t one monolayer (1015 %!ZE cm2 ) of t h e vapor atoms i s depos i t ed i n approximately 15 minutes.