EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN

Transcription

1 EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN M. Sinha, T. Lin To cite this version: M. Sinha, T. Lin. EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN. Journal de Physique Colloques, 1984, 45 (C9), pp.c9-303-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C9, supplément au n 12, Tome 45, décembre 198* page C9-303 EFFECT OF THERMIONIC CATHODES EVAPORATION PRODUCTS ON HIGH VOLTAGE VACUUM BREAKDOWN M.K. Sinha and T.Y. Lin North Dakota State University, Fargo, North Dakota 58105, U.S.A. Résumé - Nous avons étudié la conduction et le claquage électrique sous vide entre deux électrodes de molybdène en présence des produits d'évaporation délivrés soit par un "dispenser cathode" soit par un filament de tungstène thorié. Le facteur de renforcement de champ a été déterminé ainsi que le travail de sortie avant et après que les électrodes de molybdène soient exposées aux vapeurs. Les dégâts de surface dus au claquage ont été examinés au microscope électronique à balayage. 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 =2xl0 _ 9 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

3 C9-304 JOURNAL DE PHYSIQUE The Mo electrodes were outgassed several times between K till stable values of the field enhancement factor,b and the emitting area were obtained. The dispenser cathode used was Semicon type-s in which a porous tungsten matrix is impregnated with a mixture of BaO, CaO and A1203 in the mole ratio 4:l:l. The dispenser cathode was brought to face the surface of the Mo cathode and then heated to 10500C brightness. The Table 1 gives the work function and the electron emitting area of the molybdenum electrode for different exposure times to the vapors which consist of Ba, BaO and some calcium2. The work function decreased as the exposure time was increased. The minimum value obtained was 1.OleV. From the known evaporation rate2 of the impregnants we estimate that one monolayer (1015 %!ZE the vapor atoms is deposited in approximately 15 minutes. cm2 ) of TABLE 1 Molybdenum cathode exposed to dispenser cathode B = 121, dispenser cathode temperature = 1050 c (brightness) work Exposure time (mins) function (ev) Emitting area cm2) We also exposed the Mo cathode surface to the vapors of a thoria coated tungsten filament. The thoria coating was made by using the method given by lianley3. The Table 2 shows the effect of Tho2 vapors on the molybdenum cathode. The increase in work function at higher evaporation tempures ($1800K) seems to be the result of the adsorption of contaminant gas, such as oxygen on the Mo surface. TABLE 2 Molybdenum cathode exposed to Thoria coated filament Enhancement Evaporating Exposure Workfunction Emitting factor@) temp.(ok) time(mins) (ev) area (cm2) Fig. 1 shows the arc damage on a molybdenum cathode which was exposed to the 4:1:1 dispenser cathode evaporation products for 343 minutes before the breakdown experiment. Approximately 15 arcs were observed at 6.5KV. The gap distance between the electrode was 0.51 mm. The Fig. la shows that there are many circular features on the surface and some of them overlap. Each circular feature is believed to be the result of one arc. One such feature is enlarged in Fig. lb. The central region is a crater with a microparticle imbedded in it. Surrounding the rim of the crater are regions showing further damage. This arc may have been initiated due to the impact of the microparticle originating at the anode. The heat generated at the point of impact will melt the film and the material will diffuse to the

4 lower temperature area and solidify. We see that there are many pits in the annular region outside the crater. This type of damage will take place due to the ionic bombardment when arcing occurs. The Figs. lc-d show other arc spots where the film material has melted and protrusion formation occurs. Such protrusions can themselves become sites of intense field emission and initiate further breakdowns. The anode used with the cathode of Fig. 1 is shown in Fig. 2. Some microparticles are seen in Fig. 2a, and some film material detached from the cathode surface due to the high field stress is seen in Fig. 2b. The features shown in Fig. 2c are perhaps due to formation of blisters on the surface and their subsequent ru ture. The mechanism of their formation has been suggested by Sinha, Ku and Johnson z. It can be due to the electron bombardment induced diffusion of gas molecules which are already dissolved in the metal. Because of high fields and heating, these bubbles may burst and give off gas, and produce the feature as shown in Fig. 2c. The Fig.2d also shows a ruptured blister. The wavy surface is quite different from the cathode surface. It appears that the surface was heated to high temperature due to the field-emission currents. We have calculated the initial local breakdown field Eb and the results are summarized in the Table 3. TABLE 3 Vapors Vb d vb/d 6 Eb (kv) (ma) (w/cm) (MV/cm) Ba/~a Vb : breakdown voltage f3 : enhancement factor d : gap distance Eb : local field Vb/d : macroscopic field From the abovetablewe see that the local breakdown electric field for Ba/BaO coated surface is W/cm and for the surface exposed to thoria coated filament it is between 37 to "40 MY/cm. References 1. M.K. Sinha and Yee-Gee Ku, VIII International Symposium on Discharges and Electrical Insulation in Vacuum, Sandia Labs, Albuquerque, New Mexico, 5-7, September J.L. Cronin, IEE Proc. 128, Pt I 19 (1981). 3. T.E. Hanley, J. Appl. Phys. 2, 583 (1948). 4. M.K. Sinha, Yee-Gee Ku and Randall P. Johnson, J. Appl. Phys. 520, 699 (1981).

5 C9-306 JOURNAL DE PHYSIQUE Fig. 1 SEM PICTURES OF?.fo CATHODE EXPOSED TO 4:l:l DISPENSER CATHODE FOR 343 MINS

6 Fig. 2 SEM PICTURES OF Mo ANODE EXPOSED TO 4:l:l DISPENSER CATHODE FOR 343 MINS