Effects of Metal-Dielectric Junctions on Breakdown M. J. KOFOID

MEMBER AIEE

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Fig. 1. Illustration of improvement in d-c vacuum breakdown voltage obtainable by changing polarity to prevent junction discharge phenomena.

TO OBTAIN the highest breakdown strength over insulators in vacuum, the method of joining the

dielectric to the negative electrode is of high impor-tance.

In a recent paper1 it was disclosed that discharge phenomena can occur at the negative metal-dielectric junction of insulators in vacuum and that these phe-nomena can be effective in lowering the breakdown voltage. With a metal electrode in simple contact with a dielectric, electrons and radiation are released at the edge of the junction. The positive metal-dielectric junction plays no important role.

With short insulators in air at 10~4 to 10~δ millime-ters of mercury pressure, elimination of junction dis-charge phenomena was found to increase the break-down voltage by a greater amount than any other factor to which lowering of breakdown in vacuum has been normally attributed. With the arrangement shown in Fig. 1, junction discharge phenomena existed to a de-gree which led to appreciable lowering of the break-down voltage only when the metal at the simple me-chanical-contact junction was negative. As indicated, tests showed an increase by a factor of about four in the d-c breakdown voltage when the negative junction discharge phenomena were prevented.

When high-voltage insulators are operated at atmos-pheric pressure, the negative metal-dielectric junction is known to be a source of radiation and, as a result, of free electrons. However, this radiation is strongly absorbed close to the junction. The electrons, because of their very short mean free paths, cannot gain high

Digest of paper 60-1001, "Effect of Metal-Dielectric Junction Phenomena on High-Voltage Breakdown Over Insulators in Vacuum," recommended by the AIEE Electrical Insulation Committee and approved by the AIEE Technical Operations Department for presentation at the AIEE Pacific General Meeting, San Diego, Calif. Aug. 8-12, 1960. Published in AIEE Power Apparatus and Systems, Dec. 1960, pp. 999-1004. M. J. Kofoid is with the Boeing Airplane Company, Seattle, Wash.

energies by acceleration in the electric field, recombine quickly, and any effect they have in lowering the d-c breakdown strength of the gas must be very local.

In contrast, in vacuum the electrons set free at or in the immediate vicinity of the negative junction can travel distances comparable to the length of the insu-lator before making a collision. The result is that re-leased electrons can cause the production of X rays, negative ions, and further electrons. These actions are believed to lead to the lowering of the breakdown volt-age.

Two practical types of solution to the problem can be readily employed with roughly comparable improve-ments. As with the Kovar-glass seal junction of Fig. 1, the edge of the junction can be moved to a point of low electric gradients by the advantageous placement

0.65CM RADIUS

Fig. 2. Glass cylinder between electrodes designed to permit hig breakdown strength in vacuum. Lower electrode is negative.

of dielectric. Also, the electrode geometry can be made such as to reduce the electric field intensities in the gaps at the edge of the junction below a critical value.

One way to reduce the electric gradients through elec-trode geometry is by locating the edge of the junction at the bottom of a groove in the negative electrode, in the manner indicated in Fig. 2. The effectiveness of this design, using a Pyrex glass cylinder as insulation, was shown by test results; the breakdown voltage was 28 kv with the grooved electrode positive, 90 kv with it negative. With the same electrode separation but with no groove in either electrode, the breakdown volt-age was 25 kv.

R E F E R E N C E

1. Phenomena at the Metal-Dielectric Junction in Vacuum and Magnetic Field, M. J. Kofoid. AIEE Transactions, pt. I (Communication and Elec-tronics), vol. 79, Nov. 1960.

182 Kofoid—Metal-Dielectric Junction Effects on Breakdown ELECTRICAL ENGINEERING