Simultaneous time‐resolved spectroscopic and current measurements have been made during the early stages of current growth leading to dc breakdown in vacuum between plane‐parallel electrodes of dissimilar material. These measurements show that resonance radiation characteristic of anode material is emitted prior to that of cathode material from all regions of the interelectrode gap. Further, the radiation is emitted first from the cathode region of the gap. The results indicate that the initial current growth occurs in anode vapor followed at later times by the appearance of cathode vapor in the gap.

Emission of electrode vapor resonance radiation at the onset of dc breakdown in vacuum

Microparticles (MP) and thermofield emission in vacuum are mainly caused by the roughness present at the surface of electrodes holding a high voltage. They can act as a trigger for breakdown, especially under high vacuum. This theoretical study discusses the interactions between one MP and the thermofield emission electron current as well as the consequences on the MP's transit. Starting from Cranberg's assumptions, new phenomena have been taken into account such as MP charge variation due to the secondary electron emission induced by energetic electron bombardment. Hence, the present model can be solved only numerically. Four scenarios have been identified based on the results, depending on the electron emission current from the cathode roughness (tip) and the size of the MP released at the anode, namely (i) one way; (ii) back and forth; (iii) oscillation; and (iv) vaporization. A crash study of the MP on the cathode shows that the electron emission can decrease if the MP covers the thermoemissive tip, i.e., if the MP is larger than the tip size---a phenomenon often called ``conditioning''---and helping to increase the voltage holding in vacuum without breakdown.

https://link.aps.org/pdf/10.1103/PhysRevAccelBeams.20.073501

Dynamics of microparticles in vacuum breakdown: Cranberg’s scenario updated by numerical modeling

It is known that micron‐ and submicron‐sized metallic particles are released from the electrode surfaces when a vacuum gap is subjected to a high dc stress. It is also well known that larger particles (> 10 μm) are generated within the interelectrode gap when a vacuum gap is subjected to conditioning or severe prebreakdown current flow. This paper examines the role of such particles in inducing the breakdown of a vacuum gap. While the larger particles induce breakdown by way of a trigger discharge, it is shown that the smaller particles can initiate breakdown because of effects associated with impact. The various effects associated with the high‐speed impact of a metallic microparticle on a target electrode, viz., cratering, production of metal vapor, and production of thermally generated plasma and their relative significance on vacuum breakdown, are examined.

Microparticle‐initiated vacuum breakdown—Some possible mechanisms

Results are presented of an investigation into microparticle activity in a vacuum‐insulated 1‐mm electrode gap. The experiments were conducted in situ in the specimen stage of a scanning electron microscope (SEM) and confirm microparticle activity at voltages well below breakdown. The most common size of the microparticles observed was 3 μm and very few particles above 10 μm were detected.

In‐situ observation of microparticles in a vacuum‐insulated gap using a scanning electron microscope

Microparticles with radii less than 0.5 ?m have been observed in a HV vacuum gap using a twin beam laser light scattering technique. For the geometry and electrode material used it has been shown that 97% of the particles travel from anode to cathode. Investigation of the correlation between the time of cathode impact and the time of breakdown indicates that most of the breakdowns are induced by microparicles.

The Dectection of Microparticle-Induced Breakdowns Using a Twin-Beam Laser Scattering System

According to Cranberg's hypothesis, tiny aggregates of metallic matter called microparticles, emanating from the electrode surfaces, are responsible for the breakdown of a vacuum gap under high dc stress. Experiments are carried out using a scanning electron microscope to see whether such particles appear within the interelectrode gap prior to breakdown. It was observed that micron‐ and submicron‐sized particles are released in abundance well below the breakdown voltage. Investigations were continued to identify the microparticles with the electrode materials using an electron microprobe analyzer. It was found that the microparticles were composed of the electrode materials. Direct examination of the electrode surfaces, after application of the voltage, under the scanning electron microscope also showed features suggesting that the microparticles are released from the electrode surfaces.

K. D. Srivastava

Papers

Detection and identification of microparticles occurring in a high‐voltage vacuum‐insulated gap using electron optical methods