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

Abstract

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.

DC vacuum breakdown in an external magnetic field

“…field evaporation of the metal; heating of the tip, the appearance of the plasma near the cathode surface; microexplosion and breakdown (arc burning) [32].…”

“…Researchers [3,32] indicated the essential role of processes in the gap. And an ionization of the residual and diffuse gases, adsorption, metal vaporization and bombarding electrodes by accelerated ions and electrons are among them.…”

Unveiling the Nottingham Inversion Instability during the thermo-field emission from refractory metal micro-protrusions

“…Another, less frequent, possibility could be the explosion of the protrusion, namely with RF fields 11 , 12 , due to a much faster increase of the temperature and/or the maximum temperature reached below the apex, in the emitter volume 13 . On the other hand, a high thermo-field current from the cathode can also locally heat the anode and cause the detachment of adsorbed microparticles, which then collide the cathode (the micro-protrusions), following the Cranberg scenario 14 – 16 .…”

Why the Electron Energy Balance During the Thermo-Field Emission From Refractory Metal Micro-Protrusions Can Trigger a Thermal Instability ?

“…Another, less frequent, possibility could be the explosion of the protrusion, namely with RF fields [11,12], due to a much faster increase of the temperature and/or the maximum temperature reached below the apex, in the emitter volume [13]. On the other hand, a high thermo-field current from the cathode can also locally heat the anode and cause the detachment of adsorbed microparticles, which then collide the cathode (the micro-protrusions), following the Cranberg scenario [14,15,16].…”