Direct observation of vacuum arc evolution with nanosecond resolution

Abstract

Sufficiently high voltage applied between two metal electrodes, even in ultra high vacuum conditions, results in an inevitable discharge that lights up the entire gap, opening a conductive channel through the vacuum and parasitically consuming large amounts of energy. Despite many efforts to understand the processes that lead to this phenomenon, known as vacuum arc, there is still no consensus regarding the role of each electrode in the evolution of such a momentous process as lightning. Employing a high-speed camera, we capture the entire lightning process step-by-step with a nanosecond resolution and find which of the two electrodes holds the main responsibility for igniting the arc. The light that gradually expands from the positively charged electrode (anode), often is assumed to play the main role in the formation of a vacuum arc. However, both the nanosecond-resolution images of vacuum arc evolution and the corresponding theoretical calculations agree that the conductive channel between the electrodes is built in the form of cathodic plasma long before any significant activity develops in the anode. We show evidently that the anode illumination is weaker and plays a minor role in igniting and maintaining the conductive channel.

Anodic Glow and Conductive Channel Formation in Single and Double Long Vacuum Gaps

“…The anodic glow experienced an expansion and a decay, while the cathode spot kept active throughout the entire discharge process. The evolution process is similar to the small gap cases as shown in [20]. The anodic glow bridged the gap at 2000 ns in the 10 mm case, but there was only a weak glow lasting for less than 50 ns at 2000 ns in Figure 4d corresponding to the 30 mm gap.…”

“…After the cathodic glow, light emission is usually observed close to the anode surface as well [1,15,[18][19][20], which expands gradually towards the cathode. Many researchers considered the appearance and expansion of anodic glow as a necessary condition for formation of a conductive channel or the transition to the vacuum arc stage, because they observed that the voltage collapses at the moment when the expanding anodic glow met the cathodic one [18,19].…”

“…Slade et al [1] also expected that the anodic glow would play a major role already before a fully developed vacuum arc carries the full circuit current through a longer gap. In order to elucidate the formation mechanisms of conductive channels in vacuum breakdowns, we observed the evolution of the breakdown process with nanosecond resolution by adopting an ICCD (intensified charge coupled device) camera in [20]. By combining the light emission images with theoretical calculations, we found that the expansion of the anodic glow has no crucial effect on the formation of the conductive path, since the gap voltage drops to a low level long before the anodic glow bridges the gap.…”

“…Two specially designed vacuum interrupters were used in the experiments, with one of them being a single-break interrupter and the other being double-break. We extended the gap length from 1-5 mm in [20] and [21] to 10-30 mm for the single-break interrupter, and set the movable gap in the range of 10-20 mm in series with a fixed gap of 15 mm in the double-break interrupter. The electrode configuration for the gaps in the interrupters were all tip-to-plane.…”

“…A Marx Generator was adopted to provide high impulse voltage (1.2 / 50 μs) to breakdown the gaps. The spatiotemporal evolution of the breakdown process was observed by an ICCD camera using the procedure described in [20]. Figure 1 is a schematic diagram of the experimental setup.…”