  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.
