Glow-to-arc transition at the left branch of the Paschen-curve

The glow-to-arc transition in deuterium at pressures p ≤ 10 mbar has been investigated experimentally with high time resolution (10 ns) by using a high-speed image converter camera and a 1 GHz digital oscilloscope. The electrodes were made from graphite with different compositions. The following observations have been made: (i) at currents > 1 A an anode spot is formed that precedes the cathode spot. The vapor plasma of this spot expands into the gap and helps to form the cathode spot. (ii) In the presence of a transverse magnetic field (0.4 T) and in the stage of increasing current the glow discharge is constricted to the space directly between the electrodes. It has the form of a cone pointing at the anode spot. This reflects the difficulty of current conduction across the magnetic field As a consequence, the voltage V necessary for the glow-to-arc transition increases with increasing gap distance d, in contrast to the case without magnetic field and in contrast to the prediction of the Paschen law at the left branch of the curve V(pd). However, the voltage still increases with decreasing pressure. In the case of a transverse magnetic field, the discharge plasma is no longer homogeneous in axial direction. Instead, spatially constant striations appear: a sequence of luminous disks is formed that are oriented perpendicular to the axis and have increasing diameters towards the cathode. The paper discusses the mechanisms of spot formation and of the current transport, in particular in the presence of a magnetic field.