Experimental observations of the virtual anode motion and streamer breakdown mechanisms in a pseudospark discharge

A pseudospark (PS) discharge or transient hollow-cathode discharge is a high-voltage low-pressure discharge, which is characterized by the cathode being hollow and the cathode´s interior connected to the anode-cathode (A-K) gap by a hole in the center of the cathode. Some initial electron emission from the cathode region is the starting point of the different ionization processes occurring in the subsequent phases until the final breakdown. These initial electrons create an avalanche of charged particles whose tail is constituted by ions, meanwhile the head is essentially formed by electrons. An avalanche moving from the cathode toward the anode establishes physical conditions in order to create a virtual anode into the A-K region. Successive avalanches produced by spontaneous electron emission at the cathode, secondary electron emission due to ion impact in the cathode, or impact of the ionized particles with the residual gas, give access to the virtual anode arrives to the cathode leading the final electrical breakdown. During breakdown, electron and ion beam activities are present as consequence of the precedent ionization processes. In this paper, we present experimental observations of the moving virtual anode as well as the ion and electron beams emitted from a PS discharge for seven different gas pressures. The experiments were performed in hydrogen at a pressure of between 10 and 70 Pa, with a cathode aperture of 5 mm. A pulsed-charged capacitor scheme is used to produce up to a 30 kV step across the electrodes.