Emission of charged particles from laser-induced germanium ecton, vacuum spark, and vacuum arc

The highly resolved temporal evolution of laser-induced micro-explosions on a germanium surface is studied in a triode configuration for various gate charge levels and cathode currents. Electron emission from individual spots is directly imaged with a luminescence screen, showing that the opening angle of the source is about 30 degrees. Electron bunches of several nanocoulombs per pulse in a time interval of about 150 ns are directly extracted to the anode without vacuum breakdown in the cathodic gap. When breakdown occurs, a remarkable change in the arc behavior of a threshold gap potential of around 1 kV is observed, which hints at two different evaporation mechanisms that depend on the cathodic fall of an individual spot. Therefore, for voltages well above the threshold, a fast gate discharge is observed within the first 100-200 ns, followed by fundamental plasma oscillations and an electron emission of several mu C per pulse from the plasma boundary. Additionally, highly efficient emission of germanium ion clusters occurs, evidencing a stable twofold electron multiplication in the plasma, with a charge of several mu C per pulse below the threshold.