Current distribution during the breakdown in a coaxial electrode system

The breakdown stage in a coaxial electrode system (similar to Mather-type plasma focus) was studied in hydrogen for a wide range of pressures through the fast discharge of the energy stored in a parallel plate capacitor. A peak current Iep of 18.5 kA was reached in 60 ns for a 20 kV capacitor charge voltage, giving the typical initial current rise time obtained in normal plasma focus discharges. Thin current sheaths (≲2 mm thick) in the insulator region with current density ≳10 kA/cm2 were measured. These current sheaths always showed a well-defined filamentary structure with the number of filaments depending linearly on Iep , with an observed current limit per filament If depending on the pressure P as I2f∼1/P.