Evolution of the filamentary current sheath in a plasma focus device

The formation and evolution of the filamentary current sheath (CS) in the breakdown and rundown phases in a small Mather-type plasma focus device (20 kV, 18 kJ) have been examined using a high resolution laser differential interferometer. The results show that the thin and quasi-homogeneous CS initially formed along the insulator surface develops into distinct filamentary structures corresponding to the outer electrode (cathode) bars during moving towards the outer electrode driven by the Lorentz force. The sheath then develops into muddled filamentary structures once it crossed the coaxial electrodes. Later, after about 2 mu s from the discharge, the filamentary sheath reconstructs to form the distinct and homogeneous CS which accelerates towards the open end of the coaxial electrode and forms the pinch there. Such processes might be explained in terms of the specific ionization energy required to fully ionize the working gas the sheath swept.