The formative phase of a 40 kV Z pinch has been investigated for pressures from 10 to 80 mTorr in H2. The energy spectrum of the electrons on the axis of the vessel, the spatial distribution of the current at the face of the anode, and the total discharge current were monitored. At the initiation of the discharge, a current of electrons with energies in excess of 20 keV is observed on the axis. This observation is in contrast to the normally accepted mode of ionization in which ionization initiates at the wall. The current is observed to switch from the axial region to the wall where it forms a current layer which collapses in accordance with the snow-plough model. The exact nature of the switching mechanism is not understood. The formative phase of the pinch typically takes a few hundred nanoseconds. The observations indicate that the detailed process of current sheet collapse may be affected by the formative phase.
Tearing and surface preserving electron magnetohydrodynamic modes in a current layer