Imaging of Discharge Plasma Channel Evolution Process of Microsecond Wire Explosion in Air

The discharge plasma channel (DPC) evolution process is observed by taking time-series self-emission intensified charge-coupled device (iCCD) photos and simultaneous shadow images. By adding neutral density filters before iCCD, the subtle structure of the DPC self-emission is observed. It is interesting that a treelike structure is observed soon after the voltage breakdown. Long exposure time photos demonstrate that this structure existed for hundreds of nanoseconds. An inner homogeneously luminous column arises latterly, forming a two-layer self-emission structure. Then, this column gradually occupies the whole DPC. Shadow images show that the expanding DPC maintains good axial uniformity, manifesting the long-standing uneven treelike structure only has limited impact on the energy distribution and the expansion process. The calculated expansion rate reaches its maximum value (4.75 km/s) not at the power peak, but 0.24
$\mu \text{s}$
after that. Then, the expansion rate undergoes fast drop and slow decline. In addition, it seems that the distributions of related plasma parameters experience an evolution from nonuniform to uniform.