Branching of positive discharge streamers in air at varying pressures

The formation of positive streamers in a 17-mm gap in air is studied at pressures varying in the range from 1010 to 100 mbar. An intensified charge coupled device camera is used to image the discharge. At high pressures, the discharge shows many branches, while at low pressure, fewer branches arise. The structure is not simply determined by the ratio of voltage over pressure.