Positive and negative streamers in ambient air: measuring diameter, velocity and dissipated energy

Positive and negative streamers are studied in ambient air at 1 bar; they emerge from a needle electrode placed 40 mm above a planar electrode.The amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses have rise times of 30 ns or shorter.Diameters, velocities and energies of the streamers are measured.Two regimes are identified; a low voltage regime where only positive streamers appear and a high voltage regime where both positive and negative streamers exist.Below 5 kV, no streamers emerge.In the range from 5 to 40 kV, positive streamers form, while the negative discharges only form a glowing cloud at the electrode tip, but no streamers.For 5 to 20 kV, diameters and velocities of the positive streamers have the minimal values of d = 0.2 mm and v ≈ 10 5 m/s.For 20 to 40 kV, their diameters increase by a factor 6 while the voltage increases only by a factor 2. Above the transition value of 40 kV, streamers of both polarities form; they strongly resemble each other, though the positive ones propagate further; their diameters continue to increase with applied voltage.For 96 kV, positive streamers attain diameters of 3 mm and velocities of 4•10 6 m/s, negative streamers are about 20% slower and thinner.An empirical fit formula for the relation between velocity v and diameter d is v = 0.5 d 2 /(mm ns) for both polarities.Streamers of both polarities dissipate energies of the order of several mJ per streamer while crossing the gap.