The presence of ion space charges of both polarities in the anode region greatly affects the
local distribution of the field, and, consequently, the development of corona discharge at the anode.
Four different corona discharge modes having distinct electrical, physical, and visual characteristics can
be observed at a highly stressed anode, prior to flashover of the gap. These are, respectively, with
increasing field intensity (Fig. 15.5): burst corona, onset streamers, positive glow, and breakdown
streamers. An interpretation of the physical mechanisms leading to the development of these corona
modes is given below.
15.1.2.1 Burst Corona
The burst corona appears as a thin luminous sheath adhering closely to the anode surface (Fig. 15.5a).
The discharge results from the spread of ionization activities at the anode surface, which allows the
high-energy incoming electrons to lose their energy before neutralization at the anode. During this
process, a number of positive ions are created in a small area over the anode, which builds up a local
positive space charge and suppresses the discharge. The spread of free electrons then moves to another
part of the anode. The resulting discharge current consists of very small positive pulses (Fig. 15.6a),
0.5 cm 0.5 cm1.0 cm(a) (c)(b) (d)1.0 cmFIGURE 15.5 Corona modes at anode: (a) burst corona; (b) onset streamers; (c) positive glow corona;
(d) breakdown streamers. Anode spherical protrusion (d¼0.8 cm) on a sphere (D¼7 cm); gap 35 cm; time
exposure 1=4 s. (From Trinh, N.G. and Jordan, I.B.,IEEE Trans., PAS-87, 1207, 1968; Trinh, N.G.,IEEE Electr. Insul.
Mag., 11, 23, 1995a.)