The decay rate of surface charge was measured as follows:
the voltage was applied to high-voltage electrode until PD
took place, and then the surface charge distribution generated
by discharge was observed for a period of time after voltage
was removed. Hence, surface charge decay law could be
obtained, as shown in Figure 5. Obviously, the decay rate of
positive charge was much faster than that of negative charge.
The time for positive surface charge decaying from the initial
density with 0.3 nC/mm2 to zero was about 30 ms, while it
took about 150 ms for the negative charge.