Worthy to note, due to the limitation of layout, Figures 3a
and 4a only show a part of images obtained by CCD, which
caused that some charge spots were missed, e.g. there were
eight discharges in the rising part of voltage in Figure 4b, but
only six charge spots were shown in Figure 4a. However, it
had no effect on our analysis. Besides, PD may take place in
the falling part of applied voltage when the voltage cycle
increased (as Figure 4b), which may resulted from the change
of discharge time lag and surface charge decay.