The surface charge measurement system based on Pockels
effect was employed (as Figure 1). A laser light was generated
by the He-Ne laser source, and then it was expanded by the
beam expander. The output light passed through the polarized
beam splitter (PBS), 1/8 wave plate and Bismuth Silicon
Oxide (BSO) crystal. It was reflected by the top surface of
BSO crystal. The reflected light, which contained information
of surface charge distribution, passed through BSO crystal and
1/8 wave plate again, and it was reflected by PBS. Then the
light was focused on a small diameter by lens, and it passed
through pin hole, which could block the reference light due to
internal and external reflections caused by optical devices. At
last, a charge coupled device (CCD) camera with sampling
rate of 1000 frames/s obtained the image of surface charge
distribution. Based on the pulse current method, this system
could also obtain PD parameters simultaneously. The
discharge pulse current was extended by the measuring
impedance, and then it was recorded by the oscilloscope.
Synchronization control circuit was used to ensure that the
CCD camera and oscilloscope can begin to work at the same
time after the voltage was applied. The detailed description
about measurement principle and experimental setup could be
found in our previous paper [19].