Observation of low-density domain in polystyrene under nanosecond pulses in quasi-uniform electric field

Low-density domains (LDDs) developed in polystyrene (PS) samples under nanosecond pulses in a quasi-uniform electric field are observed using an on-line transmission microscope. The test samples are 2 mm in thickness with a cylindrical profile, and immersed in clean transformer oil. The nanosecond pulse is trapezoidal with a pulse width of 10 ns. Images taken by the microscope show that the LDDs always appear in the vicinity of the cone electrode, expand as the pulse number increases, and gradually vanish when the output pulses are stopped, only leaving small voids and cracks in the samples. When the pulses are imposed on the sample again, the LDDs emerge and begin to expand again, and finally lead to bulk breakdown of the samples. The observation of LDDs under this condition gives a support for the pre-breakdown and breakdown model by K. C. Kao when applyied to a nanosecond time scale.