Impulse Tree and Discharge Light in Pmma Subjected to Nanosecond Pulses

The partial breakdown of PMMA subjected to one or several successive square pulses of 7 ns rise-time and about 40 ns duration in a point-plane electrode system has been investigated by observing the discharge light using a fast photomultiplier and a close-up camera. The tree is longer and the discharge light more intense for a positive point than for a negative point. The discharge channel can initiate from a point anode when the field strength at the tip exceeds 10 MV/cm. Trees grow more rapidly with alternate application of positive and negative impulses than with successive positive impulses. However, repeated negative impulses cannot cause new extensions of a tree but they broaden the existing channel, emitting intense light. These results can be explained by space charge remaining in the channel from the preceding impulse.