Process in PE free-volume preceding electrical tree initiation

Process in polymer free volume is very important in initiation phase of electrical tree under ac stress. Electrons injected from electrode are accelerated in free volume and bombard free volume wall to produce free radical and break polymer chain. Kind of gas in free volume has a large effect on ac tree initiation. Untreated (air) specimens shows low ac tree initiation voltage. We already reported autoxidation process between polymer free radical and O₂ decreases ac tree initiation voltage. Degassed specimens and N₂ specimens show much higher value, because of absence of O₂. However, tree initiation voltages of He specimens and Ar specimens are as low as untreated specimens though absence of O₂. We consider this is attributed to Penning process where metastable He or Ar atoms are produced by collision with hot electrons and then they collide with free volume wall to break polymer chain with high efficiency