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 (10-100nm) 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 Under O₂ rich condition, free radicals react with O₂ and autoxidation process starts. Autoxidation, chain reaction with low activation energy, creates large cavity in short time, where discharge can occur.He or Ar atoms in free volume are excited to metastable level by electron impact Metastable atoms collide with free volume wall and break polymer bond with high efficiency, and creates cavity in short time.