Electrical tree initiation in polyethylene absorbing Penning gas

AC tree initiation voltage was examined in untreated LDPE, vacuum degassed LDPE and LDPE absorbing He gas (He gas was absorbed after vacuum degassing). The authors have already reported that vacuum degassed LDPE shows much higher tree initiation voltage than untreated LDPE because of the absence of oxygen. Therefore they expected that LDPE absorbing He would show the same property with vacuum degassed LDPE. However, the tree initiation voltage of LDPE absorbing He is as low as that of untreated LDPE. LDPE absorbing Ar gas shows the same tendency. He or Ar gas does not change the impulse tree initiation voltage as significantly. LDPE absorbing He was not well dyed with methylene blue after AC voltage application, which indicates that active oxidation does not occur. Low AC tree initiation voltage in LDPE absorbing He or Ar may be caused by Penning ionization in free volume