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Light emission before and during electrical breakdown from the surface of polytetrafluoroethylene (PTFE) with metallized electrodes has been investigated with ac electric field application along the polymer surface, using a photon counting method, in order to understand the initiation mechanism of the prebreakdown. Two distinct stages of light emission were observed depending on the applied voltage: a low-level stable light emission, electroluminescence (EL), before prebreakdown and an irregular intense light emission during prebreakdown. Before prebreakdown, charge injection from the electrode directly into the polymer surface layer results in EL emission and the formation of long-term electron space charges away from the electrode. The crucial factor of prebreakdown initiation is the strong modification of the local electric field near the electrode because of space charge formation in the surface layer before prebreakdown. The prebreakdown with intense light emission is initiated by detrapping the long-term trapped electrons toward the electrode via the vacuum and/or the surface layer in the positive half-cycle of ac voltage.

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Light emission from polyethylene with a needle-like microvoid has been investigated under the application of a negative impulse voltage in order to examine the temporal behavior of discharge in the microvoid at the metal-polymer interface. The light emission was observed distinctly during the voltage rise and fall phases with a period of no light emission between the two light emitting phases. The emitted light consisted of sequential light pulses which appeared regularly, according to the temporal variation rate of the voltage. The sequential light pulses corresponded to the sequential discharge in the microvoid which occurred to keep the voltage across the microvoid below the discharge inception voltage. The characteristics of light emission revealed the occurrence of intermittent discharges in the microvoid during the voltage variation phase of the impulse voltage, caused by charge deposition on the polymer surface of the microvoid.