Effect of Space Charge and Structure on Breakdown of Liquid and Solid

Space charge and structural change in solid and liquid dielectrics play important roles as positive feedback mechanisms for breakdown. In case of a KCl single crystal, introduction of F centers by γ -ray irradiation strongly modifies breakdown voltage due to the space-charge field distortion at cathode asperities: homo-type space charge (F' center) increases and hetero-type space charge (ionized F center) decreases dc breakdown voltage, while impulse breakdown is not affected. In case of a polyethylene film, space charge at the crystalline -amorphous boundary due to trapped carrier plays an important role for breakdown. These space charge effects disappear at higher temperature by detrapping. Space-charge field enhancement at streamer etips gives rise to the difference between positive and negative streamer propagations in liquids and solids, causing higher propagation velocities and longer streamers in liquids in the former case. Judging from pressure effects, negative streamers in liquid are usually strongly related to the presence of low density regions (bubbles), whereas positive streamers are not. However, liuqid He shows a peculiar polarity effect possibly due to formation of electron bubbles. Both streamer and bubble mechanisms coexist in liquid breakdown. The predominance depends on conditions such as pressure, electrode distance, voltage pulse duration, boiling point, electrode asperities, etc.