Influence of Dielectric Properties, State, and Electrodes on Electric Strength
The electric strength of all dielectric media depends on pressure (or pressure and mechanical loads in solid dielectrics), although the nature of this dependence varies. For gases this dependence is due to variations in the number density of neutral particles that are ionized during initiation and evolution of a discharge. At very low residual gas pressures (i.e., in vacuum), the pressure variations can influence near-electrode processes relating to the initiation of vacuum breakdown. For a liquid—an essentially incompressible medium at pressures of tens of megapascals—the effect of pressure on electric strength can be attributed to gas contained in the liquid and concentrated at the electrodes before applying the electric field, newly nucleated bubbles, and products of conduction current electrolysis. An increase in pressure changes the conditions of gas formation, displaces the equilibrium between molecular dissolved gas and gas bubbles toward the former, reduces gas bubble size, and increases gas pressure inside bubbles, thereby hindering the ionization processes. Increased hydrostatic pressure can alter the electrophysical parameters of the leader channel (first and foremost, its electrical conductivity) and, as a consequence, the discharge parameters.
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