Propagation velocity of cathode-initiated surface flashover

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Abstract
A polymethyl methacrylate insulator rod in vacuum is stressed by high‐voltage pulses having 3‐ns rise times, with the result that 40‐mm‐long incomplete discharges occur at the end of the rod. Time‐resolved measurements of electron emission from different areas on the insulator surface indicate that a discharge propagates toward the anode at approximately 2×107 m/s during a pulse. A velocity of this order is expected if surface flashover results from an avalanche of electrons due to secondary emission at the insulator surface. Dependence of the velocity on the angle of the electric field relative to the insulator surface is predicted.