Breakdown Voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to non-uniform gap in Dry Air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge acrossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the DC voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset.
The experimental observations have been explained qualitaively on the basis of a streamer model and precise electric field calculations of gap.