On the possibility of long path breakdown affecting the Paschen curves for microdischarges

We discuss the effect of electrode shape on Paschen curves and our ability to seal off microdischarges to prevent long path breakdown. It was found that for structured electrodes at high pressures and small gaps, the left-hand side of the Paschen curve is relatively flat, extending the minimum to lower pd values. At high pd values the curves are almost identical to those at standard pressures/gaps and the discharge runs between the top plane of the cathode and the anode. For intermediate pd values the higher electric field at the edge attracts most of the current and the discharge extends along the side wall maintaining the same low breakdown voltage. When the length of the discharge reaches the longest path the voltage starts a rapid increase. We have selected the dimension of the segmented electrode so as to have the same losses to the walls that block or allow the long path breakdown, thus being able to represent situations when the Paschen curve may be properly determined. In general, however, this shows that recording of the left-hand side for open structures (without enclosure by a dielectric) is impossible and conclusions about secondary emission should be focused on the well-defined conditions.