The Influence of Discharge Chamber Structure Upon the Initiating Mechanism the High Frequency Discharge

It is shown that a breakdown in a rarefied gas with high frequency e.m.f. of the order of megacycles may occur through three different processes. Mode a is characterized by a breakdown voltage of about half the d.c. discharge value. Mode b occurs at lower gas densities and is characterized by breakdown at a voltage somewhat above the corresponding d.c. discharge value. Mode c is the high frequency counterpart of the Paschen law phenomenon, in that when the density is too low to support an inter-electrode discharge, the latter seeks a longer path. It is shown that all three of these modes may operate in the same discharge chamber, sometimes simultaneously, and that the breakdown-process may shift from one mode to another with change in pressure or frequency. Many of the baffling complexities in earlier h.f. striking-voltage data may thus be understood. It is pointed out that mode c breakdowns are of little value for quantitative interpretation. A type of discharge chamber is described in which the breakdown may be held definitely to modes a and b.