High-Frequency Discharges: I Breakdown Mechanism and Similarity Relationship

High-frequency breakdown between wires and coaxial cylinders in air and in hydrogen was investigated over the range of frequencies f from 3.5 Mc/s. to 70 Mc/s. at pressures below 20 mm. Hg. It was found that geometrically similar systems broke down at the same potential provided that both parameters ap and f/p were invariant, a being a linear dimension. Under conditions such that the total amplitude of oscillation of the electron cloud was smaller than the electrode interspace, the electric field at the wire was the controlling factor, and breakdown was found to be independent of the nature of the surface of the cylinder. Consequently, secondary ionization processes did not appear to play an important part as in the case of static discharges. The experiments were in agreement with the view that the high-frequency discharge in air and in hydrogen is determined by primary ionization in the gas and loss of electrons to the walls by diffusion only, provided the electron mean free path is small compared with the linear dimension of the discharge tube, and the collision frequency is very much greater than the oscillation frequency. The significance of the similarity relation is briefly discussed