The Effect of Field Configuration on Gas Discharge Breakdown in Microwave Cavities at Low Pressure

Abstract

A new regime of high-frequency gas discharge breakdown in non-uniform fields at low pressures is predicted on theoretical grounds and experimentally confirmed. The controlling mechanism is the time-averaged Lorontz force on the electrons, proportional to the gradient of the square of the electric field amplitude (∇|E|2). This steady force is derivable from a potential, and field configurations are realizable in microwave cavities corresponding to potential hills or wells, exemplified by E ₀₁₀ and E ₀₁₁ cylindrical cavities respectively. In the former case the steady force acts to disperse electrons towards the walls, increasing the loss of electrons and raising the breakdown field, whereas in the latter case the steady force acts to confine electrons, inhibiting electron loss and lowering the breakdown field. The effect becomes marked at levels of field intensity for which the height of the potential hill or well becomes comparable with the ionization energy. Breakdown fields have been measured for hydrogen in E ₀₁₀ and E ₀₁₁ x-band cavities in the pressure range 30 mm Hg to 7×10^-3mmBg. The peak breakdown fields range from 400 to 30 000 v/cm. At the higher pressures and lower fields the results are in agreement with the diffusion theory. However, at the lower pressures and higher fields the behaviour of the two cavities is markedly different and qualitatively confirms the controlling effect of the time-averaged Lorentz force.