Starting potentials of high-frequency gas discharges at low pressure

Source abstract

Abstract The starting field strength of an electrodeless discharge in a uniform high-frequency electric field at pressures of order 10-3 mm. Hg in helium, mercury, hydrogen, and air, is found to be independent of the gas and only slightly dependent on its pressure. By increasing the wavelength (> 4 m.) the starting field varies first inversely with the wave-length, then becomes constant, and at a critical value rises discontinuously probably to infinity. This cut-off wavelength is proportional to the diameter of spherical and the length of cylindrical vessels respectively, the axis of the cylinder being parallel to the field. The facts that the results are independent of the gas and that at these pressures the electrons collide very rarely with gas molecules both indicate that directionization by collision with the gas cannot account for the multiplication of the few electrons originally present. A theory is worked out on the basis that secondary emission from the walls is the operative factor. The initial electrons assumed to move singly in vacuum come to life at various phase angles of the alternating field. However, only those contribute effectively to the multiplication process whose transit time between the walls corresponds to one or an odd number of half-cycles, and whose velocity is large enough to liberate from the wall more than one secondary electron. For each phase angle an appropriate transit time, that is, an appropriate wave-length, can be found to give this requisite final velocity so that discharges are possible over a limited range of wave-lengths corresponding to the allowable phase angles. From similarity follows the proportionality between cut-off wave-length and the size of the vessel. This theory agrees well with the experimental results. The dependence on secondary emission was confirmed because by coating the inner glass wall with a poor secondary emitter the starting field is increased, while the cut-off wave-length decreases; the reverse is observed when clear Pyrex glass is exchanged for soda glass, which agrees with the larger value of the coefficient of secondary emission of the latter.