Starting potentials of electrodeless discharges

Abstract

In a previous paper the starting process of electrodeless discharges in high frequency electric fields at very low pressure was shown to be controlled by the emission of secondary electrons released by oscillating electrons which hit the walls with sufficient speed. This explained why the starting field strength was found to be independent of the nature of the gas and to rise suddenly to infinity when the wave-length exceeds a critical value—the cut-off—which depends on the size of the vessel and the substance of its inner surface. This paper records measurements of the starting field which have been carried out at pressures between about 0·2 and 350 mm. Hg in hydrogen, nitrogen, neon and some in deuterium and helium covering a range of wave-lengths between 5 and 2000 m. in vessels of different sizes and types of glass. The results show that with increase in pressure the nature of the gas becomes of importance. In the above-mentioned gases the starting field rises slowly and continuously with the wave-length up to a region where a kind of cut-off occurs which is due to the growing amplitude of electron oscillation and the corresponding rise of wall losses. In nitrogen and hydrogen and deuterium sharp cut-offs persist up to the highest pressure used. In the rare gases these discontinuities occur only below about 0.2 mm. Hg in Ne and 0.5 mm. Hg in He, while at larger pressure the cut-off extends over a considerable range of wave-lengths. The different behaviour of molecular and rare gases seems to be caused by narrow and wide electron energy distributions respectively. The cut-off occurs when the amplitude of oscillation of the electrons becomes equal to the length of the tube. From values of the cut-off and the associated fields the electron drift velocity has been derived. The results are in general accord with the known data which are only available for small fields. Above the cut-off a strong field is necessary to start a discharge in hydrogen and nitrogen. In hydrogen the starting field decreases with increasing wavelength and it is shown that this decrease is due to positive molecular ions producing electrons probably by impinging on the walls. In nitrogen the picture is more complex. In deuterium the positive molecular ion is effective; its mobility is found to be about one-half of that of hydrogen. In neon at higher pressures and at wave-lengths below 100 m. the starting field rises with decreasing wave-length; this is because the electrons collide too seldom with gas atoms to attain final random velocity. The only gas investigated at pressures comparable to one atmosphere was neon. Even at these pressures a kind of cut-off occurs. At one atmosphere the starting field is about 500 V/cm., by extrapolation it would be 15,000 V/cm. in hydrogen and of the order of 2000 V/cm. in helium for wave-lengths below the cut-off.