The Child-Langmuir collision laws for the cathode sheath of glow discharge in nitrogen

This paper reports the measurements of dc cathode sheath thickness d together with the voltage drop U across it at different current I values in nitrogen. For the case of a narrow gap when the discharge consists only of a cathode sheath and a negative glow almost all voltage applied across the electrodes drops across the cathode sheath. We observe that at the nitrogen pressure p < 1 Torr, as well as for p > 1.5 Torr the Child-Langmuir collision law version characterized by the constant ion mobility is valid. Within the pressure range of 1 < p < 1.5 Torr none of Child-Langmuir law versions (with a constant mobility or a constant ion mean free path) are applicable for the description of the dc cathode sheath in nitrogen. The available references enable us to conclude that at p < 1 Torr the discharge is dominated by N+ ions whereas for p > 1.5 Torr N4+ ions dominate. These ions are not prone to charge exchange with N2 molecules and they move in the dc cathode sheath with constant mobility. Within the pressure range of 1 < p < 1.5 Torr the discharge contains N+, N2+, N3+ and N4+ ions in comparable concentrations therefore none of the Child-Langmuir law versions can be valid.