Ambipolar diffusion theory of the hot-cathode negative glow

Ambipolar diffusion theory of the negative glow (NG) and Faraday dark space (FDS) of a cylindrical gas discharge maintained by thermionic emission from a hot cathode is discussed. Electrons are divided into two groups: thermionic beam electrons and plasma electrons. Radial diffusion losses are handled by introducing the usual diffusion length. Inputs to the model include probabilities of excitation and ionization by thermionic beam electrons and by plasma electrons, electron and ion temperatures and mobilities, and discharge tube radius. The model predicts the following measured quantities with good qualitative agreement: (a) combined length of NG plus FDS, (b) axial distribution of plasma electron density, and (c) ion current to cathode. The model also predicts two reversals in axial electric field: one in the NG near the cathode where axial current is supported by ambipolar diffusion, and one near the end of the FDS at the beginning of the positive column where axial current can no longer be supported by ambipolar diffusion.