Can the negative glow plasma have a negative charge?

Summary form only given. The classical problem of the gas discharge physics is discussed: what is the sign of charge density in the negative glow region of a glow discharge. The question about the sign of the net charge in NG is important to improve and refine the classical picture of the plasma parameters distribution for a wide class of glow discharges (from the usual glow discharges, to the widely used in scientific researches and applications hollow cathode discharges consisting exclusively of the cathode sheath and the negative glow plasma).As it is well-known, in any bounded plasmas, when charges are loss at the boundaries, the diffusive fluxes of charged particles are directed from a center toward periphery. In such situation, the ambipolar field has to decelerate the electrons, aligning their flux with the ion flux. To create such electric field, the charge density of plasma must be a positive. This fact is not questioned in plasma science. At the same time, almost all textbooks on gas discharge physics indicate that the negative glow (NG) and/or Faraday Dark Space (FDS) plasmas of a DC glow discharge is negatively charged; furthermore the charge separation there is even stronger than in the positive column (see, e.g., Fig.14.1 in [1]). This statement is in contradiction with the above considerations that in a simple plasma charge must be positive. The 2D simulation of dc glow discharge in a cylindrical tube in helium is performed. The modeling was completed in the framework of 2D fluid model, in Comsol Multiphysics environment. In the investigated conditions the discharge plasma have a positive charge, including the negative glow plasma, where the axial dependence of the axial electric field is non-monotonic. It is shown that traditional interpretations in text-books on gas discharge physics, that state negative charge of the negative glow (and/or Faraday dark space) plasma, are based on analogies with simple one-dimensional model of discharge. As the real- glow discharges with positive column are always twodimensional, the transversal (radial) term in divergence of the electric field can provide a non-monotonic axial profile of charge density in the plasma with maintaining of a positive sign. The numerical calculation of glow discharge is presented, showing the positive space charge in the negative glow and Faraday Dark Space under conditions where one-dimensional model of the discharge would predict negative space charge.