  The experimentally recorded distribution of electron density along a plasma
column supported by a travelling EM wave is linearly decreasing from start to
finish; its slope depends strictly and solely on the gas pressure, the
frequency of the wave and the internal radius of the discharge tube. There are
two opposing descriptive models: a) an EM surface wave propagates along the
three-constituent media of the tubular discharge, i.e. the plasma column, the
dielectric tube and the outer wall of the latter. This model does not
faithfully reproduce the linearity of the axial distribution of the electron
density, at any rate always at the end of the column; b) an EM wave propagates
in a vacuum (ambient air) guided by the outer wall of the discharge tube, its
electric field heating the electrons present in the gas and thus ionising it.
As each electron thus obtained draws its energy from the electric field of the
wave, the axial distribution of the electron density can also be considered as
a conservation relationship between the energy dissipated by the wave and the
electron density produced. This model makes it possible in all circumstances to
find a linear axial distribution of electron density from the beginning to the
end of the column. The electron heating mechanism finally exhausts all the
energy in the wave, so there is no more energy available for it to propagate
otherwise, for example, along the dielectric tube or the plasma column. The
origin of the linear behaviour of the electron density distribution lies in the
stability condition of a discharge maintained by a travelling electromagnetic
wave, which requires the electron density to decrease monotonically axially.
