Experimental study of the electron and ion density determined by Langmuir probe is presented. The probe data have been obtained on FALP apparatus for electron (and ion) number densities ranging from 10" m-3 to loL6 at electron temperatures equal to room temperature and also roughly ten times higher and at neutral pressures from 60 Pa to 150 Pa.The number densities of electrons for all measured probe data have been deduced from the electron current to the probe at the space potential and also by means of OML method from the electron current measured for positive probe with respect to the space potential. In the paper it is shown that in the flowing afterglow plasma the values of the electron density determined according both methods are in a good agreement and it is possible to take them as the reference number density for comparison with apparent ion densities obtained from the ion current.The number densities obtained from the ion currents in the ion accelerating region (negative probe with respect to the space potential) according to different applicable probe theories have been compared with reference value. It is found that the best agreement between the number densities evaluated from the electron current and from the ion current gives the theory of ion collection by TICHL et al. [16], which takes into account the collisions of ions with neutrals in the space charge sheath around the probe provided that the conditions correspond to the OML regime.

The current/voltage characteristics of a cylindrical Langmuir probe have been studied in Ar'/electron afterglow plasmas in helium carrier gas under truly thermal conditions at 300 K using our flowing afterglow/Langmuir probe (FALP) apparatus. The orbital motion limited (oml) ion and electron current regions of the probe characteristics have been explored over a wide range of the reduced probe voltage (up to -100) and over a wide range of electron (n,) and ion (n,) number densities (1.6 x lo7 to 1.5 x loto ~r n -~) at a constant pressure of the He carrier gas of 1.2 Torr. The observed increase of the probe ion currents above those predicted by collisionless om1 theory, resulting in an apparent increase of the measured ion number density above ne in the plasma, is explained by the enhancement in the ion current collection eficiency due to collisions of ions with neutral gas atoms in the space charge sheath surrounding the probe. The continuous change in the exponent, x, of the power-law dependence, i, cc V;, of the ion current, i,, on the probe voltage, V,, from 0.5 at the highest n, (smallest sheath) towards 1.0 at the lowest n, (large sheath) indicates that the ion current collection from the plasma changes from the om1 current regime at the high n,, to the continuum regime at the low n, when the ions undergo multiple collisions with the helium atoms in the space charge sheath and thus "drift" towards the probe.