A contribution to the assessment of the influence of collisions on the measurements with Langmuir probes in the thick sheath working regime

Abstract

In the present work we investigate theoretically and experimentally the influence of elastic collisions in a probe sheath on a cylindrical Langmuir probe. The analysed probe working regime covers conditions under which the following probe characteristic parameters are comparable: the probe radius, the Debye length and both the ion and electron mean free paths.

The preliminary investigations under almost collisionless conditions show good agreement between theoretical and experimental values of the ion saturation currents and of the floating potentials only when the ion currents for the studied working regime of the cylindrical Langmuir probe are calculated according to the theory of Chen (Plasma Phys.7 (1965)47). These collisionless currents form the basis for the calculation of the collision-corrected probe characteristics according to the presented procedure by Talbot and Chou (Rarefied Gas Dynamics, Academic Press, New York, 1966, p. 1723).

The applied theoretical analysis covers the influence of the collisions on the electron and ion current of the single probe characteristic and on the estimation of the space potential. The results of the calculations are presented in graphical overviews for the series of cases of practical importance. The other working regimes can be covered using the calculating procedure presented.

For comparison of the calculated collision-corrected characteristics with those from an experiment we used the positive column plasma of the He glow discharge where the electron density is known and the space potential can be experimentally estimated from the lowest excitation potential of He. The comparison was carried out for the ion and electron currents, the floating potential and the zero-cross of the probe characteristic second derivative.

The estimation of the secondary electron current contribution to the total probe current shows that it limits the applicability of the collision-corrected probe characteristic to the plasma diagnostic in the transition to the collision-determined working regime.