The Influence of Collisions in the Space Charge Sheath on the Ion current Collected by a Langmuir Probe

Abstract

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.

PIC Model of the Ion Collection by a Langmuir Probe

“…3 we have reported the probe characteristics for three different electron number densities. The increase of the exponent from the dependence I = aφ κ P with decreasing electron number density is seen, as experimentally observed [3].…”

“…For a negatively biased probe and in a middle pressure discharge, none of the present theories regarding the ion current collected by a probe explain accurately the experimental results [1][2][3]. In particular the results show that: a) the ion current as a function of neutral gas pressure reaches a maximum which is more pronunced for smaller D λ (all the symbols used in the paper are reported in table I); in fact it has been shown that collisions of ions with neutrals produce an increase of the ion current collected by the probe at lower pressures due to the destruction of the orbital motion and a decrease at higher pressures due to the scattering effect; b) the shape and the width of the ion current peak are expected to depend on the ion temperature T i ; c) the ion current does not depend on the probe diameter with increasing pressure; d) the exponent of the dependence i ∝ η κ p increases from 0.4 to 0.9 with decreasing electron number density from 10 16 to 10 13 m −3 .…”

The Influence of Ion – Neutral Collisions in the Plasma Sheath on the Ion Current to an Electrostatic Probe: Monte Carlo Simulation

“…This divergence is probably caused by the somewhat inadequate approximation of the sheath radius by eq. (1) which fails at higher n, giving too small values of rs [8], and also because the condition rJr, 9 1 is not fulfilled at the higher n, and therefore OMLC theory is not longer strictly valid. The simulation show that collisions enhance i + above that i+omlc.…”

“…The reasons for this choice are given in detail in [8]. We assumed that beyond r, the plasma is not disturbed by the probe potential which is completely contained within the sheath.…”

“…Recent measurements of ion probe currents collected by a probe have been carried out in flowing-afterglow/Langmuir probe (FALP) apparatus at Innsbruck [8]. In this experiment afterglow plasmas were created composed largely of Ar' ions with electrons in He carrier gas.…”