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

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_e) and ion (n₊) number densities (1.6 × 10⁷ to 1.5 × 10¹⁰ cm^?3) 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 oml theory, resulting in an apparent increase of the measured ion number density above n_e in the plasma, is explained by the enhancement in the ion current collection efficiency due to collisions of ions with neutral gas atoms in the space charge sheath surrounding the probe. The continuous change in the exponent, χ, of the power-law dependence,i₊ ∞ V of the ion current, i₊, on the probe voltage, V_p 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 oml 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.