Ion velocity-locking in the neighborhood of virtual cathodes via instability enhanced collisional friction

Abstract

Axial ion motion in virtual cathodes and their connected presheaths are studied with laser-induced fluorescence. Virtual cathodes are formed using small electrodes (Aelectrode/Aloss < 1.7(2.3 me/mi)1/2) biased at higher than the plasma potential far from the electrode in multi-dipole confined filament discharges of argon gas. The virtual cathodes are electrostatic, with no magnetic fields present near the electrode to confine ions. An emissive probe is employed to measure the full potential profile from the bulk plasma up to the surface of the electrode. A planar Langmuir probe is employed to measure the electron temperature Te, the plasma density ne and to calculate the Debye length. Ions reflected from the electron sheath are only observed when the electrode is biased 1Te/e above the bulk plasma potential. When the electrode is biased more negatively, ions retain most of the kinetic energy from the potential drop along the presheath and sheath structure. When reflected ions are observed, ions retain very little of the kinetic energy from the presheath and sheath potential drop. The Baalrud–Callen–Hegna two-stream instability enhanced collisional friction theory is invoked to explain the phenomenon and provides qualitative agreement with the experimental results.