Langmuir probe measurements of weakly collisional electropositive RF discharge plasmas

Abstract

We report on Langmuir probe measurements of low-pressure (0.1-20 Pa) electropositive plasmas in an RF discharge at 13.56 MHz. From the probe I-V characteristic it is found that the electron density inferred from the ion current in the ion saturation region using radial motion (Allen, Boyd and Reynolds, ABR) theory can be up to one-half that obtained directly from the electron current at the plasma potential. The reduction in the ion current is attributed to orbital motion (OM) of the ions and also to a small number of ion-neutral collisions in the presheath. We show that if a sufficiently large probe is chosen so as to minimize the OM effects then the collisional theory developed by Shih and Levi (1971) can be used to give an appropriate correction factor over a narrow pressure range. The corrected electron density is found to agree with the knee current value to typically 10% for Ar, N2 and Kr plasmas.

Temporal Evolution of an Electron-Free Afterglow in the Pulsed Plasma Polymerisation of Acrylic Acid

“…Because of the lack of experimental data for ion-neutral collision cross sections, it is not possible to determine the degree of collisionality of the sheaths adjacent to the probe and the applicability of ion collection theories. However, it is well-known that the collisionless cold ABR theory 31 gives better agreement than the alternative orbital motion theory for cylindrical probes even in sheaths that are transitional between the collisionless (kinetic) and fully collisional (mobility limited) regimes (see ref and references therein). Therefore it has been used here.…”

Dust-Free Regions around Langmuir Probes in Complex Plasmas under Microgravity

“…We first demonstrate that the size and shape of the dust distribution can be described by a combination of the presheath theory of a probe [15,16] with two currently discussed ion-drag models [6,17]. Figure 3 shows the radius of the dust-free region measured from the probe axis at various positions along the probe shaft.…”

“…The analysis of probe characteristics shows, that, under the experimental conditions, the assumption of radial ion motion is justified, which is inherent in the presheath model [16]. It consists of ion continuity, the ion equation of motion including ion-neutral friction, and Poisson's equation…”

“…This approximation already matches the void radius around the shaft pretty well. In a second step, we compare with the spheroidal shape of the potential con- tour as suggested in [15,16]. This model gives the dotted contours in Fig.…”

“…This is a difficult problem for the prevailing situation of a probe where the floating shaft also affects the sheath geometry. Bryant et al [16] argue that the spheroidal potential contours in the presheath of a cylindrical probe become nearly spherical as soon as the radius is larger than the probe length. Following this reasoning, we discuss the force equilibrium in a presheath model for a spherical probe.…”