Potential around a small charge in the RF plasma-wall transition layer

Electrostatic potential around a small dust charged particle in the near-wall region of a low pressure rf discharge is one of the fundamental problems in the physics of complex (dusty) plasmas because the screening of charged dust particles determines interactions between them and thus governs their dynamics (e.g., formation of dust crystals). An important feature of the plasma-wall transition layer is the (high degree of) anisotropy of the ion velocity distribution. The form of the screened potential in such an anisotropic plasma is still under debate: some authors attribute the screening to electrons, some attribute the screening to ions, and others suggest that both species provide comparable contributions to the screening. The objective of the present work is to test the hypothesis that the screening under typical conditions is primarily due to ions and not electrons. Here, two different theoretical models for the ion screening in the presence of anisotropy of the ion velocity distribution are derived (for collisional and collisionless cases, respectively) and compared with existing experimental data. The first model is a kinetic model that takes the ion-neutral collisions into account and defines the zeroth-order state as a homogeneous plasma with the balance of acceleration of ions in a homogeneous external electric field and collisions of ions with neutrals. The second one is a collisionless kinetic model that defines the zeroth order state as a homogeneous plasma with the ion velocity distribution being a shifted delta-function. In both models, the first-order perturbation due to a charged dust particle (considered as a non-absorbing point charge) is self-consistently calculated. The models are found to be in agreement with experimental data of Konopka et al. (PRL 84, 891, 2000).