We compute the flux of positive ions exiting a low-pressure, planar, electronegative discharge as a function of the negative ion concentration and temperature. The positive ions are modelled as a cold, collisionless fluid, while both the electron and negative ion densities obey Boltzmann relations. For the plasma approximation, the plasma edge potential is double-valued when the negative ions are sufficiently cold. When strict charge neutrality is relaxed, spatial space-charge oscillations are observed at the edge of the plasma when the flux associated with the low (in absolute value) potential solution is less than that of the high potential solution. However, the flux is always well defined and varies continuously with the negative ion concentration. We demonstrate that the correct solution for the plasma approximation is that having the greater flux.

The area of the sheath around a thin, disk-shaped electrode that is biased below the plasma potential has been computed using a hybrid simulation with cold, collisionless ions and Boltzmann electrons. That is, the ''collecting area'' of a double-sided, planar Langmuir probe has been determined for the ion saturation current regime. Sheath areas are calculated for probe radii from 10 to 45 electron Debye lengths and for probe biases from Ϫ5 to Ϫ30 times the electron temperature. The dependence of the sheath area on probe radius and bias is parameterized using simple empirical formulas.

The effects of ion collisionality on the plasma sheath are revealed by a two-fluid model. In contrast to previous work, the ion-neutral collision cross section is modeled using a power law dependence on ion energy. Exact numerical solutions of the model are used to determine the collisional dependence of the sheath width and the ion impact energy at the wall. Approximate analytical solutions appropriate for the collisionless and collisionally dominated regimes are derived. These approximate solutions are used to find the amount of collisionality at the center of the transition regime separating the collisionless and collisional regimes. Rx-the constant ion mean-free-path case, the center of the transition regime for the sheath width is at a sheath width of five mean-free paths. The center of the transition regime for the ion impact energy is at a sheath width of about one-half of a mean-free path.

Measurements of the rf electric field have been made along the z axis of a helicon reactor using a retarding field energy analyzer. A fluid code and a simple analytical model have been developed to analyze the ion energy distribution functions, especially in the case of bimodal distributions where the ion transit time through the sheath in front of the analyzer is comparable to the rf period. A generalized curve ͑and an analytical approximation to that curve͒ has been developed from the analytical model and confirmed by the self-consistent fluid model for high, low, and intermediate ion transit time, which can be used by experimenters to quickly convert the experimental results ͑energy peak separation, plasma potential and density, electron temperature͒, which are related to rf sheath oscillations, to absolute values of the rf electric field. An analysis of the errors involved in the derivation of the field is given. The results agree qualitatively with rf pickup measured with a floating Langmuir probe.

Five distinct discharge modes are observed in a cylindrically-symmetric helicon reactor. Each mode is characterized by its plasma impedance, wave mode structure, density distribution and floating potential structure. It is shown that the lowest two modes are capacitive (E) and inductive (H), while higher modes are helicon wave (W) modes. Successive wave modes are found to correspond to helicon wave cavity resonances of the plasma-filled vacuum vessel, each with a well defined wavevector, density and impedance. Measured wavevectors and densities are in agreement with the helicon wave dispersion relation. The first helicon discharge mode is found to be an m = ±1 mode, as expected for a double saddle field antenna. Unexpectedly, the second and third helicon modes have an m = 0 azimuthal symmetry.

We investigate the one- to two-dimensional zigzag transition in clusters consisting of a small number of particles interacting through a Yukawa (Debye) potential and confined in a two-dimensional biharmonic potential well. Dusty (complex) plasma clusters with n

The ratio of the negative ion density to the electron density has been determined using a novel two-probe technique in the diffusion chamber of an SF 6 helicon reactor. The Bohm flux (as modified by negative ions) was measured using a guarded planar probe, while the electron thermal current was obtained using a small cylindrical probe. The negative ion concentration was then determined from the ratio of these two currents. Results obtained with this simple technique show that the plasma in the diffusion chamber is divided radially into three regions. The central region contains hot electrons from the source that are confined by the magnetic field, positive ions being created through impact ionization, and a large proportion of negative ions (n − /n e ≈ 5). The edge region is a positive ion-negative ion plasma having a negligible electron density. These two regions are separated by a transition layer with a potential drop of ≈3 V. This layer performs some of the functions of a sheath. Consequently, the sheath at the chamber walls may be quite small since the thermal fluxes of positive and negative ions are nearly equal. The negative ion temperature is found to be ≈0.5 eV, which is much higher than the neutral gas temperature.

The effect of ion-neutral collisions on the structure and ion flux emanating from a steady-state, planar discharge with two negative components is investigated. The positive ion component is modelled as a cold fluid subject to constant-mobility collisions, while the electrons and negative ions obey Boltzmann relations. The model includes the collisionless limit. When the negative ions are sufficiently cold three types of discharge structures are found. For small negative ion concentrations or high collisionality, the discharge is `stratified', with an electronegative core and an electropositive edge. For the opposite conditions, the discharge is `uniform' with the negative ion density remaining significant at the edge of the plasma. Between these cases lies the special case of a double-layer-stratified discharge, where quasi-neutrality is violated at the edge of the electronegative core. Double-layer-stratified solutions are robust in that they persist for moderate collisionality. Numerical solutions for finite non-neutrality verify that the plasma flux varies continuously with collisionality, although the derivative of the flux with respect to collisionality is discontinuous when the discharge structure changes from uniform to stratified. Double-layer solutions are found when the flux predicted for the plasma approximation is double-valued and the flux associated with the smaller plasma edge potential is less than that associated with the larger edge potential. A comparison with numerical non-neutral solutions confirms that the flux is correctly predicted using the plasma approximation when the larger value of the flux is taken in the two-solution regime.