With nonperturbative laser-induced fluorescence measurements of ion flow, we confirm numerical simulations of spontaneous electric double-layer (DL) formation in a current-free expanding plasma. Measurements in two different experiments confirm that the DL is localized to the region of rapidly diverging magnetic field. The measurements indicate that the trapped ion population is a single Maxwellian, that the spatial gradient of the energy of ions accelerated through the DL matches the magnetic field gradient, and that DL formation is triggered when the ion-neutral collisional mean-free path exceeds the magnetic field gradient scale length.

The parallel ion flow in a high-density helicon source plasma expanding into a region of weaker magnetic field is measured as a function of neutral pressure, magnetic field strength, rf power and rf driving frequency. The dependence of the parallel ion flow and parallel ion temperature, measured by laser induced fluorescence, on the plasma density, electron temperature and floating potential, measured with an rf-compensated Langmuir probe, is also examined. At the end of the helicon plasma source, the ion velocity space distribution changes from a single subsonically drifting Maxwellian population to a supersonic ion beam (≈15 eV) plus a cold, subsonically drifting background ion population. At 38 cm into the expansion region beyond the end of the plasma source, the supersonic ion beam is not observed.

A method of gas temperature determination in nitrogen or nitrogen doped discharges is presented. The method employs fits of numerically generated spectra of the 0-0, 1-0, and 2-0 bands of the first positive system (B Π3g→A Σ3u+) of nitrogen to experimental measurements. Excellent agreement between gas temperature values inferred by using this method and by using the 3-0 band peak ratio method [M. Simek and S. De Benedictis, Plasma Chem. Plasma Proc. 15, 451 (1995)] is demonstrated for a helicon plasma. The spectral model is available for use by the plasma spectroscopy community. The model, along with user instructions, can be downloaded from Electronic Physics Auxiliary Publication Service of American Institute of Physics. The model includes the line positions, Hönl-London factors, and provides rapid determination of gas temperature if one or more of the aforementioned emission rovibrational band spectra are available.

Measurements of the three-dimensional ion flow field and the ion temperature in a cross section of a cylindrical, argon, helicon plasma are presented. When these measurements are combined with radially resolved measurements of the plasma density, electron temperature, neutral density, and neutral temperature, the radial profiles of the ion viscosity and ion-neutral momentum transfer rate can be calculated. The ion viscosity and ion-neutral momentum transfer rate profiles are important input parameters for theoretical models of azimuthal flows arising from the nonlinear interaction of drift waves in helicon sources. The experimentally determined magnitudes and radial profiles reported in this work are significantly different than those used in recent theoretical studies. Measurements of the radial flow of argon neutrals and helium neutrals are also presented for a helicon plasma.

Observations in steady-state plasmas confirm predictions that formation of a current-free double layer in a plasma expanding into a chamber of larger diameter is accompanied by an increase in ionization upstream of the double layer. The upstream plasma density increases sharply at the same driving frequency at which a double layer appears. For driving frequencies at which no double layer appears, large electrostatic instabilities are observed. Time-resolved measurements in pulsed discharges indicate that the double layer initially forms for all driving frequencies. However, for particularly strong double layers, instabilities appear early in the discharge and the double layer collapses.