Using laser-induced fluorescence, measurements have been made of metastable argon-ion, Ar + * (3d 4 F 7/2 ), velocity distributions on the major axis of an axisymmetric magnetic-mirror device whose plasma is sustained by helicon wave absorption. Within the mirror, these ions have sub-eV temperature and, at most, a subthermal axial drift. In the region outside the mirror coils, conditions are found where these ions have a field-parallel velocity above the acoustic speed, to an axial energy of ∼ 30 eV, while the field-parallel ion temperature remains low. The supersonic Ar + * (3d 4 F 7/2 ) are accelerated to one-third of their final energy within a short region in the plasma column, ≤ 1 cm, and continue to accelerate over the next 5 cm. Neutralgas density strongly affects the supersonic Ar + * (3d 4 F 7/2 ) density.

Experimental data are presented that are consistent with the hypothesis that anomalous rf absorption in helicon sources is due to electron scattering arising from parametrically driven ion-acoustic waves downstream from the antenna. Also presented are ion temperature measurements demonstrating anisotropic heating (T( perpendicular)>T(parallel)) at the edge of the discharge. The most likely explanation is ion-Landau damping of electrostatic slow waves at a local lower-hybrid-frequency resonance.

Recently, we demonstrated that a single, tunable, low-power, diode laser can be used for laser-induced fluorescence (LIF) measurements of both argon ions and helium neutrals. We have now identified a third fluorescence scheme, for neutral argon atoms, accessible with the same tunable diode laser. Fluorescence measurements of a heated iodine cell are used to monitor the wavelength of the laser during the LIF measurement.

Electron temperature measurements in helicon plasmas are difficult. The presence of intense rf fields in the plasma complicates the interpretation of Langmuir probe measurements. Furthermore, the non-negligible ion temperature in the plasma considerably shortens the lifetime of conventional Langmuir probes. A spectroscopic technique based on the relative intensities of neutral helium lines is used to measure the electron temperature in the HELIX ͑Hot hELicon eXperiment͒ plasma ͓P. A. Keiter et al., Phys. Plasmas 4, 2741 ͑1997͔͒. This nonintrusive diagnostic is based on the fact that electron impact excitation rate coefficients for helium singlet and triplet states differ as a function of the electron temperature. The different aspects related to the validity of this technique to measure the electron temperature in rf generated plasmas are discussed in this paper. At low plasma density (n e р10 11 cm Ϫ3), this diagnostic is believed to be very reliable since the population of the emitting level can be easily estimated with reasonable accuracy by assuming that all excitation originates from the ground state ͑steady-state corona model͒. At higher density, secondary processes ͑excitation transfer, excitation from metastable, cascading͒ become more important and a more complex collisional radiative model must be used to predict the electron temperature. In this work, different helium transitions are examined and a suitable transition pair is identified. For an electron temperature of 10 eV, the line ratio is measured as a function of plasma density and compared to values predicted by models. The measured line ratio function is in good agreement with theory and the data suggest that the excitation transfer is the dominant secondary process in high-density plasmas.

A diode laser based laser induced fluorescence ͑LIF͒ diagnostic that uses an inexpensive diode laser system is described. This LIF diagnostic has been developed on the hot helicon experiment ͑HELIX͒ plasma device. The same diode laser is used to alternatively pump Ar II and He I transitions to obtain argon ion and atomic helium temperatures, respectively. The 1.5 MHz bandwidth diode laser has a Littrow external cavity with a mode-hop free tuning range up to 14 GHz (Ϸ0.021 nm) and a total power output of about 12 mW. Wavelength scanning is achieved by varying the voltage on a piezoelectric controlled grating located within the laser cavity. The fluorescence radiation is monitored with a photomultiplier detector. A narrow band interference filter is used to eliminate all but the plasma radiation in the immediate vicinity of the fluorescence wavelength. Lock-in amplification is used to isolate the fluorescence signal from noise and electron-impact induced radiation. For the Ar ion, the laser tuned at 668.43 nm is used to pump the 3d 4 F 7/2 Ar II metastable level to the 4p 4 D 5/2 level. The 442.60 nm fluorescence radiation between the 4p 4 D 5/2 and the 4s 4 P 3/2 levels is captured by the photomultiplier tube. For atomic He, the laser is tuned at 667.82 nm to pump a fraction of the electron population from the 2 1 P state to the 3 1 D upper level. Although the 2 1 P level is not a metastable, the close proximity of 2 1 S metastable makes this new He I LIF scheme possible. In this scheme, a fraction of the laser-excited electrons undergo collisional excitation transfer from the 3 1 D to the 3 1 P level. In turn, the 3 1 P state decays to the metastable 2 1 S by emitting 501.57 nm fluorescence photons.

Measurements of parallel and perpendicular ion temperatures in the Large Experiment on Instabilities and Anisotropies ͑LEIA͒ space simulation chamber display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (␤ ϭ8nkT/B 2). Fluctuation measurements indicate the presence of low frequency, transverse, electromagnetic waves with wave numbers and frequencies that are consistent with predictions for Alfvén Ion Cyclotron instabilities. These observations are also consistent with in situ spacecraft measurements in the Earth's magnetosheath and with a theoretical/computational model that predicts that such an upper bound on the ion temperature anisotropy is imposed by scattering from enhanced fluctuations due to growth of the Alfvén ion cyclotron instability.

We report measurements of electron density and perpendicular ion temperatures in an argon helicon plasma for five different rf antennas: a Nagoya type III antenna, a 'Boswell' saddle coil antenna, a 19 cm long m = +1 helical antenna, a 30 cm long m = +1 helical antenna, and a 19 cm long m = +1 helical antenna with narrow straps. The general properties of the source as a function of rf power and neutral pressure are reviewed and detailed measurements of electron density, electron temperature and ion temperature as a function of magnetic field strength and rf frequency are presented. The experimental results clearly indicate that for all antennas, the electron density is maximized when the rf frequency is close to and just above the lower hybrid frequency. The ion temperature is maximized when the rf frequency is less than 70% of the lower hybrid frequency. Ion temperatures in excess of 1 eV for 750 W of input power have been observed. These results suggest that the mechanisms responsible for coupling energy into the ions and electrons are distinct and therefore helicon sources can be configured to maximize electron density without simultaneously maximizing the perpendicular ion temperature. Enhanced ion heating is not a desirable feature of plasma sources intended for use in plasma etching, thus operational regimes that yield high plasma densities without increased ion heating might be of interest to industry.

A pulsed crossed beam technique is used to measure ionization cross-sections of metallic atoms. Relative values of cross-sections of single, double and triple ionization of magnesium have been successfully measured with good accuracy over the 0-700 eV range. Absolute values of cross-sections have been obtained by normalization to a theoretical value at high electron energy. Results are compared to previously published values and, for single ionization in particular, a comparison with theoretical cross-sections is performed,