A Langmuir probe for the study of highdensity RF discharges has been developed and tested in a helicon discharge in which the RF potential (=lOOV) is much larger than the electron temperature ( ~4 e V ) .Carbon probe tips are used to minimize erosion by ion sputtering. Miniature chokes located close to the probe tip present RF impedances of 150 kR at the operating frequency of 27.1 2 MHz and 300 kR at the first harmonic at 54.24 MHz. It is further necessary to drive the probe tip to follow the RF fluctuations by coupling it to a largerfloating electrode. We have been able to measure T . values as low as 3AeV in argon plasmas of iO'3cm-3 density; these temperatures are 1.6eV lower than ones measured by probes with chokes alone, and 2.3eV lower than measured by uncompensated probes.

The axial dependence of the plasma density, electron temperature, plasma potentials, and the 488 nm argon ion emission intensity have been measured in argon helicon discharges excited by both right helical and Nagoya III antennas for various magnitudes and directions of the magnetic field B. The plasma parameters were monitored with RF-compensated probes, while the emission line was detected with an optical emission spectrometer that incorporates an optical fibre and a miniature lens. The right helical antennas were designed to excite the m = +1 azimuthal mode when B is parallel () to the propagation vector k, and the m = −1 mode when B is antiparallel (#) to k. The plasma is found to be more dense in the former case (B k, m = +1), and the density peaks several antenna lengths downstream in the k direction. Nagoya III antennas are symmetric antennas that should excite the same azimuthal mode content in either magnetic field direction; indeed, the light profile was found to be independent of field direction. In the near field, under the antennas, the density is approximately the same for both antenna geometries and magnetic field directions. These results indicate that the m = +1 mode is preferentially excited regardless of the antenna helicity.

Traveling-and standing-wave characteristics of the wave fields have been measured in a helicon discharge using a five-turn, balanced magnetic probe movable along the discharge axis z. Helical and planepolarized antennas were used, and the magnitude and direction of the static magnetic field were varied, yielding three primary results. 1) As the density varies along z, the local wavelength agrees with the local dispersion relation. 2) Beats in the z variation of the wave intensity do not indicate standing waves but instead are caused by the simultaneous excitation of two radial eigenmodes. Quantitative agreement with theory is obtained.3)The damping rate of the helicon wave is consistent with theoretical predictions based on collisions alone.

Ion and electron densities have been measured in long, low pressure, cylindrical nitrogen and helium dc discharges using computer-controlled Langmuir probes. Cylindrical probe data have been analyzed with a variety of theories in order to determine the latter's accuracy. Electron densities were obtained from the electron saturation currents using orbital motion limited (OML) theories, and from the electron retardation region of the probe trace by integration of the second derivative of the probe current. Ion densities were obtained from both OML and radial motion analysis of the ion saturation currents. Line integrated microwave interferometry and discharge current continuity considerations in the positive column have been used to obtain two independent electron density measurements. While both probe electron density methods agree very well with each other and reasonably well with the independent density measurements, the OML theory applied to the ions overestimates the plasma density by up to a factor of 10. The radial motion theory yields ion densities that show considerably better agreement with the electron densities than the OML theory. Ion and electron densities have also been measured with planar probes, but they were found to perturb the surrounding plasma more than the cylindrical ones.4488

Abstract. Measurements of the radial and axial profiles of both the plasma parameters and the wave properties in a long, thin helicon discharge show that most of the RF power is deposited near the antenna and that a dense, cool (T e < 2 eV) plasma can be obtained in the downstream region. The density n and electron temperature T e profiles in that region can be explained quantitatively with classical collisional theory, and factor-of-two agreement can be obtained on total particle and energy balance. Spatial modulation of the helicon wave amplitude can be explained by the beating of two different radial modes launched simultaneously by the antenna. Though the helicon wave can be shown to be essential to the production of high densities, it plays little role in the downstream evolution of the plasma. These results indicate that helicon discharges can produce the cool plasmas normally associated with afterglows without the attendant loss of density.