Search

Recent experiments have demonstrated that phenomena associated with ion dynamics, such as the lower hybrid resonance, play an important role in helicon source operation. In this work, a review of recent ion heating measurements and the role of the slow wave in heating ions at the edge of helicon sources is presented. The relationship between parametrically driven waves and ion heating near the rf antenna in helicon sources is also discussed. Recent measurements of parallel and rotational ion flows in helicon sources are presented and the implications for particle confinement, instability growth, and helicon source operation are reviewed.

The West Virginia University Hot hELIcon eXperiment (HELIX) provides variable density and ion temperature plasmas, with controllable levels of thermal anisotropy, for space relevant laboratory experiments in the Large Experiment on Instabilities and Anisotropy (LEIA) as well as fundamental studies of helicon source physics in HELIX. Through auxiliary ion heating, the ion temperature anisotropy (T⊥/T∥) is variable from 1 to 20 for parallel plasma beta (β = 8πnkTi∥/B2) values that span the range of 0.0001 to 0.01 in LEIA. The ion velocity distribution function is measured throughout the discharge volume in steady-state and pulsed plasmas with laser induced fluorescence (LIF). The wavelengths of very short wavelength electrostatic fluctuations are measured with a coherent microwave scattering system. Operating at low neutral pressures triggers spontaneous formation of a current-free electric double layer. Ion acceleration through the double layer is detected through LIF. LIF-based velocity space tomography of the accelerated beam provides a two-dimensional mapping of the bulk and beam ion distribution functions. The driving frequency for the m = 1 helical antenna is continuously variable from 8.5 to 16 MHz and frequency dependent variations of the RF coupling to the plasma allow the spontaneously appearing double layers to be turned on and off without modifying the plasma collisionality or magnetic field geometry. Single and multi-species plasmas are created with argon, helium, nitrogen, krypton, and xenon. The noble gas plasmas have steep neutral density gradients, with ionization levels reaching 100% in the core of the plasma source. The large plasma density in the source enables the study of Aflvén waves in the HELIX device.

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.

Measurements of parallel and perpendicular ion temperatures in the Large Experiment on Instabilities and Anisotropies (LEIA) space simulation chamber display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B2). Fluctuation measurements indicate the presence of low frequency, transverse, electromagnetic waves with wave numbers and frequencies that are consistent with predictions for Alfven 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 Alfven ion cyclotron instability.

Laser induced fluorescence measurements of ion temperatures, parallel and perpendicular to the local magnetic field, in the Large Experiment on Instabilities and Anisotropies space simulation chamber (a steady-state, high beta, argon plasma) display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B2). These observations are consistent with in situ spacecraft measurements in the Earth’s magnetosheath and with a theoretical/computational model that predicts that such an upper bound is imposed by scattering from enhanced fluctuations due to growth of the ion cyclotron anisotropy instability (the Alfven ion cyclotron instability).

Laser induced fluorescence measurements of the parallel and perpendicular ion temperatures in a helicon source indicate the existence of a substantial ion temperature anisotropy, . The magnitude of the ion temperature anisotropy depends linearly on the source magnetic field. The parallel ion temperature is independent of magnetic field strength while the perpendicular temperature increases linearly with increasing magnetic field. Bohm-like particle confinement is proposed as an explanation for the linear dependence on magnetic field of the perpendicular ion temperature. In the helicon mode, the ion temperature components are independent of RF driving frequency and power and show a trend towards isotropy at high neutral fill pressures.

Summary form only given. An ion cyclotron resonant heating system has been developed for heating ions and controlling the ion temperature anisotropy in a helicon plasma source. Because of the very low frequency waves used, approximately 50 kHz, particular care has been taken with impedance matching of the antenna. The antenna is driven with a 1000 W source over a frequency range of 25-125 kHz. The parallel and perpendicular ion temperatures in argon plasmas are measured with a laser induced fluorescence diagnostic tuned to a metastable argon ion transition. Measurements of the efficiency of ion heating has been accomplished for several different antenna geometries. Data for both perpendicular and parallel temperatures at the fundamental and second harmonic ion cyclotron frequencies will be presented.

Summary form only given, as follows. Construction of the West Virginia University (WVU) Large Experiment on Instabilities and Anisotropies (LEIA) is now complete. LEIA is designed to investigate a wide variety of high beta, space relevant plasma phenomena. Of particular interest are electromagnetic instabilities driven by the free energy associated with non-isotropic ion distributions, i.e., different parallel and perpendicular ion temperatures. These instabilities occur at plasma betas (/spl beta/=8/spl pi/kT/B/sup 2/) of order unity and ion temperature anisotropies, perpendicular divided by parallel, over the range 0.25 to 4.0. The plasma for LEIA is generated with a steady-state helicon plasma source. By varying the source magnetic field and the magnetic field in LEIA, the ion temperature anisotropy in LEIA can be controlled. Initial measurements of the magnetic fluctuation spectra as a function of plasma parameters, e.g., plasma beta and ion temperature anisotropy, will be presented.

Summary form only given. Helicon sources are regarded as potential candidates for the next generation of plasma etching sources. The high efficiencies, steady-state operation, and high densities also make them attractive plasma sources for basic plasma physics experiments. The use of the West Virginia University (WVU) hot helicon source as part of an experiment designed to investigate ion temperature anisotropy driven instabilities under space-relevant conditions will be discussed. The WVU helicon source is "hot" because the ion temperature has been raised by the addition of ion cyclotron resonant heating. The physics objectives of the experiment, the required plasma parameters, and necessary source parameters will be reviewed.

Summary form only given. Laser induced fluorescence measurements of the ion temperature in a helicon plasma indicate that the perpendicular ion temperature scales linearly with applied magnetic field. Finite gyroradius effects play a negligible role since at the lowest magnetic fields examined, the ion gyroradius remains much smaller than the chamber diameter. In a cylindrical device, Bohm diffusion across the axial magnetic field leads to a diffusion coefficient which depends inversely on the magnetic field strength. Assuming that the diffusion coefficient determines both the particle and energy confinement times, linear scaling of the ion temperature with magnetic field suggests that the ions are heated by collisions with the electrons. Measurements of the parallel and perpendicular ion temperatures and the electron density versus magnetic field in helicon source will be presented. Ongoing attempts to demonstrate ion cyclotron resonant heating in helicon plasmas will also be discussed and the latest results presented.