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Experiments to study the effect of a second, counterpropagating shock on the growth of hydrodynamic instabilities in a convergent, compressible system have been performed on the Omega Laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester. Direct laser illumination of a cylindrical target launches a strong shock across hydrodynamically unstable interfaces formed between an epoxy ablator material on the outside, a buried aluminum marker layer and low-density CH foam on the inside. The Richtmyer–Meshkov instability mixes the marker into the two adjacent materials. Of particular interest is what happens when the mixing region is reshocked by using a second, coaxial central cylinder to reflect the incident shock back into the mixing region. These experiments have been extensively modeled, in two dimensions, using the hydrocodes NYM [P. D. Roberts et al., J. Phys. D 13, 1957 (1980)], PETRA [D. L. Youngs, Physica D 12, 32 (1984)], and RAGE [R. M. Baltrusaitis et al., Phys. Flui...

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.

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.

Standard single frequency, “fringe-counting,” microwave interferometers are of limited use for steady-state plasma experiments. We have constructed a swept frequency microwave interferometer, similar to a classic zebra-stripe interferometer, optimized for electron density measurements in steady-state plasma experiments. The key element in the system is a frequency doubled YIG oscillator capable of sweeping from 20 to 40 GHz. As the source frequency is swept, the sum of the reference and plasma leg signals exhibits a series of beats. Both the frequency shift and phase shift of the beat pattern due to the addition of plasma in one leg of the interferometer is used to determine the line-integrated electron density.

Summary form only given, as follows. West Virginia University's large experiment on instabilities and anisotropies (LEIA), is designed to study high beta space plasma phenomena. Presently, instabilities involving nonisotropic ion distributions (different parallel and perpendicular temperatures) are being investigated. Control of the temperature anisotropy is necessary to study these instabilities. Measurements of the ion temperature anisotropy in LEIA and in the steady-state helicon source (HELIX), which generates the plasma for LEIA, as a function of source RF power, source pressure, and magnetic field in both the source and LEIA will be presented.