Matthew M. Balkey

Control of ion temperature anisotropy in a helicon plasma

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.

Ion temperature anisotropy limitation in high beta plasmas

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.

Frequency dependent effects in helicon plasmas

Variations in the plasma parameters of a large volume, helicon source as a function of applied rf power (0–2 kW), driving frequency (8–18 MHz), magnetic field (0–1.4 kG) and fill pressure (2–10 mTorr) have been studied. The transitions between the capacitive, inductive, and resonant helicon mode are consistent with previous experiments. Our data indicate that the transition to the helicon mode occurs at a unique magnetic field, independent of the driving frequency. Based on the helicon wave dispersion relation, from which helicon wavelengths can be calculated, the observed variations in plasma density as a function of driving frequency suggest that the wavelength of the helicon wave is a weak function of driving frequency. Calculation of the electron energies which correspond to the phase velocity of the driving wave (i.e., Landau damping) suggest that either Landau damping cannot be responsible for the efficient ionization of helicon sources, or that the helicon portion of the discharge does not extend o...

Ion heating in the HELIX helicon plasma source

Efficient ion heating in a steady-state helicon plasma source is observed with two external loop antennae just above the ion cyclotron frequency. The ion velocity space distribution is measured by laser induced fluorescence in an argon plasma. The measured bulk ion heating is highly anisotropic (the perpendicular temperature increase is ten times the parallel temperature increase) even though the plasma is moderately collisional. Measurements of the perturbed distribution function with laser induced fluorescence suggest that an electrostatic ion cyclotron wave is launched.

Microwave interferometer for steady-state plasmas

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.

Beta-dependent upper bound on ion temperature anisotropy in a laboratory plasma

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).

A compact, intense, monochromatic, atmospheric pressure, extreme ultraviolet light source

An intense, monochromatic, extreme ultraviolet (121.6 nm) light source has been constructed for testing extreme ultraviolet light rejecting filters. The source and detection system operates at atmospheric pressure, is physically compact, and is relatively inexpensive to build. A calibrated neutral density filter with a transmission of 1×10−6 is used as a reference. The light source has been used to measure transmissions as small as 9×10−6 quickly (in less than 1 min) with negligible background levels.

Ion cyclotron resonant heating in a helicon plasma source

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.

Temperature anisotropy measurements in LEIA

Summary form only given, as follows. West Virginia Universitys 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.

High beta ion driven microinstabilities in the large experiment on instabilities and anisotropies

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.