Christopher Compton

Laser induced fluorescence in a pulsed argon plasma

A time-resolved laser induced fluorescence (LIF) technique for pulsed argon plasmas is described. A low power, tunable diode laser pumps a three level Ar II transition sequence at a wavelength of 668.6138 nm. With a standard LIF system designed for steady-state plasmas (e.g., 4 kHz optical chopper, 20 kHz band-width detector, and a lock-in amplifier), we demonstrate that the evolution of the ion velocity distribution can be resolved with a time resolution of 1 ms through a combination of time-series averaging and post-acquisition digital signal processing.

Ion dynamics in helicon sources

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.

Microparticle injection effects on microwave transmission through an overly dense plasma layer

Summary form only given. Vehicles traveling at hypersonic velocities within the Earths atmosphere, such as spacecraft during reentry and other hypersonic vehicles, are enveloped by a dense plasma layer. This plasma layer reflects and significantly attenuates GPS and S-band signals for vehicle navigation, telemetry, and voice communications, resulting in radio blackout.Injecting microparticles into a plasma discharge will reduce the free electron density via electron attachment to particles. Reducing the free electron density lowers the plasma cutoff frequency, and may allow lower frequency bands of electromagnetic signals to penetrate the plasma layer. In these studies, a linear hollow cathode produces an electron beam that is accelerated into a low pressure (50 to 150 mTorr) background of Argon gas, producing an electron beam discharge. A 170 Gauss axial magnetic field produced by two electromagnet coils in a Helmholtz configuration results in a well-collimated electron beam, producing a 2dimensional Argon plasma discharge. This discharge sheet is approximately 100 cm long by 30 cm wide by 2 cm thick, at densities as high as 1012 cm-3. The plasma sheet is intended to mimic the intense plasma layer produced and experienced by vehicles traveling at hypersonic velocities. A shaker device with fine mesh on the bottom is filled with alumina powder and fitted with a vibrating motor. When supplied with a modest voltage, the vibration drops alumina microparticles from the mesh openings, into the plasma sheet discharge, creating a dusty plasma. Varying the voltage supplied to the vibrating motor varies the flux rate and density of powder dropped into the plasma. A transmitting microwave horn is oriented normal to the dense plasma sheet while the receiving horn is mounted on a stage that can be rotated up to 180 degrees azimuthally. Results from these experiments measuring the cutoff and transmission of S-band microwaves incident on a dusty plasma layer, as well as Langmuir probe measurements assessing microparticle effects on plasma density and transparency are reported.

On the Mechanism of Pulsed Electron Beam Production From an Uninterrupted Plasma Cathode

Electron extraction from a hollow cathode plasma discharge through a primary anode, biased with a dc accelerating potential, is modulated to produce pulsed or continuous electron beams without the interruption of the plasma cathode discharge. This is achieved with the addition of a secondary anode, within the hollow cathode. When this secondary anode is electrically connected to the primary anode, beam production ceases; when the secondary anode is electrically isolated, beam production resumes. Previous works demonstrated the utility and operation of this device; however, the precise physical principles allowing beam modulation were not well characterized. In this paper, we show that beam modulation can be understood in terms of changes in plasma potential within the hollow cathode that either promote or prevent the formation of an electron sheath at the exit of the hollow cathode, which is mediated by the electrical state of the secondary anode. Maintaining a plasma within the hollow cathode during a state change of the secondary anode depends on the secondary anode meeting the criteria for global nonambipolar flow. These effects are explored with both the Langmuir probe measurements within the hollow cathode during the steady-state operation, and the high-speed current and voltage monitoring during beam pulses.

Ion velocity distribution function measurements in a helium helicon plasma

Summary form only given. The scarcity of strong absorption lines in accessible tuning ranges along with plasma saturation due to low ion population densities makes laser absorption spectroscopy of helium ions in plasma notoriously difficult. Helicon plasmas, with their characteristically high ion densities are a good candidate for helium ion spectroscopy experiments. Initial measurements of Doppler broadened ion velocity distribution functions (ivdf) involve injecting a tunable infrared diode laser, tuned to 1012.36 nm and chopped roughly at 1 kHz, along the axis of a 1.5 m long helicon plasma and a 4 m long expansion chamber. The single pass absorption as a function of laser wavelength is measured with a bandpass filtered photodiode. We will present measurements of the ivdf in helium helicon plasma as a function of RF power, source magnetic field, and neutral pressure. The calculated ion temperatures obtained from the ivdf indicate that the helium ions are at roughly room temperature, ~0.03 eV, Measurements of the ivdf of argon ions in argon doped helium plasmas will also be presented. As the partial pressure of argon is reduced to less than a few percent, the discharge switches from a dominantly argon plasma to a dominantly helium plasma. Assuming thermodynamic equilibrium, the argon ion temperature can then be used as a measure of the helium ion temperature

Ion heating due to Alfven waves in a helicon plasma

Summary form only given. Recent models for ion heating in the fast solar wind region of the Sun predicts the heating is due to MHD turbulence driven by counter propagating, low-frequency Alfven waves. Experiments to test this theory is conducted in the West Virginia University HELIX (Hot Helicon Experiment) device in helium plasma. Densities in HELIX are on the order of 10/sup 13/ cm/sup -3/ with ion temperatures of about 0.3 eV. To create counter propagating Alfven waves one of two techniques are employed. We first attempt to launch the Alfven waves from the helicon source region and generate a reflection due to an Alfven speed gradient. The HELIX device has an Alfven speed profile similar to the solar corona, a short region of increased Alfven speed followed by a rapid decrease in speed as the magnetic field expands. Should the first method prove to be unsuccessful, two waves are launched at each other from different antennas. This method has the added advantage of allowing the relative intensities of the counter-propagating waves to be varied. Temperatures of helium ions are measured using a RF compensated energy analyzer. We present information on the experimental apparatus as well as preliminary data.