Brandon Weatherford

Underwater operation of a DBD plasma jet

A plasma jet produced in water using a submerged ac excited electrode in a coaxial dielectric barrier discharge configuration was studied. Plasma jet formation was found to occur only while the source was submerged. Plasma jet operation was characterized with and without gas flow. It was found that over 60% of the discharge power was deposited into the water and did not vary appreciably with excitation frequency. Presumably the remaining power fraction went into excitation, ionization and local electrode heating. Emission spectra of the jet revealed nitrogen, hydrogen, hydroxyl and oxygen emission lines. Operation of the plasma jet in water containing the oxidation‐reduction indicator methylene blue dye resulted in a marked clearing of the water as observed visually and with a spectrophotometer, suggesting plasma-induced chemical reactivity. (Some figures in this article are in colour only in the electronic version)

Electron current extraction from a permanent magnet waveguide plasma cathode

An electron cyclotron resonance plasma produced in a cylindrical waveguide with external permanent magnets was investigated as a possible plasma cathode electron source. The configuration is desirable in that it eliminates the need for a physical antenna inserted into the plasma, the erosion of which limits operating lifetime. Plasma bulk density was found to be overdense in the source. Extraction currents over 4 A were achieved with the device. Measurements of extracted electron currents were similar to calculated currents, which were estimated using Langmuir probe measurements at the plasma cathode orifice and along the length of the external plume. The influence of facility effects and trace ionization in the anode-cathode gap are also discussed.

Characterization of the evolution of underwater DBD plasma jet

An air plasma jet formed underwater using a coaxial DBD electrode configuration with gas flow is being studied for water treatment applications. The arc-like behavior of the discharge in the absence of any obvious return electrode is not well understood. This study seeks to understand the underlying nature of the arc-like jet mode by studying the evolution of the discharge from microdischarge to jet mode. Photographic and spectroscopic data are used to develop a phenomenological model of discharge evolution. Time-averaged spectra were used to assign an average plume and electron temperature. Calculated jet temperatures were consistent with observed affects such as melting and oxide layer formation on a downstream substrate. The capacity of the microdischarge mode to decompose organic dye in water as a function of time, confirmed previously in the jet mode, was also demonstrated in the absence of the jet. (Some figures in this article are in colour only in the electronic version)

Two-dimensional laser collision-induced fluorescence measurements of plasma properties near an RF plasma cathode extraction aperture

A dense plasma structure was observed to form near the extraction aperture of a helium RF plasma cathode. Laser collision-induced fluorescence was used to generate two-dimensional spatial maps of the electron density and the effective electron temperature within the structure over a range of operating conditions. The aperture plasma reached densities nearly an order of magnitude higher than the surrounding bulk plasma. The sharp spatial change in density at the plasma structure boundary suggests the presence of a double layer sheath. Higher temperature electrons were also observed at the periphery of the plasma structure. Variations in the observed plasma structure with extracted electron current were found to be consistent with reported low pressure anode spot behavior. Measurements of plasma density within and at the boundary of the structure, and the dependence of these on the current extracted across the external gap, are compared with calculations and discussed.

Improved Performance of the ECR Waveguide Plasma Cathode with Permanent Magnets

The performance of the U-M waveguide plasma cathode is presented, with data acquired on argon, krypton, and xenon feed gases. The maximum electron current extracted from the device was 4.2 A, at 100 V and 120 W of absorbed microwave power, and either 6 sccm on argon or 2 sccm on xenon. As expected, xenon delivered the most efficient performance, due to its low mobility and low ionization potential relative to argon and krypton. The minimum amount of power consumption (microwave power plus beam extraction power) was 89.9 W/A, and the maximum gas utilization factor was 32.2 electrons per xenon atom. Spatial maps of Langmuir probe data were recorded in the region between the plasma cathode and extraction anode (on xenon), and the resulting ion density and plasma potential distributions are discussed.

Visible Plume From a Low-Power ECR Waveguide Plasma Cathode for Electric Propulsion Systems

A waveguide ECR plasma cathode is being investigated as a long-lived alternative to thermionic emitter-based electron sources for ion beam neutralization in electric propulsion systems. The present device can deliver up to 4.2 A of electron current, on argon or xenon, with low power consumption and high gas utilization.

Characterization of a Waveguide ECR Plasma Source

The design and preliminary testing of a waveguide electron cyclotron resonance (ECR) heated plasma source is presented in this paper. The design utilizes resonant absorption of traveling microwaves to heat the discharge, avoiding any lifetime constraints on the device due to barium depletion or ion bombardment in more conventional devices. Results from magnetostatic calculations are presented, which are used to design the magnetic field geometry with samarium cobalt permanent magnets. The results from microwave solver simulations used to finalize the waveguide design are presented. The source, which operates in the circular TE11 mode, produces a discharge through either ECR or upper hybrid heating, depending on the magnet geometry. Preliminary tests show that the self-starting source is capable of delivering 226 mA of current at an absorbed power level of 80 W and argon flow rate of 2.6 sccm. The strong magnetic field perpendicular to the desired direction of plasma flow confines the plasma quite well. However, it appears to be the limiting factor on the amount of extractable current along with the plasma density at the extraction electrode. Langmuir probe traces at the location of the extraction electrode suggest that the plasma density is highly peaked at low flow rates, while the electron temperature is nearly independent of flow rate over the gas flow range investigated.

Two-dimensional laser collision-induced fluorescence mapping of electron density and temperature near plasma cathode apertures

Laser collision-induced fluorescence (LCIF) diagnostics are used to generate two-dimensional images of electron density and temperature inside a RF helium plasma cathode setup, near the electron extraction aperture. During electron current extraction, a region of dense plasma is seen at the aperture, and this aperture plasma is often surrounded by a layer of high-temperature electrons. Density and temperature profiles are imaged at variable gas pressure, applied bias, and aperture size. The properties of the aperture plasma are compared with those in the bulk plasma, using LCIF and probe diagnostics. Because this aperture plasma acts as an interface between the internal bulk plasma and the downstream extraction anode, an understanding of these behaviors may prove useful when optimizing plasma cathodes for more efficient current extraction.

Two-Dimensional LCIF Images of Electron Density and Temperature Within an ECR Plasma Cathode

Two-dimensional mappings of electron density and temperature within an electron cyclotron resonance (ECR) plasma cathode are presented. As electron current is extracted, a secondary plasma is formed on the upstream side of the extraction aperture, and the spatial distribution of the ECR plasma is affected. The aperture plasma is surrounded by a region of “hot” electrons, suggesting the possibility of a double layer near the aperture.

Evolution of Underwater DBD Plasma Jet

The evolution of an underwater dielectric barrier discharge from microdischarge to plasma jet has been investigated. Images of the discharge evolution with voltage are discussed.