Eric Gillman

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)

Observation of Cathode-Spot Phenomena at the Surface of a Dielectric Powder-Covered Electrode in the Presence of a Background Plasma and Applied Magnetic Field

An electrode containing a layer of micrometer-sized ceramic powder was exposed to a low-pressure background argon plasma. Negative biasing of the electrode resulted in the formation of localized arcs which manifested attributes similar to that observed in vacuum cathode-spot discharges. The arcing action on the electrode in the presence of an applied magnetic field ejected particles and metal vapor, eventually etching repeatable tracks into the electrode surface. The regular tracks left by the action of the cathode spots on the electrode surface suggest J times B effects. The cathode-spot waveforms and erosion tracks with and without an applied magnetic field were documented using a fast frame rate camera and oscilloscope.

Electron depletion via cathode spot dispersion of dielectric powder into an overhead plasma

The effectiveness of cathode spot delivered dielectric particles for the purpose of plasma depletion is investigated. Here, cathode spot flows kinetically entrain and accelerate dielectric particles originally at rest into a background plasma. The time variation of the background plasma density is tracked using a cylindrical Langmuir probe biased approximately at electron saturation. As inferred from changes in the electron saturation current, depletion fractions of up to 95% are observed. This method could be exploited as a means of communications blackout mitigation for manned and unmanned reentering spacecraft as well as any high speed vehicle enveloped by a dense plasma layer.

Investigation of an RF plasma afterglow with a cathode-spot driven quenching method for spacecraft reentry blackout mitigation

Communications blackout, which is experienced by spacecraft re-entering the atmosphere at hypersonic velocities, is caused by the formation of a dense plasma envelope produced by shock heating. Communication signals at frequencies below the plasma cutoff frequency cannot propagate through this layer. Methods suggested for mitigating blackout have included aerodynamic shaping, magnetic windows, and the use of quenchants to reduce plasma densities. The Gemini 3 mission in 1965 successfully used water as a quenchant to cool the reentry plasma and increase communication signal strength.

Laser Imaging and Velocimetry of Cathode-Spot-Ejected Ceramic Powder

Velocimetry of ceramic particles ejected via cathode spot eruption on the surface of a biased electrode in the presence of a low-density plasma has been carried out. Witness plate measurements indicated the rate of powder removal to be greater than 3.0 mg/s. An expanded laser beam and fast frame-rate camera were used to perform laser-illuminated particle imaging velocimetry measurements. The velocity distribution for 45- μm-diameter ceramic particles was found to range between 0.2 and 1.5 m/s in the plane of view as they are ejected from the electrode surface. Nonuniform powder ejection in the horizontal plane is shown to be related to the presence of an applied magnetic field.

Characterization of electron density depletion in a cathode spot driven dusty plasma for reentry vehicle communications applications

Communications blackout, which is experienced by spacecraft re-entering the atmosphere at hypersonic velocities, is caused by the formation of a dense plasma envelope produced by shock heating. Communication signals at frequencies below the plasma cutoff frequency cannot propagate through this layer. Methods suggested for mitigating blackout have included aerodynamic shaping, magnetic windows, and the use of quenchants to reduce plasma densities. The Gemini 3 mission in 1965 successfully used water as a quenchant to cool the reentry plasma and increase communication signal strength.

A Magnetically Enhanced Inductive Discharge Chamber for Electric Propulsion Applications

A magnetically enhanced inductive discharge was investigated for electric propulsion applications. The high plasma density produced by the source makes it attractive as an ion source for a gridded ion thruster or possibly a standalone ambipolar thruster. The discharge plasma is produced by a compact ldquostovetoprdquo spiral antenna that is similar to that used in plasma processing sources. Operation on argon to pressures as low as 1 mtorr was demonstrated at powers ranging from 100 to 250 W. Ion current as high as 1 A was extracted from a 10-cm-diameter device. Plasma properties and ion production efficiency are reported and commented upon.

Magnetically Enhanced Inductive Source for Electric Propulsion Applications

[Abstract] A magnetically enhanced inductive discharge was investigated for electric propulsion applications. The high plasma density produced by the source makes it attractive as an ion source for a gridded ion thruster or possibly a stand-alone ambipolar thruster. Plasma is produced by a compact “stovetop”, spiral antenna similar to that used in plasma processing sources. Operation on Argon to pressures as low as 1 mTorr were demonstrated at powers ranging from 100 to 250 W. Ion current as high as 1 A were extracted from the 10 cm diameter device. Plasma properties and ion production efficiency are reported and commented upon.

Dust Ejection and Stream Formation via Cathode Spot Ignition at the Surface of a Dielectric-Covered Electrode

Cathode spots have been observed to form on an electrode surface covered with dielectric powder when biased at large negative voltages in the presence of a background plasma. The ignition of cathode spots leads to the ejection of the powder particles, which subsequently form dust streams. Particle imaging velocimetry (PIV) techniques are used to track the ejected particles and obtain the distribution of particle ejection velocities.

Heating and chemical reactivity of a low frequency, DBD plasma jet in liquid water

A coaxial, gas fed dielectric barrier discharge was used to produce a plasma jet in liquid water. It was found that a significant fraction of the discharge power was deposited into the water as heat. Actual deposited power was determined via the Manley method.1 The water heating rates showed a weak dependence on frequency. The plasma discharge, which consisted of streamers inside the source, appeared as a directed plasma jet inside the liquid water, suggesting significant excitation of the gas entering the water. Emission spectra of the air jet revealed nitrogen, hydrogen and oxygen emission lines in deionized water. Methylene Blue (MB) was used a chemical reactivity indicator. It was found that the water discharge could significantly reduce the concentration of MB in the water as verified using a spectrophotometer. A comparison of plasma treated MB solution and those treated with simple boiling suggests that MB destruction is a plasma induced-effect, not a thermal one.