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The volumetric photon emission from a pulsed, RF-driven atmospheric pressure plasma jet was monitored from ultraviolet to the near infrared part of the spectrum. When a small admixture of dry air, as low as ∼0.01%, is added to the helium flow, the helium line intensities decrease, while those of oxygen increase dramatically. To understand the emission trends, a non-equilibrium, collisional-radiative model is developed to estimate He and O excited state densities as a function of relative air concentration. The model is based on a numerical solution of the electron Boltzmann equation for the electron energy distribution function self-consistently coupled to the balance equations for helium and air species. The spectral analyses reveal a good agreement between experiments and modeling, with the latter indicating that a complex interplay between electron kinetics and plasma chemistry is responsible for the substantial changes in emission profiles when He is diluted with a small amount of air.

The energy flux to a surface during plasma exposure and the associated surface heating are of long standing interest since both contribute to the physicochemical changes associated plasma-based materials processing. The unique feature of plasmas compared to other methods of materials synthesis and processing is that the energy flux is delivered and absorbed at or very near the surface over short time scales, and thus requires fast, surface-sensitive techniques to fully appreciate the dynamics of the plasma-surface interface. To achieve this, we employ pump-probe Time-Domain Thermoreflectance (TDTR) to measure the electron and phonon excitation and energy transport dynamics in thin metal films during exposure to an atmospheric pressure plasma jet. The results show the energy delivered by the plasma jet causes a localized thermal spike that is dissipated radially from the point of contact. More specifically, energy delivered via the flux of particles and photons causes the kinetic energy of the electrons within the material to increase over an area commensurate with the plasma jet radius. These results, in conjunction with voltage and current measurements, will be discussed in an effort to develop a first order understanding of energy transfer and relevant kinetics during plasma jet-surface interactions.

Atmospheric pressure plasmas have certain advantage in materials synthesis and processing that are not available with other approaches including low-pressure plasmas. In particular, the breadth of reactions afforded by non-equilibrium, low temperature plasmas is unique; plasmas produced in full density air allows one to extend the application space to systems and materials that are not vacuum compatible. Non-equilibrium, atmospheric pressure plasma jet devices are well-suited for such applications given their relatively simple design and modest power requirements. However, their size tends to limit their utility to small scale processes and treatments. In this work, we describe approaches to extend the volume of non-equilibrium, atmospheric pressure plasma jets and thus, surface area that can be treated. In particular, we consider geometric and gas flow solutions to increase volume. We use high-speed cameras, optical emission spectroscopy (OES), current and voltage measurements, and simulations to characterize the results and understand the potential for and/or limitations to scale-up. This work is supported by the Naval Research Laboratory base program.

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.

A small RF signal is applied to the anode of a low pressure (P ≤ 200 mTorr), low temperature (Te ≤ 3 eV) direct current (DC) glow discharge operating at an electron density of ne ∼ 106 cm-3. The discharge is modeled as a collection of capacitive, resistive, and inductive circuit elements that have resonances at particular frequencies, much like an RLC circuit. The location of these resonances in frequency space provides information about the plasma parameters. In this work, an electrode that is used to sustain a DC glow discharge is also used to probe the impedance of the discharge. The benefit of this approach is that it is not necessary to insert a physical probe that could introduce perturbations or contaminate the discharge. Experiments were performed to demonstrate this non-invasive impedance probing method for extracting the plasma discharge density at various neutral gas pressures and discharge voltages and currents from changes to the input impedance of the anode. Comparisons between densities extracted with this method and Langmuir probe measurements showed overall good agreement.

The energy flux to a surface during plasma exposure and the associated surface heating are of long standing interest as they contribute to the physico-chemical changes that occur during plasma-based materials synthesis and processing. Indeed, the energy delivered to the surface, via a flux of particles and photons, in concert with a flux of reactive species serves to chemically modify, etch, and/or deposit materials, with an efficacy that depends on the plasma processing environment. A unique feature of plasma synthesis and processing is that most of the delivered energy is absorbed at or very near the surface over short (picosecond) time scales. The dissipation of thermal energy proceeds through electron-electron and/or electron-phonon interactions as they propagate through the material, with relaxation time scales that can be orders of magnitude slower. Typically then, the surface is not in thermal equilibrium with the bulk material. Fast, surface-sensitive techniques are thus required to fully appreciate the dynamics of the plasma-surface interaction. In this work, we employ pump-probe Time-Domain Thermoreflectance, a surface sensitive technique typically used to measure thermal properties of thin films, to determine electron heating of thin metal films during exposure to an atmospheric pressure plasma jet. The results, in conjunction with current measurements, are used to develop a first order understanding of plasma jet-surface interactions. The results show that the energy delivered by the plasma jet causes a localized increase in electron energy within the thin film over an area commensurate with the plasma jet radius.

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.

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.

Vehicles traveling at hypersonic velocities within the Earth's 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.

Vehicles traveling at hypersonic velocities within the Earth's atmosphere, such as spacecraft during reentry and weapons systems, 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. 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 relatively constant 170 Gauss axial magnetic field is produced by two electromagnet coils arranged in a Helmholtz configuration. This results in a well-collimated electron beam, producing a 2-dimensional Argon plasma discharge in a sheet configuration. This discharge sheet is approximately 40 cm long by 30 cm wide by 1 cm thick, and can produce electron 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. The electron beam is accelerated vertically towards a grounded beam dump electrode. This electrode is modified to include an array of six piezo buzzers modified and filled with alumina powder. When supplied with a modest voltage, the piezoelectric shakers uniformly drop dust particles into the plasma sheet discharge directly below at a constant rate, creating a dusty 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. Microwave cutoff, transmission, reflection, and scattering measurements of the plasma sheet are made in the S-band and X-band range. These measurements are relevant for applications related to communications blackout and over-the-horizon communications.

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.

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 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.

A 10 cm stand-alone gridded ion thruster discharge chamber based on a laboratory demonstration model that utilized a planar inductive antenna was designed and fabricated. The source features multipole confinement which has been shown to dramatically improve performance based on significant increases in ion density observed in demonstration tests. The thruster design considerations and the performance goals are discussed. Preliminary thruster discharge operation will be commented upon as well as the future development work necessary to complete the performance assessment. Nomenclature E = electric field B = magnetic flux Vsh = (capacitive) voltage across plasma sheath Vrf = RF voltage sm = sheath thickness t = quartz window thickness e = electron charge ns = plasma number density uB

[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.

Microparticles injected into a plasma have been shown to deplete the free electron population as electrons are collected through the process of microparticles charging to the plasma floating potential. However, these charged microparticles can also act to scatter electromagnetic signals. These experiments investigate microwave penetration through a previously impenetrable overly dense plasma layer as microparticles are injected and the physical phenomena associated with the competing processes that occur due to electron depletion and microwave scattering. The timescales for when each of these competing processes dominates is analyzed in detail. It was found that while both processes play a significant and dominant role at different times, ultimately, transmission through this impenetrable plasma layer can be significantly increased with microparticle injection.

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.

These experiments utilize a linear hollow cathode to create a dense, rectangular plasma sheet to simulate the plasma layer surrounding vehicles traveling at hypersonic velocities within the Earth's atmosphere. Injection of fine dielectric microparticles significantly reduces the electron density and therefore lowers the electron plasma frequency by binding a significant portion of the bulk free electrons to the relatively massive microparticles. Measurements show that microwave transmission through this previously overly dense, impenetrable plasma layer increases with the injection of alumina microparticles approximately 60 μm in diameter. This method of electron depletion is a potential means of mitigating the radio communications blackout experienced by hypersonic vehicles.

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)