Brian L. Sands

Role of Penning ionization in the enhancement of streamer channel conductivity and Ar(1s5) production in a He-Ar plasma jet

Plasma jet devices that use a helium gas flow mixed with a small percentage of argon have been shown to operate with a larger discharge current and enhanced production of the Ar(1s5) metastable state, particularly in the discharge afterglow. In this experiment, time-resolved quantitative measurements of He(23S1) and Ar(1s5) metastable species were combined with current and spectrally resolved emission measurements to elucidate the role of Penning ionization in a helium plasma jet with a variable argon admixture. The plasma jet was enclosed in a glass chamber through which a flowing nitrogen background was maintained at 600 Torr. At 3%–5% Ar admixture, we observed a ∼50% increase in the peak circuit current and streamer velocity relative to a pure helium plasma jet for the same applied voltage. The streamer initiation delay also decreased by ∼20%. Penning ionization of ground-state argon was found to be the dominant quenching pathway for He(23S1) up to 2% Ar and was directly correlated with a sharp increas...

Ozone generation in a kHz-pulsed He-O2 capillary dielectric barrier discharge operated in ambient air

The generation of reactive oxygen species using nonequilibrium atmospheric pressure plasma jet devices has been a subject of recent interest due to their ability to generate localized concentrations from a compact source. To date, such studies with plasma jet devices have primarily utilized radio-frequency excitation. In this work, we characterize ozone generation in a kHz-pulsed capillary dielectric barrier discharge configuration comprised of an active discharge plasma jet operating in ambient air that is externally grounded. The plasma jet flow gas was composed of helium with an admixture of up to 5% oxygen. A unipolar voltage pulse train with a 20 ns pulse risetime was used to drive the discharge at repetition rates between 2–25 kHz. Using UVLED absorption spectroscopy centered at 255 nm near the Hartley-band absorption peak, ozone was detected over 1 cm from the capillary axis. We observed roughly linear scaling of ozone production with increasing pulse repetition rate up to a “turnover frequency,” b...

Current scaling in an atmospheric pressure capillary dielectric barrier discharge

Abstract : Current scaling in an atmospheric pressure capillary dielectric barrier discharge, comprising a structured rare gas flow that extends into ambient air, is characterized by electrical and optical measurements. In the transient glow mode, two current scaling regimes were identified that are separated by the static free shear flow boundary. The peak current was sensitive to cathode placement relative to this flow structure and could be scaled from ~300 mA to over 5 A. Applying a Boltzmann equation solver, it was found that ~1% air entrainment into the flow and an E/N could account for the observed trends.

Decay of the electron number density in the nitrogen afterglow using a hairpin resonator probe

A hairpin resonator was used to measure the electron number density in the afterglow of a nitrogen glow discharge (p=0.25–0.75Torr). Electron number densities were measured using a time-dependent approach similar to the approach used by Spencer et al. [J. Phys. D 20, 923 (1987)]. The decay time of the electron number density was used to determine the electron temperature in the afterglow, assuming a loss of electrons via ambipolar diffusion to the walls. The electron temperature in the near afterglow remained between 0.4 and 0.6eV, depending on pressure. This confirms the work by Guerra et al. [IEEE Trans. Plasma. Sci. 31, 542 (2003)], who demonstrated experimentally and numerically that the electron temperature stays significantly above room temperature via superelastic collisions with highly vibrationally excited ground state molecules and metastables, such as AΣu+3.

Effect of Gas Mixture on Plasma Jet Discharge Morphology

The morphology of plasma emission from a streamerlike atmospheric pressure plasma jet initiated inside a glass capillary has been measured with a high spatial resolution for a gas flow of pure helium and He with a 5% Ar admixture using 5-ns time-gated intensified charge-coupled device imaging. In the pure helium plasma jet, emission remains annular and converges gradually along the core gas-air interface. With a 5% argon admixture, however, the plasma emission is initially annular inside the glass capillary but converges to the flow axis within 5 mm outside the capillary tip.

Femtosecond, two-photon laser-induced-fluorescence imaging of atomic oxygen in an atmospheric-pressure plasma jet

Femtosecond, two-photon-absorption laser-induced-fluorescence (fs-TALIF) spectroscopy is employed to measure space- and time-resolved atomic-oxygen distributions in a nanosecond, repetitively pulsed, externally grounded, atmospheric-pressure plasma jet flowing helium with a variable oxygen admixture. The high-peak-intensity, low-average-energy femtosecond pulses result in increased TALIF signal with reduced photolytic inferences. This allows 2D imaging of absolute atomic-oxygen number densities ranging from 5.8???×???1015 to 2.0???×???1012cm?3 using a cooled CCD with an external intensifier. Xenon is used for signal and imaging-system calibrations to quantify the atomic-oxygen fluorescence signal. Initial results highlight a transition in discharge morphology from annular to filamentary, corresponding with a change in plasma chemistry from ozone to atomic oxygen production, as the concentration of oxygen in the feed gas is changed at a fixed voltage-pulse-repetition rate. In this configuration, significant concentrations of reactive oxygen species may be remotely generated by sustaining an active discharge beyond the confines of the dielectric capillary, which may benefit applications that require large concentrations of reactive oxygen species such as material processing or biomedical devices.

Electron and metastable state interactions in two-step ionization waves

We demonstrate the use of a microwave hairpin resonator to measure the time-dependent, phase-resolved electron number density in ionization waves. Under our argon glow discharge conditions, the instability was caused by two-step ionization; and the wave frequency depended on the volume quenching rate of the metastable states. We measured the 1s5 metastable state density using diode laser absorption. The peak electron number density lagged behind the peak metastable state density by 60°. This phase shift reveals the nonlocal nature of the electron kinetics due to two-step ionization.

Temporally Resolved Imaging of Jet-Type Dielectric Barrier Discharge Using He and Ar/Acetone Crossed Gas Flows

We report on temporally resolved images in a jet-type dielectric barrier discharge with two crossed gas flows obtained by a spectrally filtered intensified charge-coupled device (ICCD) camera. The crossed-flow configuration of He and Ar gases and a small acetone impurity added into the Ar flow simulated a discharge configuration and an organic precursor used for the practical application of material processing, respectively. From the ICCD images, streamer and transient glow discharges could be distinguished between the electrode and the substrate. Filtered emission from excited N2+, CH, C2, He, and Ar also revealed the excitation and decomposition processes in the discharge.

Diode laser absorption and emission spectroscopy of a streamer discharge in an atmospheric pressure plasma jet

Research into atmospheric pressure plasma jets (APPJs) that are initiated via a self-sustaining streamer discharge has recently been driven by both their potential for applications to nonthermal material processing, and fundamental questions regarding the basic discharge mechanisms that drive this remarkably stable atmospheric pressure discharge. We have characterized a streamer-initiated atmospheric pressure plasma jet in a 5% Ar / 95% He carrier gas flowing into ambient air using tunable diode laser absorption spectroscopy together with photomultiplier-coupled optical emission spectroscopy and current/voltage measurements. Improvements in the stability of this APPJ have allowed us to isolate the streamer discharge component with a higher precision than we have previously reported, as the shot-to-shot jitter is now comparable to the optical detector bandwidths. This APPJ configuration is similar to that used in previous workl, with the difference that only a single ring electrode is used here. Optical transitions from He, Ar, and various air species were used to characterize the spatiotemporal evolution of the discharge. The optical detector was aligned with the output from an 811.53 nm tunable diode laser that was used to measure the line integrated density of the Ar ls5 metastable.