Bradley Sommers

Observations of electric discharge streamer propagation and capillary oscillations on the surface of air bubbles in water

The propagation of electric discharge streamers inside bubbles in liquids is of interest for the remediation of toxins in water and plasma-based surgical instruments. The manner of streamer propagation has an important influence on the production of reactive species that are critical to these applications. Streamer propagation along the surface of electrode-attached bubbles of air in water, previously predicted by numerical simulations, has been experimentally imaged using a fast frame-rate camera. The successive pulsing of the streamer discharge inside the bubbles produced oscillations along the air‐water interface. Subsequent streamers were observed to closely follow surface distortions induced by such oscillations. The oscillations likely arise from the non-uniform perturbation of the bubble driven by the electric field of the streamer and were found to be consistent with Kelvin’s equation for capillary oscillations. For a narrow range of applied voltage pulse frequencies, the oscillation amplitude increased over several pulse periods indicating, potentially, resonant behaviour. We also observed coupling between bubbles wherein oscillations in a second bubble without an internal discharge were induced by the presence of a streamer in a fixed bubble. (Some figures in this article are in colour only in the electronic version)

Perspectives on the Interaction of Plasmas With Liquid Water for Water Purification

Plasma production or plasma injection in liquid water affords one the opportunity to nonthermally inject advanced oxidation processes into water for the purpose of purification or chemical processing. Such technology could potentially revolutionize the treatment of drinking water, as well as current methods of chemical processing through the elimination of physical catalysts. Presented here is an overview of current water treatment technology, its limitations, and the future, which may feature plasma-based advanced oxidation techniques. As such, this field represents an emerging and active area of research. The role that plasma-driven water chemistry can play in addressing emerging threats to the water supply is discussed using case study examples. Limitations of conventional plasma injection approaches include limited throughput capacity, electrode erosion, and reduced process volume. At the University of Michigan, we are investigating two potential approaches designed to circumvent such issues. These include direct plasma injection using an underwater DBD plasma jet and the direct production of plasmas in isolated underwater bubbles via a pulsed electric field. These approaches are presented here, along with the results.

Plasma formation in underwater gas bubbles

The generation of plasma in underwater gas bubbles offers the potential to produce large volume plasma in water while minimizing electrode erosion. Such attributes are desirable for the design of plasma-based water purification systems. In this work, gas bubbles of diameter 0.4‐0.7mm were trapped in the node of a 26.4kHz underwater acoustic standing wave and pulsed with voltages in the range 10‐14kV. Plasma formation in trapped, isolated bubbles was observed to occur through two separate pathways: (1) plasma generated in the bubble through impact by a liquid streamer and (2) plasma generated in the bubble due solely to the applied electric field. The former case demonstrates the mechanism of so-called streamer hopping in which the discharge transitions from a water streamer to a gaseous surface streamer. Perturbations of the bubble’s fluid boundary due to the streamer are also discussed.

Nonlinear oscillations of gas bubbles submerged in water: implications for plasma breakdown

Gas bubbles submerged in a dielectric liquid and driven by an electric field can undergo dramatic changes in both shape and volume. In certain cases, this deformation can enhance the distribution of the applied field inside the bubble as well as decrease the internal gas pressure. Both effects will tend to facilitate plasma formation in the gas volume. A practical realization of these two effects could have a broad impact on the viability of liquid plasma technologies, which tend to suffer from high voltage requirements. In this experiment, bubbles of diameter 0.4?0.7?mm are suspended in the node of a 26.4?kHz underwater acoustic standing wave and excited into nonlinear shape oscillations using ac electric fields with amplitudes of 5?15?kV?cm?1. Oscillations of the deformed bubble are photographed with a high-speed camera operating at 5130?frames?s?1 and the resulting images are decomposed into their axisymmetric spherical harmonic modes, , using an edge detection algorithm. Overall, the bubble motion is dominated by the first three even modes l?=?0, 2 and 4. Electrostatic simulations of the deformed bubbles internal electric field indicate that the applied field is enhanced by as much as a factor of 2.3 above the nominal applied field. Further simulation of both the pure l?=?2 and l?=?4 modes predicts that with additional deformation, the field enhancement factors could reach as much as 10?50.

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)

Towards understanding plasma formation in liquid water via single bubble studies

Plasma-in-water based technological approaches offer great potential to addressing a wide range of contaminants threatening the safety of freshwater reserves. Widespread application of plasma-based technologies, however require a better understanding of plasma formation processes in water and the nature of the plasma-driven chemistry in solution. In this paper, we survey the scope of the threat to freshwater via contamination from a variety of sources, the status of conventional treatment technologies, the promise of plasma-based water purification, and the pathway to understanding plasma formation in water through the study of single bubble breakdown physics. Plasma formation in bubbles lie at the heart of plasma formation in liquid water. We present findings from ongoing research at the University of Michigan aimed at understanding the nature of plasma formation in bubbles, which provides an avenue for not only understanding breakdown conditions, but also insight in reducing the magnitude of the breakdown voltage. These experiments also establish an approach to a standardized apparatus for the study of plasma discharges in bubbles. We also discuss approaches to controlling plasma-induced chemistry in liquid water.

Plasma Production in Isolated Bubbles

The production of plasma in isolated (i.e., unattached to a physical electrode) air bubbles in liquid water is investigated. Bubble size along with bubble position relative to the pulsed high voltage electrode is considered.

A comparison of gas temperatures measured by ultraviolet laser scattering in atmospheric plasma sources

A laser scattering system utilizing an ultraviolet laser with a triple grating spectrometer has been assembled in order to measure gas temperature in atmospheric plasma sources. Such laser scattering interactions offer a non-invasive technique for investigating atmospheric microplasma sources, which have potential applications in remote optical sensing, materials processing, and environmental decontamination. This particular system is unique in that it utilizes a ultraviolet laser line (266 nm), which increases the cross section for Rayleigh and Raman scattering by a factor of 16 in comparison to the more common 532 nm laser operating in the visible range. In this work, the laser scattering system is used to directly compare the rotational gas temperature (T r) and gas kinetic temperature (T g) in two different atmospheric plasma sources [1]: a direct current plasma jet operating on nitrogen and [2] a conventional pin–pin glow microdischarge in air. Results show agreement between T r and T g both in the low temperature afterglow of the plasma jet (300–700 K) and the hot center of the atmospheric glow (1500–2000 K). These observations lend credence to the common assumption of rotational relaxation in atmospheric plasmas and validate the ultraviolet laser diagnostic for future application in atmospheric microplasma sources.

Characterization of Ion Acceleration Processes in a Surface ECR Plasma Source

A microwave ECR ion source is developed using distributed ECR antenna excitation over the cusp of a permanent multipole magnetic configuration. The ion source employs a combination of natural nozzle acceleration effects occurring within the cusp and the application of an external DC bias to extract both electron and ion current from the device. The ECR discharge is shown to be stable over the pressure range 0.5-100 mTorr and at low pressure (1.4 mTorr), beam energies up to 80 eV are observed. In this paper, we present both current extraction and ion beam energy measurements taken over the pressure range 1-20 mTorr and power levels between 20 and 40 W. In addition, beam energy and beam energy spread are documented and correlated to discharge voltage.

Plasma ignition in free standing gas bubbles

Summary form only given. Achieving efficient plasma production in water is limited by two main factors: (1) the high breakdown strength of water, which increases the required voltage or energy input to produce plasma and (2) the high permittivity of water, which limits the penetration of the applied electric field. A recent approach used to circumvent these issues is to ignite plasma in gas bubbles injected into the water. These bubbles provide the high reduced fields (E/N) needed to maintain the discharge. To date, such discharges have been restricted to bubbles in contact with the electrode. In this study we investigate the formation of plasma inside free standing gas bubbles that are completely isolated from the driving electrode. Observation of discharge mechanisms in this geometry has yielded a host of new, complex phenomena. Chief among them is a multi-phase streamer that originates in the liquid but later makes contact with the bubble, igniting a secondary discharge inside the gas. Accompanying this interaction is an intense transfer of energy that destabilizes the bubbles shape. We also report on the successful ignition of plasma that is confined solely within the isolated gas bubble. In our experimental approach, the gas bubble, air or helium, is isolated by trapping it in the node of an ultrasonic acoustic wave. A pair of electrodes are positioned around the bubble using a 3-D translation stage. The electrodes are pulsed at 13.6 kV with a 1 us pulse length. Images of the resulting plasma formation are captured using a PIMAX ICCD camera with gate times in the 50-500 ns range.