@article{sponsel_gershman_stapelmann_2023, title={Electrical breakdown dynamics in an argon bubble submerged in conductive liquid for nanosecond pulsed discharges}, volume={56}, ISSN={["1361-6463"]}, url={https://doi.org/10.1088/1361-6463/acfb1b}, DOI={10.1088/1361-6463/acfb1b}, abstractNote={Abstract
               This study delves into the dynamics of cold atmospheric plasma and their interaction within conductive solutions under the unique conditions of nanosecond pulsed discharges (22 kV peak voltage, 10 ns FWHM, 4.5 kV ns−1 rate-of-rise). The research focuses on the electrical response, breakdown, and discharge propagation in an argon bubble, submerged in a NaCl solution of varying conductivity. Full or partial discharges were observed at conductivities of 1.5 µS cm−1 (deionized water) to 1.6 mS cm−1, but no breakdown was observed at 11.0 mS cm−1 when reducing the electrode gap. It is demonstrated that at higher conductivity electric breakdown is observed only when the gas bubble comes into direct contact with the electrode and multiple emission nodes were observed at different timescales. These nodes expanded in the central region of the bubble over timescales longer than the initial high-voltage pulse. This work offers a temporal resolution of 2 ns exposure times over the first 30 ns of the initial voltage pulse, and insight into plasma formation over decaying reflected voltage oscillations over 200 ns.}, number={50}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Sponsel, Nicholas L. and Gershman, Sophia and Stapelmann, Katharina}, year={2023}, month={Dec} }
 @article{sponsel_gershman_quesada_mast_stapelmann_2022, title={Electric discharge initiation in water with gas bubbles: A time scale approach}, volume={40}, ISSN={["1520-8559"]}, url={https://doi.org/10.1116/6.0001990}, DOI={10.1116/6.0001990}, abstractNote={High voltage nanosecond pulse driven electric discharges in de-ionized water with an argon bubble suspended between two electrodes were experimentally investigated. Two electrode configurations were used to temporally resolve the time scales of the discharge from the applied voltage rise time (7 ns), through the end of the first pulse (∼30 ns), and longer (>50 ns). We found that, in positive and negative applied voltage polarities, discharge initiates in the water at the tip of the anode. The discharge in the water rapidly extends (∼104 m/s) to the apex of the bubble and light emitted from inside the bubble begins to form. The steep rate of rise of the applied voltage (dV/dt<4 kV/ns) and the short time for the development of discharge in the water suggest that cavitation is a likely mechanism for discharge initiation and propagation in water. In addition, the short duration of the applied voltage pulse results in only a partial Townsend discharge inside the bubble.}, number={6}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Sponsel, Nicholas L. and Gershman, Sophia and Quesada, Maria J. Herrera J. and Mast, Jacob T. and Stapelmann, Katharina}, year={2022}, month={Dec} }
 @article{pillai_sponsel_mast_kushner_bolotnov_stapelmann_2022, title={Plasma breakdown in bubbles passing between two pin electrodes}, volume={55}, ISSN={["1361-6463"]}, url={https://doi.org/10.1088/1361-6463/ac9538}, DOI={10.1088/1361-6463/ac9538}, abstractNote={Abstract
               The ignition of plasmas in liquids has applications from medical instrumentation to manipulation of liquid chemistry. Formation of plasmas directly in a liquid often requires prohibitively large voltages to initiate breakdown. Producing plasma streamers in bubbles submerged in a liquid with higher permittivity can significantly lower the voltage needed to initiate a discharge by reducing the electric field required to produce breakdown. The proximity of the bubble to the electrodes and the shape of the bubbles play critical roles in the manner in which the plasma is produced in, and propagates through, the bubble. In this paper, we discuss results from a three-dimensional direct numerical simulation (DNS) used to investigate the shapes of bubbles formed by injection of air into water. Comparisons are made to results from a companion experiment. A two-dimensional plasma hydrodynamics model was then used to capture the plasma streamer propagation in the bubble using a static bubble geometry generated by the DNS The simulations showed two different modes for streamer formation depending on the bubble shape. In an elliptical bubble, a short electron avalanche triggered a surface ionization wave (SIWs) resulting in plasma propagating along the surface of the bubble. In a circular bubble, an electron avalanche first traveled through the middle of the bubble before two SIWs began to propagate from the point closest to the grounded electrode where a volumetric streamer intersected the surface. In an elliptical bubble approaching a powered electrode in a pin-to-pin configuration, we experimentally observed streamer behavior that qualitatively corresponds with computational results. Optical emission captured over the lifetime of the streamer curve along the path of deformed bubbles, suggesting propagation of the streamer along the liquid/gas boundary interface. Plasma generation supported by the local field enhancement of the deformed bubble surface boundaries is a mechanism that is likely responsible for initiating streamer formation.}, number={47}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Pillai, Naveen and Sponsel, Nicholas L. and Mast, J. T. and Kushner, Mark J. and Bolotnov, Igor A. and Stapelmann, Katharina}, year={2022}, month={Nov} }
 @article{pillai_sponsel_stapelmann_bolotnov_2022, title={Direct Numerical Simulation of Bubble Formation Through a Submerged "Flute" With Experimental Validation}, volume={144}, ISSN={["1528-901X"]}, DOI={10.1115/1.4052051}, abstractNote={Abstract
               Direct numerical simulation (DNS) is often used to uncover and highlight physical phenomena that are not properly resolved using other computational fluid dynamics methods due to shortcuts taken in the latter to cheapen computational cost. In this work, we use DNS along with interface tracking to take an in-depth look at bubble formation, departure, and ascent through water. To form the bubbles, air is injected through a novel orifice geometry not unlike that of a flute submerged underwater, which introduces phenomena that are not typically brought to light in conventional orifice studies. For example, our single-phase simulations show a significant leaning effect, wherein pressure accumulating at the trailing nozzle edges leads to asymmetric discharge through the nozzle hole and an upward bias in the flow in the rest of the pipe. In our two-phase simulations, this effect is masked by the surface tension of the bubble sitting on the nozzle, but it can still be seen following departure events. After bubble departure, we observe the bubbles converge toward an ellipsoidal shape, which has been validated by experiments. As the bubbles rise, we note that local variations in the vertical velocity cause the bubble edges to flap slightly, oscillating between relatively low and high velocities at the edges.}, number={2}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Pillai, Naveen and Sponsel, Nicholas L. and Stapelmann, Katharina and Bolotnov, Igor A.}, year={2022}, month={Feb} }