Allen L. Garner

Scaling laws for gas breakdown for nanoscale to microscale gaps at atmospheric pressure

Electronics miniaturization motivates gas breakdown predictions for microscale and smaller gaps, since traditional breakdown theory fails when gap size, d, is smaller than ∼15 μm at atmospheric pressure, patm. We perform a matched asymptotic analysis to derive analytic expressions for breakdown voltage, Vb, at patm for 1 nm ≤ d ≤ 35 μm. We obtain excellent agreement between numerical, analytic, and particle-in-cell simulations for argon, and show Vb decreasing as d → 0, instead of increasing as predicted by Paschens law. This work provides an analytic framework for determining Vb at atmospheric pressure for various gap distances that may be extended to other gases.

A universal theory for gas breakdown from microscale to the classical Paschen law

While well established for larger gaps, Paschens law (PL) fails to accurately predict breakdown for microscale gaps, where field emission becomes important. This deviation from PL is characterized by the absence of a minimum breakdown voltage as a function of the product of pressure and gap distance, which has been demonstrated analytically for microscale and smaller gaps with no secondary emission at atmospheric pressure [A. M. Loveless and A. L. Garner, IEEE Trans. Plasma Sci. 45, 574–583 (2017)]. We extend these previous results by deriving analytic expressions that incorporate the nonzero secondary emission coefficient, γSE, that are valid for gap distances larger than those at which quantum effects become important (∼100 nm) while remaining below those at which streamers arise. We demonstrate the validity of this model by benchmarking to particle-in-cell simulations with γSE = 0 and comparing numerical results to an experiment with argon, while additionally predicting a minimum voltage that was mask...

Plasma membrane temperature gradients and multiple cell permeabilization induced by low peak power density femtosecond lasers

Calculations indicate that selectively heating the extracellular media induces membrane temperature gradients that combine with electric fields and a temperature-induced reduction in the electropermeabilization threshold to potentially facilitate exogenous molecular delivery. Experiments by a wide-field, pulsed femtosecond laser with peak power density far below typical single cell optical delivery systems confirmed this hypothesis. Operating this laser in continuous wave mode at the same average power permeabilized many fewer cells, suggesting that bulk heating alone is insufficient and temperature gradients are crucial for permeabilization. This work suggests promising opportunities for a high throughput, low cost, contactless method for laser mediated exogenous molecule delivery without the complex optics of typical single cell optoinjection, for potential integration into microscope imaging and microfluidic systems.

Electric pulse shape impact on biological effects: A modeling study

Electric pulses (EPs) can permeabilize cell membranes and intracellular organelles through pore formation. Changing the pulse duration and the shape of the pulses can alter the biological effects. Here, simulation results based upon the coupling of the asymptotic Smoluchowski equation for pore formation and the Nernst-Planck equation for ion motion show that a single 10ns or 600 ns EP permeabilizes the cell membrane on the side of field exposure to facilitate electrophoresis of calcium ions into the cell. Following the EP, the pores partially reseal and calcium concentration initially declines before increasing again across the cell due to diffusion. Calcium concentrations increase to ~5 mM due after approximately 1 ms. Preliminary simulations for other pulse durations and discussed and extensions for bipolar pulses and novel pulse shapes are discussed.

Compact solid state pulsed power architecture for biomedical workflows: Modular topology, programmable pulse output and experimental validation on Ex vivo platelet activation

We describe a system based on a Marx generator approach using all solid state components in a modular topology that enables higher voltages by simply adding more modules. This novel system maximizes parameter flexibility, including electric pulse amplitude, duration and repetition rate. Example of pulses generated by this all solid state generator of ~20 kV/cm, ~ 600 ns pulse width, and more than 300 A per pulse are shown to activate platelets by stimulating human PRP (platelet rich plasma) in a 2 mm cuvette. Two growth factors released during platelet activation were identical whether nsPEFs (nanosecond pulsed electric fields) or bovine thrombin, the standard clinical platelet activator, was used. Future in vivo studies will assess the effectiveness of nsPEFs activated platelet gels compared to those activated by bovine thrombin.

Demonstration of field emission driven microscale gas breakdown for pulsed voltages using in-situ optical imaging

While multiple studies have explored the mechanism for DC and AC microscale gas breakdown, few have assessed the mechanism for pulsed voltage gas breakdown at the microscale. This study experimentally and analytically investigates gas breakdown for gap widths from 1 μm to 25 μm. Using an electrical-optical measurement system with a spatial resolution of 1 μm and a temporal resolution of 2 ns, we measure the breakdown voltages and determine breakdown morphology as a function of the gap width. An empirical fit shows that the breakdown voltage varies linearly with the gap distance at smaller gaps, agreeing with an analytical theory for DC microscale gas breakdown coupling field emission and Townsend avalanche that shows that the slope is a function of field emission properties. Furthermore, the curved breakdown paths captured between 5 μm and 10 μm demonstrate a similar effective length (∼11.7 μm) independent of the gap width, which is consistent with a “plateau” in breakdown voltage. This indicates that Townsend avalanche alone is insufficient to drive breakdown for these gaps and that ion enhanced field emission must contribute, in agreement with theory. The overall agreement of measured breakdown voltage with theoretical predictions from 1 μm to 25 μm indicates the applicability of DC microscale gas breakdown theory to pulsed breakdown, demonstrating that pulsed voltages induce a similar transition from Townsend avalanche to field emission as DC and AC voltages at the microscale.While multiple studies have explored the mechanism for DC and AC microscale gas breakdown, few have assessed the mechanism for pulsed voltage gas breakdown at the microscale. This study experimentally and analytically investigates gas breakdown for gap widths from 1 μm to 25 μm. Using an electrical-optical measurement system with a spatial resolution of 1 μm and a temporal resolution of 2 ns, we measure the breakdown voltages and determine breakdown morphology as a function of the gap width. An empirical fit shows that the breakdown voltage varies linearly with the gap distance at smaller gaps, agreeing with an analytical theory for DC microscale gas breakdown coupling field emission and Townsend avalanche that shows that the slope is a function of field emission properties. Furthermore, the curved breakdown paths captured between 5 μm and 10 μm demonstrate a similar effective length (∼11.7 μm) independent of the gap width, which is consistent with a “plateau” in breakdown voltage. This indicates that Tow...

Scaling laws for AC gas breakdown and implications for universality

The reduced dependence on secondary electron emission and electrode surface properties makes radiofrequency (RF) and microwave (MW) plasmas advantageous over direct current (DC) plasmas for various applications, such as microthrusters. Theoretical models relating molecular constants to alternating current (AC) breakdown often fail due to incomplete understanding of both the constants and the mechanisms involved. This work derives simple analytic expressions for RF and MW breakdown, demonstrating the transition between these regimes at their high and low frequency limits, respectively. We further show that the limiting expressions for DC, RF, and MW breakdown voltage all have the same universal scaling dependence on pressure and gap distance at high pressure, agreeing with experiment.

Assessment of efficacy and reactive gas species generation for orange juice decontamination using high voltage atmospheric cold plasma

Summary form only given. While multiple studies have demonstrated atmospheric cold plasma as an effective non-thermal technology for eliminating bacteria, spores, and biological contaminants from food and non-food surfaces1, few report the application of this technique to liquid food within a package2,3. In this study, we use plasma spectroscopy to characterize the high voltage atmospheric cold plasma (HVACP)2 generated by a dielectric barrier discharge (DBD) in orange juice (OJ) within a package containing either air or modified atmosphere (MA65). We also evaluate the effectiveness of HVACP for decontaminating Salmonella enterica serovar Typhi (S. Typhi) in OJ. We exposed 50 ml of OJ both directly and indirectly to 90 kV for up to 120 s and used optical emission spectroscopy (OES) and optical absorption spectroscopy (OAS) to characterize the reactive gas species (RGS). The sample was stored in a refrigerator at 4°C for 24 h following treatment. Conductivity, pH and hydrogen peroxide content was measured before and after the treatments. Treating 50 ml of OJ containing S. Typhi for 120 s with direct treatment resulted in a 2.9 log10 reduction in air and a 4.2 log10 reduction in MA65 24 h after treatment. Using indirect treatment resulted in a 3.8 log10 reduction in MA65. OES and OAS indicated that the levels of OH, N2, N2+, and O- generated in OJ varied with package gas and whether the application was direct or indirect. These differences in RGS between fill gases and contact methods may impact the decontamination efficiency of S. Typhi in OJ. The implications of RGS on microorganism decontamination and potential plasmas systems will be discussed.

Optical absorption spectroscopy of high voltage, cold atmospheric pressure plasmas

Summary form only given. High voltage cold atmospheric plasmas (HVCAPs) offer a novel method for enhancing food safety and shelf-life1. HVCAPs eliminate contaminants by combining charged radical chemical species that readily oxidize microorganisms with surface ion impingement that essentially sputters cell walls2. The athermal nature of HVACPs allows treatment of numerous materials, ranging from metals to plastics. Research to date has quantified the concentrations of N2O5, NO2, N2O4 and O3; however, HVACPs generate numerous additional species that could contribute to the observed mechanisms and should be considered, such as HONO, HO2NO2, and HNO3. Previous work did not consider absorption cross-sections of these species despite their presence at the wavelengths studied3. In this study, we use optical absorption spectroscopy (OAS) to quantify the species created by exposing a food packaging container containing various fill gases to voltages from 70 kV to 88 kV. We further assess these concentrations as a function of applied voltage and fill gas humidity, which directly influences the generation of plasma species concentration. Voltage and current measurements provide information on the power applied to system and are correlated to plasma species generated. The potential implications of these gas species on cell membrane interactions and microorganism eradication will be discussed.

Sensitivity of modeled microscale gas breakdown voltage due to parametric variation

Device miniaturization increases the importance of understanding and predicting gas breakdown and electrical discharge thresholds. At gap sizes on the order of ten microns at atmospheric pressure, field emission drives breakdown rather than Townsend avalanche. While numerical and analytical models can demonstrate this transition, a quantitative understanding of the relative importance of each parameter remains unclear. Starting from a universal model for gas breakdown across the field emission and Townsend avalanche regimes [A. M. Loveless and A. L. Garner, Phys. Plasmas 24, 113522 (2017)], this paper applies the concept of error propagation from ionizing radiation measurements to determine the relative impact of each factor on the predicted breakdown voltage. For limits of both large and small products of the dimensionless ionization coefficient, α ¯ , and gap distance, d ¯, the electrode work function has the largest relative effect on the predicted breakdown voltages with a deviation of 50% in the work function resulting in an uncertainty in the calculated breakdown voltage of ∼84% for both α ¯ d ¯ ≫ 1 and α ¯ d ¯ ≪ 1. This quantifies the significance of nonuniformities in material surfaces and changes in the surface structure during multiple electric field applications and help predict the breakdown voltage for small gaps, motivating better electrode characterization both initially and during repeated operation.Device miniaturization increases the importance of understanding and predicting gas breakdown and electrical discharge thresholds. At gap sizes on the order of ten microns at atmospheric pressure, field emission drives breakdown rather than Townsend avalanche. While numerical and analytical models can demonstrate this transition, a quantitative understanding of the relative importance of each parameter remains unclear. Starting from a universal model for gas breakdown across the field emission and Townsend avalanche regimes [A. M. Loveless and A. L. Garner, Phys. Plasmas 24, 113522 (2017)], this paper applies the concept of error propagation from ionizing radiation measurements to determine the relative impact of each factor on the predicted breakdown voltage. For limits of both large and small products of the dimensionless ionization coefficient, α ¯ , and gap distance, d ¯, the electrode work function has the largest relative effect on the predicted breakdown voltages with a deviation of 50% i...