Bulletin of the American Physical Society
73rd Annual Gaseous Electronics Virtual Conference
Volume 65, Number 10
Monday–Friday, October 5–9, 2020; Time Zone: Central Daylight Time, USA.
Session KT3: Plasmas in LiquidsLive

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Chair: David Staack, Texas A&M 
Tuesday, October 6, 2020 3:00PM  3:30PM Live 
KT3.00001: Electron transport and streamer propagation in gases and nonpolar liquids and their applications in modelling of particle detectors Invited Speaker: Sasa Dujko In this work, we discuss the physics of resistive plate chambers (RPCs), including the electron transport and streamer propagation in the gas filled gaps, and signal induction in the system of electrodes. RPCs are used for timing and triggering purposes in many highenergy physics experiments at CERN, including ATLAS, ALICE and CMS. Using the pulsedTownsend measurements and calculations of the electron drift velocity, longitudinal diffusion coefficient and effective ionization coefficient in pure C$_{\mathrm{2}}$H$_{\mathrm{2}}$F$_{\mathrm{4}}$ and its mixtures with Ar, we propose a complete and consistent set of cross sections for electron scattering in C$_{\mathrm{2}}$H$_{\mathrm{2}}$F$_{\mathrm{4}}$. Electron transport coefficients, required as an input in fluidequation based models, are calculated from numerical multi term solutions of Boltzmann's equation and Monte Carlo simulations in a variety of RPC gas mixtures, as a function of the reduced electric field. We have developed a 1.5D classical fluid model with photoionization to investigate the streamer development in various RPCs at CERN and elsewhere. Among many important points, it is found that the electron absorption on the anode has a large influence on the space charge effects and positive streamer formation. The classical fluid model is extended by considering the model in which the source term in the equation of continuity is expanded in terms of the powers of the number density gradient operator. The expansion coefficients are calculated over a wide range of the reduced electric fields using a Monte Carlo simulation technique. Both fluid models are developed to demonstrate how the nature of transport data affects the results of an RPC modelling. Transport of electrons and propagation of streamers, are also considered in liquid noble gases. Solutions of Boltzmann's equation and Monte Carlo method for electrons in dilute neutral gases, are extended and generalized to consider the transport processes of electrons in liquid Ar and liquid Xe by accounting for the coherent and other liquid scattering effects. We focus on the way in which electron transport coef?cients and streamer properties are in?uenced by a representation of the inelastic energy losses, highlighting the need for a correct representation of elementary scattering processes in modeling of liquid discharges. The present work has been done in collaboration with D. Bosnjakovic, I. Simonovic, Z.Lj. Petrovic and R.D. White. [Preview Abstract] 
Tuesday, October 6, 2020 3:30PM  3:45PM Live 
KT3.00002: Fast pulsed electrical breakdown of water: electron multiplication in liquid ruptures Zdenek Bonaventura, Petr Bilek, Jan Tungli, Milan Simek Liquid water under the action of sharp pulse of the electric field may be disrupted so that cavities of nanometer scale would eventually appear and expand. Electric field forces these cavities to rapidly elongate to the form of long needlelike ruptures. We propose a scenario for electron multiplication inside of these ruptures: Electrons are accelerated by the electric field inside the ruptures and can create a multitude of secondary electrons by sequence of bouncelike collision events with the surface of water. This results in electron avalanche that is confined inside of the cavity. For electron transport and electron waterinteractions we use Monte Carlo model based on Geant4DNA simulation toolkit. We show that there exists a minimum value of the product of electric field strength and the cavity radius, when this electron multiplication scenario can be put in action. We will also discuss the avalanche dynamics and present its characteristic time and spatial scales. Conclusions of our work shed light on one of the crucial steps behind the nanosecond electrical breakdown of liquid water. [Preview Abstract] 
Tuesday, October 6, 2020 3:45PM  4:00PM Live 
KT3.00003: Electron generation and multiplication at the initial stage of nanosecond breakdown in water Xuewei Zhang, Mikhail Shneider Pulsed breakdown in water has important applications as potential plasma sources and is fundamental to pulsed power systems design. There have been many theoretical and experimental studies on pulsed breakdown in water under inhomogeneous nanosecond electric fields. Electrostrictive cavitation has been proposed as the mechanism of breakdown initiation at nanosecond timescale. There are still missing links between the cavitation inception and the formation of the first plasma channel. We first analyze the generation of the primary or seed electrons. Among the possible sources, the electron dissociation from hydroxide is the most probable process to release electrons from the cathodeside pole to the interior of a nanocavity, especially when the hydroxide concentration is increased at the pole. Next, consider a linear chain of nanocavities along the electric field line, we theoretically model the processes of electron gaining energy traversing the nanocavity, hitting the opposite cavity wall, and generating more electrons before entering the next nanocavity. The rate of electron multiplication is a function of the nanocavity size, density, as well as the background field. Although the work mainly focuses on breakdown with positive impulse applied to a needle tip, similar electron multiplication processes also exist in negativepolarity breakdown. [Preview Abstract] 
Tuesday, October 6, 2020 4:00PM  4:15PM Live 
KT3.00004: New Raman Spectroscopy Results of Singleelectrode Pulsed Plasma Branches in Water for Interrogation of HighPressure LiquidSolid Phase Transition Christopher Campbell, David Staack Pulsed plasmas in liquids are broadly useful phenomena, already employed for chemical conversion and sterilization among other applications. However, these plasmas exhibit complex multiphase behavior over short timescales ($<$20 ns) which is not welldescribed by conventional plasma theory. Using a lowjitter lasertriggered voltage pulse (30 kV, 5 mJ), resulting plasmas achieve high instantaneous power density ($\sim$1 TW/cm$^2$), which in liquids causes local isotropic and isochoric behavior. In water, phase transitions which exist at pressures above 1 GPa (Ice VI and VII) are achievable via such thermodynamic processes. Prior results provided tentative evidence of this type of local phase transition during pulsed plasmas in water, using timeresolved Raman spectroscopy of the OH stretching mode of H$_2$O. Here we present updated imaging and Raman spectroscopy results at higher time resolutions and larger sample sizes, further investigating the local presence of a transient highpressure solid phase. Such a phase may limit achievable energy densities, which has ramifications across several fields interested in producing highenergydensity plasma processes in liquids. [Preview Abstract] 
Tuesday, October 6, 2020 4:15PM  4:30PM Live 
KT3.00005: Spectrum of cavitation luminescence generated by a bioinspired mechanical device Xin Tang, Matthew Burnette, David Staack Collapsing cavitation is an effective energy focusing method to generate hightemperature and highpressure singularities at its minimum volume, which result in plasma formation accompanied with strong acoustic waves and light emission. A bioinspired mechanical device was designed and manufactured to generate cavitation repetitively as a plasma source in liquid media. An optical system consisting of an achromatic lens, transmission gratings, and an intensified CCD camera are utilized to probe the cavitation luminescence spectrum nonintrusively. The recorded spectrum for each collapsing cavitation event is corrected by the overall optical system efficiency contributed by all of the components in the optical diagnostic system. The corrected broad band spectrum is not close to a blackbody radiation, and the peak intensity wavelength varies stochastically due to RayleighTaylor instabilities during the cavitation collapsing process. [Preview Abstract] 
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