Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session NO8: Basic Plasma Studies of Low-temperature Plasmas |
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Chair: Truell Hyde, Baylor University Room: OCC C120-122 |
Wednesday, November 7, 2018 9:30AM - 9:42AM |
NO8.00001: A comparative study of continuum and kinetic simulations of GHz - THz microwave microplasmas Abhishek Kumar Verma, Arghavan Alamatsaz, Venkattraman Ayyaswamy Microplasmas ignited using high-frequency excitation are becoming an active area of research due to the myriad applications and the ability to be sustained at low powers ($\approx$ mW) and longer lifetimes. A majority of the computational studies reported thus far for microwave microplasmas have utilized a fluid approach using a drift-diffusion approximation with the concept of an effective electric field often recommended for better accuracy. Our group has previously reported kinetic (using PIC-MCC simulations) and fluid simulations (using full-momentum equations) for argon microwave microplasmas ignited at excitation frequencies up to 4 GHz and a specified current density with good agreement between the two methods at the chosen operating conditions. The main goal of this talk is to report our recent work on extending this comparative study to frequencies up to 320 GHz. Specifically, one-dimensional kinetic and continuum simulations at 1 mW, 10 mW, 50 mW and 100 mW are presented to compare and contrast the frequency response and electron dynamics predicted by the two simulations techniques. Phase plots of electron heating will be presented demonstrating the importance of stochastic heating even in atmospheric pressure plasmas at excitation frequencies as high as 20 GHz. |
Wednesday, November 7, 2018 9:42AM - 9:54AM |
NO8.00002: Electron Emission and Gas Breakdown from Schrodinger’s Equation to Paschen’s Law Amanda Loveless, Allen L Garner Historically, gas breakdown has been predicted by Paschen’s law; however, deviations from Paschen’s law characterized by the absence of a minimum breakdown voltage arise due to field emission. Recently derived universal, closed form equations predict breakdown voltage from field emission to the classical Paschen’s law for gap distances above the electron mean free path and below the transition to streamer discharge (A. M. Loveless and A. L. Garner, Phys. Plasmas, 24, 113522 (2017).). Continuing reduction in devices sizes motivates assessing the impact of gap size and device pressure for submicroscale gaps. This study extends the previous theory by defining common scaling parameters to derive universal relationships for the transition from field emission to the Child-Langmuir law and from the Child-Langmuir law to Schrodinger’s equation as we reduce gap distance. We report the resulting gap sizes corresponding to each transition up to the classical, universal Paschen’s law as a function of pressure. The implications of these transitions and implications of nanoscale features on this behavior will be discussed. |
Wednesday, November 7, 2018 9:54AM - 10:06AM |
NO8.00003: Propagation and Evolution of the Electron Cyclotron Drift Instability in a Hall Thruster: Fitting Experimental Data and Analytical Models Zachariah Alexander Brown, Benjamin Jorns Plasma oscillations in the acceleration region and near-field plume of a magnetically shielded Hall thruster are identified as the electron cyclotron drift instability. Using a high speed electrostatic probe diagnostic, spatially resolved dispersion relations are measured. At the acceleration region, distinct cyclotron resonances are observed between 3-20 MHz. In the near-field plume the resonances are still evident, but spectral power becomes dominant around 100-500 kHz, associated with the sharp appearance of a linear ion acoustic like wave. The experimental dispersion is fitted to linear analytical models and agreement between the measured and theoretical dispersion is demonstrated. The growth of the low frequency oscillations is characterized and shown to agree with recent non-linear simulations of the electron cyclotron drift instability. |
Wednesday, November 7, 2018 10:06AM - 10:18AM |
NO8.00004: Model for the anomalous electron mobility in a Hall effect discharge Benjamin Jorns The mechanism for driving the anomalously high cross-field electron current is one of the most dominant but least understood processes in Hall effect discharges. Recent numerical and experimental studies have suggested that the onset of micro-instabilities---specifically the electron cyclotron drift instability---may be the driving mechanism for this process. There is an outstanding question, however, as to how to relate this mechanism directly to the observed electron transport in these devices. It is not clear if and how this inherently kinetic process can be represented in a fluid-framework, i.e. if there is a form of closure. This work presents a model for this closure inspired by data-driven regression techniques as well recent experimental measurements performed on a 9-kW Hall effect accelerator operated at the University of Michigan’s Large Vacuum Test Facility. |
Wednesday, November 7, 2018 10:18AM - 10:30AM |
NO8.00005: Characterization of standing plasma instabilities in a hollow cathode plume Marcel P Georgin, Benjamin A Jorns, Alec D Gallimore Hollow cathodes are thermionic electron current sources commonly used in electric propulsion devices such as Hall thrusters. Although these cathodes have been used in space for decades, limited understanding of the fundamental plasma physics in the plume of the device has prevented the development of predictive numerical models. The low temperature plasma (3 eV) in the plume of the cathode is subject to a variety of plasma instabilities that dictate the operation of the device. For example, there exists a low frequency oscillation that is known to empirically onset at high current or low flow rate. This wave has been associated with the generation of ions with energies that exceed the discharge voltage, leading to erosion of the cathode and limiting its lifetime. Recent measurements of this plasma oscillation have indicated that a localized, standing instability generates structures that can be described as wave packets and solitons that propagate away from this source. A clear description of the source is currently unavailable. Presented is an experimental characterization of this standing wave and an analytical description of the onset criterion for this instability. |
Wednesday, November 7, 2018 10:30AM - 10:42AM |
NO8.00006: Incorporating collisions and resistance into the transition from field emission to the space charge regime regime Samuel David Dynako, Adam Darr, Allen Garner In electron emission, emission current increases with surface electric field and causes a buildup in space charge that limits the surface electric field, eventually resulting in a transition from the Fowler-Nordheim (FN) equation to the Child-Langmuir (CL) law at vacuum. Coupling single-particle motion to Poisson’s equation and the conservation of energy, one finds universal curves showing this transition from FN to CL (Y. Y. Lau, Y. Liu, and R. K. Parker, Phys. Plasmas 1, 2082 (1994)). One may also model the transition from CL to Mott-Gurney (MG) at non-vacuum pressures by incorporating electron mobility into a fluid model (M. S. Benilov Plasma Sources Sci. Technol. 18, 014005 (2009).). This presentation combines these approaches to predict the transitions from FN to CL and MG, deriving the condition for a new triple point where the asymptotic equations for FN to CL and CL to MG intersect. This presents the impact of leakage and gap distance on vacuum electronics and the transition to microscale breakdown dynamics, driven by FN. We assess dissipation by introducing an external resistor on these asymptotes and the triple point. The implications on the sensitivity of these transitions on device physics will be discussed. |
Wednesday, November 7, 2018 10:42AM - 10:54AM |
NO8.00007: Potential Distribution Between a Hot Cathode and an Anode when the Current is Limited Michael D. Campanell Space-charge limited (SCL) cathode sheaths have been predicted to limit the thermionic current flow into the plasma in many devices that rely on hot cathodes. But recently, we showed that SCL sheaths are unstable and unlikely to exist because charge-exchange ion trapping in the virtual cathode forces a transition to a current mode with an inverse cathode sheath. The unique physics of the inverse mode has now been modeled for the first time [1]. Moving outward from the cathode, the potential first decreases, then flattens across the quasineutral plasma region, and then rises across the anode sheath to reach the bias value. All plasma ions are confined between the electrode sheaths. Inverse modes have current flow, electrode erosion, and power dissipation properties that strongly differ from conventionally theorized modes with classical or SCL cathode sheaths. Experimental evidence of inverse modes in devices will be discussed. Recent work [2] predicts that inverse sheath formation leads to ion-ion plasma formation in negative ion sources. [1]Campanell, PRE 97, 043207 (2018). [2]Zhang et al., PSST 27, 06LT01 (2018). |
Wednesday, November 7, 2018 10:54AM - 11:06AM |
NO8.00008: Sheath dynamics during the vapor shielding at a plasma-facing wall Kenzo Ibano, Yusuke Kikuchi, Akito Tanaka, Satoshi Togo, Heun Tae Lee, Yoshio Ueda, Tomonori Takizuka Erosion of wall materials during transient events is a large concern for ITER and future devices with high fusion power. Enormous heat loads at the transient event cause melting, vaporization, and ablation of wall materials. The generated vapor from the surface strongly interacts with the incoming plasma and dissipates its energy. Then, vapor shielding can be an inherent protection for this wall erosion. We have developed a particle-in-cell simulation code, called PIXY, and have applied it to simulations of the vapor shielding in fusion devices. During the vapor shielding, the vapor emission rate significantly spreads over a very wide range. Thus, in order to treat the sufficient numbers of numerical super-particles, a weighted particle model is adopted in the PIXY code. Behaviors of incoming plasma, generated vapor, and ionized impurities are solved in a consistent manner with the sheath and pre-sheath formation. The sheath potential affects the heat transmission to the wall and resultantly the vapor emission. In this presentation, we focus on the dynamics of the sheath during the vapor shielding, and highlight the influence of sheath dynamics on the shielding efficiency. Overall simulation model and estimated total erosion during the transient heat loads will also be presented. |
Wednesday, November 7, 2018 11:06AM - 11:18AM |
NO8.00009: understanding the plasma potential as a confinement method Lutfi Oksuz, Noah Hershkowitz, Peter Li
In a multi-dipole DC hot filament argon discharge, a transition from positive to negative plasma potentials were examined. A new confinement method in negative plasma potential were obtained Emissive probes, Langmuir probes, ion acoustic waves and laser induced fluorescence were employed in order to measure the plasma parameters. Potential dips and wells and also ion sheaths, electron sheaths were obtained and investigated theoretically and experimentally. It is found that primary electron energy plays important role in plasma potential. Experimental results and theory will be discussed. |
Wednesday, November 7, 2018 11:18AM - 11:30AM |
NO8.00010: Standoff detection of isotopes using laser-induced fluorescence of laser-produced plasmas Sivanandan S Harilal, Mark C Phillips The standoff and rapid detection are the most desired capability for an analytical tool for identifying dangerous substances in the field such as high-energy explosives and radioactive nuclear materials. In this work, we demonstrate the use of laser-induced fluorescence (LIF) of laser-produced plasma for standoff applications of bulk and trace elements including isotopes. A laser-produced plasma was generated at a distance ~1-10 m using focused ns laser pulses. A continuous wave (cw) LIF laser was propagated nearly collinear to the ablation laser and preferentially excited selected transitions in the plasma. For obtaining excitation spectrum, the LIF laser was tuned across the selected optical transitions and direct fluorescent emission was collected to avoid resonant scattering. Our results show that significant improvement in the magnitude and persistence of emission from selected atomic transitions using LIF. 2D fluorescence spectroscopy (2D-FS) is used to record simultaneous absorption and emission spectra of selected transitions in the plasma plume.
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Wednesday, November 7, 2018 11:30AM - 11:42AM |
NO8.00011: Design Analysis of 10kW Magnetohydrodynamic Generator using Off-gas Plasma GeunHyeong Lee, Hee Reyoung Kim The design variables of plasma based magnetohydrodynamic(MHD) generator is analyzed to produce electricity directly using the off-gas which is made by the thermal power generator. In the thermal power generator, the combustion gas is used for the making electricity through the gas-turbine system. The off-gas comes out after the 1st electric production, and this gas is used for generating 2nd electricity through the MHD generator. The plasma state off-gas is used to make electric conductivity at fluid, and its conductivity is increased to add the alkali metal seed. The magnetic field from the external magnet is applied to off-gas, and electricity is generated due to its conductivity and velocity. The magnetic flux density from magnet, electric conductivity from plasma and temperature are analyzed in terms of enthalpy extraction rate at the electric production condition of 10 kW. |
Wednesday, November 7, 2018 11:42AM - 11:54AM |
NO8.00012: Progress made in kinetic trajectory simulation model for the understanding of plasma sheath Roshan Chalise, Raju Khanal Due to sharp gradients of physical parameters in the plasma sheath region, that forms whenever plasma is confined using material walls, kinetic treatment becomes necessary. The kinetic trajectory simulation model that was initially developed for studying bounded plasmas and applied to simple one dimensional electrostatic plasma sheath has been improved by including various situations like oblique magnetic field, collision, interactive walls, multi-component plasmas, etc. Progress made in developing the kinetic trajectory simulation model and its application for the understanding of magnetized plasma sheath will be presented. In particular, the response of carbon and tungsten surfaces to hydrogen plasma and the effect of temperature in two-ion species magnetized plasma sheath will be discussed. Being a kinetic model, it can be used to gain better insight in various bounded plasma problems, where fluid theory is not reliable or even shows singularity. |
Wednesday, November 7, 2018 11:54AM - 12:06PM |
NO8.00013: Abstract Withdrawn
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Wednesday, November 7, 2018 12:06PM - 12:18PM |
NO8.00014: Development of the high repetition frequency non-equilibrium plasma source based on solid-state device hongtao li An all solid-state pulse power source based on the novel photoconductive semiconductor switches (PCSS) and the solid-state pulse forming line (PFL)was designed and tested. The Si-PCSS with mirror cavity combined quantum-trap structure and the solid-state PFLs which were made of ferroelectric ceramic-glass composition and high frequency ceramic with curve electrode structure were proposed. Then, A compact 20 stages Marx generator was developed. Combined with the well designed charging system, circulatory cooling system and laser triggering system, the durative time of the prepared generator which works with output voltage 113kV, pulse width 50ns, and repetition frequency 1kHz, is more than 6 minutes. As it work with pulse width several nanoseconds,it can work continuously for more than 1 hour without any performance degradation。 And then, a plasma gun was excited by the high frequency all solid-state pulsed power source,the characteristics of the non-non-equilibrium plasma was measured. The distribution of the plasma in the jet was improved significantly. |
Wednesday, November 7, 2018 12:18PM - 12:30PM |
NO8.00015: Spatiotemporally Resolved Ion Velocity Distribution Measurements in the 12.5 kW HERMeS Hall Thruster Vernon H Chaplin, Robert B Lobbia, Alejandro Lopez Ortega, Ioannis G Mikellides, Richard R Hofer Non-invasive measurements of the ion velocity distribution function (IVDF) obtained using laser-induced fluorescence (LIF) are playing a critical role in the life qualification of NASA’s 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS), which will be accomplished through a combination of limited duration wear testing and computational modeling validated by experiments. Previous LIF measurements on HERMeS have revealed bimodal time-averaged IVDFs in the acceleration region of the thruster, suggestive of oscillations in the acceleration region’s position, as well as time-averaged velocity vectors that are difficult to reproduce in simulations. In order to understand these phenomena in more detail, we are making time-resolved LIF measurements using the transfer function averaging technique, which employs phase-sensitive detection and averaging in Fourier space to enable measurements resolving both periodic and aperiodic oscillations at typically Hall thruster breathing mode frequencies (10-60 kHz). The first time-resolved IVDFs measured in HERMeS will be presented, along with time-averaged 2D velocity vector maps spanning a finer spatial mesh than in previous studies. Implications for performance and life modeling of HERMeS using the Hall2De code will be discussed. |
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