Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session BO7: Diagnostics & Partially Ionized Plasmas |
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Chair: Erik Tejero, Naval Research Laboratory Room: 212 AB |
Monday, October 31, 2016 9:30AM - 9:42AM |
BO7.00001: A two-photon laser induced fluorescence diagnostic with improved sensitivity, localization, and measurement rate. Drew Elliott, Earl Scime, Zachary Short A two-photon absorption laser induced fluorescence diagnostic has been developed for measuring neutrals in fusion plasmas. Implementation of this diagnostic on the HIT-SI3 spheromak has demonstrated the sensitivity of the diagnostic and shown that measurements taken over several plasma pulses are possible. These measurements yielded an unexpected loss of signal when complex collection optics were utilized. Simulations show that this loss of signal can be explained by chromatic aberrations caused by the disparate Kr and D emission. This loss of signal has been addressed with the development of a new calibration scheme involving xenon gas. The Xe calibration scheme emission occurs at 656.00 nm while the deuterium emission is 656.09 nm. This nearly identical emission allows for advanced optical techniques such as confocal collection/injection and spatial filtering to be employed without loss of signal. Spatial filtering has been demonstrated to decrease noise while improving measurement localization, while confocal collection/injection allows for probing and measuring to occur through one viewport. The Xe scheme also allows for a Doppler-free hydrogen measurement. Doppler-free measurements eliminate the need to scan the laser spectrally thus greatly increasing the rate of measurement. [Preview Abstract] |
Monday, October 31, 2016 9:42AM - 9:54AM |
BO7.00002: Spectroscopic Studies of Laser Produced Plasma Metasurfaces Roberto Colon Quinones, Thomas Underwood, Mark Cappelli In this presentation, we describe the spatial and temporal plasma characteristics of the dense plasma kernels that are used to construct a laser produced plasma metasurface (PM) that is intended to serve as a tunable THz reflector. The PM is an n x n array of plasmas generated by focusing the light from a 2 J/p Q-switched Nd:YAG laser through a multi-lens array (MLA) and into a gas of varying pressure. A gated CCD camera coupled to a high-resolution spectrometer is used to obtain chord-averaged H$\alpha$ broadening data for the cross section of a single plasma element at the lens focal point. The data is then Abel inverted to derive the radial plasma density distribution. Measurements are repeated for a range of pressures, laser energies, and lens f-number, with a time resolution of 100 ns and a gate width of 20 ns. Results are presented for the variation of plasma density and size over these different conditions. [Preview Abstract] |
Monday, October 31, 2016 9:54AM - 10:06AM |
BO7.00003: Gamma Ray Imaging of Inertial Confinement Fusion Experiments Carl Wilde, Petr Volegov, Verena Geppert-Kleinrath, Christopher Danly, Frank Merrill, Raspberry Simpson, David Fittinghoff, Gary Grim Experiments consisting of an ablatively driven plastic (CH) shell surrounding a deuterium tritium (DT) fuel region are routinely performed at the National Ignition Facility (NIF). Neutrons produced in the burning fuel in-elastically scatter with carbon atoms in the plastic shell producing 4.4 MeV gamma rays. Providing a spatially resolved distribution of the origin of these gammas can inform models of ablator physics and also provide a bounding volume for the cold fuel (un-burned DT fuel) region. Using the NIF neutron imaging system hardware, initial studies have been performed of the feasibility of imaging these gamma rays. A model of the system has been developed to inform under which experimental conditions this measurement can be made. Presented here is an analysis of the prospects for this diagnostic probe and a proposed set of modifications to the NIF neutron imaging line-of-site to efficiently enable this measurement. [Preview Abstract] |
Monday, October 31, 2016 10:06AM - 10:18AM |
BO7.00004: Spatial and Temporal Investigations of Laser Ablation Plasma Plume Density and Composition Jeremy Iratcabal, Bernhard Bach, Cuyler Beatty, Eric Dutra, Timothy Darling, Piotr Wiewior, Aaron Covington Laser ablation of solid targets with laser intensities of the order of 10$^{8}$-10$^{11}$ W/cm$^{2}$ provides a rich platform for investigating the density and composition of coexisting molecular, atomic, and ion species in the resulting plasma plume. Experiments measuring the spatial- and temporal-evolution of laser ablation plumes have been performed to simultaneously characterize the multiple parameters related to the energy and momentum partitioning of the incident laser energy as the ablation process occurs. The temperature, density, and relative populations of different molecular, atomic, and ion species can be determined by the simultaneous measurement of optical and charged particle spectroscopy, fast imaging cameras, and optical interferometric diagnostics. Additionally, background gas pressure, density, and species were carefully varied. A comparison of density measurements obtained with multiple interferometric, spectroscopic, and fast imaging diagnostics for a carbon ablation plume expanding into vacuum and into background gases with different Reynolds numbers will be presented. Atomic, molecular, and ion species population evolution will be presented as measured with optical and charged particle spectroscopy. [Preview Abstract] |
Monday, October 31, 2016 10:18AM - 10:30AM |
BO7.00005: ABSTRACT WITHDRAWN |
Monday, October 31, 2016 10:30AM - 10:42AM |
BO7.00006: An Electrothermal Plasma Source Developed for Simulation of Transient Heat Loads in Future Large Fusion Devices. Trey Gebhart, Larry Baylor, Leigh Winfrey The realization of fusion energy requires materials that can withstand high heat and particle fluxes at the plasma material interface. In this work, an electrothermal (ET) plasma source has been designed as a possible transient heat flux source for a linear plasma material interaction device. An ET plasma source operates in the ablative arc regime, which is driven by a DC capacitive discharge. The current travels through the 4mm bore of a boron nitride liner and subsequently ablates and ionizes the liner material. This results in a high density plasma with a large unidirectional bulk flow out of the source exit. The pulse length for the ET source has been optimized using a pulse forming network to have a duration of 1ms at full-width half maximum. The peak currents and maximum source energies seen in this system are 2kA and 5kJ. The goal of this work is to show that the ET source produces electron densities and heat fluxes that are comparable to transient events in future large magnetic confinement fusion devices. Heat flux, plasma temperature, and plasma density were determined for each test shot using infrared imaging and optical spectroscopy techniques. This work will compare the ET source output (heat flux, temperature, and density) with and without an applied magnetic field. [Preview Abstract] |
Monday, October 31, 2016 10:42AM - 10:54AM |
BO7.00007: Influence of emitter temperature on the energy deposition in a low-pressure plasma. Dmitry Levko, Laxminarayan Raja The influence of emitter temperature on the energy deposition into low-pressure plasma is studied by the self-consistent one-dimensional Particle-in-Cell Monte Carlo Collisions model. Depending on the emitter temperature, different modes of discharge operation are obtained. The mode type depends on the plasma frequency and does not depend on the ratio between the densities of beam and plasma electrons. Namely, plasma is stable when the plasma frequency is small. For this plasma, the energy transfer from emitted electrons to plasma electrons is inefficient. The increase in the plasma frequency results first in the excitation of two-stream electron instability. However, since the thermal velocity of plasma electrons is smaller than the electrostatic wave velocity the resonant wave-particle interaction is inefficient for the energy deposition into the plasma. Further increase in the plasma frequency leads to the distortion of beam of emitted electrons. Then, the electrostatic wave generated due to two-stream instability decays into multiple slower waves. Phase velocities of these waves are comparable with the thermal velocity of plasma electrons which makes possible the resonant wave-particle interaction. This results in the efficient energy deposition from emitted electrons into the plasma. [Preview Abstract] |
Monday, October 31, 2016 10:54AM - 11:06AM |
BO7.00008: Generation of Currents in Weakly Ionized Plasmas through a Collisional Dynamo Yakov Dimant, Meers Oppenheim, Alex Fletcher Intense electric currents called electrojets occur in weakly ionized magnetized plasmas. An example occurs in the Earth's ionosphere near the magnetic equator where neutral winds drive the plasma across the geomagnetic field. Similar processes take place in the Solar chromosphere and MHD generators. We argue that not all convective neutral flows generate electrojets and it introduces the corresponding universal criterion for the current formation, $\nabla\times (\vec{U}\times\vec{B})\neq\partial\vec{B}/\partial t$, where $\vec{U}$ is the neutral flow velocity, $\vec{B}$ is the magnetic field, and $t$ is time. This criterion does not depend on the conductivity tensor, $\hat{\sigma}$. For many systems, the displacement current, $\partial\vec{B}/\partial t$, is negligible, making the criterion even simpler. This theory also shows that the neutral-dynamo driver that generates electrojets plays the same role as the DC electric current plays for the generation of the magnetic field in the Biot-Savart law. [Preview Abstract] |
Monday, October 31, 2016 11:06AM - 11:18AM |
BO7.00009: Long pulse Soft X-ray Emission from Laser Generated Irradiated Gold Foils Joshua Davis, Yechiel Frank, Erez Raicher, Moshe Fraenkel, Paul Keiter, Sallee Klein, R. P. Drake, Dov Shvarts Long pulse soft x-ray sources (SXS) allow for flexibility in high-energy-density experimental designs by providing a means of driving matter to the high temperatures needed, for example to study radiation waves in different materials. SXSs can be made by using lasers to heat a high-Z thin foil, which then acts as a quasi-blackbody emitter. Previous studies of the x-ray emission characteristics of gold foils have focused on laser pulses of \textasciitilde 1ns or less. We performed experiments using a 6.0ns laser pulse with energy of 2kJ on the Omega-60 system to generate and characterize multi-ns laser heated Au foils of thicknesses between 0.5-2.0$\mu$ m. We measured the 2D spatial profile of the emission with a soft x-ray camera and the time history of the emission with the Dante photodiode array . Effective temperatures for the emission were then calculated using the Dante measurements. Discussion of experimental results~and~a comparison with 1-D Rad-Hydro NLTE simulations [see~Y. Frank et. al, Phys. Rev. E 92, 053111 (2015)] will be presented. [Preview Abstract] |
Monday, October 31, 2016 11:18AM - 11:30AM |
BO7.00010: Contraction of Surface Barrier Discharge at Positive and Negative Polarities in Atmospheric Air. Sergey Leonov, Alec Houpt This experimental study of surface dielectric barrier discharge examines the morphology and charge transfer dynamics depending on the supplied voltage pattern: single polarity vs alternating polarity. Diagnostics included electrical measurements, camera imaging, optical emission spectroscopy, and a set of original charge sensors [S. Leonov et al J. Phys. D: Appl. Phys., vol. 47, p. 465201, 2014]. Two basic modes were analyzed: diffusive and filamentary. The key factor of the discharge dynamics is the development of ionization instability causing the contraction of the discharge current and formation of the filamentary, highly conductive plasma during both positive and negative polarities. A main criterion of the discharge contraction is the generation of a zone with a high level of longitudinal electric field, not less than 15 kV/cm, realized during the alternating of the sign of surface charge. It is shown that the alternating polarity of the supplied voltage accompanied with the process of discharge contraction gives a significant benefit in the surface area covered by the discharge and in the power deposition, increasing it 2-4 times. [Preview Abstract] |
Monday, October 31, 2016 11:30AM - 11:42AM |
BO7.00011: Investigation of Partially Ionized Plasma Chemistry of Hydroxylammonium Nitrate Forrest Kidd, Kristina Lemmer Future space missions require an efficient and versatile method of propulsion. One possibility is the use of dual-mode-propulsion. Dual-mode-propulsion combines the high specific impulse of electric propulsion with the high thrust of chemical propulsion. However, to effectively implement dual-mode-propulsion a propellant that can be used in both electric and chemical propulsion is required. Ionic liquids are one class of propellants currently being investigated for their use in dual-mode-propulsion systems. Hydroxylammonium Nitrate (HAN) is the ionic liquid of interest in this study. HAN’s chemistry and decomposition have been thoroughly investigated at pressures above atmospheric pressure; however, the chemistry of partially ionized HAN is not well understood. Ab-initio modelling is used to determine the chemistry of key ion-neutral reactions and to develop a reaction network for HAN plasma. Calculations using Density Functional Theory and Coupled Cluster Singles Doubles with approximate Triples corrections are performed. This will allow for accurate modelling of HAN’s interaction with thruster components and prevent premature failure. [Preview Abstract] |
Monday, October 31, 2016 11:42AM - 11:54AM |
BO7.00012: Effect of transverse magnetic field on the steady state properties of Plasma Diodes Sourav Pramanik, Victor Kuznetsov, Nikhil Chakrabarti A study of the steady-states of the Plasma Diodes (e.g., Bursian diode, Pierce diode etc.) driven by a cold electron beam is presented in presence of an external transverse magnetic field is presented. Both the regimes of no electron-reflection and electron-reflection are taken into account. Steady state solutions are evaluated using emitter electric field as a characteristic parameters, for fixed values of the diode length, applied voltage, and magnetic field strength. For our purpose, both the Eulerian and Lagrangian descriptions are employed. It is shown that transverse magnetic field has profound influences on the space charge limit, maximum diode current, aperiodic instability and other characteristic parameters. An external magnetic field can be used to design fast electronic switches based on its effects on the Plasma diode. [Preview Abstract] |
Monday, October 31, 2016 11:54AM - 12:06PM |
BO7.00013: Electrical description of N$_2$ capacitively coupled plasmas with the global model Ming-Lu Cao, Yi-Jia Lu, Jia Cheng, Lin-Hong Ji N$_2$ discharges in a commercial capacitively coupled plasma reactor are modelled by a combination of an equivalent circuit and the global model, for a range of gas pressure at 1\~{}4 Torr. The ohmic and inductive plasma bulk and the capacitive sheath are represented as LCR elements, with electrical characteristics determined by plasma parameters. The electron density and electron temperature are obtained from the global model in which a Maxwellian electron distribution is assumed. Voltages and currents are recorded by a VI probe installed after the match network. Using the measured voltage as an input, the current flowing through the discharge volume is calculated from the electrical model and shows excellent agreement with the measurements. The experimentally verified electrical model provides a simple and accurate description for the relationship between the external electrical parameters and the plasma properties, which can serve as a guideline for process window planning in industrial applications. [Preview Abstract] |
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