Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session PP9: Poster Session VI: General and NSTX Spherical Torus; Other ICF and HEDP Physics and Diagnostics; Pinches, Plasma Focus, CRASH; Low Temperature Plasmas |
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Room: Hall A |
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PP9.00001: GENERAL SPHERICAL TORUS AND NSTX SPHERICAL TORUS |
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PP9.00002: Plasma Startup via Local Helicity Injection in Pegasus R.J. Fonck, J.L. Barr, M.W. Bongard, M.G. Burke, E.T. Hinson, A.J. Redd, N.L. Schoenbeck, D.J. Schlossberg, K.E. Thome Magnetic helicity injection from localized current sources at the outboard plasma edge and poloidal field induction produced plasma currents $I_{p} \leq 0.17$ MA using $\sim4$ kA injected current $I_{inj}$, consistent with a model invoking helicity balance and Taylor relaxation. Varying the edge current density via source realignment raised the attainable $I_{p}$, as expected. A double-layer sheath describes the impedance of the current injectors, setting the helicity injection rate for a given $I_{inj}$. This suggests the helicity input and discharge evolution can be manipulated by the edge density. MHD activity during plasma growth correlates with rapid equilibrium shifts and current redistribution into the plasma interior. Impurity ion spectroscopy indicates strong heating during helicity drive. Plasmas can also be smoothly driven through the growth stage using passive, gas-fueled electrodes as the helicity sources, after initial tokamak formation with the active current sources. Such electrodes hold promise for optimization of both the Taylor limit that sets the maximum $I_{p}$ and the achievable helicity input rates needed to attain it. Developing a fully predictive model of this startup technique will allow application to next-step fusion experiments. [Preview Abstract] |
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PP9.00003: Peeling Instability in the Pegasus ST M.W. Bongard, J.L. Barr, R.J. Fonck, A.J. Redd, D.J. Schlossberg Ohmic plasmas in \textsc{Pegasus} are often initially unstable to peeling modes, an instability underlying deleterious edge localized mode (ELM) activity in fusion-grade plasmas. These edge-localized instabilities are observed under conditions of high parallel edge current density ($J_\parallel \sim 0.1$ MA/m$^{2})$ and low magnetic field ($B\sim 0.1$ T) present at near-unity aspect ratio, corresponding to high peeling instability drive ($\propto J_\parallel /B)$. They generate electromagnetic MHD activity with low toroidal mode numbers $n\le 3$ and ELM-like, field-aligned edge filaments with high poloidal coherence that detach from the plasma and propagate outward. The modest edge temperatures and short pulse lengths of \textsc{Pegasus} discharges permit time-resolved measurements of the edge current density profile $J_{edge} $ using an insertable Hall probe. Peeling MHD fluctuation amplitudes scale strongly with measured $J_\parallel /B$, consistent with theory. Ideal stability analysis of Hall-constrained equilibrium reconstructions with DCON finds instability to peeling modes. Filaments form from an initial $J_{edge} $ ``current-hole'' perturbation and carry currents $\sim $100-250 A. Their radial trajectories feature transient acceleration due to magnetostatic repulsion followed by constant-velocity motion, consistent with models of ELM dynamics. [Preview Abstract] |
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PP9.00004: Impedance Model for Outboard Midplane Helicity-Injection Sources E.T. Hinson, R.J. Fonck, B.T. Lewicki, A.J. Redd Plasma cathode current injectors are being investigated at the \textsc{Pegasus} Toroidal Experiment for application to point source helicity injection start-up. Knowledge of the physics governing the impedance of the current-injecting circuit is needed for a predictive model of the helicity injection rate. Neutral fueling, particularly near the injector site, reduces the voltage necessary for a fixed injected current. For nominally fixed plasma conditions, $J \propto V^{3/2}$ is observed, up to a saturation value $J_{sat} \sim 1$ kA/cm$^{2}$. This is well in excess of estimated ion saturation current density to the injector in the plasma edge region. When the internal plasma arc is terminated, the injectors operate as simple, passive electrodes that continue to drive toroidal plasma current. Fast camera images show intermittent cathode spots on the electrode surfaces, and the voltage required by the larger electrode area remains well described by $J \propto V^{3/2}$. These observations are consistent with a model invoking a strong double layer sheath in the vicinity of the plasma cathode or electrode surfaces. Current saturation is hypothesized to be due to either the Langmuir condition for double layer sheaths, or electron saturation within the injector. [Preview Abstract] |
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PP9.00005: MHD Evolution in Point-Source Helicity Injection Driven Plasmas on Pegasus J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, A.J. Redd Point-source helicity injection for non-solenoidal startup on \textsc{Pegasus} produces plasmas with $I_p \le 0.17$ MA consistent with Taylor relaxation. The helicity injection supplies an effective loop voltage $V_{eff} $ inversely proportional to the plasma toroidal flux $\Psi _T $. Accurate measurement of the $V_{eff} $ evolution requires equilibrium reconstructions. Helicity injection-driven plasmas originate on the outboard, low-field side and expand inward to fill the vessel. This evolution increases $\Psi _T $, reducing $V_{eff} $ from $\ge $ 10 V to $\le $ 2 V. Supplemental loop voltage from poloidal field induction is used to obtain higher plasma current. I$_{p}$ growth is accompanied by bursts of $n=1$ magnetic activity with frequencies between 10--150 kHz, abrupt inward motion of the plasma, and a drop in internal inductance. This magnetic activity persists during helicity injection. Afterward, MHD quiescence is obtained and persists in discharges subsequently sustained by ohmic induction. The spectral content of these magnetic fluctuations measured with a scanning Mirnov probe does not differ significantly with distance from the plasma edge. [Preview Abstract] |
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PP9.00006: Ohmic Confinement Studies in the Pegasus Toroidal Experiment K.E. Thome, J.L. Barr, M.W. Bongard, M.G. Burke, A.S. Dowd, R.J. Fonck, A.J. Redd, D.J. Schlossberg A key process in enhancing energy and effective particle confinement and plasma performance in tokamak discharges is particle recycling. \textsc{Pegasus} discharges indicate that a low-recycling regime is obtained through the use of titanium gettering and cryogenic pumping. The energy confinement and effective particle confinement times, $\tau _e $ and $\tau _p^\ast $, respectively, are determined using: magnetic diagnostics to perform equilibrium reconstructions, a 32-channel AXUV bolometer diode array to measure $P_{RAD} $, and a heterodyne Michelson microwave interferometer to measure $\bar {n}_e $. A fast wide-angle view D-$\alpha $ camera observes and measures recycling. Motivated by earlier results that indicate a decrease in $\tau _p^\ast $ with decreased wall pumping, systematic studies of confinement and wall conditioning are in progress. Measurements of the instantaneous density decay rate after the termination of the external gas supply during an established Ohmic discharge with low-MHD activity indicate $2<\tau _p^\ast <5$ ms. Scans of $\bar {n}_e $ with and without titanium gettering in stable Ohmic discharges are used to study $\tau _e $, $\tau _p^\ast $, and recycling on \textsc{Pegasus}. [Preview Abstract] |
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PP9.00007: Development of a Multichannel Spectrometer for the Thomson Scattering Diagnostic on Pegasus N.L. Schoenbeck, A.S. Dowd, R.J. Fonck, D.J. Schlossberg, G.R. Winz To explore electron transport in helicity-driven discharges and investigate edge stability, a multi-point Thomson scattering diagnostic is being developed. Red-shifted scattered light from the Nd:YAG laser, 532-632 nm, is imaged using a custom lens coupled to fiber optic cables capable of imaging 1.4 cm along the length of the laser beam. Initially 1 spectrometer, containing up to 8 radial spatial points will be available for detection, with an upgrade to 3 spectrometers planned in the near future. New high efficiency volume phase holographic gratings, with $>$ 75{\%} transmission, allow for a simplified spectrometer design. This provides high optical throughput and readily couples to new high quantum efficiency ($\sim $45{\%}) image intensified CCD cameras for multichannel design. These cameras can be gated to as low as 2 ns. The two gratings fabricated for this system (2971 lines/mm and 2072 lines/mm) cover the design temperature range of 10 eV to 1 keV. Completing the spectrometer are high quality lenses with focal lengths of 130 cm on the collimating lens and 85 cm on the exit lens. This design has a spectral range compatible with the blue shift from a conventional ruby laser and allows for a compact, simplified system. [Preview Abstract] |
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PP9.00008: Implementation of a Thomson Scattering Diagnostic on Pegasus D.J. Schlossberg, A.S. Dowd, R.J. Fonck, J.I. Moritz, N.L. Schoenbeck, G.R. Winz The multipoint Thomson scattering system on \textsc{Pegasus} will diagnose point-source helicity-driven plasmas, including dominant particle transport mechanisms and sources of helicity dissipation. Helicity-driven plasmas are estimated to have $\left\langle {T_e } \right\rangle \sim 50$ eV for stochastic field line confinement and $\sim $200 eV for standard Ohmic closed flux surface confinement. To accurately characterize these regimes, a novel system is being designed, installed, and calibrated. A Nd:YAG laser is frequency doubled to provide a 9 ns, 2 J pulse radially across the plasma. Remote alignment of steering mirrors can be performed between shots along the 6 m length of the external laser beam-line. The 532 nm laser beam is focused to a $\le $ 3 mm diameter beam within the plasma. Plasma background measurements are made simultaneously with data collection. A custom optical system collects signal from $>$70{\%} of the plasma cross-section with 1.4 cm radial resolution. Optical fibers relay light to a high-efficiency volume phase holographic grating spectrometer coupled to a high quantum efficiency image intensified CCD camera, gated at $\ge $ 2 ns. Signal levels for plasmas with $n_e >10^{18}$ m$^{-3}$ and 10 eV $< T_e <$ 1 keV are predicted at 10$^{3}$ photons/pulse before background subtraction. [Preview Abstract] |
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PP9.00009: Numerical Simulation of Non-inductive Startup in the Pegasus Toroidal Experiment J.B. O'Bryan, C.R. Sovinec Nonlinear numerical computation is used to investigate the relaxation of non-axisymmetric current channels from washer-gun plasma sources into ``tokamak-like" plasmas in the Pegasus Toroidal Experiment. Resistive MHD simulations with the NIMROD code utilize fully 3D, anisotropic, temperature-dependent thermal conductivity corrected for regions of low-magnetization [Braginskii, 1965], temperature-dependent resistivity, and ohmic heating. Our modeling of injection has been improved by implementing a gaussian toroidal shape function for the current drive source, which has a more rapidly converging Fourier expansion than the original half-sine-wave shape, while retaining a similar current profile. The thermal boundary conditions have also been modified to allow a conducting path to form all the way to the electrodes. With sufficient localized vertical magnetic field reversal, the current channel oscillation frequency doubles. After this transition, parallel current profiles suggest the occurrence of magnetic reconnection when the current channel nearly makes contact with itself. [Preview Abstract] |
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PP9.00010: Results from the Lithium Tokamak eXperiment (LTX) R. Majeski, T. Abrams, L. Berzak, D.P. Boyle, M. Cassin, E.M. Granstedt, C.M. Jacobson, R. Kaita, T. Kozub, B.P. LeBlanc, D.P. Lundberg, M. Lucia, A. Ryou, J. Squire, D.P. Stotler, L. Zakharov, L.R. Baylor, T.M. Biewer, T.K. Gray, R. Maingi, K. Tritz, C.E. Thomas, V. Soukhanovskii In 2010 LTX operated with two new lithium evaporation systems. No other wall conditioning techniques or low-Z limiters are employed. Early discharges in LTX were impurity-dominated, with plasma current 10 - 15 kA, lasting 4-6 msec. Operation with cold lithium coatings increased plasma currents to 70 kA, and discharge duration to 20 msec. Operation with hot lithium coated (300 C) walls produced rapid passivation of the lithium. Indications are that oxygen and other impurities segregate to the surface when the lithium is liquefied. For the 2011 run, bakeout and active cooling of the vacuum vessel is being implemented, along with a novel set of lithium getter pumps. A new liquid lithium filling system for the lower shell segments is currently being fabricated. These new systems will all be operable in 2011, and preliminary results will be presented. [Preview Abstract] |
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PP9.00011: The design and status of a ChERS diagnostic for LTX T.M. Biewer, D. Boyle, T.K. Gray, R. Kaita, R. Maingi, R. Majeski There has been a long-standing collaboration between ORNL and PPPL in the area of edge and boundary layer plasma physics. As part of this collaboration, ORNL has a large role in the instrumentation and interpretation of the edge physics in the Lithium Tokamak Experiment (LTX). In particular, a charge-exchange recombination spectroscopy (ChERS) diagnostic is being designed and is undergoing a staged implementation on LTX. This year passive spectroscopy measurements have been made on LTX, in anticipation of active spectroscopy measurements, which will be enabled by the installation of a diagnostic neutral beam in FY2012. The LTX ChERS diagnostic will consist of both toroidal and poloidal lines of sight, allowing for profile measurement of all the plasma parameters (T$_{i}$, n$_{Li}$, v$_{P}$, v$_{T})$ required for the calculation via force balance of the radial electric field profile (E$_{r})$, when combined with the magnetic field profile from equilibrium reconstructions. The effect of lithium on the E$_{r}$ profile, as well as the fundamental plasma parameters, is a major topic of interest for LTX and the plasma physics community. Preliminary data will be presented. [Preview Abstract] |
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PP9.00012: Results from the LTX High-Speed Digital Holography System C. E. (Tommy) Thomas Jr., L.R. Baylor, S.K. Combs, S.J. Meitner, D.A. Rasmussen, E.M. Granstedt, D.P. Lundberg, C.M. Jacobson, R. Majeski, R. Kaita A high-speed CO$_{2}$ laser digital holography system (500 frames per second (FPS) at 256 x 256 pixels, 1500 FPS at 128 x 128 pixels, etc., to a maximum of 43,000 FPS at 64 x 4 pixels) has been built for high-resolution imaging of electron density on the Lithium Tokamak Experiment (LTX). The laser operates at 9.1 microns by using an Oxygen-18 isotope, and has a power output up to 20 W. A FLIR SC4000 IR camera is used to capture the digital holograms. An acousto-optic modulator (AOM) is used to ``shutter'' the laser so that effective camera integration times down to less than one microsecond are possible. The system will be used for examining profile modifications on LTX with molecular cluster injection (MCI), supersonic gas injection (SGI), and external gas puffing. Results of measurements will be presented along with a discussion of system design, including noise-reduction techniques developed during system testing and initial operation. [Preview Abstract] |
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PP9.00013: Accurate Physical and Electromagnetic Tokamak Model Developed with a Response Function Technique L. Berzak Hopkins, R. Kaita, R. Majeski, L. Zakharov Accurate models which incorporate diagnostic responses, vacuum vessels, and the other conducting structures required by tokamaks are critical for designing machine operations and for calculating highly constrained equilibrium reconstructions to analyze plasma performance. A novel Discharge Development Code (Cbddc) has been developed for the purpose of building such a model. This code links an infinitely thin, finite conductivity, three-dimensional triangular mesh with a response function technique to yield a millimeter-scale match between the simulated model and the as-built physical construction of the tokamak. This technique has been utilized to calibrate magnetic diagnostics and to construct an accurate model of the Lithium Tokamak eXperiment (LTX), based on a triangular mesh with 83,500 elements. Cbddc has also been implemented to simulate three-dimensional eddy currents in the non-axisymmetric, conducting structures incorporated into LTX. The flexibility of the code permits it to be generally applicable to new machines, such as ITER, as well as non-axisymmetric experiments, such as stellarators. [Preview Abstract] |
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PP9.00014: First Electron Temperature Profile Measurements on the Lithium Tokamak Experiment C.M. Jacobson, R. Kaita, B.P. LeBlanc, R. Majeski, Q. Zang The Lithium Tokamak Experiment (LTX) is a spherical tokamak designed to study the low-recycling regime through the use of a liquid-lithium coated shell conformal to the last closed flux surface. A low recycling rate is expected to flatten core electron temperature profiles, raise edge temperatures, and strongly affect electron density profiles. A Thomson scattering diagnostic uses a 15 J, 30 ns FWHM pulsed ruby laser (694.3 nm) to measure $T_{e}$ and $n_{e}$ at 9 radial points on the horizontal midplane, spaced from the plasma axis to the edge at a single temporal point for each discharge, with two background light channels. Scattered light is imaged though a spectrometer into an intensified CCD. $T_{e}$ values have been observed from 50 to 150 eV. $T_{e}$ and $n_{e}$ profiles under various wall conditions are presented. Calibrated $n_{e}$ and $P_{e}$ profiles are used to constrain equilibrium reconstructions. Details and progress regarding an upcoming 5 channel, 5 mm resolution edge polychromator system are presented. [Preview Abstract] |
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PP9.00015: Effect of Molecular Cluster Injector Fueling on Lithium Tokamak Experiment Plasmas with Lithium-Coated Walls D.P. Lundberg, E. Granstedt, R. Kaita, R. Majeski Lithium Tokamak Experiment (LTX) plasmas with lithium-coated walls have demonstrated low-recycling conditions, with substantially higher fueling requirements and reductions in edge neutral emission. Most fueling systems, such as wall-mounted gas puffers or supersonic gas injectors, are ill-suited for use with low-recycling plasmas, as they primarily source low-density gas into the plasma edge. A Molecular Cluster Injector (MCI) has been installed to improve fueling efficiency by increasing the penetration of neutrals into the plasma core. The MCI molecular density has been measured with an electron beam, with n$_{H2 }$exceeding 10$^{16}$cm$^{-3}$ more than 15cm from the nozzle. These densities are 100-1000 the LTX n$_{e}$, making the MCI suitable for testing high-density fueling. By varying the MCI pressure, temperature, and location relative to the plasma, the relative importance of the molecular density and the degree of cluster formation within the supersonic jet can be studied. The effects of MCI fueling on LTX n$_{e}$ profiles is discussed. [Preview Abstract] |
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PP9.00016: Spectroscopic Measurements on the Lithium Tokamak eXperiment E.M. Granstedt, R. Kaita, R. Majeski, T.K. Gray, R. Maingi, K. Tritz, V.A. Soukhanovskii The Lithium Tokamak eXperiment (LTX) is a spherical torus designed to investigate the very low-recycling, lithium wall regime for magnetically confined plasmas. Since lithium surfaces primarily influence plasma performance through their effect on wall recycling, comprehensive measurements of hydrogen fluxes from the wall are necessary. Three instruments measure Lyman-$\alpha$ emission around most of the poloidal cross-section: two arrays view the inboard shell and outboard shell, and a single diode views a molybdenum limiter. These measurements will be used with a neutral transport code to calculate recycling and the fueling profile. Lithium wall conditioning also affects plasma performance through modifying wall impurity fluxes. A visible survey spectrometer and filterscope measurements of lithium, carbon, and oxygen emission lines are used to quantify the fluxes of light impurities ejected from the walls. Trends in the core penetration of these light impurities are measured by an XUV spectrometer, which is also used to examine high-Z impurity emission. Finally, an AXUV array is used as a radiometer to quantify the radiation emission profile. [Preview Abstract] |
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PP9.00017: Simulations of Diffusive Lithium Evaporation onto the Lithium Tokamak eXperiment (LTX) Vessel Walls T. Abrams, D.P. Stotler, R. Majeski The evaporation of lithium (Li) onto the shells of the LTX vacuum vessel has been shown to dramatically increase the magnitude and duration of plasma current pulses. This improvement in performance is believed to be due in large part to a decrease in the sputtering rate of impurities from the vacuum vessel wall, leading to a smaller concentration of impurities in LTX plasma discharges. Reduction in the strength of impurity sources with thicker Li coatings motivates maximizing the toroidal and poloidal uniformity of evaporative Li coverage. This optimization can be achieved by Li evaporation into a helium (He) background and exploiting the inverse scaling of He pressure with the mean free path of Li atoms. Three-dimensional DEGAS 2 neutral transport simulations of this process have been performed to determine an optimal sequence of He background pressures. The results of this optimization will be presented. [Preview Abstract] |
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PP9.00018: Equilibrium reconstruction and interpretive edge modeling for LTX J. Squire, E. Granstedt, C.M. Jacobson, M. Jaworski, R. Kaita, D. Lundberg, R. Majeski, J. Menard The Lithium Tokamak eXperiment (LTX) is a medium sized spherical tokamak intended to advance understanding of discharges in very low recycling regimes. To create the necessary absorbing boundary conditions, lithium is evaporated onto a heated conducting shell designed to be conformal to the plasma edge. We present various important plasma parameters from the most recent run, as well as the first axisymmetric equilibrium reconstructions of LTX plasmas. The calculation of these equilibria is confounded by the strong eddy currents generated in the shell, as well as 3-D field effects from toroidal shell gaps. With the aim of characterizing important edge processes, equilibria are used as input for the OEDGE interpretive edge modeling suite.\footnote{Stangeby, P.C., et al., J. Nucl. Mater. 313-316 (2003) 883.} OEDGE utilizes an onion-skin method to model a fluid background plasma and then employs Monte Carlo methods to solve the hydrogenic neutral and impurity species dynamics. This allows a large number of atomic physics processes to be included in a straightforward way. To improve the reliability of the model and reduce the influence of experimental uncertainties, many different LTX diagnostic data sets are utilized simultaneously. This work was supported by USDOE Contract DE-AC02-09CH11466. [Preview Abstract] |
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PP9.00019: The Spherical Tokamak MEDUSA for Mexico C. Ribeiro, M. Salvador, J. Gonzalez, O. Munoz , A. Tapia, V. Arredondo, R. Chavez, A. Nieto, J. Gonzalez, A. Garza, I. Estrada, E. Jasso, C. Acosta, C. Briones, G. Cavazos, J. Martinez, J. Morones, J. Almaguer, R. Fonck The former spherical tokamak MEDUSA (Madison EDUcation Small Aspect.ratio tokamak, R $<$ 0.14m, a $<$ 0.10m, B$_{T}<$ 0.5T, I$_{p}<$ 40kA, 3ms pulse) is currently being recomissioned at the Universidad Aut\'{o}noma de Nuevo Le\'{o}n, Mexico, as part of an agreement between the Faculties of Mech.-Elect. Eng. and Phy. Sci.-Maths. The main objective for having MEDUSA is to train students in plasma physics {\&} technical related issues, aiming a full design of a medium size device (e.g. Tokamak-T [1]). Details of technical modifications and a preliminary scientific programme will be presented. MEDUSA-MX will also benefit any developments in the existing Mexican Fusion Network. Strong liaison within national and international plasma physics communities is expected. New activities on plasma {\&} engineering modeling are expected to be developed in parallel by using the existing facilities such as a multi-platform computer (Silicon Graphics Altix XE250, 128G RAM, 3.7TB HD, 2.7GHz, quad-core processor), ancillary graph system (NVIDIA Quadro FE 2000/1GB GDDR-5 PCI X16 128, 3.2GHz), and COMSOL Multiphysics-Solid Works programs.\\[0pt] [1] M.Salvador et al., Fus. Energy Conf., S. Korea, 2010, FTP/P6-36 [Preview Abstract] |
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PP9.00020: Design and Construction of Versatile Experiment Spherical Torus (VEST) at Seoul National University YoungHwa An, Kyoung-Jae Chung, BongKi Jung, HyunYeong Lee, ChoongKi Sung, Hyun-Seok Kim, Yong-Su Na, Yong-Seok Hwang A new spherical torus, named as VEST (Versatile Experiment Spherical Torus), has been built at Seoul National University to investigate versatile research topics such as double null merging start-up, divertor engineering and non-inductive current drive. VEST is characterized by two partial solenoid coils installed at both vertical ends of a center stack, which will be used for double null merging start-up schemes. A poloidal field (PF) coil system including the partial solenoids for break-down and a long solenoid for the sustainment of merged plasma has been designed by solving circuit equations for the PF coils and vacuum vessel elements in consideration of required volt-second, null configuration and eddy current. To supply required currents to the PF coils and solenoids, power supplies based on double-swing circuit have been designed and fabricated with capacitor banks and thyristor switch assemblies. Also a power supply utilizing cost-effective commercial batteries has been developed for toroidal field( TF) coils. Detailed descriptions on the design of VEST and some initial test results will be presented. [Preview Abstract] |
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PP9.00021: Overview of NSTX Research Operations and Facility Upgrades Masayuki Ono The National Spherical Torus eXperiment (NSTX) facility has been routinely operating with lithium wall coating, yielding record plasma parameters and improved operational efficiency. In 2010, its Liquid Lithium Divertor (LLD) was commissioned. The interactions of the outer SOL plasma with lithium evaporated on the molybdenum surface of the LLD have been investigated for a wide range of surface temperatures. The carbon core accumulation was observed to be reduced with the LLD surface above the lithium melting temperature but there was also no significant molybdenum influx observed, even with the strike point directly on the LLD. For the 2011-12 experimental campaign, a row of molybdenum tiles was installed inboard of the LLD for experiments with both strike points on lithiated molybdenum surfaces. Other new facility and diagnostic systems were implemented to support the research program in all plasma science areas. For the longer term, a new center stack and associated structural enhancements are being prepared, together with a second NBI system, to achieve 5 s operation at Bt = 1 T and Ip = 2 MA to enable low-collisionality, non-inductively driven scenarios to provide the database needed for a compact fusion neutron science facility. [Preview Abstract] |
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PP9.00022: Control Development for NSTX and the Effects of Strong Shaping Egemen Kolemen, D.A. Gates, S.P. Gerhardt, D.A. Humphreys, D. Mueller, V. Soukhanovskii, M.L. Walker New shape control implementations and the effects of the strong shaping on the NSTX are summarized. Spherical Tokamak devices all operate at high elongation in order to maximize the bootstrap fraction and $q^*$. In addition, the location of the outer strike point must often be fixed for effective divertor operation. As a result, neither the plasma elongation nor the triangularity can be modified greatly. An additional shape parameter that can help optimize plasma stability is the plasma squareness. Squareness control was implemented in 2010 and in this paper the effects of squareness variation on stability and plasma performance are presented. NSTX by design has a thin center column, which does not allow placing polodial field (PF) coils to directly control the inner plasma boundary. Controlling the full plasma boundary therefore needs the combined effort of all the PF coils. A multi-input-multi-output (MIMO) control law for the full plasma was developed. A snowflake divertor configuration is currently being tested to enable higher-power operations in NSTX-Upgrade and a new control algorithm to enable these operations was developed. Experiments are planned to test the MIMO and snowflake controls and the effects of the enhanced shape control on the plasma performance and stability. Work supported by U.S. DOE Contract DE-AC02-09CH11466. [Preview Abstract] |
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PP9.00023: System Modeling, Validation, and Design of Shape Controllers for NSTX M.L. Walker, D.A. Humphreys, N.W. Eidietis, J.A. Leuer, A.S. Welander, E. Kolemen Modeling of the linearized control response of plasma shape and position has become fairly routine in the last several years. However, such response models rely on the input of accurate values of model parameters such as conductor and diagnostic sensor geometry and conductor resistivity or resistance. Confidence in use of such a model therefore requires that some effort be spent in validating that the model has been correctly constructed. We describe the process of constructing and validating a response model for NSTX plasma shape and position control, and subsequent use of that model for the development of shape and position controllers. The model development, validation, and control design processes are all integrated within a Matlab-based toolset known as TokSys. The control design method described emphasizes use of so-called decoupling control, in which combinations of coil current modifications are designed to modify only one control parameter at a time, without perturbing any other control parameter values. [Preview Abstract] |
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PP9.00024: Overview of NSTX Liquid Lithium Divertor Performance and Divertor Upgrade Plans R. Kaita, H. Kugel, M.G. Bell, R. Bell, M. Diallo, S. Gerhardt, M. Jaworski, J. Kallman, S. Kaye, B. LeBlanc, D. Mansfield, J. Menard, D. Mueller, S. Paul, A.L. Roquemore, F. Scotti, C.H. Skinner, L. Zakharov, J.P. Allain, A. McLean, R. Maingi, R. Nygren, R. Raman, S. Sabbagh, V. Soukhanovskii NSTX experiments were conducted in 2010 with a Liquid Lithium Divertor (LLD) surface that covered the outer part of the lower divertor. It was designed to study the deuterium retention properties of a static liquid lithium surface, refreshed by lithium from evaporators to approximate a surface renewed by flowing liquid lithium. The LLD surface temperature ranged from below to above the lithium melting point, depending on the amount of applied and plasma heating. Noteworthy improvements in plasma edge conditions were obtained, and analysis is in progress to compare them with previous lithiated graphite results. Following the end of 2010 plasma operations, repairs were made to the mechanical damage, apparently from plasma current disruptions, to the LLD supports and other hardware, and the LLD was reinstalled. A row of molybdenum tiles was also installed inboard of the LLD. Since the LLD substrate is porous molybdenum, experiments with both inner and outer strike points on lithiated molybdenum will be possible in 2011-12. [Preview Abstract] |
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PP9.00025: Plasma facing surface composition during Li evaporation in NSTX and LTX C.H. Skinner, M. Jaworski, H.W. Kugel, R. Majeski, R. Kaita, V. Surla, R. Sullerberger, B. Koel Evaporated lithium coatings can react with water in the base vacuum to produce lithium hydroxide and hydrogen. Such coatings can significantly reduce the implantation length of incident deuterium. Since tokamaks typically do not have ultrahigh vacuum (UHV) conditions, surface reactions can occur in the time interval between lithium evaporation and the next discharge resulting in a PFC surface that should be considered as a mixed material rather than a pure ``lithium coating.'' We present calculations of the flux of water from the residual vacuum to PFCs in NSTX and LTX. To avoid reactions with residual vacuum gases over a period of a few minutes UHV conditions are required and a new UHV surface science facility at PPPL is being commissioned. Investigations of the reactions of freshly evaporated lithium with controlled introduction of trace gases will be presented. [Preview Abstract] |
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PP9.00026: Measurements of core lithium concentration in NSTX R.E. Bell, M. Podesta, B.P. Leblanc, A. Diallo, F. Scotti The core lithium concentration in NSTX was measured using charge exchange recombination spectroscopy during the FY2010 experimental run, which featured routine lithium conditioning of plasma facing surfaces. Both active and background Li III emission at 5167{\AA} (n=7-5) and C VI emission at 5291 {\AA} (n=8-7) were monitored with spatially-interleaved vertical-viewing sightlines over the outer major radius (120-157 cm) of the plasma. These line-integrated Li and C measurements were inverted to recover profiles and account for the differences in the charge exchange cross sections between Li and C. No significant accumulation of lithium was observed in the core plasma with $\Delta $Z$_{eff}$ due to lithium $\le $ 0.006. A persistently low lithium concentration was observed, with N$_{LI}$/N$_{e} \quad <$ 0.1 {\%}, despite heavy lithium conditioning. The ratio of lithium to carbon density remained roughly proportional, depending on plasma radius, with N$_{Li}$/N$_{C} \quad \le $1{\%} over a wide range of plasma parameters. [Preview Abstract] |
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PP9.00027: 2-D thermal response calculations of the liquid lithium divertor on NSTX K. Gan, A.G. McLean, J-W. Ahn, T.K. Gray, R. Maingi The liquid lithium divertor (LLD) in NSTX was installed for particle and impurity control in NSTX, and its effectiveness was predicted to vary with the lithium surface temperature. It is therefore important to know the temperature evolution of the LLD during plasma discharges. A 2-D implicit finite difference code (``Li{\_}enthalpy'') was written to simulate the lithium temperature with an accurate description of the LLD components, which include a surface lithium layer, a porous molybdenum mesh that is $\sim $ 50{\%} filled with lithium, a thin stainless steel layer, and a thick underlying copper substrate. The heat flux on the graphite was measured with a recently developed dual-band infrared camera; we use the same heat flux profile on the LLD at the same major radius, because of toroidal symmetry. The code ``Li{\_}enthalpy'' computes the LLD thermal response to this heat flux profile; a Gauss-Seidel iterative procedure was implemented to solve the phase-change problem as lithium melted in response to plasma heating. The computed LLD temperature response is then compared and calibrated with the measured surface temperature on the LLD with the dual-band camera. From this the dynamics of the spatially and time varying liquid lithium layer thickness are extracted. Analysis from a number of plasma discharges is presented. *Supported in part by U.S. DoE contracts DE-AC05-00OR22725 and DE-AC02-09CH11466. [Preview Abstract] |
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PP9.00028: L-H power threshold scaling with magnetic geometry on NSTX and the role of ion orbit loss D.J. Battaglia, C.-S. Chang, S.M. Kaye, S. Ku, R. Maingi The L-H power threshold (P$_{LH})$ on the National Spherical Torus Experiment varies with X-point radius (R$_{X})$, plasma current (I$_{p})$, the direction of the ion grad-B drift and the amount of lithium evaporated on the divertor surfaces. The edge T$_{e}$ and T$_{i}$ (where T$_{e} \sim $ T$_{i})$ just prior to the time of the L-H transition vary with the magnetic geometry, but are fairly independent of the neutral fueling rate and lithium conditioning. These observations are consistent with the X-transport theory, which describes the mean edge radial electric field (E$_{r})$ profile required to prevent non-ambipolar ion loss in a diverted plasma. A guiding-center orbit calculation in the absence of electric fields, collisions and flows provides insight into the dependence of the ion loss, and thus E$_{r}$, on the magnetic geometry and edge T$_{i}$. For example, the number of ion loss orbits remains constant as R$_{X}$ is reduced from 0.64m to 0.47m only if the edge T$_{i}$ increases by 60{\%}. This is in agreement with self-consistent calculations of E$_{r}$ using the neoclassical XGC0 code and experiments that measured edge T$_{e}$ and T$_{i}$ to be 40 -- 60{\%} larger. Similar agreement is also observed between guiding-center calculations, XGC0 results and the measured P$_{LH}$ versus I$_{p}$ and ion grad-B direction. [Preview Abstract] |
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PP9.00029: Edge turbulence and flow behavior preceding L-H and H-L transitions in NSTX Y. Sechrest, T. Munsat, S.J. Zweben Recent Gas Puff Imaging (GPI) observations on the National Spherical Torus Experiment (NSTX) have revealed a quasi-periodic oscillation in the plasma edge preceding the L-H transition in a limited set of neutral beam heated plasmas [1-2]. These $\sim $3 kHz flow oscillations exhibit both long wavelength and long correlation lengths, suggesting they are zonal-flow-like. The flow oscillations are strongly correlated with modulations of the level of edge turbulence, thus the system appears to undergo a predator-prey type limit-cycle preceding the L-H transition. However, a clear trigger for the L-H transition was not observed. In addition to these results, analysis of the Reynolds stress profiles obtained from image velocimetry for L-mode periods preceding the L-H transition will be discussed. Imaging data from the GPI diagnostic has also captured several L-H and H-L transitions in RF heated plasmas near the L-H input power threshold. These observations show a very distinct $\sim $25 kHz feature present during H-mode that appears to precede large ejections of plasma into the scrape-off-layer. This feature is also seen to grow in amplitude preceding an H-L transition. A detailed characterization of this feature will be presented in addition to the previous results on zonal-flow-like oscillations. \\[0pt] [1] S.J. Zweben et al, Phys. Plasmas 17, 102502 (2010) \\[0pt] [2] Y. Sechrest et al, Phys. Plamsas 18, 012502 (2011) [Preview Abstract] |
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PP9.00030: Effect of collisionality on energy transport in NSTX Stanley Kaye, Stefan Gerhardt, Rajesh Maingi, Ron Bell, Ahmed Diallo, Benoit LeBlanc Lithium (Li) coating of the plasma facing components in NSTX has led to modifications of plasma profiles and the underlying energy transport. With lithium-coated walls, the electron temperature profiles are broader than without lithium, and this is reflected by a sharp reduction in the electron thermal diffusivity. This reduction in electron transport is further manifest as an increase in the thermal energy confinement time overall, as well as H-mode confinement enhancement. The energy confinement scaling of discharges with lithium-coated walls shows a much stronger I$_{p}$ scaling and weaker B$_{T}$ scaling than those without lithium coating, similar to the ITER98y,2 scaling trends. The change of confinement with lithium coatings is associated with a concomitant change in collisionality, either through changes in the plasma temperature profiles themselves or the impurity levels. Furthermore, the relative amount of pedestal to core stored energy increases with increasing Li deposition (decreasing collisionality). [Preview Abstract] |
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PP9.00031: Density fluctuation measurement by FIReTIP for the Enhanced Pedestal H-mode on NSTX K.C. Lee, C.W. Domier, N.C. Luhmann, Jr., R. Kaita, R. Maingi The multi-channel Far Infrared Tangential Interferometry/Polarimetry (FIReTIP) system has been used to measure changes in the electron density fluctuation spectrum for the Enhanced Pedestal H-mode (EPH-mode) on the National Spherical Torus Experiment (NSTX). Data shows dramatic density fluctuation suppression as the EPH-mode is triggered, similar in nature to the turbulence reduction present at the conventional L$\backslash $H transition. Coherent fluctuations are observed by FIReTIP during the EPH-mode with frequencies greater than 10 kHz. Density fluctuation measurements from FIReTIP edge channels with different tangency radii during the EPH-mode are compared with L-mode and H-mode cases, and are presented together with a discussion of a possible EPH-mode triggering mechanism based on the gyro-center shift (GCS) theory. [Preview Abstract] |
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PP9.00032: Turbulence suppression and poloidal flow dynamics at the L-H transition in NSTX D.S. Thompson, R.J. Fonck, G.R. McKee, D.R. Smith, I.U. Uzun-Kaymak Long-wavelength density fluctuations are suppressed at the L-H transition in both the edge and core regions of National Spherical Torus Experiment (NSTX) plasmas. The magnitude of reduction varies among operational regimes. A beam emission spectroscopy (BES) system installed on NSTX measures these ion gyroscale fluctuations from r/a $\sim $ 0.1 to the scrape off layer. The system includes four poloidal arrays and high throughput optics aligned to the magnetic field pitch angle at the neutral beam. Radial and poloidal correlation lengths are measured near r/a $\sim $ 0.85 across the LH transition and typically decrease at the transition. These edge fluctuations have frequencies up to 100 kHz and radial and poloidal correlation lengths of approximately 10 cm. High frequency poloidal flow velocity fluctuations of the density turbulence are measured using the dynamic programming method of time delay estimation. These velocity fluctuations will be investigated for signatures of zonal flows or changes in the equilibrium velocity that may precede the transition. [Preview Abstract] |
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PP9.00033: Edge turbulence changes with increasing lithium coating in NSTX Bin Cao, Stewart Zweben, Daren Stotler, Ahmed Diallo, Benoit LeBlanc Lithium wall coating improves energy confinement and suppresses edge localized modes in NSTX, but the mechanism of this improvement is not yet well understood. We have investigated whether lithium wall coating is correlated with changes in the edge turbulence by analyzing data from the NSTX gas puff imaging (GPI) diagnostic. This diagnostic provided two dimensional fast camera images of the D-alpha light emitted by a deuterium gas puff near the outer midplane. So far we have not found any systematic changes of the time-averaged radial profile of GPI D-alpha light with increased lithium wall coating. However, we did observe a clear reduction in the GPI intensity fluctuations, autocorrelation times, and radial correlation lengths with this increased lithium coating, but with no obvious change in poloidal correlation length. The time-averaged GPI profile results will be interpreted using the DEGAS 2 code, with input from the edge density and temperature data from Thomson scattering. The fluctuation changes will be interpreted using models based on shear-stabilization of edge turbulence, including possible effects of the neutral density. This work was supported by DOE Contract DE-AC02-09CH11466. [Preview Abstract] |
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PP9.00034: Studies of impurity transport in NSTX with the edge multi-energy SXR diagnostic D.J. Clayton, K. Tritz, M. Finkenthal, D. Kumar, D. Stutman, R.E. Bell, B.P. LeBlanc A high-resolution multi-energy soft-x-ray (ME-SXR) diagnostic is utilized for transport measurements in the NSTX plasma edge. In particular, it is well suited for measuring impurity particle transport from trace impurity injections. Four 20-channel photodiode arrays, each with a different x-ray filter, measure emission from different charge states of the injected impurity. A fifth, unfiltered array acts as a bolometer. It measures emission primarily from the lowest charge states, thus providing the source term when modeling transport of the higher charge states. The ME-SXR arrays have a mid-plane tangential view of the plasma edge from $\rho /a \sim 0.6$ to the SOL, with a spatial resolution of 1 cm. Variable-gain preamplifiers provide a good signal-to-noise ratio with a time resolution $> 10$ kHz. The STRAHL 1D radial impurity transport code, in conjunction with a synthetic x-ray diagnostic, is used to obtain the radial diffusive and convective transport coefficient profiles from measurement. Results will be presented from impurity injection experiments in various plasma edge conditions, including the application of 3D fields. [Preview Abstract] |
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PP9.00035: Study of C and Li neoclassical transport in NSTX Li-conditioned ELM-free H-mode discharges F. Scotti, V.A. Soukhanovskii, S. Kaye, R. Andre, S. Gerhardt, R.E. Bell, A. Diallo, B.P. Leblanc, M. Podest\`a, S.A. Sabbagh, H.W. Kugel, R. Kaita, M.G. Bell ELM-free H-mode discharges are routinely achieved with lithium wall conditioning in NSTX, with a concomitant core impurity accumulation. Z$_{eff}$ generally increases up to 4-5 (due to C) and core P$_{rad}$ ramps up to several MWs (due to metals). In contrast, Li is efficiently screened from the core, where it is present at about 1\% of C densities. C and Li density profiles show similar time evolutions, with the early formation of a higher density ``ear'' and a slower diffusion to the core. In this work, the neoclassical transport code NCLASS is used to study neoclassical multi-ion transport in NSTX plasmas. In particular, possible effects leading to a change in C transport due to Li conditioning are analyzed in discharges with and without applied Li coatings; these include the presence of a low Z collisional background ion (Li) and changes in the D ion temperature and density profiles. Neoclassical predictions are tested with the MIST impurity transport code to check consistency with experimentally measured core impurity density profiles. Work supported by U.S. DOE Contracts DE-AC02-09CH11466 and DE-AC52-07NA27344. [Preview Abstract] |
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PP9.00036: Effect of lithium on the power exhaust channel in NST Travis Gray, Adam McLean, Rajesh Maingi, Michael Jaworski, Tom Osborne, Vlad Soukanouvskii NSTX has been shown [1] to have peak heat fluxes up to 15 MW/m$^2$ where the heat flux width, when magnetically mapped to the midplane, decreases strongly with Ip (lambda$_q^{mid}~$I$_P^{-1.6}$) during high performance (I$_p$ = 1.2 MA, P$_{NBI}$ = 6 MW, $\delta$~0.7) H-mode discharges with boronization. However, with lithium wall conditioning, the effect on NSTX discharges has been to improve energy confinement [2], reduce ohmic flux consumption therefore extending the discharge lifetime, and the elimination of Edge Localized Modes (ELMs) when sufficient lithium is applied [3]. However, with significant lithium wall deposition, lambda$_q^{mid}$ contracts by 50-60\% and the dependence on I$_P$ is reduced (lambda$_q^{mid}~$I$_P^{- 0.4}$). A similar trend is observed in the profile width of divertor D$_\alpha$ profiles with significant lithium deposition. Measurements from flush mounted divertor Langmuir probes show a slight increase in electron density but electron temperature, T$_e$ is unchanged at $\sim$20eV. The scaling of the heat flux width will be reported as a function of pedestal parameters under lithiated conditions. [Preview Abstract] |
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PP9.00037: Heuristic Drift-Based Model for the Power Scrape-off Width in H-Mode Tokamaks at Low Gas Puff Robert Goldston An heuristic model for the plasma scrape-off width in H-mode plasmas is presented, in which magnetic drifts into the SOL are balanced against c$_{s}$/2 parallel flows to the divertor plates. The overall particle flow pattern is a modification for open field lines of Pfirsch-Shl\"{u}ter flows, including sinks to the divertors. This model results in an estimated SOL width of $\sim $2a$\rho $/R. It also results in a first-principles calculation of the particle loss rate from low-gas-puff H-mode plasmas, given n$_{sep}$ and T$_{sep}$, and thus the global particle confinement time. Using measured values of n$_{sep}$ and T$_{sep}$, these are in reasonable agreement with experiment. It is next assumed that anomalous perpendicular electron thermal diffusivity is the dominant source of heat flux across the separatrix, investing the SOL particle width derived above with heat from the main plasma. The separatrix temperature is then calculated self-consistently, based on a two-point model balancing power input to the SOL against Spitzer-H\"{a}rm parallel thermal conduction losses to the divertor. This results in a closed-form prediction for the power scrape-off width that is in reasonable quantitative agreement both in absolute magnitude and scaling with recent experimental data. [Preview Abstract] |
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PP9.00038: Determination of Sheath Heat Transmission Coefficient in NSTX Discharges with Applied Lithium Coatings J. Kallman, M.A. Jaworski, R. Kaita, H. Kugel, A. McLean, V. Surla Recycled particle flux can be a significant contributor to tokamak edge plasma density, and lead to reductions in edge temperature. Previous measurements have shown that lithium PFC coatings can lead to lowered edge recycling, corresponding decreases in edge plasma density, and a radial broadening of the electron temperature profile. During the 2010 run campaign, The National Spherical Torus Experiment operated with both solid and liquid lithium coatings on its plasma-facing components. A 99-tip dense Langmuir probe array was installed in the NSTX outboard divertor to measure scrape-off layer density and temperature. A dual-band fast IR camera was also installed to provide surface temperature and heat flux measurements. The present study compares the derived heat fluxes from these diagnostics to determine the sheath heat transmission coefficient $\gamma $. The value of $\gamma $ was measured to be 2.49 +/- 0.04, smaller than the expected classical result of $\sim $7. Implications and possible mechanisms will be discussed. [Preview Abstract] |
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PP9.00039: Effect of 3-D magnetic perturbations on pedestal structure and transport in NSTX J.M. Canik, J.-W. Ahn, R. Maingi, R.E. Bell, A. Diallo, S.P. Gerhardt, B.P. LeBlanc, J.E. Menard, J.-K. Park, M. Podesta, S.A. Sabbagh The application of non-axisymmetric magnetic perturbations has varied effects on the pedestal structure in the National Spherical Torus Experiment. Although the resonant part of the perturbation is sufficient to ergodize the magnetic field provided screening is weak, the expected reduction in the electron temperature is not typically observed. Rather, in initial experiments to measure the effect of 3-D fields, the pedestal electron temperature was observed to increase by 30{\%}. During later experiments that used thick lithium coatings on the plasma-facing components, this increase was not observed, but instead a flattening in the electron temperature and density profiles was measured slightly inside the pedestal. The impact of 3-D fields on pedestal structure under varying operating conditions will be presented. The edge transport rates inferred from these experiments will be compared to several theoretical predictions. These include stochastic transport in magnetic fields that include plasma response, as well as neoclassical transport due to the 3-D magnetic fields, calculated under the assumption of full screening of the resonant perturbations. [Preview Abstract] |
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PP9.00040: Amplifying and Transforming of External Error Field by Scrape-Off-Layer Current (SOLC) in Tokamaks Hironori Takahashi, Eric Fredrickson, Stefan Gerhardt, Stewart Zweben External error fields may induce a thermoelectrically driven SOLC, if the fields produce Te differences between the two ends of open field lines. SOLC may in turn generate an error field of its own that is greater in amplitude and more destructive in nature than the external fields. The SOLC-generated field tends to be symmetrized in the face of non-axisymmetry in the SOL plasma because of ``phase mixing effect'' arising from strong shear in the field line pitch angle. Near a ``sweet spot,'' midway between primary and secondary separatrices, however, field lines tend to stay bundled together under opposite influences of the two separatrices, and current along these field lines can generate a low toroidal-harmonic error field in spite of the axisymmetry of the background field. A unit line SOLC (kA) in NSTX can generate an n=1 harmonic of amplitude in the magnetic axis plane of $\sim$ 3 mT/kA at inboard q95, rapidly decaying to $\sim$ 0.2 mT/kA at the axis, and to $\sim$ 0.03 mT/kA at outboard q95, which may play a role in rotation slow-down/locking and other MHD phenomena. [Preview Abstract] |
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PP9.00041: Characterization of small transport events triggered with n=3 fields below the ELM destabilization threshold in NSTX J.D. Lore, J.M. Canik, R. Maingi, J.W. Ahn, E.D. Fredrickson, A.G. McLean, F. Scotti, V. Soukhanovskii, K. Tritz ELMs can be reliably triggered in NSTX with the application of n=3 fields above a threshold level [1]. The destabilization threshold is a combination of the applied Amp-turns in the external coils, the length of the applied pulse, and the edge safety factor. Even for pulses below the threshold, however, a small perturbation in the divertor $\mbox{D}_\alpha $ emission is observed. We refer to this as a ``transport response,'' characterized by small perturbations in the soft X-ray emission, divertor fluxes, and other diagnostic signals. Unfortunately these events appear to be insufficient to reduce the impurity accumulation observed in lithium-enhanced ELM-free H-modes. A characterization of these events will be presented, for applied n=3 fields of various amplitudes, pulse lengths and periods. Work supported by DE-AC05-00OR22725, DE-AC02-09CH11466 and by the Magnetic Fusion Energy fellowship from ORISE.\\[4pt] [1] Canik, J.M., \textit{et al.}, Nucl. Fusion \textbf{50} (2010) 034012. [Preview Abstract] |
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PP9.00042: Observations of ELM Magnetic Precursors and Harmonic Oscillations in NSTX F. Kelly, E. Frederickson, R. Bell, K. Tritz, H. Takahashi, R. Maingi Recent experiments on NSTX have shown n=1 dominant and n=2 mode ELM magnetic precursors with mode frequency in the 30 to 90 kHz range. The growing magnetic oscillations measured with the NSTX high-n Mirnov diagnostic occurred simultaneous with the onset of the increase in fast D$\alpha $ signal. These bursts of dominantly n=1, some n=2 and fewer higher modes resemble the predictions of a model simulation of ELMs by T. Evans in which a feedback amplification mechanism causes explosive growth of the separatrix topology driven by thermoelectric currents in flux tubes connecting the divertor plates. The n=1 mode remained dominant as wall recycling was reduced with lithium conditioning and n=3 RMP was applied, suggesting the trigger mechanism remained the same. Sufficient lithium suppressed ELMs and made the occurrence of low-frequency, low-n Harmonics Oscillations (HOs) more frequent. The HOs are consistent with modes localized in the edge with the frequency of the n = 1 harmonic near the rotation frequency of the edge plasma. [Preview Abstract] |
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PP9.00043: Measurements of Divertor Particle and Energy Deposition Profiles During Transient Events in NSTX Vijay Surla, Michael Jaworski, Vlad Soukhanovskii, Travis Gray, Robert Kaita, Josh Kallman, Henry Kugel, Adam McLean, David Ruzic, Filippo Scotti Transient events like edge localized modes (ELMs) or disruptions pose a serious problem to the plasma facing component (PFC) material and knowledge of the power loads on PFCs is important for the design considerations of NSTX upgrade and next generation devices. Thus, it is important to obtain particle and energy deposition profiles on the divertor target during an ELM. In this study, the particle and energy flux profiles arriving at the divertor plate are determined by the use of a high density Langmuir probe (HDLP) array, which when operated in triple probe configuration gives a time resolution of 4 $\mu $s. Typically, it is found that the evolution of an ELM from the probe is characterized by a steep rise and a gradual decrease of current signal. This burst like structure is seen by the Langmuir probes as a rise in the ion saturation current with a width of a few milliseconds. In this presentation, the utility of the Langmuir probe for characterizing ELMs is presented. In addition, analysis from diagnostics like IR camera and D$_{\alpha}$ camera during transient events are presented for comparison. Work supported by DOE contract No. DE-PS02-07ER07-29, DE-AC02-09CH11466, DE-AC05-00OR22725, and DE-AC52-07NA27344. [Preview Abstract] |
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PP9.00044: Correlation Between ELM Average SOL Velocities, ELM Sizes, and Pedestal Temperatures and Gradients in NSTX R.Q. Maingi, T.K. Gray, R. Maingi Edge Localized Modes (ELMs) are an area of concern for devices operating in an H-mode regime, as they result in high periodic heat and particle fluxes that can damage plasma facing components. ELMs are believed to be caused by the peeling and /or ballooning MHD instabilities, which eject hot particles from the top of the H-mode pedestal into the scrape-off layer (SOL). Electrons ejected by ELMs travel to the targets in $<$ 10 $\mu $sec, which is difficult to measure. On the other hand, ions ejected by the ELMs are transported through the SOL in 100-1000 $\mu $sec, at velocities near their sound or acoustic speed, which should be dependent on the pedestal ion temperature, T$_{i}^{ped}$. The average speed parallel to the magnetic field can be computed from experimental data by dividing the difference in the magnetic connection length from the outer midplane to the outer and inner targets in the SOL by the ELM in-out D$\alpha $ delay time, i.e. the difference in time of the ELM flux reaching the outer and inner diverter strike points. Here, we compare the calculated average speeds of the ELM fluxes to the T$_{i}^{ped}$ and its gradient. We further compare these pedestal parameters and in/out delay times to measures of the ELM size, e.g. percent changes in density and stored energy. *Supported in part by U.S. DoE contracts DE-AC05-00OR22725 and DE-AC02-09CH11466. [Preview Abstract] |
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PP9.00045: Progress toward Triggering ELMs with Injected Li Granules on NSTX D. Mansfield, A.L. Roquemore, H.W. Kugel, L.R. Baylor, R. Maingi, P. Parks, W. Wu Efforts to trigger/pace low amplitude ELMs at high frequencies using injected Lithium granules have been undertaken at PPPL. A dropper/impeller injection technology is being developed and is scheduled for installation on NSTX during the 2011-12 run campaign. Using this technology, millimeter-scale spherical Lithium granules can potentially be injected at ``pacing'' frequencies up to several hundred Hz and injection speeds approaching 100m/s. [Preview Abstract] |
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PP9.00046: The role of kinetic dissipation in modifying RWM eigenfunctions Jonathan Menard, Yueqiang Liu Kinetic resonances have been identified to play an important role in the stability of the resistive wall mode (RWM) in tokamaks. One approach to computing RWM stability is a ``pertubative'' approach in which the mode stability is computed assuming the ideal kink eigenfunction is not modified by kinetic damping. The so-called ``self-consistent'' approach includes the dissipation in the perturbed force balance and computation of the mode eigenfunction as embodied in the MARS-F and MARS-K codes. Initial MARS-F and MARS-K simulations for NSTX plasmas find that there can be substantial changes between the predicted fluid (non-kinetic) limit of the RWM eigenfunctions and the self-consistent eigenfunctions computed with full kinetic damping included. These changes can be most pronounced near the plasma edge where the dissipation can be large for NSTX plasmas. These comparisons and the possible implications for the perturbative and self-consistent approaches will be described. [Preview Abstract] |
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PP9.00047: Validation of RWM Kinetic Stability Model and Physics Implications in NSTX J. Bialek, J.W. Berkery, S.A. Sabbagh, R. Betti, R.E. Bell, A. Diallo, S.P. Gerhardt, B.P. LeBlanc, J. Manickam, M. Podesta The resistive wall mode (RWM) instability may limit the promise of disruption-free operation in future tokamaks unless a reliable stabilization mechanism is found. Kinetic effects in the plasma can passively stabilize the mode by dissipating its energy. The change in potential energy by kinetic effects is calculated for experimental plasma equilibria with the MISK code. Further improvements of the theoretical model are presently being investigated to refine the quantitative agreement between computed RWM marginal stability points and experimental results. These include the role of collisions in both dissipating the mode energy and also in damping the resonant kinetic effects, and the inclusion of anisotropy of neutral beam injected energetic ions to correctly account for their stabilizing effects on RWM stability. Recent experiments in NSTX with reduced plasma internal inductance that have decreased RWM stability are consistent with MISK calculations. These kinetic stability calculations are being benchmarked through comparison with the results of other codes such as MARS-K and HAGIS. The implications of this physics for the stability of future devices, such as ITER, are also discussed. $^{*}$Work supported by U.S. DOE contracts DE-FG02-99ER54524, DE-AC02-09CH11466, and DE-FG02-93ER54215. [Preview Abstract] |
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PP9.00048: Onset conditions for disruptions during VDEs Janardhan Manickam, Stefan Gerhardt, Allen Boozer The Vertical Displacement Event, VDE, evolves on a slow timescale relative to the ideal MHD Alfvenic timescale. At some point the kink mode is strongly destabilized, leading to the final disruption. In most cases this occurs when q-edge is slightly less than two. Non-axisymmetric halo currents are often observed, well before the disruption. The evolution of the plasma during the VDE is modeled as a sequence of shrinking equilibria, where the core current profile remains constant so that the safety-factor at the axis, q-axis, remains fixed and the q-edge systematically decreases. Stability analysis shows that the plasma is indeed stable or has a small growth-rate until q- edge drops below 2, at which point the growth-rate rises rapidly, approaching $\gamma T_A =1$. The kink mode is characterized by ${m/n=2/1}$, where ${m}$ and $n$ are the poloidal and toroidal mode numbers. The surface currents associated with the MHD perturbation are computed. These may be related to the non-axisymmetric component of the halo currents and may be providing stability at modest growth-rates. This model is compared with observations on NSTX. [Preview Abstract] |
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PP9.00049: ABSTRACT WITHDRAWN |
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PP9.00050: Detailed drive balance for NTMs on the MAST tokamak Jack Snape, Kieran Gibson, Thomas O'Gorman, Howard Wilson The neoclassical tearing mode (NTM) is a performance limiting instability for tokamaks. Though well established, many uncertainties remain in the modified Rutherford equation for NTM evolution. We build on previous work [1,2], focussing on 2/1 NTMs (the most detrimental) and making use of a large suite of diagnostics, including upgraded Thomson scattering and charge exchange systems. Novel diagnostic triggering techniques are used to capture temperature and density profiles at specific NTM phases and amplitudes. Using this data as an input for the Fitzpatrick transport model we directly measure the transport threshold width [2], usually obtained by a simple heuristic formula. We extend this model to include the asymmetric nature of the NTMs that are observed on MAST. We combine these measurements with information from equilibrium codes and present a detailed drive balance calculation to determine the relative significance of different terms in the Rutherford equation. \\[4pt] [1] R J Buttery et al, PRL, 88 (2002) 125005 \\[0pt] [2] K J Gibson et al, PPCF, 52 (2010) 124041 [Preview Abstract] |
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PP9.00051: MHD Simulations of Disruptions in NSTX J.A. Breslau, H.R. Strauss, R. Paccagnella, S.C. Jardin Research tokamaks such as ITER must be designed to tolerate a limited number of disruptions without sustaining significant damage. It is therefore vital to have numerical tools that can accurately predict the effects of these events. The 3D nonlinear extended MHD code M3D [1] has been augmented with models of the vacuum/halo region and a thin axisymmetric resistive shell that allow it to simulate disruptions and calculate the associated wall currents and forces [2]. Its reliability, however, must be assessed with careful validation studies against disruption databases from existing experiments. Here we compare M3D VDE/kink disruption calculations with data from NSTX. The results of high-resolution numerical simulations at realistic Lundquist numbers show reasonable agreement with experimental data and provide confidence that M3D will be a useful tool for future ITER calculations. The effects of different choices of plasma outflow boundary conditions will also be reported. \\[4pt] [1] W. Park, \textit{et al., Phys. Plasmas} \textbf{6} (1999) 1796.\\[0pt] [2] H.R. Strauss, \textit{et al., Phys. Plasmas} \textbf{17} (2010) 082505. [Preview Abstract] |
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PP9.00052: Formation of long-lived phase space structures by high frequency Alfv\'en Eigenmodes through the Doppler-shifted cyclotron resonance E.D. Fredrickson, N.N. Gorelenkov, E. Belova, N.A. Crocker, G. Kramer Super-Alfv\'enic ion populations, like the fusion-a's on ITER, can excite instabilities a broad range of instabilities. The resonance condition for the higher frequency Alfv\'enic modes (GAE and CAE) is predominantly through a Doppler-shifted cyclotron frequency resonance, although short wavelength CAE have been excited through a simple parallel resonance. There is evidence that the GAE, and possibly CAE, create relatively long-lived, phase space structures, suggesting fast-ion trapping in the wave field. The strong gradients in mod(B) intrinsic to low aspect ratio devices like NSTX, and the large orbit excursions of fast ion raise the question of how resonances may be maintained for many wave periods when the cyclotron frequency varies substantially over fast ion orbits. The question is investigated by looking at the resonant fast ions found in non-linear simulations of the mode with the initial value code HYM. [Preview Abstract] |
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PP9.00053: Comparison of measurements and modeling of beam ion loss during TAE avalanches in NSTX Douglass Darrow, N. Crocker, E. Fredrickson, N. Gorelenkov, M. Podesta, L. Shi, R. White Brief `avalanches' of toroidal Alfv\'{e}n eigenmodes (TAEs) are observed in NSTX plasmas having several different n numbers simultaneously present. All are accompanied by a drop in the neutron rate, indicating likely loss of beam ions. However, the scintillator fast ion loss detector at the wall on NSTX registers losses only for some avalanches and not others. When losses are seen, they cover a wide range of pitch angles, suggesting that the modes cause stochasticity in the beam ion phase space. A 16 channel array of microwave reflectometers provides a radial profile of the structure of each n TAE, which can then be used to produce an absolute amplitude for a TAE mode structure computed by the NOVA code for the plasma at the time of interest. Using the equilibrium plus the TAE modes structures in the guiding center code ORBIT then allows modeling of the effect of the avalanches on beam ion orbits. The modeling does reveal stochastic behavior and loss of some classes of orbits that are populated by the neutral beams on NSTX, with an amplitude threshold for loss that is close to that observed in the experiments. [Preview Abstract] |
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PP9.00054: Modeling of the effect of TAEs and TAE avalanches on neutral beam ion orbits in NSTX L. Shi, N. Crocker, D. Darrow, E. Fredrickson, N. Gorelenkov, M. Podesta, R. White Fast ion loss caused by MHD modes is an important issue for magnetically confined plasmas. Using the guiding center approximation, in the unperturbed axisymmetric magnetic field, particle's energy $E$, magnetic moment $\mu$, and toroidal canonical momentum $P_\zeta$ are conserved. In principle, the existence of magnetic perturbations will break the conservation of these quantities. However, the Toroidal Alfven Eigenmodes (TAEs) typical frequencies are much lower than the ion gyro- frequency. Thus, the magnetic moment $\mu$ is still conserved. In this case, the redistribution of particles within the $P_\zeta -E$ plane is the key for understanding particle transport due to TAEs. We use an EFIT equilibrium, with TAE mode structures computed by NOVA, as input to the guiding center code ORBIT to calculate the particle orbits evolution and thus the mode induced redistribution in $P_\zeta -E$ plane. Results with different mode structures and amplitudes are shown in this presentation. In addition, using the initial beam ion distribution given by TRANSP code, we show that this method can predict the ion loss in the observed energy and pitch angles. [Preview Abstract] |
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PP9.00055: Novel Alfv\'{e}n eigenmode structure measurements in NSTX via reflectometry N.A. Crocker, W.A. Peebles, S. Kubota, J. Zhang, R.E. Bell, E.D. Fredrickson, N.N. Gorelenkov, B.P. LeBlanc, J.E. Menard, M. Podest\`a, S.A. Sabbagh, K. Tritz, H. Yuh Novel measurements of toroidicity-induced (TAE), global (GAE) and compressional (CAE) Alfv\'{e}n eigenmode structure have been obtained in NSTX with a recently upgraded 16 channel array of fixed frequency quadrature reflectometers. Measurements of TAE ($\sim $ 50 kHz $<$ f $< \quad \sim $ 150 kHz) phase show significant radial variation in the midplane, indicating radial propagation. This distinctly contrasts with expectation from ideal MHD since the plasma is up-down symmetric. The structure of GAEs and CAEs (f $< \quad \sim $ 400 kHz) have been obtained for the first time in the core of NSTX high power beam-heated H-mode plasmas. The GAEs have core-peaked, broad radial structure, while the CAEs are strongly core-localized, with substantially higher amplitude. GAEs, which are observed to correlate with enhanced thermal electron transport in such plasmas [D Stutman, PRL 102 115002 (2009)], are hypothesized to be the cause due to their core localization and capacity to resonantly modify electron orbits. The observed CAE structures indicate they must also be considered. [Preview Abstract] |
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PP9.00056: Fast Ion Redistribution and CAE Destabilization in Presence of Low Frequency MHD, in NSTX H-mode Plasmas Alessandro Bortolon, Eric Fredrickson, Gerrit Kramer, Mario Podesta, William Heidbrink In NSTX H-mode plasmas, the confinement of injected energetic beam ions is often deteriorated by early low frequency kink modes ($10-20$~kHz, $n=1-3$). After the mode onset, FIDA measurements show a collapse of fast ion density profile, with central values reduced by as much as 30\%. Early kinks are often accompanied by clusters of high frequency Compressional Alfv\`en Eigenmodes ($1.5-2$~MHz, $n=9-12$). Observations suggest that fast ion redistribution from core to edge associated with the kink may enhance the CAE drive to destabilization. We investigate this hypothesis simulating the fast ion distribution function in a kink perturbed equilibrium, with the full orbit code SPIRAL. The mode structure is derived from combined measurements (Mirnov arrays, SXR, interferometry), and checked against ideal stability calculations. Results are compared to FIDA measurements, with the support of preliminary observations from the new tangential view, sensitive to co-going passing fast ions. [Preview Abstract] |
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PP9.00057: Comparison of Fast-ion D-alpha Measurements with Classical Theory W.W. Heidbrink, A. Bortolon, E. Ruskov, M. Podesta The fast-ion D-alpha (FIDA) diagnostic technique measures profiles of Doppler-shifted light from energetic ions that neutralize in a heating beam. The TRANSP NUBEAM module calculates the fast-ion distribution function $f$, which is then used in a simulation code to predict the FIDA spectra. In MHD-quiescent plasmas in conventional tokamaks, measured FIDA data compare well with predictions [1] but, to date, similar agreement has not been obtained for NSTX. In a spherical tokamak, the large fast-ion gyroradius should cause asymmetries between the intensity of blue-shifted and red-shifted light; also, the large field-line pitch should produce additional spectral asymmetries. Data from a vertically-viewing FIDA diagnostic show smaller spectral asymmetries than expected. Comparisons with data from a newly installed tangentially-viewing FIDA diagnostic are reported, as well as modifications in $f$ that improve agreement with experiment.\\[4pt] [1] Y. Luo {\it et al.}, Phys. Pl. {\bf14} (2007) 112503; B. Geiger {\it et al.}, Pl. Phys. Cont. Fusion {\bf53} (2011) 065010. [Preview Abstract] |
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PP9.00058: Transient energetic charge exchange flux enhancement observed in NSTX neutral-beam-heated H-mode discharges S.S. Medley, G.J. Kramer, R.E. Bell, E. Belova, E.D. Fredrickson, S.P. Gerhardt, B.P. LeBlanc, M. Podest\'a, Y. Ren, A.L. Roquemore, N.A. Crocker Large increases in the E$\vert \vert $B Neutral Particle Analyzer (NPA) charge exchange neutral flux localized at the Neutral Beam Injection (NBI) full energy are observed in the National Spherical Torus Experiment (NSTX). Termed the High-Energy Feature (HEF), it appears only at the NBI full energy, exhibits growth times $\sim $ 20-80 ms, seldom develops a slowing down distribution and arises only in discharges where NTM modes (f $<$ 30 kHz) are absent, TAE activity (f $\sim $ 30-150 kHz) is weak and GAE/CAE activity (f $\sim $ 400-1200 kHz) is robust. The HEF occurs only in H-mode discharges with P$_{b} \quad \ge $ 3 MW and v$_{\vert \vert }$/v $\sim $ 0.7-0.9; i.e. only for passing ions. The HEF appears to be caused by a GAE wave-particle interaction that modifies of the NB fast ion distribution, f$_{i}$(E,v$_{\vert \vert }$/v,r). This proposed mechanism was studied using the SPIRAL code that imports a TRANSP-calculated f$_{i}$(E,v$_{\vert \vert }$/v,r) distribution and evolves it under drive from GAE wave-particle resonances. [Preview Abstract] |
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PP9.00059: Progress on CHI and MGI Experiments on NSTX R. Raman, T.R. Jarboe, B.A. Nelson, D. Mueller, S.P. Gerhardt, H.W. Kugel, G. Taylor Discharges initiated by Transient Coaxial Helicity Injection (CHI) in NSTX have attained peak toroidal plasma currents up to 300 kA. When induction from the central solenoid is then applied, these discharges develop up to 300 kA additional current compared to discharges initiated by induction only. CHI initiated discharges in NSTX have achieved 1 MA of plasma current using only 258 mWb of solenoid flux whereas standard induction-only discharges require about 50{\%} more solenoid flux to reach 1 MA. In addition, the CHI-initiated discharge has lower plasma density and a low normalized internal plasma inductance of 0.35, as needed for achieving advanced scenarios in NSTX. In support of ITER disruption mitigations studies, two Massive Gas Injection (MGI) assemblies have been installed on NSTX. These would allow for a comparison of the benefits of injecting gas from the lower divertor region into the private flux region and the high field side region versus injection from the conventional mid-plane region. This work supported by U.S. DOE Contracts DE-AC02-09CH11466~and DE-FG02-99ER54519 AM08. [Preview Abstract] |
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PP9.00060: Comparing resistive MHD simulations and experiments of Coaxial Helicity Injection (CHI) in NSTX E.B. Hooper, C.R. Sovinec, R. Raman, J.E. Menard NSTX generates CHI plasmas with current, density, and temperature appropriate for ST startup.\footnote{R. Raman, et al., PRL \textbf{104}, 095003 (2010).} Whole-device simulations of CHI using the NIMROD MHD code\footnote{C.R. Sovinec, et al., J. Comp. Phys \textbf{195}, 355 (2004).} extend the HIT-II model.\footnote{A. Bayliss, et al., submitted for publication.} A model power supply generates time-dependent voltage and current at the injection gap. Absorber gap voltage maintains a constant vacuum toroidal flux. Simulation physics includes ohmic heating and thermal conductivity along and across the magnetic field and generation of nonaxisymmetric fields and flows. A flux bubble expands in the simulation with current and plasma temperature similar to experiment; an n=1 mode is observed to generate an helical ribbon of current and velocity vortices on the flux bubble surface. Time-dependent poloidal-field boundary conditions for interesting NSTX discharges are used for quantitative comparisons with experiments. [Preview Abstract] |
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PP9.00061: Performance of High Non-Inductive Current Fraction H-Modes Generated by HHFW Heating in NSTX G. Taylor, N. Bertelli, J.C. Hosea, B.P. LeBlanc, R. Perkins, C.K. Phillips, M. Podesta, E.J. Valeo, J.R. Wilson, P.T. Bonoli, J.C. Wright, R.W. Harvey, E.F. Jaeger, P.M. Ryan High-harmonic fast wave (HHFW) heating in the National Spherical Torus Experiment (NSTX) has generated H-mode plasmas that have a significant non-inductive current fraction ($f_{NI} )$. These experiments are part of a long-term strategy on NSTX to develop H-mode plasmas that are fully non-inductive ($f_{NI} \ge 1)$ and that do not use the central solenoid. Initial experiments in 2010 achieved $f_{NI} \sim 0.65$ in a $I_p =300kA$, $B_T (0)=0.55T$ deuterium H-mode plasma with only 1.4 MW of HHFW power [1]. HHFW power could not be increased above 1.4 MW in 2010 due to poor antenna conditioning. Experiments are planned this year that will use 3-4 MW of HHFW power, with the goal of achieving $f_{NI} \ge 1$. This paper will present results for these recent high $f_{NI} $ experiments. \\[4pt] [1] G. Taylor, \textit{et al.}, 38th EPS Conf. on Plasma Physics, Strasbourg, France (2011) Paper P5.095. [Preview Abstract] |
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PP9.00062: Effects of the Spatial Extent of Multiple Cyclotron Harmonic Layers J.W. Burby, G.J. Kramer, C.K. Phillips, E.J. Valeo An analytic model for single particle motion in the presence of a wave field and multiple cyclotron harmonics is developed and investigated. The model suggests that even in the absence of Doppler broadening, cyclotron harmonic layers have finite spatial extent. This allows for particles to interact with more than one harmonic layer simultaneously, provided the layers are tightly packed. Evidence that certain NB particles in NSTX experience this effect is presented. A model is then developed for the interaction of a magnetized particle with a pair of cyclotron harmonics. An analytic condition for the onset of strong stochasticity in this setting is then derived. The theory is checked numerically using the full-orbit code SPIRAL. Furthermore, a means for testing its validity experimentally based on the analytic stochasticity threshold is proposed. [Preview Abstract] |
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PP9.00063: Evaluation of the High Power Performance of the Upgraded NSTX HHFW Antenna P.M. Ryan, A. McLean, J.C. Hosea, B.P. LeBlanc, R.J. Perkins, G. Taylor, J.R. Wilson, R.I. Pinsker The end-grounded straps of the NSTX HHFW antenna array were replaced in 2009 with center-grounded straps to reduce the interior voltages and electric fields in the plasma/Faraday shield region. After accumulated lithium deposits were removed from the antenna surface by plasma conditioning, reliable HHFW power was increased to 3-4 MW from the 2-3 MW levels of 2008. However, in 2010 reliable operation was limited to less than 2 MW, primarily due to changes in the antenna environment associated with the Liquid Li Divertor. The antennas have been cleaned of Li coatings and Li$_{2}$CO$_{3}$ dust in preparation for the 2011 campaign. The HHFW system will be operated early in the campaign to minimize the effects of Li accumulation in ascertaining the efficacy of reducing internal fields to increase reliability. A visible light camera covering the complete array will observe Li ablation from the powered antenna, and a newly installed IR camera covering $\sim $40{\%} of the array will monitor localized hot spot formation. [Preview Abstract] |
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PP9.00064: MPTS Operation and Recent Upgrade B.P. LeBlanc, A. Diallo, G. Labik, D.R. Stevens NSTX's Multi-Point Thomson Scattering (MPTS) diagnostic has supported plasma operation for over ten years, during which time a phased implementation has been pursued. The measurements span the horizontal midplane covering around 90 percent of the full-bore confined plasma and the scrape-off layer (SOL). While beginning with one 30-Hz Nd:YAG laser and 10 radial positions, MPTS has operated with a second laser -- combined frequency of 60 Hz -- and 30 radial positions during the past six years. A recent upgrade brings the total number of radial positions to 42. While most of the 12 new channels are set to improve spatial resolution in the pedestal and internal transport barrier (ITB) regions, a limited number of extra channels have been added to the inner edge and the SOL. Many of the new channels resulted from the splitting of existing fiber bundles, an option that had been left open in MPTS's original design. The 42-channel configuration is planned to begin operation during the 2011 NSTX experimental run. Experimental results will be presented. Future plans for the upcoming NSTX center-stack upgrade will be discussed. [Preview Abstract] |
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PP9.00065: Initial results from the NSTX Real-Time Velocity diagnostic M. Podesta, R.E. Bell A new diagnostic for fast measurements of plasma rotation through active charge-exchange recombination spectroscopy (CHERS) was installed on NSTX. The diagnostic infers toroidal rotation from carbon ions undergoing charge-exchange with neutrals from a heating Neutral Beam (NB). Each of the 4 channels, distributed along the outer major radius, includes active views intercepting the NB and background views missing the beam. Estimated uncertainties in the measured velocity are $<$5{\%} at the maximum sampling rate of 5000 Hz (or $<$1{\%} at 1000 Hz), to be compared with $<$0.5{\%} and 100 Hz of the main NSTX CHERS system. Signals are acquired on 2 CCD detectors, each controlled by a dedicated PC. Spectra are fitted in real-time through a C++ processing code and velocities are made available to the Plasma Control System for future implementation of feedback on velocity. Results from the initial operation during the 2011 run are discussed, emphasizing the fast dynamics of toroidal rotation, $e.g.$ during L-H mode transition and breaking caused by instabilities and by externally-imposed magnetic perturbations. [Preview Abstract] |
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PP9.00066: Absolute Calibration of the Neutron Monitoring System for NSTX A.L. Roquemore, D.S. Darrow, S.S. Medley NSTX has a complement of six neutron detectors consisting of two fission chambers, one NE-451 ZnS scintillator and 3 plastic BC-400 scintillators. The primary purpose of the fission chambers is to provide an absolute calibration of the neutron emission rate, while the scintillator detectors monitor fast excursions in the neutron yield, due for instance to MHD events. Initially, a point-wise calibration was performed by introducing a 252Cf source on the midplane of NSTX at 10 toroidal locations. The more recent calibrations employed a commercial G-gauge model train and three different diameter circular tracks as a source transport. This method provided the most accurate calibration to date, as well as information on detector sensitivity with changes in plasma position. The results of the four insitu calibrations are presented and the technique of cross-calibration from pulse counting to current mode using low-yield plasma discharges will be discussed. [Preview Abstract] |
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PP9.00067: The ME-SXR fast electron temperature diagnostic for NSTX K. Tritz, D. Clayton, M. Finkenthal, D. Kumar, D. Stutman A novel, compact multi-energy soft X-ray (ME-SXR) diagnostic has been designed, built, and installed on the National Spherical Tokamak eXperiment (NSTX). The system is comprised of 5 photodiode arrays viewing the plasma tangentially on the mid-plane through independent, vertically-stacked pinholes covered with different filters. By viewing the same plasma volume through filters with various transmission profiles, the ME-SXR diagnostic provides a coarse sub-sampling of the X-ray spectrum. This method allows the determination of fast changes in electron temperature, density, and impurity content by comparing the emission collected by the set of filtered diode arrays. The ME-SXR system views the plasma from 0.6 $<$ r/a $<$ 1.1 with $\sim $1cm spatial resolution and $>$10kHz time resolution, and provides profiles of the changes in T$_{e}$, n$_{e}$, n$_{z}$ relevant for studies of transport, low-f MHD instabilities, and ELM dynamics. With a careful selection of filters, the measurements are insensitive to uncertainties in the plasma composition, while remaining sensitive to changes in the T$_{e}$, n$_{e}$, and impurity profiles. Results of algorithm development for modeling profile changes will be presented. [Preview Abstract] |
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PP9.00068: SXR to XUV imaging diagnostics for NSTX Upgrade D. Stutman, K. Tritz, D.J. Clayton, D. Kumar, M. Finkenthal A suite of SXR to VUV imaging diagnostics for transport and MHD measurements in NSTX-Upgrade are being designed by the JHU group. Two toroidally displaced tangential multi-energy SXR arrays will image the edge and core plasma with 1 cm and 3 cm resolution respectively. The applications of the diagnostic will include perturbative impurity and electron transport, fast Te and nexnz profile measurements, and non-magnetic MHD mode identification. The system is complemented by a tangential Transmission Grating Imaging Spectrometer measuring the spatial distribution of XUV impurity emission from the edge to the core and by a repetitive laser-blow off system aimed at enabling multiple transport measurements during a discharge. For the divertor, we are designing a dual grating Imaging Radiometer for measurements of the radiated power in multiple SXR and VUV spectral bins with high space and time resolution. These measurements will assist the experimental and modeling studies planned for the NSTX-U divertor. Lastly, an ultrafast (4 MHz) dual-energy SXR imaging system is being designed for diagnostic of *AE modes and the associated Te, nexnz perturbations. Work supported by US DOE Grants DE-S0000787 and DE-FGO2-86ER53214. [Preview Abstract] |
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PP9.00069: Dual-band infrared observation of temperature dynamics on the NSTX liquid lithium divertor (LLD) A.G. McLean, J-W. Ahn, T.K. Grey, R. Maingi, J.H. Nichols, H.W. Kugel, A.L. Roquemore The combined presence of lithium, with low emissivity, and ATJ graphite, with high emissivity, introduce significant error in traditional single-band infrared (IR) observation for measurement of surface temperatures and calculation of impinging heat flux. To account for variable emissivity especially for observation of the liquid lithium divertor starting in 2010, and Molybdenum tiles starting in 2011, a high speed IR camera on NSTX has been adapted to dual-band, and a new wide-angle IR camera is being implemented with dual-color. In 2010, heating of the LLD surface by repeated plasma exposure was shown to incrementally increase the melted area fraction, but that fraction was found not to change significantly through any single discharge. The surface temperature near the outer strike point at toroidal locations on the LLD was found to be clamped compared to the same major radius on graphite suggesting a possible role of vapor shielding. These observations and new capabilities for the 2011/2012 campaign are presented. [Preview Abstract] |
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PP9.00070: Design and Deployment of a Wide-Angle Two-Color Infrared Camera with Optical Relay on NSTX J.H. Nichols, A.G. McLean, R. Maingi, J.-W. Ahn, A.L. Roquemore, B.C. Stratton, R. Kaita, H.W. Kugel, K. Gan, M. Benjamin A new 30 Hz infrared camera featuring a wide-angle view of the lower divertor has been installed on the National Spherical Torus Experiment (NSTX). This camera utilizes a dichroic beamsplitter to project two IR channels (a 7-10 $\mu $m wavelength band and a 10-14 $\mu $m wavelength band) of the same image side by side on its uncooled microbolometer detector; taking the ratio allows the camera to make temperature measurements that are nearly independent of surface emissivity, an important quality because of the use of highly reflective lithium coatings on top of highly emissive graphite tiles. The camera looks through a reentrantly-mounted chemical vapor deposited (CVD) diamond window, chosen because of its high infrared transmission and ability to withstand a 350\r{ }C vessel bakeout. A cost-effective reflective relay has also been built to remove the camera from regions of strong magnetic field while avoiding chromatic aberrations. First diagnostic results from the 2011 NSTX run are presented. [Preview Abstract] |
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PP9.00071: Implementation of millimeter-wave polarimetry on NSTX Jie Zhang, William Peebles, Neal Crocker, Troy Carter, Shigeyuki Kubota, Walter Guttenfelder Measurements of internal magnetic fluctuations on NSTX, a high-$\beta $ machine, are important to understand stability, fast-ion and thermal transport. A 288 GHz polarimeter operating along a major radial chord in a retroreflection geometry has been developed and installed on NSTX. This will provide the first direct measurement of internal magnetic fluctuations (1--100 kHz) in a high-performance spherical tokamak. Laboratory tests indicate $\le $ 1\r{ } phase resolution. Calculations using a simplified tearing mode model indicate the feasibility of measurement of magnetic fluctuation levels $\ge $ 0.2{\%}. The sensitivity of polarimetry to microtearing modes is assessed using gyrokinetic simulations. The polarimetry phase fluctuations are calculated using the predicted magnetic ($\le $ 1{\%}) and density ($\sim $ 1{\%}) fluctuations in addition to the input equilibrium profiles. It is shown that the system is primarily sensitive to magnetic fluctuations, as long as the propagation chord lies within $\pm $10 cm of the plasma midplane. Initial measurement results from NSTX will be presented. [Preview Abstract] |
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PP9.00072: High-resolution Time-resolved Extreme Ultraviolet Spectroscopy on NSTX J.K. Lepson, P. Beiersdorfer, J. Clementson, M. Bitter, K. Hill, R. Kaita, L. Roquemore, C.H. Skinner, G. Zimmer We report on high-resolution, time-resolved spectroscopy in the extreme ultraviolet spectral region (10-200 \AA) on the NSTX tokamak. This work utilizes two flat-field spectrometers on loan from LLNL's electron beam ion trap facility. XEUS, installed in 2004, has a 2400 line/mm flat-field grating with field of view of $\sim$50 \AA\ that can be positioned to survey 5 - 135 \AA\ with an instrumental resolution of $\sim$0.1 \AA\ and $\lambda/\Delta\lambda \sim$100 at 10\AA\ to $\sim$1000 at 100 \AA. LoWEUS, installed in 2008, utilizes a 1200 line/mm grating with field of view of $\sim$180\AA, is typically positioned to survey 60-280 \AA\ with an instrumental resolution of $\sim$0.3 \AA\ and $\lambda/\Delta\lambda \sim$300 at 100\AA to $\sim$600 at 200\AA. New cameras have achieved a time resolution of 12-13 ms for both instruments. We can now examine time dependence and evolution of both intrinsic and extrinsic impurities on NSTX in the EUV band. Of particular interest is monitoring the entry of molybdenum into the plasma after installation of Mo tiles for the 2011 run. \\[4pt] Work supported by DOE General Plasma Science program. Part of this work performed under the auspices of DOE by LLNL under contract DE-AC52-07NA27344 and PPPL under contract DE-AC02-09CH11466. [Preview Abstract] |
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PP9.00073: OTHER ICF AND HEDP PHYSICS AND DIAGNOSTICS |
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PP9.00074: Low-density foam characterization using monochromatic soft x-ray sources J.M. Taccetti, P.A. Keiter, N. Lanier, B.M. Patterson We present the characterization of low-density foam components using two monochromatic x-ray sources in the soft x-ray regime. The foams, with densities ranging from 30 to 125~mg/cm$^{3}$, are SiO$_{2}$ or CH foams used in targets for HEDP experiments at NIF and Omega laser facilities. An important issue that arises when fabricating such low-density foams is the uniformity of the foam sample density. Experience has shown that a skin-effect results when SiO$_{2}$ foam is cast inside a metal mold, due to more rapid cooling of the aerogel at the metal surface. And when machined, the machining process causes foam cells at the surface to collapse, also leading to a higher density at the surface. It is extremely important that we quantify any variation in density for each target to be used if we want to successfully compare results with simulations. For this reason, we have set up two soft x-ray sources, at 2.3 and 5.4~keV, to characterize the density of our foams by measuring the transmission through the sample. Our flexible setup, which will be described, should allow us to determine the density of a sample to within 2\%. [Preview Abstract] |
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PP9.00075: Manipulating nonstationary plasmas through wave-particle interactions Paul Schmit, Ilya Dodin, Nathaniel Fisch Through both analytic theory and particle-in-cell simulations, the evolution of linear and nonlinear plasma waves is examined in plasma undergoing nonstationary processes such as compression and expansion. Plasmon conservation results in gradual amplification of linear wave energy in plasmas compressed longitudinally, while nonlinear waves embedded in compressing plasma are found to demonstrate even stronger amplification than linear waves. Different collisionless damping mechanisms for linear and nonlinear waves in nonstationary plasmas result similarly in the sudden switch-like conversion of wave energy into kinetic energy of fast particles. The new phenomenology encountered is shown to produce a number of unique and potentially useful effects, like the sudden selective heating of the plasma, the sudden generation of electrical current and magnetic field energy, and the effective reduction of plasma compressibility. [Preview Abstract] |
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PP9.00076: Using Monoenergetic Proton Radiography to Probe Magnetic Reconnection of Laser-Generated Plasma Bubbles M. Rosenberg, C. Li, F.H. Seguin, J. Frenje, M. Manuel, R. Petrasso, C. Stoeckl, V. Glebov, J. Kilkenny, A. Nikroo The magnetic topology changes due to reconnection of laser-generated magnetic fields, which can affect energy transport inside hohlraums and around inertial confinement fusion (ICF) capsule implosions. To probe and better understand this effect, a series of experiments were conducted in which the interaction of azimuthal magnetic fields surrounding laser-produced plasma bubbles was imaged using face-on monoenergetic proton radiography. The timing of the interaction beams with respect to each other and to the backlighter was varied to provide snapshots of the reconnection both during and after the laser-foil interaction. Images and quantitative field-mapping demonstrate a change in magnetic topology during the bubble interaction. Future work will entail side-on radiography of interacting plasma bubbles to probe for Hall fields associated with fast magnetic reconnection. This work was performed at the LLE NLUF, and was supported in part by the US DOE, LLNL, and LLE. [Preview Abstract] |
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PP9.00077: Adiabatic compression and expansion of a plasma jet Chiping Chen The rms envelope equation of a plasma jet has been developed to determine the profiles of plasma jets undergoing adiabatic compression and expansion. Recently, experimental data have become available for comparison between theory and experiment. In a comparison with the fast pressure probe measurements [S. Messer, A. Case, R. Bomgardner, M. Phillips, and F.D. Witherspoon, Phys. Plasmas \textbf{16}, 064502 (2009)] in the Plasma Jet Driven Magnetoinertial Fusion (PJMIF) experiment at HyperV Technologies Corporation [F.D. Witherspoon, A. Case, S.J. Messer, R. Bomgardner II, M. W. Phillips, S. Brockington, R. Elton, Rev. Sci. Instrum. \textbf{80}, 083506 (2009)], good agreement is found between the rms envelope theory and the PJMIF experiment performed at HyperV Technologies Corporation. Although the plasma jet underwent adiabatic expansion in the PJMIF experiment at HyperV Technologies Corporation, an important implication of the rms envelope theory is that a plasma jet can be compressed adiabatically, resulting in an increase in the energy density of the plasma jet. [Preview Abstract] |
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PP9.00078: Rarefaction Waves in Van der Waals Fluids Albert Yuen, John Barnard, Richard More As the simplest description of material that exhibits a liquid- vapor two-phase state, the Van der Waals' fluid model can be used to obtain qualitative (and sometimes quantitative) information about the fluid dynamics of material in the two-phase regime. We apply the general one-dimensional self-similar solution of a rarefaction wave in an initially semi-infinite liquid, uniform in temperature and density, to the specific case of a Van der Waals' fluid. We obtain a set of profiles for the fluid density, temperature and velocity, that describes the fluid for a wide range of space, time, initial conditions, and Van der Waals' parameters. These results may be used to interpret experiments in which a material is rapidly isochorically heated. In particular, ``plateaus'' in temperature and density as a function of position are observed characterizing entrance into the two-phase regime. [Preview Abstract] |
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PP9.00079: Electrical conductivity and equation of state data from Ohmically exploded tamped planar foils E.L. Ruden, D.J. Amdahl, R.H. Cooksey, T.P. Fleming, F.T. Analla, D.J. Brown, M.R. Kostora, J.F. Camacho Design details and preliminary results are presented for an effort to produce homogeneous Warm Dense Matter (WDM) in a regime characteristic of single-shot pulsed power devices, and diagnose dynamic surface conditions to infer intrinsic bulk properties. The WDM is produced by Ohmically exploding a metal foil placed between transparent tampers using a 36~$\mu $F, 40~kV capacitor bank. Electrical conductivity, pressure, density, specific energy, and temperature time histories are inferred from a combination of pyrometer, polarimeter, and photonic doppler velocimeter measurements of the foil surface, and a B-dot probe array around the foil. 3-D MHD simulations of the overall dynamics and diagnostic response, which include the results of quantum many body simulations of tamper properties, facilitate interpretation. The ultimate goal is to validate theoretical ab initio models of matter encountered in single-shot pulsed power devices. [Preview Abstract] |
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PP9.00080: Measurements of Sound Velocity of Diamond at the Pressure Around the Melt K. Shigemori, K. Shimizu, Y. Asakura, D. Hayashi, Y. Nakamoto, T. Kagayama, T. Sakaiya, T. Kondo, H. Sumiya, T. Irifune, K. Kurosawa, S. Sugita, T. Kadono, Y. Hironaka We have measured the sound velocity of the diamond foils at around the melting pressures (500 -- 1500 GPa). Experiments were done on GEKKO-XII glass laser system with HIPER irradiation facility. Single crystal diamond foils (Ia) of 30$\sim $40 $\mu $m thickness were irradiated at intensities of 0.2 -- 1.5 $\times$ 10$^{14}$ W/cm$^{2}$. We measured the sound velocity by side-on x-ray backlighting technique. Trajectories of foil surfaces were observed by x-ray streak camera. We also measured the shock velocity by two VISARs (velocity interferometer system for any reflector), and shocked temperature by an SOP (streaked optical pyrometer) in order to determine the pressure and the temperature at around the melting. Experimental data were compared with the previous theoretical studies and the percolation theory. [Preview Abstract] |
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PP9.00081: Modeling of EOS and Ion Coupling Experiments on NDCX-II* J.J. Barnard, F.M. Bieniosek, A. Friedman, E. Henestroza, B.G. Logan, W. Liu, R.M. More, P.A. Ni, A. Yuen, N.A. Tahir The Neutralized Drift Compression Experiment II (NDCX-II) is an induction accelerator currently under construction at LBNL, scheduled for completion by March, 2012. The baseline design for NDCX-II will accelerate $\sim $0.03 $\mu $C of singly charged Li ions to 1.2 MeV (with planned upgrades up to 3.1 MeV), delivered in sub-ns pulses with sub-mm rms beam radii. The beam is predicted to heat metal foils several microns thick in a timescale comparable to the hydrodynamic expansion timescale of the target for experiments that infer material properties from measurements of the rarefaction wave. Experiments using metallic foam targets several tens of microns thick will infer ion energy coupling into kinetic energy of fluid flow. Geometries with multiple layers may be used to adiabatically compress target materials. We have carried out detailed hydrodynamic simulations of targets for several configurations, exploring how optical intensity measurements (from IR to UV), laser doppler measurements (VISAR), and X-ray density measurements can be used to distinguish EOS, and measure beam energy coupling in ion driven shock experiments. *Work performed under the auspices of the U.S. DOE under contract DE-AC52-07NA27344 at LLNL, and UC contract DE-AC02-05CH11231 at LBNL. [Preview Abstract] |
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PP9.00082: Modeling droplet breakup effects with diffuse interface methods in ALE-AMR code with application in modeling NDCX-II experiments Wangyi Liu, John Barnard, Alex Friedman, Nathan Masters, Aaron Fisher, Alice Koniges, David Eder In this paper we describe an implementation of a single-fluid diffuse interface model in the ALE-AMR hydrodynamics code to simulate surface tension effects. This model works for 2D and 3D. We show simulations and compare them to other surface tension models. We benchmark this code against analytic models that incorporate surface tension (showing agreement with Laplace's equation describing the pressure difference between the interior and exterior of a droplet, for example). We also show how this simulation can be used for modeling the NDCX-II ion beam heated target experiments planned to begin in 2012. [Preview Abstract] |
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PP9.00083: A possible 12C(n,2n)11C total cross section measurement Andrew Evans, Keith Mann, Mark Yuly Tertiary neutron production can be used as an indicator of the burn fraction of a deuterium-tritium pellet in inertial confinement fusion reactions. One way to monitor tertiary neutrons is by carbon activation using the 12C(n,2n)11C reaction, which has a threshold of 20.3 MeV and so is insensitive to primary neutrons produced in the DT reaction. However, the cross section for this reaction is not well known. Several different experimental techniques for measuring 12C(n,2n) have been examined, with an activation experiment being the most feasible. [Preview Abstract] |
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PP9.00084: Radiative diagnostics for sub-Larmor scale magnetic turbulence S.J. Reynolds, M.V. Medvedev The radiation produced by particles moving through small-scale magnetic field turbulence, such as may be produced in laser-plasma instabilities, relativistic collisionless shocks, and reconnection outflow, can be used to analyze the features of the magnetic field distribution in such regions. Particles propagating through such turbulence encounter locally strong magnetic fields, but over lengths much shorter than a particle gyroradius (that is, $lambda_B \ll \gamma mv/q \langle B_\perp\rangle$). Consequently, the particle is accelerated but not deviated substantially from a straight line path. We develop the general jitter radiation solutions for this case and show that the resulting radiation is directly dependent upon the spectral distribution of the magnetic field through which the particle propagates. We demonstrate the power of this approach in considering the radiation produced by particles moving through a region in which a (Weibel-like) filamentation instability grows magnetic fields randomly oriented in a plane transverse to counterstreaming particle populations. We calculate the spectrum as would be seen from the original particle population and as could be seen by using a quasi-monoenergetic electron beam to probe the turbulent region at various angles to the filamentation axis. [Preview Abstract] |
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PP9.00085: Electron density and temperature measurements of laser-produced plasmas from electron-feature Thomson-Scattering Jena Meinecke, Bradley Pollock, Chris Stoafer, James Ross, Laurent Divol, George Tynan, Siegfried Glenzer We present a comparison of the electron temperature and density from electron plasma wave Thomson-Scattering measured for foil targets in laser-produced plasmas. The experiments were performed at the Jupiter Laser Facility using a 2ns square, 1054 nm, 300J laser focused onto CH and Al thin foil targets. A 200ps, 527 (2$\omega )$, 45J laser pulse was used as the Thomson scattering probe for spatial and temporal resolution. The plasma conditions were measured from 0.1 to 1 mm from the foil surface with a spatial resolution of 15$\mu $m. The densities of the blowoff plasmas were about the same for each foil target which was independently measured at the Omega Laser Facility with a 1ns, 3$\omega $, 500J drive laser pulse. Measurements were made 500 $\mu $m from CH and V foil targets using a 1ns, 4$\omega $, 200J scattering probe. These measurements will be compared to hydrodynamic simulations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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PP9.00086: High Resolution Shadowgraphy and Interferometry at 266nm for 1-MA Z-Pinches Sara Altemara, Vladimir Ivanov, Daniel Papp, Austin Anderson, Alexey Astanovitskiy A UV laser probing channel at the wavelength of 266 nm was implemented for investigations of wire-array Z-pinches at the Zebra generator. The UV channel has provided images of the dense plasma inside the lower-density trailing plasma, allowing the observation of details and structures in the pinch. Kink instabilities, loops, areas of disruption and micro-pinching were observed in Z-pinches. A high spatial resolution UV shadowgraphy and interferometry channel allows for investigation of the micron-scale instabilities. Experiments with high resolution shadowgraphy and interferometry are presented. The fine structure of the stagnated Z-pinches in cylindrical and star wire arrays was unfolded, and new information about instabilities was obtained. A two-frame UV shadowgraphy was tested for investigation of plasma dynamics. Shadowgraphy results show a very inhomogeneous pinch with instabilities on the microscopic level, strong kink instabilitiy, and micro-pinches. Interferometery results show plasma gradients on the edge of the pinch and a density of the trailing mass. [Preview Abstract] |
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PP9.00087: On Optimizing K-Shell X-ray Conversion Efficiencies with New Nano-structured Laser Targets Jeffrey Colvin, Supakit Charnvanichborikarn, Tom Felter, Chad Flores, Kevin Fournier, Dustin Gilbert, Sergei Kucheyev, Kai Liu We have begun developing new nano-fabrication techniques to make suitable laser targets that can form highly uniform high-Z non-LTE plasmas when illuminated by high-intensity laser light. In this presentation we first discuss progress in developing very low-density pure Cu foams via a four-step ion-lithography process and progress in mechanically trapping Cu nanowires in a silica aerogel foam. We then discuss the radiation-hydrodynamics and non-LTE atomic physics modeling of various targets we could assemble from these foams, and what the modeling reveals about how best to optimize Cu K-shell x-ray conversion efficiencies via target design. [Preview Abstract] |
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PP9.00088: ABSTRACT WITHDRAWN |
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PP9.00089: Upgrade of a Theta Pinch Plasma Source for Energetic Plasma Flow Generation Soonwook Jung, Daniel Andruczyk, David Ruzic DEVeX is a theta pinch device used to investigate fusion-related material interaction such as vapor shielding and ICRF antenna interactions with plasma-pulses in a laboratory setting. However, recent upgrades of the device indicate that guiding magnet and crowbar operation hinder plasma flow to the target when the magnetic field at the theta coil is reversed from that of the guiding magnet. This reversal occurs mainly due to finite inductance and resistance at the crowbar switch. Therefore, an upgrade for more suitable fusion-related material study is required. In this study, several upgrades to produce higher-temperature plasma have been carried out. Major modification of theta coil is carried out and its effects on plasma parameters are theoretically predicted with simulation. The results will be compared with experiments including voltage/current measurement at the coil, plasma parameter measurement with triple Langmuir probe and time of flight technique, and incident energy measurement with thermocouples. [Preview Abstract] |
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PP9.00090: Optimization of Shielded Scintillator for Neutron Detection Patrick Belancourt, John Morrison, Kramer Akli, Richard Freeman The High Energy Density Physics group is interested in the basic science of creating a neutron and gamma ray source. The neutrons and gamma rays are produced by accelerating ions via a laser into a target and creating fusion neutrons and gamma rays. A scintillator and photomultiplier tube will be used to detect these neutrons. Neutrons and photons produce ionizing radiation in the scintillator which then activates metastable states. These metastable states have both short and long decay rates. The initial photon count is orders of magnitude higher than the neutron count and poses problems for accurately detecting the neutrons due to the long decay state that is activated by the photons. The effects of adding lead shielding on the temporal response and signal level of the neutron detector will be studied in an effort to minimize the photon count without significant reduction to the temporal resolution of the detector. MCNP5 will be used to find the temporal response and energy deposition into the scintillator by adding lead shielding. Results from the simulations will be shown. Optimization of our scintillator neutron detection system is needed to resolve the neutron energies and neutron count of a novel neutron and gamma ray source. [Preview Abstract] |
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PP9.00091: Advances in compact proton spectrometers for diagnosing ICF experiments F.H. Seguin, N. Sinenian, M. Manuel, H.G. Rinderknecht, M. Rosenberg, A. Zylstra, J. Frenje, C.K. Li, R. Petrasso, S. Roberts, T.C. Sangster The compact proton spectrometer (or WRF, for Wedge-Range-Filter proton spectrometer) measures the spectra of protons in the energy range $\sim $ 3 to 20 MeV for diagnosing ICF experiments. It utilizes CR-39 for detecting individual protons and their energies, after they pass through a ranging filter with a continuously varying thickness, and appropriate algorithms for reconstructing the incident spectrum. It has now been in use for a decade at OMEGA, and is currently being used at the NIF, for measuring spectra of primary D$^{3}$He protons in D$^{3}$He implosions, secondary D$^{3}$He protons in DD implosions, and ablator protons in DT implosions. These spectra are used to determine proton yields, shell areal density at shock-bang time and compression-bang time, fuel areal density, and implosion symmetry. During the decade of use there have been significant changes in fabrication and in analysis algorithms. An overview will be given here of the historical development, current analysis methods, and measurement accuracy. This work was supported in part by DOE and LLE. [Preview Abstract] |
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PP9.00092: Detecting charged fusion products in high-fluence conditions on OMEGA and the NIF C. Waugh, M. Rosenberg, A. Zylstra, H. Rinderknecht, N. Sinenian, M. Manuel, D. Casey, M. Gatu Johnson, C.K. Li, F. Seguin, J. Frenje, R. Petrasso, V. Glebov, T.C. Sangster, S. Pape, R. Bionta, A. Mackinnon, O. Landen, Y. Kim, H. Hermann, J. Kilkenny, A. Nikroo CR-39 solid state nuclear track plastic, used as charged particle detectors on the ``back-end'' of OMEGA and NIF diagnostics/spectrometers, is ideally suited to record particle fluences up to $\sim $ 3x10$^{4}$ / cm$^{2}$. However, conditions on OMEGA and the NIF can often result in fluences two orders of magnitude greater. By using shorter etch times than the standard (6 hrs), and cross calibrating to CR39 shot on the MIT accelerator to the equivalent (ICF) fluence, the dynamic range of the CR39 can be significantly extended. Specific examples of this analysis from both OMEGA and the NIF will be presented for the case of D$^{3}$He exploding pushers. This work was supported in part by LLE, the NLUF, the FSC, the US DOE, LLNL, and GA. [Preview Abstract] |
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PP9.00093: New Imaging Geometry for Measuring Ion Temperatures with a Crystal Spectrometer on NIF P. Beiersdorfer, E. Wang, A. Graf, M. Schneider, R. Shepherd, M. Bitter, K.W. Hill Measurements of the ion temperature utilizing very high-resolution crystal spectrometers to resolve the Doppler broadening of x-ray lines of high-Z dopants embedded in laser-heated plasma typically suffer from a lack of photons. This lack requires limiting (or even eliminating) time resolution or increasing the dopant amount to a non-perturbative level. Here we present a new measurement geometry that increases the photon density on the detector by at least two orders of magnitude over standard geometries. The increase in the photon flux per detector resolution element is achieved by using a spherically bent crystal and placing the source at the sagittal focus. A crystal spectrometer based on this geometry can, in principle, measure the ion temperature of a laser-heated capsule on NIF with picosecond temporal resolution and a low (<0.01\%) amount of dopant (Kr). The geometry also provides imaging, albeit with demagnification. Part of this work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA-27344 and by PPPL under Contract DE-ACO2-76-CHO-3073. [Preview Abstract] |
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PP9.00094: Optimization of Neutron Activation of Carbon at the NIF S. Padalino, D. Polsin, M. Russ, T. Sangster To determine the rhoR of ignition scale targets at the NIF, a carbon activation diagnostic is being developed to measure tertiary neutron yield. It has been shown theoretically that the ratio of the tertiary yield to the primary yield is directly related to rhoR and is nearly independent of hot-spot electron temperature. Due to carbon's 20.3 MeV reaction threshold, it is insensitive to 14.7 MeV primary neutrons which are measured by other means and allows for an unambiguous determination of the tertiary to primary ratio. The energy distribution of the 20 to 30 MeV DT neutrons folded with the (n,2n) cross section in this energy region determines the degree in which carbon will be activated. However, the published 12C(n,2n) cross sections in this energy range are bifurcated. To set upper and lower limits on the sensitivity of the activation diagnostic, a finite element calculation was used to determine the limits of the method's usefulness at differing primary yields and solid angles for the NIF chamber. It was further used to verify MCNPX activation calculations. This work was funded in part by the USDOE through LLE. [Preview Abstract] |
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PP9.00095: Multiple Monochromatic Imaging (MMI) Status and Plans for LANL Campaigns on Omega and NIF F.J. Wysocki, S.C. Hsu, I.L. Tregillis, M.J. Schmitt, G.A. Kyrala, D.D. Martinson, T.J. Murphy, R.C. Mancini, T. Nagayama LANL's DIME (Defect Implosion Experiment) campaigns on Omega and NIF are aimed at obtaining improved understanding of defect-induced mix via experiments and simulations of directly driven high-Z doped plastic capsules with DD or DT gas fill. To this end, the MMI diagnostic has been identified as a key diagnostic for providing space and time-resolved density, temperature, and mix profiles. The high Z shell dopants used on Omega are Ti and V, and to be used on NIF are Ge and Se. This poster will discuss the following four areas of MMI-related work at LANL, in collaboration with UNR: (1) data and preliminary analysis of MMI data from FY11 Omega campaigns, (2) development of a capability to generate simulated MMI data from radiation- hydrodynamic simulations of ICF implosions, (3) design of an MMI instrument for NIF that will cover the photon energy range 9.5-16.9 keV which includes the Ge/Se, H- like/He-like, $\alpha$/$\beta$ lines, and (4) the development of MMI data post- processing and spectroscopic analysis tools. [Preview Abstract] |
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PP9.00096: A versatile facility for advanced diagnostics development for OMEGA, Z and the NIF D.T. Casey, N. Sinenian, M. Manuel, M. Rosenberg, A. Zylstra, H.G. Rinderknecht, C. Waugh, H. Sio, M. Gatu Johnson, J. Frenje, C.K. Li, F.H. Seguin, R. Petrasso, R. Leeper, C.L. Ruiz, T.C. Sangster The MIT Linear Electrostatic Accelerator generates D-D and D-3He fusion products for the development of nuclear diagnostics for OMEGA, Z, and the NIF. Fusion reaction rates as high as 10$^{7}$ s$^{-1}$ are achieved with a new ion source and gas control system. The fluence and energy of the fusion products has been accurately characterized to allow for the development of new nuclear diagnostics on OMEGA, Z and the NIF. In-situ measurements of the on-target beam profile are used to determine the metrology of the fusion products source for particle counting applications. In addition, neutron diagnostics development is facilitated by detailed MCNP simulations used to correct for scattering within the system. These recent improvements have resulted in a versatile platform suitable for advanced diagnostics development. This work was supported in part by SNL, DOE, LLE and LLNL. [Preview Abstract] |
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PP9.00097: Ion temperature measurements in shock-driven implosions on OMEGA and the NIF H. Sio, A. Zylstra, M. Rosenberg, C. Waugh, H. Rinderknecht, N. Sinenian, M. Manuel, D. Casey, M. Gatu Johnson, C.K. Li, F. Seguin, J. Frenje, R. Petrasso, V.Yu. Glebov, P.B. Radha, J. Delettrez, P. Mckenty, C. Stoeckl, T.C. Sangster, S. Pape, R. Bionta, A. Mackinnon, O. Landen, J. Kilkenny, A. Nikroo Y. KIM, H. HERMANN, LANL Shock-driven ``exploding pusher'' implosions are commonly used at the OMEGA and the NIF facilities as test platforms for calibrating and validating diagnostics. We present extensive data on temperatures in exploding pushers obtained through methods: measurement of Doppler broadening of fusion products (from both DD and D$^{3}$He reactions), and measurement of temperature sensitive yield ratios of DD and D$^{3}$He yields. Since burn-averaged nuclear observables depend on density and temperature gradients and the time evolution after the spherical shock collapse, it is not evident a priori that these methods measure the same Ti. We compare experimental results to both radiation hydrodynamics simulations and an analytic Guderley shock model. This work was supported in part by LLE, the NLUF, the FSC, the US DOE, LLNL, and GA. [Preview Abstract] |
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PP9.00098: Dilation X-ray Imager (DIXI): A Sub-10ps \mbox{X-ray} Framing Camera for the NIF T.J. Hilsabeck, J.D. Kilkennny, T. Chung, B.S. Sammuli, J.D. Hares, A.K.L. Dymoke-Bradshaw, P.M. Bell, D.K. Bradley, S.R. Nagel We have constructed a microchannel plate based \mbox{x-ray} framing camera which utilizes pulse-dilation technology [1] to achieve temporal resolution below 10~ps. The design is suitable for fielding at the National Ignition Facility and can operate in a high yield neutron environment. Here, we present the instrument design specifications and construction details along with data from calibration experiments performed with femtosecond laser pulses in the ultra-violet. We will also discuss the capabilities of pulse-dilation imaging and further applications in high energy density plasma physics experimentation.\par \vskip6pt \noindent [1] T.J.\ Hilsabeck, et al., Rev.\ Sci.\ Instrum.\ {\bf 81}, 10E317 (2010). [Preview Abstract] |
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PP9.00099: Bang Time and Burn Width Analysis at the NIF E. Grafil, H.W. Herrmann, W. Stoeffl, Y. Kim, N.M. Hoffman, C.S Young, J.M. Mack, P.W. Watts, A.C. Carpenter, J. Church, L. Bernstein, J. Liebman, M. Rubery, C.J. Horsefield, E.K. Miller The time of peak fusion reactivity with respect to the impingement of laser light on an Inertial Confinement Fusion capsule is known as Nuclear Bang Time (BT). The width of this peak is the Nuclear Burn Width (BW). Accurately measuring BT \& BW is essential for constraining hydrodynamic simulations as it is a measure of energy coupling to the target. BT \& BW measurements have been performed using a variety of instruments at the National Ignition Facility (NIF). The Gamma Reaction History (GRH) diagnostic is designed to measure fusion gamma-rays in determining BT \& BW to within 30 ps precision and accuracy. The GRH consists of four Gas Cherenkov Detectors having variable energy-thresholding capability. For BT measurements, the threshold is typically set $>$8 MeV so as to isolate DT fusion gamma-rays at 16.75 MeV while thresholding out the lower energy (n,n') gammas. The GRH experimental setup, data and comparison to BT \& BW from other diagnostics methods are presented. [Preview Abstract] |
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PP9.00100: Gamma Reaction History Backgrounds at the NIF J.A. Church, W.S. Stoeffl, P.W. Watts, A.C. Carpenter, J. Liebman, H.W. Herrmann, Y.H. Kim, E. Grafil The Gamma Reaction History (GRH) diagnostic at NIF detects gamma-rays, emitted directly from DT fusion reactions (DT$\gamma )$, through the use of four Gas Cherenkov detectors with adjustable gamma-ray energy thresholds. It is primarily used to determine bang time, burn width and total DT yield of the implosion. Background interference to the signal is insignificant when capsules are driven directly by the lasers, but can be significant during indirect-drive using a hohlraum, forming an approximately 20 ns plateau under the narrow $\sim $200 ps FWHM fusion signal. This background is independent of fusion yield and most likely the result of laser-plasma interaction (LPI) induced hot electron bremsstrahlung radiation. These hard x-rays stream out target chamber ports and take multiple scatter paths to reach the GRH photomultiplier tubes (PMT), where they then bypass the Cherenkov conversion process and generate signal by direct interaction with the PMT microchannel plates. An examination of this background contribution to the GRH signal and possible mitigation strategies will be presented. [Preview Abstract] |
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PP9.00101: Neutron imaging development for ICF experiments Olivier Landoas, T. Caillaud, B. Rosse, M. Briat, I. Thfoin, S. Kime, S. Laffite, F. Philippe, J.L. Bourgade, L. Disdier, V. Yu Glebov, F.J. Marshall, T.C. Sangster Various failure mechanisms may limit the fuel compression during Inertial Confinement Fusion experiments with MegaJoule class lasers. Implosions asymmetries can be observed using a Neutron Imaging System (NIS) which can record both a primary (14MeV) and a down-scattered (5-10 MeV) neutron image with significant spatial resolution ($<$10$\mu $m) and Signal to Noise Ratio (SNR). For 10 years, we have developed the LMJ NIS diagnostic using penumbral or annular coded aperture technique. The annular imaging technique demonstrated to maintain a large SNR but this technique is very sensitive to misalignment ($<$50$\mu $m) compared to the penumbral aperture. A new alignment method was used in 2011 on OMEGA allowing the on-line replacement from the penumbral to the annular aperture. Spherical, prolate and oblate 14 MeV neutron core images were obtained by varying the laser energy balance and compared to X-ray images and simulations. The SNR is in the 40-50 range and the system resolution is 15$\mu $m with the large neutron camera (LMJ scaled) placed at 13m from target. Because the resolution contribution is equally limited by detector and aperture, neutron images can be corrected from the scintillator Point Spread Function (PSF) to decrease the system resolution down to about 10$\mu $m. [Preview Abstract] |
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PP9.00102: PINCHES, PLASMA FOCUS, CRASH |
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PP9.00103: PIC Simulations of Dense Plasma Focus Z-pinch A. Schmidt, D. Blackfield, V. Tang, D. Welch, D. Rose Dense Plasma Focus (DPF) Z-pinches are abundant sources of radiation, including neutrons, x-rays, and energetic electron and ion beams. Energetic protons and deuterons up to 10 MeV have been observed from cm-scale-length pinches, implying average acceleration gradients up to 1 GV/m. Gradients of this magnitude could potentially be exploited in the design of a compact accelerator. However, the physical mechanisms behind these immense electric fields are not well understood and thus DPF design cannot currently be optimized to maximize these gradients. At LLNL, we have assembled a DPF Z-pinch experiment and will be using a 4 MV ion probe beam to directly measure pinch-induced gradients. LSP, a fully relativistic electromagnetic Particle-In-Cell (PIC) code is used to perform time-dependent simulations of the pinch phase of the DPF and to gain insight into the origin and evolution of the large accelerating fields. LSP can be used in 2D or 3D geometries and can model the ions kinetically with fluid electrons (hybrid model) or model both species kinetically (fully kinetic model). We present results from both pressure and sheath width scans using LSP. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Laboratory Directed Research and Development Program (11-ERD-063) at LLNL. [Preview Abstract] |
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PP9.00104: Cavitation and hollow foam-like structure formation in the dense core of exploded wire S.A. Pikuz, T.A. Shelkovenko, P.F. Knapp, C.L. Hoyt, D.A. Hammer, V.V. Zhakhovsky Complex hollow foam-like structures in the exploded wire cores have been observed with the help of high resolution x-ray radiography and x-ray K-shell absorption spectroscopy with single and multi-wire loads in the load current range from 2 kA/wire to 250 kA/wire. The processes of hollow structures formation have been studied depending on wire material, load configuration and discharge parameters. Large-scale molecular-dynamics (MD) simulations of aluminum and nickel wires are being carried out to try to develop an understanding of the observed structures. Density profiles in the simulations show qualitative agreement with the experimental X-ray radiographs in both single wire explosions and the explosions of twisted wire pairs. The MD simulations predict that the exploding wire cores should experience cavitation and the formation of vapor bubbles within the liquid phase. [Preview Abstract] |
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PP9.00105: Improved Non-Local Radiation Coupling Model for Mach2-TCRE Joseph Thornhill, John Giuliani, Young Chong, John Apruzese Mach2-TCRE is a 2D (R-Z) radiation-MHD code with non-LTE population kinetics and radiation transport using ray tracing. The code is designed to simulate the plasma dynamics and equation of state of high temperature K-shell, radiating Z-pinch plasmas. In the past, the radiation coupling that occurs between remote plasma regions was treated within the ``on-the-spot-approximation (OTS)'',\footnote{D. E. Osterbrock, Astrophysics of Gaseous Nebulae (Freeman, San Francisco, 1974).} i.e., if a photon is absorbed anywhere in the plasma it is assumed to be absorbed in the emitting zone. While this simplifies the energy accounting it does not properly treat the energy transfer between different regions of the pinch. Here we extend the radiation transport capabilities of MACH2-TCRE by following the zone-to-zone radiation coupling using an escape probability method and account for plasma heating by the non-local radiation field. Verification of the new approach is made through 1D comparisons between this method, the OTS approximation, and a full radiation transport calculation. [Preview Abstract] |
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PP9.00106: Stagnation of a Gas Puff Z Pinch Henry Strauss Two dimensional MHD computer simulations of the stagnation of a gas puff Z pinch were carried out using an adaptation of the M3D code [1]. The implosion of the Z pinch is driven by a magnetic piston. The piston front is Rayleigh Taylor unstable. Ahead of the magnetic piston is a shock wave, which is stable. The laminar, stable shock reaches the geometric axis and reflects. The high density core plasma expands until it collides with the incoming magnetic piston at the stagnation radius. Thereafter the plasma column contracts, and turbulent motion is transferred to the plasma column, causing it to break up. The kinetic energy in the plasma column decays on the time scale in which the plasma expands to the stagnation radius. Stagnation occurs for $\beta \le 1,$ where $\beta$ is the ratio of plasma pressure in the column to magnetic pressure outside the column. A simple radiation model is introduced, and the dependence of stagnation radius on radiation and $\beta$ is determined. Recent measurements [2] are consistent with these results. \\[4pt] [1] W. Park, E.V. Belova, G.Y. Fu, X. Tang, H.R. Strauss, L.E. Sugiyama, Phys. Plasmas 6, 1796 (1999). \\[0pt] [2] E. Kroupp, D. Osin, A. Starobinets, V. Fisher, V. Bernshtam, Y. Maron, I. Uschmann, E. F\"orster, A. Fisher, and C. Deeney, Phys. Rev. Lett. 98, 115001 (2007). [Preview Abstract] |
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PP9.00107: Magneto Rayleigh Taylor Diagnostic Experiments on a MA-LTD David Chalenski, Ronal Gilgenbach, Jacob Zier, Yue Ying Lau, Sonal Patel, Adam Steiner, Andrew McKelvey, Nicholas Holmes Recent work on the 1-MA Michigan Linear Transformer Driver, MAIZE, has focused on the Magneto Rayleigh-Taylor (MRT) instability and validation of analytic theory, developed at UM. MAIZE is a nominal 1-MA, 100 ns, 100 kV driver, capable of driving 0.1 $\Omega $ matched loads. We present here the continuing results of diagnostic development on experiments on planar and pseudo-planar foils. Some of the results will include various techniques used to seed the MRT instability on the foil. This work was conducted on 400-nm thick, 1-cm wide aluminum foils placed between two planar or pseudo-planar current return plates. The driver charge was limited to $\pm $70 kV, giving $\sim $700 kA with a risetime of $\sim $180 ns. Experiments were performed employing various methods to seed the MRT instability on either the foil (cathode) or return current plates (anode). Cathode seeding was performed by imposing a periodic ripple in the foil. Anode seeding was performed by using electrodes with a periodic structure machined into them. The progress of these experiments is presented here. Analysis of MRT was derived from laser shadowgraphic images, obtained using a sub-ns, frequency-doubled Nd:YAG laser. [Preview Abstract] |
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PP9.00108: Spectroscopic Analysis of First Experiments of Al Planar Foils and Single Planar Wire Arrays on Zebra at UNR* H.A. Zunino, M.E. Weller, A. Stafford, A.S. Safronova, V.I. Kantsyrev, A.S. Chuvatin, I. Shrestha, G.C. Osborne, V.V. Shlyaptseva, S.F. Keim A spectroscopic analysis of experiments with Al planar foils and single planar wire arrays (SPWA) performed on the Zebra generator is presented. Both types of experiments were performed at standard (1.0MA) and enhanced (1.7MA) currents each with the required masses to attain implosion close to peak current. As a general trend, the loads performed at enhanced current are shown to have nearly double the energy output than loads performed at standard current. Data from a full diagnostic set was analyzed with major focus on x-ray spectroscopic data. A non-LTE kinetic model was used to derive plasma parameters of electron temperature and density and estimate opacity effects. Similarities and differences in K-shell spectroscopic features radiated from Al foils and SPWAs are identified and discussed. [Preview Abstract] |
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PP9.00109: Decomposition of neutron signals into separate pulses and estimation of the proportion of scattered neutrons at PF-1000 Jiri Hitschfel, P. Kubes, D. Klir, K. Rezac, M. Paduch, M. Scholz On the plasma focus facility PF-1000 at the deuterium filling the hard x-rays and neutrons from the DD fusion reaction were registered with scintillation detectors in axial and radial directions. The signals of neutrons from the plasma are generated in a few pulses and each pulse contains neutrons going directly, neutrons scattered from the components of chamber and objects out of chamber. This work deals with a separation of neutron signals into individual pulses, determining of the proportion of slowed and delayed neutrons and calculation the mean energy of neutrons and deuterons. The components of mean energy of neutrons were determined from the difference of signals registered downstream, side-on and upstream, with assumption of contemporary of hard x-ray and neutron production. The components of mean energy of deuterons were calculated from mean energy of neutrons. [Preview Abstract] |
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PP9.00110: Hybrid X pinches T.A. Shelkovenko, S.A. Pikuz, P.F. Knapp, A.D. Cahill, C.L. Hoyt, D.A. Hammer, S.N. Mishin, A.R. Mingaleev, V.M. Romanova, I.N. Tilikin, A.E. Ter-Oganesyan A hybrid X-pinch configuration consisting of solid conical electrodes connected by a wire has been tested on four different generators with currents varying from 270 kA to 1 MA and risetimes ranging from 50 ns to 170 ns. Wires of different materials were loaded through holes in the cones; wire lengths were varied from 0.6 to 2 mm. It was possible to optimize the wire material, length and diameter so that most of these hybrid X pinches generated an intense single burst of soft x-rays with energy yield comparable with the one in standard X pinches. In such cases, the single hot spot that was of micron-scale size. Hybrid X pinches generate less hard x-ray intensity than standard X pinches. Early stage of the wire explosion in the hybrid X pinches was studied. [Preview Abstract] |
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PP9.00111: Neutron Diagnostics of a Deuterium Gas-Puff Z-pinch on the Level of 3 MA Karel Rezac, Daniel Klir, Pavel Kubes, Jozef Kravarik, Alexander Shishlov, Aleksey Labetsky, Nicolai Ratakhin The diagnostics of a deuterium gas-puff Z-pinch (outer shell with diam. of 100 or 80 mm, inner annular with diam. of 30 mm or solid-fill shell with diam. of 20 mm with linear mass varied in each shell in the range of 25 -- 40 $\mu$g/cm) is presented. The experiments were carried out on the GIT-12 generator at IHCE in Tomsk (2.5 MJ bank energy, load current of 2.8 MA with the rise time of 250 ns) during the April-May campaign in 2011. Results from the neutron time-of-flight diagnostics including the determination of the neutron production time and reconstructed radial energy spectra are shown. Several methods which provided measurement of the total neutron yield indicated the number of neutrons in order of $10^{11}$ per one shot. The time correlations with other diagnostics such as electrical characteristics, soft X-rays, hard X-rays and a visible streak camera are also presented. [Preview Abstract] |
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PP9.00112: Cylindrical Liner Z-pinch Experiments on MAGPIE Guy Burdiak, Sergey Lebedev, Adam Harvey-Thompson, George Swadling, Francisco Suzuki-Vidal, Philip de Grouchy, Jonathon Skidmore, Gareth Hall, Essa Khoory, Louisa Pickworth, Lee Suttle, Simon Bland The formation of plasma in thin-walled metal tube (liner) experiments on the MAGPIE generator (1.4 MA, 240 ns rise) was investigated using axial and side-on laser probing and self-emission imaging. Liners with a wall thickness comparable to the initial current skin depth allow magnetic field diffusion through the wall, driving a cylindrically converging plasma flow from the inner surface at a speed of approximately 100 km s$^{-1}$. Initial flow has good azimuthal symmetry, with radial striations appearing later. The growth and azimuthal correlation of plasma instabilities on the outer liner surface is also observed, in conditions where no bulk motion of the liner occurs. Finally, the effect upon ablation dynamics of filling the interior of the liner with gas prior to application of the current pulse is presented. [Preview Abstract] |
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PP9.00113: The effect of electron beams on emission line ratios used in spectroscopic diagnostics Nicholas Ouart, John Giuliani, Arati Dasgupta, John Apruzese, Robert Clark Emission line ratios have been used as an effective diagnostic for the determination of the plasma parameters. Prior methods have assumed that the electron distribution function, EDF, is Maxwellian. However, previous experiments have measured electron beams and K$\alpha $ line emission. The K$\alpha $ emission was produced as a result of the electron beams ionizing an inner-shell electron. The presence of such hot electrons suggests that the EDF is simply not Maxwellian and requires a modification in the high energy tail. These high energy electrons can produce doubly excited ions with inner-shell vacancies, alter the ionization kinetics, and furthermore impact line ratios used to determine the plasma parameters. The effect of electron beams on synthetic spectra from a non-LTE copper pinch model will be presented and discussed in light of experimental data. [Preview Abstract] |
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PP9.00114: Spectroscopy of Ablated Aluminum Foil Plasmas Driven by MA-LTD S.G. Patel, J.C. Zier, D.A. Chalenski, R.M. Gilgenbach, A.M. Steiner, Y.Y. Lau Spectroscopic analysis has been performed on Al foil plasmas ablated by the Linear Transformer Driver (LTD) at the University of Michigan. The MAIZE LTD can supply 1- MA, 100 kV pulses with 100 ns risetime into a matched load. The plasma load consisted of a 400 nm thick Al foil (cathode) placed between two, planar, current-return anode posts. An optical fiber was placed 1 cm away from the load; plasma light passed through a 0.75-m optical spectrograph and was gated for 10 ns by an intensified CCD detector. The density of the edge plasma was determined through Stark broadening of the H-alpha line. The Fourier transform was taken of the Voigt profile, which was then used to approximate the density of the Al plasma. This method resulted in a density of approximately 10$^{15}$ cm$^{-3}$ in the outer regions of the Al plasma at peak current. Spectra taken midway in the current rise yielded 1-2 eV plasma temperatures from the slope of the continuum emission. These data will be shown as well as planned future experiments. [Preview Abstract] |
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PP9.00115: Radiation characteristics of Al wire arrays on Z* C.A. Coverdale, D.J. Ampleford, B. Jones, M.E. Cuneo, S. Hansen, C.A. Jennings, N. Moore, S.C. Jones, C. Deeney Analysis of mixed material nested wire array experiments at Z have shown that the inner wire array dominates the hottest regions of the stagnated z pinch. In those experiments, substantial free-bound continuum radiation was observed when Al was fielded on the inner wire array. Experiments with Al (5{\%} Mg) on both wire arrays have also been fielded, with variations in the free-bound continuum observed. These variations appear to be tied to the initial mass and diameter of the wire array. The results presented here will investigate the trends in the measured emission (Al and Mg K-shell and free-bound continuum) and will compare the measured output to more recent Al wire array experimental results on the refurbished Z accelerator. *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. $^{+}$current address: NNSA/DOE Headquarters, Washington D.C. [Preview Abstract] |
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PP9.00116: Ablation and Implosion Dynamics Of Coiled Wire Arrays Gareth Hall, Sergey Lebedev, Simon Bland, Jeremy Chittenden, Francisco Suzuki-Vidal, Philip de Grouchy, Adam Harvey-Thompson, George Swadling, Guy Burdiak, Louisa Pickworth, Essa Khoory, Jonathan Skidmore, Lee Suttle, Nicolas Niasse, Kwek Hiang Coiled arrays, a cylindrical array in which each wire is formed into a single helix, suppress the modulation of ablation at the fundamental wavelength. Instead, ablation flow is modulated at the wavelength of the coil, and arrays with large coil wavelength produce an organized mode of implosion in which the global instability can be controlled. The ablation and implosion dynamics of coiled arrays in this regime were studied using a combination of resistive voltage probes and Faraday rotation. These experiments were carried out on the MAGPIE generator at Imperial College. This research was sponsored by the DOE under Cooperative Agreements DE-F03-02NA00057 and the Imperial College Junior Research Fellowship scheme. [Preview Abstract] |
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PP9.00117: Interaction of radiatively cooled plasma jets with static gas and collimated supersonic gas jets F. Suzuki-Vidal, S.V. Lebedev, J. Skidmore, M. Bocchi, S.N. Bland, G. Burdiak, J.P. Chittenden, P. de Grouchy, G.N. Hall, A.J. Harvey-Thompson, E. Khoory, L.A. Pickworth, L. Suttle, G.F. Swadling, M. Krishnan, R.E. Madden, K.W. Elliot, P.L. Coleman, A. Ciardi The MAGPIE generator (1.4 MA, 240 ns) drives an ablative plasma from a thin Al foil. The convergence of the plasma from the foil on its axis leads to the formation of a supersonic plasma jet, with a typical velocity of $\sim $100 km/s. Jet-ambient interactions are studied either by injecting a highly-collimated, supersonic, cold gas jet above the foil using a nozzle, or by enclosing the foil inside a cell filled with neutral, static gas. In both cases, the plasma jet creates several shock features in the ambient medium. Experimental results from varied ambient parameters such as gas pressure and composition are presented together with 3-D MHD simulations. [Preview Abstract] |
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PP9.00118: New Exact MHD Solutions Describing the Stagnating Z-pinch plasma A.L. Velikovich, J.L. Giuliani, J.W. Thornhill, S.T. Zalesak, Y. Maron, A. Starobinets, E.P. Yu Recent 3D RMHD simulations at Sandia and experiments at Weizmann Institute of Science have demonstrated that axially and azimuthally averaged dynamics of a strongly radiating stagnated Z-pinch column resembles a self-similar, cylindrically symmetric motion. The cold, rapidly imploding plasma transforms into the hot stagnated plasma heated and compressed in the diverging shock wave that propagates from the pinch axis. The simplest analytical solution describing such flow was given by Noh. Here we discuss generalizations of the classical Noh's solution, which take into account the non-uniform density and velocity profiles in the incident plasma, as well as the presence of azimuthal magnetic field in it. These new solutions are found to be surprisingly close to the observations and simulation results. They have also been used for verification tests of MHD codes. [Preview Abstract] |
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PP9.00119: Neutron and X-ray emissions in the FN-II dense plasma focus with needle implanted electrodes Julio Herrera, Fermin Castillo, Jose Rangel, Isabel Gamboa, Guillermo Espinosa, Jose-Ignacio Golzarri Plasma foci are efficient plasma based X-ray and neutron sources, when deuterium is used as the filling gas. The dense plasma focus FN-II is a small device (4.7 kJ), in which the emission of deuterium fusion neutrons (2.45 MeV) are studied. The system produces an averages neutron yields of (5.3 $\pm $ 0.5) x 10$^{8}$ neutron/shot into 4$\pi $ sr at $\sim $ 350 kA peak discharge current and 2.75 torr deuterium operation. Three methods are currently used to detect them; silver activation counters and CR-39 nuclear track detectors, for time integrated and angular distribution studies, and BC 400 scintillators coupled to photomultiplier tubes for spectra studies. Soft X-rays are studied with a pin-hole camera, as well as with arrangements of three PIN detectors filtered by different materials, such as Be, Ti, Ni, Mo, and Al. In this work we show the results obtained when needle implanted electrodes are used, and compare them with those of a flat electrode. There has also been a dosimetric study of the laboratory with TLD dosimeters, which will be presented in this report. [Preview Abstract] |
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PP9.00120: Study of magnetic field evolution in planar wire Z-pinch experiments Maria Pia Valdivia, Joohwan Kim, Derek Mariscal, Gilbert Collins IV, Farhat Beg We report on experiments with two planar arrays consisting of four nickel wires in each array to study advection of current to the precursor plasma. The linear transformer driver GenASIS was used to deliver a peak current of 200 kA with 150 ns rise-time. Laser probing along and across the wire arrays provided time evolution information of the variation of density, individual wire expansion, and precursor plasma formation on-axis. It is found that plasma dynamics differ significantly from aluminum arrays as the plasma precursor formation occurs earlier and is m=1 unstable, indicating a fraction of current flows through the precursor column. Results will be discussed in the context of current flow and general wire array plasma dynamics. [Preview Abstract] |
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PP9.00121: Analysis of Ag Single-Wire Explosion Experiments on 1 MA Zebra pulsed power generator at UNR S.F. Keim, M.E. Weller, V.L. Kantsyrev, A.S. Safronova, A.A. Esaulov, I. Shrestha, G.C. Osborne, V.V. Shlyaptseva In order to better understand implosion and radiative characteristics of wire arrays, single wire (SW) loads can be employed due to their uncomplicated configuration. Ag has been a recent focus for wire arrays as the highest atomic number element (z = 47) to be investigated for L-shell radiative properties on the Zebra generator and has been shown to produce radiation yields up to 29kJ in planar wire arrays (PWA). The results of two experiments with Ag wire loads, both consisting of a single 30$\mu $m wire (M $\sim $ 150$\mu $g), are analyzed using a full time gated and time integrated diagnostic suite, from 10eV to 50keV. A detailed comparison with results from implosions of Ag PWAs is accomplished. Both generally and specific to SWs, mechanisms of ablation and heating as well as radiative properties during and after the stagnation phase are discussed. [Preview Abstract] |
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PP9.00122: Laser Cut X-pinches Gilbert Collins, Maria Pia Valdivia, Joohwan Kim, Jun Yi, Kanchana Gunasekera, Farhat Beg, Rich Stephens We present experimental data from laser cut X-pinches in an effort to develop a reliable and efficient X-ray backlighting source for the characterization of ICF capsules. Temporal and spatial resolution of the pinch is resolved using wideband Si diodes and time integrated x-ray pinhole cameras respectively. A comparison is made between laser cut and traditionally arranged X- pinches in terms of X-ray emission intensity, point-source coherence, and reproducibility. [Preview Abstract] |
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PP9.00123: The LLNL High-gradient Z-pinch Ion Probe Experiment V. Tang, S. Falabella, G. Guethlein, A. Schmidt, D. Blackfield, E. Cook, S. Hawkins, B. Rusnak, M. Adams, G. Akana, E. Anaya, C. Holmes, H. McLean, T. Houck, J. Watson, Y-J. Chen, G. Caporaso, D. Welch, D. Rose Dense Plasma Focus (DPF) Z-pinches are copious sources of neutrons and MeV level particle beams. Deuterons up to 10 MeV have been observed from centimeter-scale long pinches indicating gradients up to 1 GV/m. These beams contribute significantly to the DPF's radiation output. The mechanisms behind these gradients are not understood and a true predictive capability required for optimization or application is not currently available. At LLNL we are assembling a DPF experiment with a 4 MeV ion probe beam to measure these gradients directly. These unique data can be used to validate kinetic simulations. Here we discuss how the probe beam can measure the acceleration gradients in the plasma and present first experimental DPF results. [Preview Abstract] |
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PP9.00124: Estimating the inductance at the PF-1000 plasma focus Jiri Kortanek, Pavel Kubes, Josef Kravarik, Karel Rezac, Daniel Klir, Marian Paduch, Marek Scholz, Eva Zielinska The plasma focus PF-1000 in the IPPLM in Warsaw, operating with 2 MA with 10 to the power of 11 neutron gain, was investigated with interferometry. 16 interferometric pictures with millimeter spatial resolution were obtained, representing the layout of plasma density. Their time step is 10-20 ns during a 220 ns period. Laser Nd:YLF with pulse duration below 1ns and a set of mirrors was used, splitting the main ray in 16 mutually delayed beams. Interferograms were processed with developed applications in order to calculate the inductance of plasma column under the assumption that the current flows straight along a thin skin layer at the surface of the current sheath. Known inductance enables estimation of the energy transformations in the plasma. [Preview Abstract] |
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PP9.00125: A comparison of discrete-ordinates and flux-limited-diffusion methods for modeling radiation transport in radiative shock tubes Eric S. Myra, Wm. Daryl Hawkins The Center for Radiative Shock Hydrodynamics (CRASH) seeks to improve the predictive capability for models of Omega laser experiments of radiative shock waves. The laser is used to shock, ionize, and accelerate a beryllium plate into a xenon-filled shock tube. These shocks, when driven above a threshold velocity of about 60 km/s, become strongly radiative and convert most of the incoming energy flux into radiation. Radiative shocks have properties that are significantly different from purely hydrodynamic shocks and, in modeling this phenomenon numerically, it is important to compute radiative effects accurately. In this presentation, we examine approaches to modeling radiation transport by comparing two methods: (i) a computationally efficient approximation (multigroup flux-limited diffusion), currently in use in the CRASH code, with (ii) a more accurate discrete-ordinates treatment that is offered by the code PDT. We present a selection of results from a suite of comparison tests, showing both idealized problems and those that are representative of conditions found in the CRASH experiment. [Preview Abstract] |
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PP9.00126: Shock Capturing Anomalies and the Jump Conditions in One Dimension Daniel Zaide, Philip Roe In this work we examine how the nonlinearity of the Rankine-Hugoniot jump conditions dictates the behavior of shock capturing methods, particularly of Godunov-type schemes. Here we present four related one-dimensional examples of the artifacts caused by this: sub-cell shock position in the stationary shock, the slowly moving shock, the wall heating problem, and the carbuncle phenomenon. Each one of these well known problems is shown to be directly related to the nonlinearity of the Hugoniot and numerical experiments are performed to verify the connection. Lastly, a system with a straight Hugoniot is described and shown not to su{\_}er from any of these phenomena. [Preview Abstract] |
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PP9.00127: Investigation of Mixed Cell Treatment via the Support Operator Method Nick Patterson, Katsuyo Thornton A support operator method (SOM) discretization of the diffusion equation is used to examine treatment of mixed cells. The diffusion equation is used to simulate radiation transport in optically thick system. Multiple fluids or species can be simulated by assigning distinct diffusivities to different regions of the computational domain. A mixed cell occurs when the boundaries of the fluids do not align with the boundary of the mesh cells. The SOM discretizes the diffusion equation into a symmetric and positive-definite matrix system, which allows for more efficient solvers. The SOM is spatially second-order accurate for isotropic, anisotropic, continuous, or discontinuous diffusion coefficients. The use of anisotropic diffusion tensor is explored as a means of simulating the material interface in a mixed cell by rotating a diagonal diffusion tensor. The results are compared with more typical mixed cell treatments, such as global or local grid refinement, or setting the diffusivity equal to that of the fluid occupying the largest volume of the cell. [Preview Abstract] |
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PP9.00128: The Modeling of Turbulent Radiative Shocks with Applications to High-Energy-Density Physics and Astrophysics Tiberius Moran-Lopez, James Holloway, Oleg Schilling Radiation transport, shock physics, and turbulence are coupled in many high-energy-density environments including supernovae, stellar life cycles, high-energy laser experiments, and black hole evolutions. A turbulent radiative gas dynamics model for examining such phenomena is developed. A four-equation Reynolds-averaged Navier--Stokes model for the turbulent kinetic energy, turbulent kinetic energy dissipation rate, density variance, and temperature variance is used to describe the effects of turbulence while an equilibrium diffusion model describes the radiative transfer. The mean radiative flux introduces additional terms addressed by modeled transport equations. This model will contribute to an improved understanding of high energy density and astrophysical phenomena in which radiation and turbulence are important. [Preview Abstract] |
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PP9.00129: Laser Energy Deposition Package for CRASH B.R. Torralva, I.V. Sokolov, B. van der Holst, G. Toth, M.J. Grosskopf, K.G. Powel, R.P. Drake A package has been implemented into the Center for Radiative Shock Hydrodynamics (CRASH) code to self-consistently model laser energy transport and deposition. This allows one to use CRASH to simulate a complete radiative shock wave experiment in which all forms of energy present in the calculations are computed and evolved in time using a single, multi-physics model -- the CRASH model. The laser energy transport is simulated via the implementation of an efficient parallel ray-tracing algorithm based on the geometric optics approximation, whereas, the laser energy deposition is calculated via the inverse bremsstrahlung mechanism along the ray's path. Results will be presented for tests in which laser energy is transported and deposited in a gas with a linear density profile, along with convergence tests of intense laser initiated shock waves. Initial test simulations of a complete radiative shock wave experiment will also be presented. [Preview Abstract] |
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PP9.00130: Validation and Verification of the multi-material EOS and Opacity Model used in CRASH Igor Sokolov, R. Paul Drake The laser target for the CRASH project has a complicated geometry and is manufactured from four materials. The beryllium foil, and, partly, the gold washer are directly irradiated with the laser beams, the physical conditions in them vary from very high temperatures and low densities in the laser corona till multi-hundred MBar pressure in the pusher driving the strong shock wave in xenon. Shock-compressed xenon intensely radiates, the radiation transport being responsible for the most of the observed phenomena: wall shock, generated from the irradiated tube wall, perturbations in the shocked layer of xenon, wavy xenon-beryllium interface. The capillary tube is made of two different sorts of plastic: polyimide and acrylic. To perform coupled simulations of the hydrodynamic motion and radiation transport one needs to incorporate the EOS and multi-group opacity data. For a long time we were employing a simple thermodynamic model, based on direct calculation of statistical sums and the Helmholtz free energy and accounting for the partial Fermi degeneration for electrons as well as the Coulomb interactions. In the present simulation we present the results of the validation and verification research, for the EOS and multi- group opacity model used in CRASH. First goal is a direct comparison between the EOS data tables as produced using the CRASH code with those obtained from different databases (ARTEP, PRISM and some others). We analyze the difference in the results and discuss, which of the data can be better used in simulations. [Preview Abstract] |
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PP9.00131: Effects of Opacity Uncertainties on Simulations of Radiative Shock Experiments Bruce Fryxell, Eric Myra It is straightforward to generate radiative shocks in the laboratory using high-energy lasers. In one such experiment, a thin Be disk is attached to a shock tube filled with Xe gas. The laser pulse accelerates the Be disk, which acts like a piston, driving a strong shock into the Xe. Radiation produced in the hot, post-shock Xe, heats the tube walls ahead of the shock. Material then ablates from the tube walls, and a ``wall shock'' is driven inward toward the center of the shock tube. This results in a complex shock structure in which the primary shock and the wall shock intersect at a triple point. To date, attempts to simulate this morphology have produced shock structures far more complex than what has been seen in the experiments. One possible explanation for this discrepancy is uncertainty in the opacities of Xe and polyimide, which is used for the shock tube walls. A series of simulations is discussed in which these opacities are varied in order to see what effects they have on the flow morphology. The goal is to determine what opacity values produce results that are the most consistent with the experimental data. [Preview Abstract] |
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PP9.00132: Computer Model Calibration Using Outputs From Multi Fidelity Simulators Joslin Goh, Derek Bingham, James Paul Holloway, Mike J. Grosskopf, Forrest W. Doss, Erica Rutter, Carolyn C. Kuranz In some experiments, there are multiple computer codes that can be used to describe a physical process of interest. In this work, we consider a statistical model that combines data from field experiments and multiple computer simulations to calibrate parameters of interest and make prediction in the physical system. Our approach is Bayesian and will be illustrated through a simple example and CRASH experiment with outputs from multiple simulators. [Preview Abstract] |
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PP9.00133: Incorporating Gradient Information for Uncertainty Quantification using Response Surface Methods Johann Dahm, Colin Miranda, Krzysztof Fidkowski, Kenneth Powell Response surfaces provide a nonintrusive framework for quantifying the effects of uncertainties in the inputs on resulting computational outputs. Using gradient information can in turn provide substantially improved representations of the response surface, which may enable significant computational savings. Here we assess various ways that gradient information based on discrete adjoints can be used to obtain such computational efficiencies within the context of a radiation hydrodynamics simulation. These include interpolation-based schemes as well as predictor-based schemes. Additionally, we examine effects of the discretization level used in representing the response surface, both by changing the spatial discretization directly and by increasing the sparseness of the basis set used to represent the surface. Results show that among the interpolation schemes examined, piecewise cubic interpolants usually provide the best combination of accuracy and computational efficiency. In general, predictor-based schemes impose substantially greater computational loads than do interpolation-based schemes having similar levels of accuracy. Regarding sparse basis representations of the surface, the choice of retained basis elements can significantly affect the resulting computational efficiency. This work is supported by DOE DE-FC52-08NA28616. [Preview Abstract] |
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PP9.00134: Target Fabrication for OMEGA Campaigns at the University of Michigan S.R. Klein, R.P. Drake, C.C. Kuranz, M.J. Grosskopf, D.C. Marion, C.M. Huntington, C. di Stefano, C.M. Krauland, E.J. Gamboa, R.S. Gillespie The University of Michigan has been fabricating targets for OMEGA campaigns since 2003. These experiments explore supernova-relevant high-energy-density physics. Using an automated system of stages and through the ever innovated designs of University of Michigan graduate students, our targets have become increasingly sophisticated. Our systematic approach has earned us repeatability that is reflected in our successful data. A large portion of our target fabrication is accomplished by our undergrads, whose involvement has proven to be invaluable. We present improvements in our capability techniques and their results from recent campaigns. [Preview Abstract] |
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PP9.00135: Evolution of a radiative shock system on the Omega laser facility Carolyn Kuranz, R.P. Drake, M.J. Grosskopf, C.M. Krauland, F.W. Doss, C.M. Huntington, B. Torralva, E. Rutter, S.R. Klein, D.C. Marion Radiative shocks, which are in a regime where most of the incoming energy flux is converted into radiation, can be created in a laboratory using a high-powered laser. We have performed experiments on the Omega laser facility that irradiate a 20 $\mu $m thick Be disk with about 4 kJ of laser energy in a 1 ns pulse. This shocks and accelerates the disk into a Xe or Ar gas at 1.1 atm. These radiative shocks can reach up to 130 km/s. A 3D, MHD code with a radiation solver is being developed at the Center for Radiative Shock Hydrodynamics (CRASH) that will model this experiment. Diagnostics for this experiment have included x-ray radiography, x-ray Thomson scattering, optical pyrometry, and UV Thomson scattering. Experimental results include observations ranging from shock breakout of the Be disk until 26 ns after the laser pulse is initiated. These results will be presented. [Preview Abstract] |
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PP9.00136: Imaging X-Ray Thomson Scattering E.J. Gamboa, C.C. Kuranz, C.M. Huntington, M.R. Trantham, R.P. Drake, D.S. Montgomery, J.F. Benage, S.A. Letzring In many laboratory astrophysics experiments, intense laser irradiation creates novel material conditions with large, one-dimensional gradients in the temperature, density, and ionization state. X-ray Thomson scattering (XRTS) is a powerful technique for measuring these plasma parameters. However, the scattered signal is typically measured with little or no spatial resolution, which limits the ability to diagnose these inhomogeneous plasmas. We report on an experiment at the Omega laser to diagnose a radiation-driven heat wave in a low density carbon foam [1]. The temperature profile is resolved spatially using a new imaging x-ray Thomson scattering diagnostic. Diffraction of scattered x-rays from a toroidally curved crystal creates high-resolution images that are spatially resolved along a one-dimensional profile in the target while simultaneously spectrally resolving the scattered radiation. \\[4pt] [1] G. Gregori \textit{et al.}, Phys. Rev. Lett. 101, 045003 (2008). [Preview Abstract] |
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PP9.00137: A Data-Model Comparison Using a Novel X-Ray Thomson Scattering Diagnostic M.R. Trantham, E.J. Gamboa, R.P. Drake We have developed a novel X-ray Thomson scattering diagnostic capable of creating high-resolution spatially resolved one-dimensional profiles while spectrally resolving the scattered radiation (see poster by Gamboa, et al). This instrument will collect spatially-resolved data profiling the temperature, density, and ionization state of a radiation-driven wave in low-density carbon foam. This data set will be compared to results from CRASH, a numerical model for radiation-hydrodynamics. We expect this study to be very important in analyzing the overall model performance, but also potentially important in refining equation of state and opacity information. [Preview Abstract] |
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PP9.00138: Modeling the Kelvin-Helmholtz Instability in High-Energy-Density Experiments Using CRASH Code Erica Rutter, R.P. Drake, K.I. Gasior, M.J. Grosskopf, C.C. Kuranz, B. Fryxell The Center for Radiative Shock Hydrodynamics (CRASH) at the University of Michigan is developing an AMR radiation-hydrodynamics code, which can model laser-driven experiments. One such experimental campaign on the OMEGA Laser at LLE is designed to replicate and diagnose the Kelvin-Helmholtz instability in a high-energy-density system. Experimental results have been reported using a design with CRF foam layered on top of polyamide-imide plastic, which have a sinusoidal perturbation at their interface and are encased in beryllium [1]. CRASH simulations have modeled this experiment to validate the code in this regime, as well as to aid in the design of future Kelvin-Helmholtz experiments. We present the results of a series of CRASH simulations of the Kelvin-Helmholtz instability. \\[4pt] [1] E. C. Harding, et al, Phys Rev Lett, 2009 [Preview Abstract] |
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PP9.00139: Simulation of Experiments Generating Collisionless Shocks With Intense Lasers Using the CRASH Code M.J. Grosskopf, R.P. Drake, C.C. Kuranz, E.M. Rutter, H.S. Park, N.L. Kugland, S. Pollaine, J.S. Ross, B.A. Remington, D. Ryutov, A. Spitkovsky, L. Gargate, G. Gregori, A. Bell, C. Murphy, Y. Sakawa, Y. Kuramitsu, H. Takabe, D.H. Froula, G. Fiksel, F. Miniati, M. Koenig, A. Ravasio, E. Liang, N. Woosley Collisionless shocks, shocks generated by plasma wave interactions in regions where the collisional mean-free-path for particles is long compared to the length scale for shock interaction, are found ubiquitously in astrophysics. Experiments to investigate collisionless shocks in a laboratory-scale system are being carried out on intense lasers; measuring the density, temperature, magnetic field, and velocity of counter-streaming flows generated by laser ablation. This poster reports hydrodynamic simulations modeling the ablative flow of plasma generated in order to assess potential designs and infer properties of collected data from previous single foil experiments. [Preview Abstract] |
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PP9.00140: Magnetic Rayleigh-Taylor Instability with Biermann Battery Effect Chuan-Chih (Jason) Chou, Bruce Fryxell, R. Paul Drake, Bart van der Holst Recently, unexpected morphology has been observed in high-energy Rayleigh-Taylor experiments. In these experiments with 3D initial perturbations, the spikes lack the mushroom cap observed in 2D or low-energy counterparts. It is suspected that magnetic field generated by Biermann battery may be responsible for this unusual morphology. In order to estimate the magnitude of the magnetic field due to Biermann battery effect under the experimental circumstances, we performed preliminary simulations using CRASH and its recent implementation of Biermann battery term. Although limited by the lack of corresponding dissipation term, we are able to obtain the upper limit of the magnetic field present in the experiment. We will discuss its implication in the context of the plausibility of the Biermann battery hypothesis. [Preview Abstract] |
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PP9.00141: Turbulence characteristics of the mixing region in the planar Richtmyer-Meshkov Instability Pooya Movahed, Sreenivas Varadan, Eric Johnsen The development of the turbulent mixing region at late times in the Richtmyer-Meshkov instability plays an important role in high energy density phenomena, specifically in Inertial Confinement Fusion and supernova explosion. A numerical investigation of the turbulence characteristics of the single-mode Mach 1.21 air(acetone)/SF6 shock tube experiment of Collins and Jacobs [JFM 2002] with reshock is performed using a second-order MUSCL-Hancock and several high-order WENO schemes. The effects of the grid-dependent numerical dissipation on small-scale features and efficiency of the methods are discussed for the multi-component Euler equations by considering the turbulent kinetic energy, turbulent Mach number, and enstrophy. Resolved simulations of the Navier-Stokes equations are performed at practical Reynolds numbers to evaluate the role of physical diffusion. A similar analysis of the Rayleigh-Taylor instability will be discussed. This research was supported in part by the DOE NNSA under the Predictive Science Academic Alliance Program by grant DEFC52- 08NA28616. [Preview Abstract] |
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PP9.00142: LOW TEMPERATURE PLASMAS |
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PP9.00143: Standoff Detection of Volatile Organic Compounds In Air Using Laser Induced Fluorescence Jerry Clark, Alonzo Alexander, Delonia Wiggins, Sydney Williams, Charlemagne Akpovo, Ephrem Mezonlin, Joseph A. Johnson III The use of laser-induced fluorescence has proven to be an excellent method of detecting important intermediates in turbulent systems. However, Acetylene detection in air at ambient temperatures has proven more challenging. Molecular spectra were collected in laser induced acetylene plasmas using a 250 mJ Nd:YAG laser and an optical parametric oscillator (OPO) to achieve the 260 nm wavelength and greater than 3 mJ energy necessary to excite acetylene molecules. The acetylene laser-induced fluorescence excitation was observed at the 228 nm wavelength. Using various concentration ratios, acetylene was mixed with air to specifically determine the capabilities of standoff acetylene detection at atmospheric pressure. These results will lead to further research and development of turbulence based battlefield ready detection devices. [Preview Abstract] |
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PP9.00144: Measurement of Electron Temperature and Plasma Density via Thomson Scattering and Electric Probe in Low Temperature Plasmas Hyun-Jong Woo, Kyu-Sun Chung, Taihyeop Lho Laser Thomson scattering system has been developed for the measurements of electron temperature and plasma density in low temperature plasma by means of the Q-switched Nd:YAG laser with 2nd Harmonics (250 mJ at 532 nm, repetition rate of 20 Hz), triple grating spectrometer and ICCD camera. The triple grating spectrometer is composed of 3 grating (1800 gr/mm and 100 x 100 mm dimensions), 6 achromatic doublet lens (f=400 mm and 100 mm diameter) for reducing the abbreviation effect, two slits (entrance and exitance), opto- mechanical instruments, etc. The alignment and calibration of TGS system were performed by a diode laser and diffraction optics, Ne lamp (wavelength) and metal halide lamp (intensity), respectively. The LTS measurements were done wide ranges of $1< T_e < 10$ and $10^{11} < n_e < 10^{13} cm^{-3}$ without different magnetic fields and neutral pressures in Divertor Plasma Simulator-II (DiPS-II) and it compared with the single probe measurements. [Preview Abstract] |
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PP9.00145: ABSTRACT WITHDRAWN |
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PP9.00146: A spectroscopic study of the initial stages of pulsed glow discharges in noble gases V.I. Demidov, S.F. Adams, E.A. Bogdanov, A.A. Kudryavtsev, J.M. Williamson The dynamics of the rapid growth of electron and metastable atom densities at the beginning of a pulsed dc discharge in argon was explored by investigating the relationship between the strength of the applied electric field and the measured emission line intensities at 419.8 and 420.1 nm. Data showed that when the electric field strength was low, the growth of the metastable atom density began before the growth of the electron density. The opposite relationship was observed when a higher strength electric field was applied. This observed dependence can be confirmed by modeling the argon dc discharge. Furthermore, similar measurements for the 345.4 and 347.3 nm spectral lines in neon and the 480.7 and 467.2 nm spectral lines in xenon suggest analogous behaviors in these noble gases. Thus, spectral measurements of the above lines could be a sensitive indicator of the presence of noble gas metastable atoms in plasmas. This relationship could be used to control plasma properties, a tool that would be useful for many technological applications. [Preview Abstract] |
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PP9.00147: Numerical studies of helicon plasmas in the lower hybrid resonance frequency range Suwon Cho In attempt to analyze hollow density profiles observed near the lower hybrid resonance condition, we investigate absorption of the radiofrequency waves and equilibrium profiles of plasma parameters numerically for a helicon discharge with a lower hybrid resonance layer. As the wave approaches the resonance layer, it undergoes heavy damping so that most of the rf power is absorbed outside the resonance layer and the peak absorption occurs near the resonance at a low collision frequency. The strongly localized heating leads to a temperature profile which is different from that of usual discharge at lower magnetic fields. Considering that the collisional energy loss is dominant, the electron temperature is determined from the local power balance. The diffusion equation is used to find the plasma density profile iteratively which is consistent with the temperature and the power deposition profiles. [Preview Abstract] |
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PP9.00148: Low Pressure High Density Plasma Development on a Small Helicon Plasma Experiment (HPX)* R.W. James, L.A. Allen, R.N. Paolino, N. Thayer, B. Romano, B.S. Stutzman, C. Welicka Small helicon plasmas have been employed in various capacities from industry to spacecraft propulsion. At the Coast Guard Academy Plasma Lab (CGAPL), a small Helicon Plasma Experiment (HPX) is being developed to utilize the reputed high densities (10$^{13}$ cm$^{-3}$ and higher) at low pressure (.01 T) [1], in high temperature and density diagnostic development for future laboratory investigations. HPX is designed to operate at these high densities and pressure to create repeatedly stable Capacitively Coupled Plasma (CCP) and Inductively Coupled Plasma (ICP) plasmas induced by an RF frequency in the 10 to 70 MHz range. Progress on the development of the RF coupling system, and qualitative observations from the optical and electric diagnostics are to be reported.\\[4pt] [1] K. Toki, \textit{et al}., \textit{Thin Solid Films} \textbf{506-507} (2005) [Preview Abstract] |
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PP9.00149: Production of H, D, and He Plasmas in the ORNL High Flux Helicon Plasma Source R.H. Goulding, T.M. Biewer, J.B.O. Caughman, G.C. Chen, L.W. Owen, D.O. Sparks The ORNL high particle flux helicon source is has been operated with various light ions at power levels up to 30 kW. It is being studied as an electrodeless source for a linear plasma materials interaction (PMI) test facility that will generate particle fluxes $\Gamma_p > 10^{23}m^{-3}s^{-1}$, and utilize additional ion and electron cyclotron heating to produce high heat fluxes $\sim 10 MW/m^2$. The maximum magnetic field strength $|B|$ in the plasma production region for which high density operation is possible at the present power level has been found to increase with increasing ion mass. Operation with $|B| \sim 0.5 T$ has been achieved with He as the working gas. The radial density profile is found to be strongly dependent on the axial magnetic field geometry, and both strongly centrally peaked and flat profiles have been obtained. Maximum plasma densities $> 4\times 10^{19} m^{-3}$ have been achieved with He, and $> 2.5\times 10^{19} m^{-3}$ with H. The device has been modeled using the EMS2D (G. Chen et al., Phys Plasmas 13 (2006) 123507) and SOLPS (R. Schneider, X. Bonnin et al., Contrib. Plasma Phys. 46 (2006) 3) codes. The latest results will be presented. [Preview Abstract] |
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PP9.00150: EBW and Whistler propagation and damping in a linear device S.J. Diem, J.B.O. Caughman, R.W. Harvey, Yu. Petrov Linear plasma devices are an economic method to study plasma-material interactions under high heat and particle fluxes. ORNL is developing a large cross section, high-density helicon plasma generator with additional resonant electron heating to study plasma-material interactions in ITER like conditions. The device will produce a heat flux of 10-20 MW/m$^{2}$ and particle flux of 10$^{24}$ /m$^{2}$/s in a high recycling plasma near a target plate with a magnetic field of $\sim$1 T. As part of this effort, heating of overdense plasma is being studied using a microwave-based plasma experiment. The plasma is initiated with a high-field launch of 18 GHz whistler waves producing a moderate-density plasma of n$_{e}$ $\sim$10$^{18}$ m$^{-3}$. Electron heating of the overdense plasma can be provided by either whistler waves or EBW at 6 and 18 GHz. A modified GENRAY (GENRAY-C) ray-tracing code has been used to determine EBW and ECH whistler wave accessibility for these overdense plasmas. These results combined with emission measurements will be used to determine launcher designs and their placement. [Preview Abstract] |
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PP9.00151: Operation of a microwave plasma source for electron heating and antenna testing J.B.O. Caughman, T.S. Bigelow, S.J. Diem, R.H. Goulding, D.A. Rasmussen, C.R. Schaich, T.L. White One of the major challenges for magnetic fusion is the interaction of the plasma with materials. Linear plasma-material interaction test stands can benefit from additional electron heating of the high-density source plasma to increase the total plasma heat flux at the target to better simulate fusion reactor conditions (10-20 MW/m$^{2}$). A microwave-based plasma experiment has begun at ORNL to study electron heating of over-dense plasmas and to provide a plasma environment for antenna testing. The plasma is generated by high-field launched whistler waves at 18 GHz to create a moderate-density plasma (n$_{e}$$\sim$10$^{18}$/m$^{3}$). Electron heating of the over-dense plasma is provided by either whistler waves or electron Bernstein waves at 6 GHz. In addition, a single strap mockup antenna, designed to operate at 40-50 MHz, is being constructed to study near-field plasma interactions. The antenna will be placed in the experiment's central vacuum chamber, which will act as an rf test facility. [Preview Abstract] |
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PP9.00152: Cold plasma selectivity in the interaction with various types of the cells Olga Volotskova, Mary Ann Stepp, Michael Keidar Present research in the area of cold atmospheric plasma (CAP) demonstrates great potential in various areas including medicine and biology. Depending on their configuration they can be used for wound healing, sterilization, targeted cell/tissue removal, and cancer treatments. Here we explore potential mechanisms by which CAP alters cell migration and influences cell adhesion. The migration studies are focused on the CAP interaction with fibroblasts and corneal epithelial cells. Data show that various types of cells have different thresholds (treatment times) required to achieve maximum inhibition of cell migration which is around $\sim $30-40{\%}. Studies to assess the impact of CAP treatment on the activation state of integrins and focal adhesion size by immunofluorescence showed more active b1 integrin on the cell surface and large focal adhesions after CAP treatment. Based on these data, a thermodynamic model is presented to explain how CAP leads to integrin activation and focal adhesion assembly. Also responses of the various types of the cells to the cold plasma treatment on the example of the epithelial keratinocytes, papilloma and carcinoma cells are studied. Cell cycle, migration and cell vitality analysis were performed. The goal of this study is to understand the mechanism by which the CAP jet alters cell migration, influences adhesion and cell survival. [Preview Abstract] |
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PP9.00153: Application of electrostatic Langmuir probe to atmospheric arc plasmas Michael Keidar, Alexey Shashurin, Jian Li, Taisen Zhuang, Yevgeny Raitses, Isak Beilis Relatively high pressure (about several hundred Torr) anodic arcs demonstrated its efficiency for synthesis of different types of nanostructures, such as single and multi-wall carbon nanotubes, fullerenes and graphene. Extensive interest to arc synthesized nanostructures stimulates active recent studies of high pressure anodic arcs. In this paper we apply the Langmuir probe technique for plasma parameter measurements in atmospheric pressure arcs. V-I curves of single probe were analyzed and it was observed that ratio of saturation current on positively biased probe to that on negatively biased was about 1-4, which is significantly lower than $\sim $100 predicted by conventional collisionless theory. This result can be explained by secondary electron emission from the probe due to the de-excitation of excited background gas atoms at the negatively biased probe surface. The shape of V-I curve for potentials more negative than plasma potential as well the value of saturation current are significantly deviated from that in conventional collisionless case. In this case the plasma electron temperature and plasma density no longer can be precisely determined using the standard expressions utilizing the slope of V-I curve and the ion saturation current. [Preview Abstract] |
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PP9.00154: Characteristics of line pulsed glow discharge plasma at atmospheric pressure for preparation of amorphous carbon films K. Fukui, M. Miyamae, Y. Kikuchi, Y. Matsuo, Y. Horiguchi, Y. Nishimura, M. Nagata, M. Yatsuzuka Diamond-like carbon (DLC) films have excellent physical and chemical properties for applications such as high mechanical hardness, low friction, chemical inertness, electrical insulation, optical transparency, and biological compatibility. For preparation of large-area amorphous carbon film, the line glow discharge plasma (LGDP) at atmospheric pressure was produced with a high-voltage, high-repetition bipolar pulse using the three-electrode configuration in our experiment. The DBD source plasma was generated by a high-voltage, high-repetition bipolar pulse with a fast rise time using the parallel-plate geometry with a gap width of 2 mm. Two quartz glasses were placed between two electrodes made of stainless-steel to produce DBD. A mixed gas of He as a carrier gas and CH$_{4}$ as a precursor was supplied to the discharge region. Applying another pulsed bias voltage to the gap between the parallel plate electrode and the bias electrode (stainless-steel substrate), the LGDP was extracted on the substrate for a film deposition. When the pulsed bias voltage was applied to the substrate at several $\mu $s later, the largest intensity of LGDP was observed at the both polarity of bias voltage. We will show the detail characteristics of the LGDP. [Preview Abstract] |
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PP9.00155: Plasma parameter measurements in a small-size atmospheric plasma jets Alexey Shashurin, Michael Keidar Recently a great attention is attracted to the creation of the small size atmospheric plasma jets and their interaction with living tissue. This facilitates the development of appropriate tools for diagnostics of plasma parameters in small-size plasma jets. Two main diagnostic tools are traditionally utilized for jet characterization, namely, photographing by intensified charge-coupled device (ICCD) cameras and optical emission spectroscopy. It is observed that streamer (``plasma bullet'') propagating along with gas flow is generated immediately after the breakdown. Recently a new method for temporally resolved measurements of absolute values of plasma density in the plasma column of small-size atmospheric plasma jet utilizing Rayleigh microwave scattering was proposed. In this work we present a new method for measurements of plasma potential in the jet. Method utilizes application of external electrostatic fields to control propagation of streamer. The distribution of plasma potential along the jet was determined. [Preview Abstract] |
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PP9.00156: ABSTRACT WITHDRAWN |
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PP9.00157: Laser-based measurement of 3-dimensional dust trajectory in plasma Kil-Byoung Chai, Wonho Choe Dusts, produced by plasma-wall interactions or gas phase chemical reactions in various plasmas including tokamak plasmas and processing plasmas, brings about many negative concerns in the operational and safety aspects [1]. Therefore, many efforts on developing dust diagnostics and control/removal techniques of dusts from the plasma environments have been made during the last decade [2,3]. Among various dust diagnostics adopted in plasma experiments, the laser based diagnostics are attractive because they are non-intrusive and time-resolved in-situ. In this study, a laser diagnostic technique for 3-dimensional dust trajectory by utilizing two video cameras and a sheet laser beam with a multiple beam pass method is presented, which results in improved sensitivity than previously developed methods. In addition, 3-dimensional dust trajectories are measured with different neutral drag and thermophoresis varied by neutral gas flow rates and neutral temperature gradient, respectively. \\[0pt] [1] S. H. Hong, C. Grisolia, V. Rohde, P. Monier-Garbet, Tore Supra Team, and ASDEX Upgrade Team, Nucl. Fusion 50, 035002 (2010). [2] A. L. Roquemore, W. Davis, R. Kaita, C. H. Skinner, R. Maqueda, and N. Nishino, Rev. Sci. Instrum. 77, 10E526 (2006). [3] W. P. West, B. D. Bray, and J. Burkart, Plasma Phys. Control. Fusion 48, 1661 (2006). [Preview Abstract] |
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PP9.00158: Modeling and simulation of arc discharges for nanoparticle synthesis Madhusudhan Kundrapu, Jian Li, Alexey Shashurin, Michael Keidar Arc discharge can produce nanoparticles with fewer topological defects. In order to improve the controllability of synthesis, a comprehensive understanding of the growth mechanism of nanoparticles is required. Experimental investigation has limitations in measuring the required parameters especially, species density and temperature distribution. Hence, a self-consistent numerical model is developed to simulate arc discharge chamber. Simulations are performed for arc current varying from 10 to 100 A with 4 mm electrode-gap in Helium background at 68 kPa. The sublimation rate and current-voltage characteristics are compared with the experiments. Good agreement is observed between simulation and experiment results. The probable growth region of nanoparticles is identified and Gibbs free energy model is then used to estimate the critical cluster size and further growth of nanoparticles. The catalyst cluster sizes are compared with those obtained from the TEM images of the experiments. Parametric studies are carried to indentify the inputs for which better growth is achieved. [Preview Abstract] |
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PP9.00159: Ion Acceleration in Rotating Magnetized Plasma N.J. Fisch, Y. Raitses, A. Fruchtman The cylindrical Hall thruster represents a basic magnetic nozzle configuration; it also appears to work far better than it should. While its performance has been reported in considerable detail, the ability of this kind of thruster to propel ions with such a narrow plume has remained a mystery. Moreover, in the discharge overrun current regime, cylindrical Hall thrusters display increased electric fields, plume narrowing, reduction of noise, and differences in temperature and density profiles. The unexpected, unusual, and fortunate behavior of the cylindrical Hall thruster, can now be approached by considering certain self-organizing features of supersonically rotating electron plasma. [Preview Abstract] |
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PP9.00160: Azimuthal Dynamics and Transport in a Two Dimensional Hall Thruster Hybrid Model Eduardo Fernandez, Mark Cappelli Current Hall thruster simulation efforts aimed at modeling the global plasma discharge typically employ hybrid schemes in order to circumvent the stiff requirements of fully kinetic approaches. While such hybrid schemes capture the basic features of the plasma seen experimentally, they do not generate sufficient electron transport needed to sustain the discharge. As a result, the electron mobility is enhanced in a somewhat ad-hoc manner with models that employ adjustable coefficients. In this paper we present a hybrid model that aims to capture electron transport by resolving the azimuthal dynamics self consistently. Simulations show that transport-producing fluctuations are naturally excited in the plasma. The features of the model will be presented and compared with the more standard hybrid model, which does not resolve azimuthal physics. Initial steps of a three dimensional, hybrid simulation for Hall thrusters will be shown. [Preview Abstract] |
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PP9.00161: Conditions and growth rate of Rayleigh instability in a Hall thruster under the effect of ionization Sukhmander Singh, Hitendra Kumar Malik Rayleigh instability is investigated in a Hall thruster under the effect of ionization . The instability occurs only when the frequency of the oscillations $\omega $ falls within a frequency band, described by $\omega _{\min } <<\omega <<\omega _{\max } $ where $\omega _{\max } =\frac{\alpha k_y u_0 +\sqrt {(\alpha k_y u_0 )^2-4\alpha (\alpha +v)\left( {\alpha k_y u_0 +\frac{\omega _{pi} ^2\Omega ^2}{\omega _{pe} ^2}} \right)} }{2(\alpha +v)}$, $\omega _{\min } =\frac{\alpha }{2}\left( {1-\frac{v}{k_y u_0 }} \right)+\alpha \sqrt {\frac{1}{4}-\frac{v}{2k_y u_0 }-\frac{\omega _{pi} ^2V_{thE} ^2\left( {\frac{\partial ^2u_0 }{\partial x^2}+\frac{\Omega }{n_0 }\frac{\partial n_0 }{\partial x}} \right)}{\alpha ^2\omega _{pe} ^2u_0 }} $ ,$u_0 $ is the drift velocity of the electrons, $\Omega $ is their gyration frequency under the effect of magnetic field, $k_y $ is the wave propagation constant, $n_0 $ is the plasma density together with ${\partial n_0 } \mathord{\left/ {\vphantom {{\partial n_0 } {\partial x}}} \right. \kern-\nulldelimiterspace} {\partial x}$ as the density gradient, and $V_{thE} $, $\alpha $, $v$ and $\omega_{p_i} (\omega_{p_e} )$ are the electron thermal velocity, ionization rate, collision frequency and plasma frequency of the ions (electrons), respectively. A relevant Rayleigh equation is derived and solved numerically using fourth-order Runge-Kutta method for investigating the perturbed potential under the effect of ionization rate and collision frequency. It is obtained that the growth rate of the instability gets reduced with the ionization rate and collision frequency. [Preview Abstract] |
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PP9.00162: Generalized Causes and Consequences of Sheath Instability in Hall Thruster Simulations Michael Campanell, Alexander V. Khrabrov, Igor D. Kaganovich, Dmytro Sydorenko A generalized sheath instability condition is presented for low collisionality planar plasmas bounded by walls with secondary electron emission (SEE). It is shown that the sheath becomes unstable when the SEE coefficient of the trapped electrons bordering the depleted loss cone reaches unity. Instability causes a large potential drop, allowing an intense flux of previously trapped electrons to reach the wall. This produces an intense emitted beam flux. In the first incidence of instability, the system restores to a state in which the sheath potential is much lower than before instability. The new state has much larger beam fluxes and the beams become the dominant component of the wall flux and axial conductivity. Once the beam flux is already large, further instabilities return the system roughly to its pre-instability potential, performing relaxation sheath oscillations (RSO). The periodic feature of RSOs is found to be caused by velocity space diffusion of electrons from the hot bulk towards the loss cone, driven by two-stream instability. [Preview Abstract] |
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PP9.00163: Experimental determination of plasma detachment from the diverging magnetic nozzle of the VASIMR VX-200 Electric Thruster Christopher Olsen, Jared Squire, Benjamin Longmier, Maxwell Ballenger, Leonard Cassady, Mark Carter, Andrew Ilin, Paul Cloutier, Edgar Bering, Matthew Giambusso Theories of magnetized plasma detachment in an expanding magnetic field have been lacking detailed experimental evidence. Recent experiments using a 200 kW class electric rocket (VX-200), run at 100 kW using argon and a peak magnetic field of 2 T, produced ion energies greater than 100 eV with a flux of 2x10$^{22}$ ions/s in a 150 m$^{3}$ vacuum facility. Ion-neutral charge exchange effects were reduced and the resultant data show evidence of plasma detachment in a diverging magnetic field on a scale length of 2 m. The detachment is confirmed using multiple plasma diagnostics and magnetic nozzle topologies. Spatial maps of the data are compared to simulations from a particle detachment model, ParTraj, as well as MHD detachment theory. ParTraj, when compared to experiment, is shown to be more consistent in describing the data. Unless the MHD models are modified to incorporation two-fluid effects, single fluid MHD theory is inconsistent with the observations. [Preview Abstract] |
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PP9.00164: Cross-field electron transport through a rotating spoke in Hall thrusters Yevgeny Raitses, Leland Ellison, Nathaniel J. Fisch A rotating spoke was observed in cylindrical and annular Hall thrusters [1, 2]. In the cylindrical Hall thrusters with a cusp-shape magnetic field topology, the spoke rotates with a speed of 1-3 km/s in the ExB direction, which is substantially less than the local E$\times $B speed of 30 km/s [2]. In recent experiments, the cross-field electron transport induced by the spoke was directly measured for the first time using a segmented anode. Approximately 50{\%} of the total current is found to pass through the spoke. The cross-field transport mechanism has been explored using emissive and biased electrostatic probes and high speed imaging. The findings reveal a perturbed electric field which enhances electron transport across the field.\\[4pt] [1] G. S. Janes and R. S. Lowder, Phys. Fluids \textbf{9}, 1115 (1966)\\[0pt] [2] J. B. Parker, Y. Raitses, and N. J. Fisch, Appl. Phys. Lett. \textbf{97}, 091501 (2010). [Preview Abstract] |
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PP9.00165: Isotope Exchange Experiments in Divertor and First Wall Materials in PISCES Joseph Barton, Thomas Schwarz-Selinger, Yongqiang Wang, Russell Doerner, George Tynan D and H isotope exchange experiments in W and Be samples were conducted in the PISCES linear plasma devices to simulate Tritium retention in first wall and divertor materials in toroidal confinement devices. The one inch diameter samples were first exposed to D plasma in typical tokamak divertor conditions to a fluence of 10$^{22}$ ions/cm$^2$ while maintaining a sample temperature around 300 K. The samples were subsequently exposed to H plasma at varying fluences (10$^{20}$ to 10$^{22}$ ions/cm$^2$) and sample temperatures. Intensities of D-alpha and H-alpha optical emission spectrum (OES) signals from the plasma track the isotope exchange in front of the metal samples in situ, while thermal desorption spectroscopy (TDS) after exposure show what amount of D remains in the bulk sample relative to H. It is shown that W and Be tend to exchange isotopes at the same rate and retain similar fractional amounts of D after exposure. Retention decreases with higher sample temperatures during H plasma exposure, as expected. Nuclear reaction analysis (NRA) with He-3 ions will be conducted to obtain D depth profiles in exposed W to understand how this exchange process varies the bulk D concentration profile. [Preview Abstract] |
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PP9.00166: Rotating plasmas as mass filters for nuclear waste Abraham Fetterman, Nathaniel Fisch High-throughput mass filters can be a critical tool in the disposal of high level nuclear waste.~ The fission product is the most radioactive part of the nuclear waste and is responsible for most of the heat generation, although it is only a small fraction of the mass.~ Rotating plasmas can efficiently separate fission product from nuclear waste because every species in a particular mass group can be removed simultaneously.~ We compare quantitatively the performance of the plasma centrifuge, Ohkawa mass filter, and asymmetric centrifugal trap in separating fission product from nuclear waste. [Preview Abstract] |
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PP9.00167: A New Optical Method to Measure Electron Impact Excitation Cross Sections of Atoms in a Metastable State Peeyush Sahay, Susan T. Scherrer, Chuji Wang We report a new method, based on plasma-cavity ringdown spectroscopy (P-CRDS), to measure electron impact excitation cross-sections (EIECS) of atoms. Measurements of EIECS of mercury atoms in two regular and one metastable states, 6s6p $^{3}$P$_{1}$, 6s7s $^{3}$S$_{1}$, and 6s6p $^{3}$P$_{0}$, respectively, have been conducted. The measured EIECS values of these energy levels, (9.0 $\pm $ 2) $\times $ 10$^{-17}$ cm$^{2}$, (2.5 $\pm $ 0.6) $\times $ 10$^{-18}$ cm$^{2}$, and (1.7 $\pm $ 0.4) $\times $ 10$^{-17}$cm$^{2}$, are in agreement with the reported literature values. As a part of the measurements, the highly sensitive CRDS technique is employed to determine the absolute population density of Hg atoms, with an atmospheric plasma acting as the atomization and excitation sources. This new approach can be an alternative technique to the optical emission spectroscopy (OES) and electron energy loss techniques to measure EIECS. The method can extend EIECS measurements to both high and low pressure conditions. [Preview Abstract] |
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PP9.00168: Electron-ion recombination in laser-produced plasmas using optical interferometry Nathan Heilmann, Justin Peatross, Scott Bergeson We are developing methods to measure electron-ion recombination in laser-produced plasmas. A high intensity fs laser pulse is focused into a gas jet and forms a plasma. A weaker probe beam first passes through a slightly mis-aligned Michelson interferometer and is also focused into the plasma. The probe ``beam'' is actually two temporally coincident but spatially offset laser beams. One of the laser beams passes through the plasma and the other does not. These beams expand and produce interference fringes in the far field, similar to a Young's double slit experiment. The spatial position of these fringes depends on the differential phase shift in the two probe beams. This differential shift is due to the electron density in the plasma, which is probed by only one beam. By measuring the fringe shift as a function of time after the plasma is formed, we should be able to measure the time-evolving electron density. At sufficiently high densities, three-body recombination will become important. In that regime, the measured recombination rate can be used to determine the electron temperature. [Preview Abstract] |
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PP9.00169: Plasma-Surface Interactions in 2D and 3D Simulations for Materials Processing Paul Moroz A computational model of plasma-surface interactions implemented in the FPS-3D feature profile code [1] is discussed. The code uses cellular model for solid materials and is capable of treating etching, deposition, and ion implantation, both in 2D and 3D. It allows modeling of materials processing at very different scales, ranging from a few nanometers to a few micrometers. As input parameters, FPS-3D requires that the fluxes of all reactive species to the surface be provided. Incoming fluxes are represented by particles, each characterized by the kind of species as well as by the energy and direction of flight. The Monte Carlo launcher generates those species in correspondence with specified fluxes. A size of a Monte Carlo particle is typically significantly smaller than the size of a material cell, so numerical statistical artifacts could be reduced. The Monte Carlo treatment of gas and ion reactions might depend on particle energy and angle of incidence to the cell's normal, as well as on the surface temperature. The finite penetration depth of ions into solid materials is included in FPS-3D, which allows treatment of more complex situations such, for example, as etching through the deposited polymer layers. \\[4pt] [1] P. Moroz, APS-DPP, NP8, Atlanta, GA, 2009. [Preview Abstract] |
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PP9.00170: On the exploration of molten salt (FLiNaK) interaction with plasma Yong-Sup Choi, HyonJae Park, Taihyeop Lho We investigated possibility of application of molten salt as a liquid wall in plasma confinement device. Plasma interactions of molten salt - FLiNaK (LiF 46.5 mol{\%} + NaF 11.5{\%} + KF 42 mol{\%}) were investigated by OES(Optical Emission Spectroscopy) and RGA(Residual Gas Analyzer). The plasma was generated with ECR source and the molten salt was maintained as liquid with SUS 316 mold-heater. Optical and mass spectrums were measured during hydrogen plasma interaction with the molten salt and qualitative analysis of resultant species was done. Chemical/physical erosion of FLiNaK after interaction of hydrogen plasma was studied with ICP-MS and IC (Ion Chromatography). Viscosity change of FLiNaK after plasma interaction was measured. Based on the preliminary measurement result, a plasma interaction system with flowing molten salt was designed. [Preview Abstract] |
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PP9.00171: Study on hydrogen retention produced from reaction of FLiNaK in ECR plasma Hyunjae Park, Yong-sup Choi, Taiheop Lho In this study, hydrogen retention from the interaction between an electron cyclone resonance (ECR) plasma and a molten salt, has been experimentally investigated. FLiNaK, which is comprised of LiF (46.5 mol{\%}), NaF (11.5 mol{\%}), and KF (42 mol{\%}), has been used as a molten salt. The molten salt was prepared at the base pressure of 2 $\times$ 10$^{-6}$ torr and 500$^{\circ}$C which is above the melting point. However, the chamber pressure was increased up to 3 $\times$ 10$^{5}$ mTorr owing to the outgassing from the mixture. The experiments have been perfromed at the working pressure of 1 mTorr at different reaction times. The electron temperature and the ion density in hydrogen ECR plasma were measured by making use of Langmuir probe. Retention of hydrogen in FLiNaK was investigated by means of TDS (Thermal Desorption Spectrometry) method. The chemical species produced from the interaction between the plasma and the molten salt by an optical emission spectroscopy (OES) and a residual gas analyzer (RGA) have been also observed. [Preview Abstract] |
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PP9.00172: Rate Change Graph Technology: Absolute Value Point Methodology Ken Strickland, Michael DuVernois Absolute Value Point Methodology (AVPM) is a new theoretical tool for science research centered on Rate Change Graph Technology (RCGT). The modeling techniques of AVPM surpass conventional methods by extending the geometrical rules of mathematics. Exact geometrical structures of matter and energy become clearer revealing new ways to compile advanced data. RCGT mechanics is realized from geometrical intersections that are the result of plotting changing value vs. changing geometry. RCGT methods ignore size and value to perform an objective analysis in geometry. Value and size are then re-introduced back into the analytical system for a clear and concise solution. Available AVPM applications reveal that a massive amount of data from the Big Bang to vast super-clusters is untouched by human thought. Once scientists learn to design tools from RCGT Mechanics, new and formidable approaches to experimentation and theory may lead to new discoveries. In the creation of AVPM, it has become apparent there is a particle-world that exists between strings and our familiar universe. These unrealized particles in their own nature exhibit inflation like properties and may be the progenitor of the implements of our universe. Thus space, time, energy, motion, space-time and gravity are born from its existence and decay. This announcement will be the beginning of many new ideas from the study of RCGT mechanics. [Preview Abstract] |
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PP9.00173: An Alpha Schottky Junction Power Source Marc Litz, James Carroll, Stan Henriquez Isotope batteries present solutions for long-lived low power sources. Compact sensors, and electronic circuit boards can be powered for the lifetime of infrastructure. Alpha sources are practical for safety reasons because of the limited distance before energy absorption in materials, and the high energy ($\sim $5MeV) per particle. Damage to materials from the alphas limits the practical use. A Schottky diode geometry is created from an alpha foil on a diamond-like crystal. A power source is proposed that takes advantage of the radiation damage tolerance of diamond, combined with the short range of the alpha radiation. The internal field of the Schottky barrier creates a current through the diode from electron-hole pairs created by alpha bombardment in the gap. Calculations of the expected current, circuit model results, and design parameters for a device are described. [Preview Abstract] |
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