Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session UP11: Poster Session VIII: MST; DIII-D Tokamak; SPARC, C-Mod, and High Field Tokamaks; HBT-EP; Transport and LPI in ICF Plasmas, Hydrodynamic Instability; HEDP Posters; Space and Astrophysical Plasmas (2:00pm-5:00pm) |
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Room: OCC Exhibit Hall A1&A |
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UP11.00001: Local Measurement of helical flows in the Single Helical Axis state J. Boguski, M. D. Nornberg, B. E. Chapman, M. Cianciosa, D. J. Den Hartog, D. Craig, U. Gupta, K. J. McCollam, T. Nishizawa, J. S. Sarff, C. R. Sovinec, Z. A. Xing Results from local flow measurements in helical RFP plasmas challenge previous assumptions that a global m=1 poloidal flow profile is the dominant flow structure. Charge Exchange Recombination Spectroscopy (CHERS) provides the first core-localized measurements of the 3D ion flow structure in Single Helical Axis (SHAx) plasmas. In high current and low density RFP plasmas, the island associated with the innermost resonant tearing mode can grow to sufficient width to envelop the magnetic axis, resulting in a helical equilibrium. The orientation of the helical structure relative to the fixed CHERS diagnostic is controlled using an RMP. Toroidal flow measurements are dominantly m=1 throughout the plasma; however, the axisymmetric component of the flow has two characteristic regions, a flat flow profile in the core, and a highly sheared flow profile outboard from r/a ~ 0.3. Poloidal flows have a small m=1 structure in the core, while the mid-radius has a larger, more complicated structure, with inboard/outboard asymmetry and a dominantly m=2 structure at outboard r/a=0.62. These observations are compared with results from toroidal NIMROD computations with limited toroidal periodicity, which also produce complicated flow patterns in SHAX-like conditions. |
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UP11.00002: Identification and Modeling of Plasma Edge Helicity in QSH states on MST Ryan J Norval, John Goetz, Oliver Schmitz Edge modeling efforts are enabled by knowledge of the plasma parameters in the edge and scrape-off layer plasma. The quasi-single helicity (QSH) state necessitates a 3D model of the edge due to its helical nature. Recent measurements with probes in MST 425 kA QSH plasmas have provided needed plasma parameters, and revealed an edge pressure bulge that matches the orientation of the core helical mode. The edge plasma temperature ranges from 75 eV in the edge to 40 eV in the SoL when the mode is locked to the measurement location, while the temperature profiles are flat at 20 eV otherwise. Density drops off an order of magnitude from edge to SoL in both cases, with the absolute value higher when the QSH mode is locked over the probe. Probes measure the plasma flux to the limiters and, combined with ongoing camera measurements, constrain the recycling coefficient on the limiters to 0.85 ± 0.07. These measurements now constrain EIRENE simulations used to obtain energy losses of the edge plasma via neutral interactions. Simulations show that neutrals can represent a significant minority of total plasma energy losses, of about 20 − 45% under various QSH conditions. Exhaust pathways will be presented in detail. |
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UP11.00003: Toroidal simulations of forced Quasi-Single Helicity in RFPs with the NIMROD code U. Gupta, C. R. Sovinec, J. Boguski, M. D. Nornberg, J. S. Sarff, K. J. McCollam The Quasi-Single Helical (QSH) state of Reversed Field Pinches (RFPs) is dominated by one of the m=1 helical modes with non-dominant modes also existing at finite amplitudes. Toroidal 3D NIMROD computations of MST-like RFPs with high current, no reversal and restricted five-fold toroidal periodicity were performed to promote an equilibrium QSH state with a dominant m=1, n=5 mode at saturation. Our results show characteristic Single Helical Axis (SHAx) behavior with preservation of closed helical flux surfaces near the core, surrounded by field line stochasticity due to low amplitude secondary modes. Poloidal flow profiles have a more complicated mode structure than the global m=1 flow pattern expected from the dominant mode alone. Recent core-localized 3D ion flow measurements in the SHAx state of MST also show complex flow patterns, indicating the role of toroidal and non-linear processes in sustaining the SHAx state. Inclusion of two-fluid effects and complete toroidal resolution in NIMROD simulations is the subject of ongoing work that can give us further insights into mechanisms at play in the QSH regime. |
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UP11.00004: Determining the resistivity profile of a toroidal plasma using Integrated Data Analysis Mark D Nornberg, Daniel J Den Hartog, Lisa M Reusch, Karsten J McCollam, Stephanie Z Kubala, Darren J Craig The resistivity profile of a toroidally-confined plasma, one of the governing parameters of nonlinear MHD dynamics, is determined from a synthesis of measurements of the electron temperature, density, impurity density, and soft-x-ray emission profiles. In particular, corrections to the Spitzer resistivity due to non-uniform ion effective charge (Zeff) are comparable to neoclassical corrections due to trapped particle effects. The Zeff profile arises from the transport of highly-charged ions from the core to the mid-radius by either thermal screening (enhanced confinement) or magnetic stochasticity (standard plasmas). A self-consistent Integrated Data Analysis framework incorporates both the local information of charge exchange impurity density measurements and Thomson Scattering measurements with non-local information like neutral beam attenuation and line-integrated soft-x-ray brightness from impurity emission. The resulting resistivity profile and confidence intervals provide required information for validating nonlinear MHD simulations that predict the scaling of magnetic fluctuations and stochasticity due to resistive tearing instabilities with Lundquist number. |
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UP11.00005: The physics of m=0 modes and the RFP sawtooth crash D. Craig, A. M. Futch, R. Hesse, C. M. Jacobson In the RFP, poloidal mode number m=0 fluctuations are stable but important for nonlinear coupling of m=1 modes. We study the energy flow in/out of different modes and the role of resistivity and viscosity in the rise and fall of m=0 amplitude at a sawtooth crash. Nonlinear, visco-resistive MHD simulations using DEBS show that resistivity, viscosity, and their radial profiles all play a role. For example, increased resistivity near the m=0 resonance in the edge reduces crash duration and the m=0 rise and fall times. Energy flow into m=0 from other modes is initially positive but reverses sign early in the crash. The mean current profile becomes the energy source for the explosive increase even though the modes are linearly stable. The drop in m=0 amplitude at the end of the crash occurs because energy input from the mean current profile fades while energy transfer to other modes persists. Analysis of MST experiments yields m=0 rise times consistent with the code results but the decay is generally faster in experiment and more weakly dependent on Lundquist number. Temporal variation of the dissipation profiles in the experiment may partially explain the differences in m=0 fall time. This work was supported by the U.S.D.O.E. |
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UP11.00006: Edge impurity ion heating from toroidally localized magnetic reconnection in a reversed field pinch plasma Zichuan Anthony Xing, Mark D Nornberg, Dylan T Adams, John C Boguski, Daniel J Den Hartog, Takashi Nishizawa Edge impurity ion heating associated with toroidally local magnetic reconnection events have been observed in MST, and show characteristics of ion cyclotron damping of Alfven waves or stochastic heating. MST is capable of operating in an improved confinement regime that reduces m = 1 tearing instabilities and associated global reconnection events. In these periods of reduced tearing activity,there are occasionally short bursts of radially and toroidally localized $m = 0$ magnetic activity, referred to as $m = 0$ bursts. These $m = 0$ bursts excite turbulence that quickly travels around the torus. Heating of C and Al impurities are observed within an sample time ($20\mu s$), independent of toroidal location. The impurity heating further demonstrates significant anisotropy and charge to mass ratio dependence, as expected for ion cyclotron or stochastic heating. Unlike previous observations of impurity ion heating associated with global reconnection events, this work show global heating resulting from local reconnection. This research is supported by US DOE. |
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UP11.00007: Helical plasma-wall interaction in the RFX reversed-field pinch: toroidal effects, localization and role of sidebands Gianluca Spizzo, Matteo Agostini, Paolo Scarin, Susanna Cappello, Lionello Marrelli, Monica Spolaore, David Terranova, Marco Veranda, Nicola Vianello, Roscoe B White, Oliver Schmitz The magnetic topology of the stochastic edge of a helical reversed-field pinch in the quasisingle helicity state (QSH)\footnote{R. Lorenzini et al., Nature Phys., \textbf{5}, 570574, (2009).}, which is characterized by a dominant tearing mode with helicity $m/n = 1/7$, shows to be deeply influenced by higher harmonics $(m \pm 1)/n$, with the same $n=7$, due to the toroidal coupling of the dominant mode with the Shafranov shift. Recent analyses\footnote{M. Agostini et al., Nucl. Fusion, \textbf{57}, 076033, (2017).} show that even a modest amount ($\sim 5$~mT total) of higher $7 < n \le 23$ harmonics (the so-called ``secondary'' modes) can have sizable effects on the PWI pattern, as it is evident both in measurements of radiated power and particle influxes, and in simulations of 3D maps of parallel field-connection length to the wall $L_{c,w}$. This is a caveat for MP application in tokamaks, showing that toroidal and poloidal sidebands, though smaller than the base mode by a factor $\epsilon = a/R$, can influence the kinetic response and related issues (i.e. ELM suppression). On the contrary, an expected halvening of the amplitude of the secondary modes with $n > 7$ in the modified RFX-mod2 experiment could result in a dramatic reduction of PWI in this device. |
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UP11.00008: Anisotropic and asymmetric fast ion distribution generated by magnetic reconnection in MST plasmas Jungha Kim, Jay K. Anderson, Phillip J Bonofiglo, Robert W. Harvey, John Stephen Sarff Magnetic reconnection drives ion energization in both astrophysical and laboratory plasmas. In the reversed-field pinch (RFP), reconnecting plasma with a turbulent spectrum creates an ion distribution with distinct asymmetry and anisotropy. Both the energization mechanisms and the diffusion of the accelerated particles are crucial in creating and sustaining the distribution, which produces fusion neutron flux several orders higher than a typical thermal distribution. The resulting anisotropic fast ion distribution has been measured in MST using a neutral particle analyzer and neutron detectors. A collimated neutron diagnostic is also being developed for localized spatial measurements of neutron flux. Transport modeling can adequately explain the measurements, with the caveat that the perpendicular heating mechanism of thermal ions isn’t yet completely understood. A full orbit particle tracer is quantifies the asymmetric diffusion in velocity-space, compared with neutron decay rate data using neutral beam blips. Fokker-Planck calculations mimic the time evolution of fusion neutrons when taking into account asymmetric drive and diffusion, compared with NPA data along three lines of sight. Work supported by US DOE. |
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UP11.00009: Alfvén waves in RFP plasmas: nonlinear 3D MHD modeling and comparison with RFX-mod D. Bonfiglio, A. Kryzhanovskyy, M. Zuin, S. Cappello, M. Veranda We report a recent modeling study on Alfvén waves in reversed-field pinch plasmas, and the comparison with experimental findings in the RFX-mod device. The nonlinear 3D MHD cylindrical code SpeCyl [1] has been used to analyze configurations with increasing level of complexity. First of all, numerical solutions have been compared with analytical ones in the most simple case of a uniform axial magnetic field: an excellent agreement is obtained for both the shear Alfvén wave (SAW) and the compressional Alfvén eigenmodes (CAEs). Then, the RFP configuration has been studied by assuming perturbations with a single space periodicity. Phenomena such as phase mixing of SAW, resonant absorption of CAEs and the appearance of the global Alfvén eigenmode (GAE) are reported. Finally, the fully 3D RFP case with realistic magnetic reconnection events [2] has been investigated, showing for the first time in nonlinear RFP simulations the excitation of Alfvén waves by magnetic reconnection. Modeling results are in good quantitative agreement with the experimental characterization of Alfvénic activity observed in RFX-mod [3]. [1] S. Cappello and D. Biskamp, Nucl. Fusion 36 (1996) 571 [2] D. Bonfiglio et al., Phys. Rev. Lett. 111 (2013) 085002 [3] S. Spagnolo et al., Nucl. Fusion 51 (2011) 083038 |
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UP11.00010: Poloidal and Toroidal Programmable Power Supplies on MST D. J. Holly, B. E. Chapman, I. R. Goumiri, K. J. McCollam, J. C. Morin, A. A. Squitieri For the first time, MST has been operated with two separate IGBT-based Programmable Power Supplies, giving precise, simultaneous control of MST's poloidal (BP) and toroidal (BT) fields via pre-programmed waveforms or real-time feedback control. |
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UP11.00011: Simultaneous feedback control of the toroidal field and plasma current on MST using advanced programmable power supplies Imene R Goumiri, Karsten J McCollam, Alexander A Squitieri, Donald J Holly, John S Sarff Programmable control of the inductive electric field enables advanced operations of reversed field pinch (RFP) plasmas in the MST device. MST’s poloidal and toroidal magnetic fields (Bp and Bt) can be sourced by IGBT-based programmable power supplies (PPSs). In order to provide real-time simultaneous control of both Bp and Bt circuits, a time-dependent integrated model is developed. The actuators considered for the control are the Bp and Bt primary currents sourced by the PPSs. The control system goal will be tracking two particular demand quantities, in this case, the plasma current Ip, directly related to Bp, and the RFP reversal parameter F, closely related to both Bp and Bt. To understand the responses of Ip and F to the actuators and to enable systematic design of control algorithms, dedicated experiments are run in which the actuators are modulated, and a linearized dynamic data driven model is generated using the system identification method. We perform a series of real time experiments to test the designed controller and validate the derived model predictions. |
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UP11.00012: Measuring the Poynting flux produced by coherent field fluctuations in a reversed-field pinch plasma Derek J Thuecks, Karsten J McCollam In a reversed-field pinch driven by a toroidal electric field, the equilibrium profile is maintained by a net EMF generated by tearing modes. These tearing modes are also expected to redistribute energy in the plasma through an outwardly-directed Poynting flux. In the experiments reported on here, insertable edge probes are used to measure the Poynting flux associated with coherent fluctuations in electric and magnetic fields. Our results indicate that this outward flux is a significant fraction of the total input power, peaking during discrete magnetic relaxation events (or sawtooth crashes). The flux is also observed to reach a maximum near the magnetic reversal surface, suggesting that electromagnetic energy is deposited there by nonlinear mode coupling. These results are consistent with estimates of global power balance from time-resolved experimental equilibrium reconstructions and are compared to expectations from fluid modeling of a resistive, incompressible plasma. |
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UP11.00013: Measurement of density and magnetic fluctuations in MST tokamak plasmas using an advanced interferometer-polarimeter James R Duff, Weixing Ding, Brett Edward Chapman, Mihir D Pandya, John Goetz, Karsten J McCollam Diagnosis of the internal MHD dynamics of tokamak plasma disruptions is needed for improved understanding of disruptions and for comparison with nonlinear MHD computational modeling. We have initiated measurements of both density and magnetic fluctuations in MST tokamak plasmas, before and during disruptions. The measurements are made with an advanced, far-infrared interferometer-polarimeter comprised of 11vertical chords spanning the plasma diameter. The diagnostic can detect fluctuations with several hundred kHz bandwidth. Initial analysis of interferometry data before the disruption reveals a ~6 kHz density fluctuation, as well as a ~30 kHz feature in the toroidal spectral coherence between core chords not visible in the density fluctuation spectrum. Analysis of polarimetry data reveals an ~18 kHz magnetic fluctuation leading up to the disruption. An increase in broadband fluctuations in both the density and magnetic fluctuation spectra are observed. Correlation of these fluctuations with MHD dynamics will be performed. |
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UP11.00014: Updated and expanded S-scaling and validation studies on the MST RFP S. Z. Kubala, D. J. Den Hartog, C. M. Jacobson, K. J. McCollam, J. S. Sarff Disruptions in tokamaks and self-organization in RFPs are governed by nonlinear MHD processes that are encoded into computational models. The objectives of the study are to perform MHD validation experiments on RFPs in the MST device, focusing on magnetic fluctuation behavior and its scaling with Lundquist number, S, and to make validation metric comparisons between experimental and nonlinear extended MHD simulation results. Nonlinear MHD simulations have been run with single-fluid NIMROD and DEBS codes. RFP experiments were performed on MST including at newly-accessible low-current values using a programmable power supply. The results from both simulations and experiment were fit to b=cSα, where b is the magnetic field fluctuation amplitude. These fits give α values around -0.2, within statistical uncertainties, and respective c values match within uncertainties. New experiments will scan over S∼Te3/2, where Te, is the electron temperature. Straylight mitigation has recently enabled Thomson scattering temperature measurements across an entire minor radial profile, including at the plasma edge. Validation metric comparisons and integrated data analysis techniques are applied to these data. |
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UP11.00015: Design, installation and first measurements of a multi-energy soft x-ray (SXR) pinhole camera in the Madison Symmetric Torus (MST) Luis F. Delgado-Aparicio, John P Wallace, Patrick VanMeter, Lisa M Reusch, Mark D Nornberg, Daniel J Den Hartog, Novimir A Pablant, Manfred Ludwig Bitter, Kenneth Wayne Hill A multi-energy soft x-ray (SXR) pinhole camera has been designed and built for the Madison Symmetric Torus (MST) Reversed Field Pinch (RFP) to aid the study of particle and thermal transport, as well as MHD stability physics. This novel imaging diagnostic technique employs a pixelated x-ray detector in which the lower energy threshold for photon detection can be adjusted independently on each pixel. The local x-ray emissivity can be measured in multiple energy ranges simultaneously, from which it is possible to infer profile measurements of core electron temperature ($T_{e}$) and impurity density ($T_{e}$) with no \textit{a priori} assumptions of plasma profiles, magnetic field reconstruction constraints, high-density limitations or need of shot-to-shot reproducibility. The maximum detector frame rate is 500 Hz with expected time and space resolutions of $\sim2$ ms and $<$1 cm, respectively. Brightness measurements, and inferred spectral emissivity, during improved confinement (PPCD) and helical core (SHAx) scenarios are presented. |
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UP11.00016: Development of a forward model for a new multi-energy soft x-ray diagnostic on MST Patrick D VanMeter, Lisa M Reusch, Luis F. Delgado-Aparicio, Mark D Nornberg, Daniel J Den Hartog The multi-energy soft x-ray (ME-SXR) diagnostic installed on MST utilizes a novel calibration of the DECTRIS PILATUS3 100k x-ray detector to achieve a combination of spatial and spectral sensitivity. Forward modeling is required to extract valuable information on underlying plasma parameters (Te, ne, etc.) from data collected by the diagnostic. A full forward model is under development which calculates emission from the given plasma profiles using information from the Atomic Data and Analysis Structure (ADAS) code. ADAS calculates ionization state fractions for impurity ions, including the effects of charge-exchange with neutral particles. ADAS is also used to calculate x-ray emissivity due to free-free, free-bound, and bound-bound electron-ion interactions. The model also incorporates the response of the detector based on calibration results, including effects from charge-sharing between neighboring pixels. A method for using this model to infer plasma properties using Bayesian probability theory is presented. This model will be added to an existing integrated data analysis framework to make use of all available information. Work supported by the US DOE. |
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UP11.00017: Spatial Calibration of a New Pilatus3-Based Multi-Energy Soft X-ray Camera on MST Lisa Reusch, Abdulgader F. Almagri, Luis F. Delgado-Aparicio, Mark D Nornberg, Patrick VanMeter, Daniel J Den Hartog We present a spatial calibration for the new multi-energy soft x-ray pinhole camera based on the PILATUS3 detector installed on the Madison Symmetric Torus (MST). The detector has a two-dimensional array of 487 x 195 pixels for each of which lower-threshold cutoff energies can be independently set. The large numbers of pixels and individual energy settings provide a unique combination of spatial and spectral resolution to aid the inference of important plasma parameters (e.g. Te, nZ, Zeff). Spatial position and resolution for the 487 poloidal lines of sight were calibrated in situ using an Iron-55 x-ray source that was inserted into MST under vacuum, and scanned vertically. This poloidal calibration shows that the camera can view almost the entire minor cross section of MST with a resolution of approximately 1.0 cm (~ρi), making high spatial resolution studies of islands and helical structures feasible. The 195 toroidal lines of sight can be calibrated using target emission from in-vessel fiducials, since lack of an appropriate port means use of the Iron-55 is not feasible. |
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UP11.00018: Development of a multi-channel capacitive probe for electric field measurements with fine spatial resolution Abdulgader Almagri, T. Nishizawa, W. Goodman, J. Sarff A capacitive is one of a few diagnostics that is directly sensitive to the plasma potential. A Multi-Channel Linear Capacitive Probe (MLCP) for turbulence measurements has been developed. The MLCP has 10 spatial channels and provides 9 points of radial electric field measurements simultaneously with the spatial separation of 7 mm. The electrodes are stainless steel cylinders 2 mm long and 11 mm in diameter. These 10 electrodes are separated by 5 mm resulting into 5.7 mm of radial sensitivity to plasma potential. A BN cylinder with 2 mm wall thickness is the dielectric. The time response of this probe has been improved by a new readout circuit. A correction technique for low frequency attenuation is also developed to achieve the required time resolution. The performance of the MLCP is tested using reversed field pinch plasma; and to confirm that the MLCP resolves sub-centimeter structures and plasma potential fluctuations up to 680 kHz. Two of these probes, separated by about 5 m, were used for zonal flow study, long wave length structure in plasma potential. Probe design, read out circuit, spatial resolution, and gain calibration will be presented. Spectra of electric field fluctuation in standard and improved confinement, PPCD plasma will be presented. |
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UP11.00019: Supersonic gas injection for achieving higher density regime in RELAX RFP Satoshi Yoshioka, Sadao Masamune, Akio Sanpei, Haruhiko Himura RELAX is a low-aspect-ratio (R/a=2=0.5 m/0.25 m) reversed field pinch (RFP) machine to explore the geometrical optimization in the RFP. Typical discharge parameters are Ip~100 kA with flat-topped phase of ~2 ms. The electron density ne is around 1019m-3 and temperature around 100 eV. The electron beta values have reached higher than 10% at the normalized density n/nG~0.3 where nG is the Greenwald density. Our scenario to achieve higher density at high current is the use of supersonic gas injection after achieving high current with low density. We have developed a gas injection system consisting of a fast-acting electromagnetic valve combined with a simulated Laval nozzle. The gas flow velocities were estimated by using the time-of-flight technique. We will report initial experimental results, basic data of the gas injection system, and detailed discussion on our supersonic gas injection scenario. |
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UP11.00020: The role of neutrals for power dissipation in the Small Angle Slot divertor (SAS) in the DIII-D tokamak Livia Casali, Brent M Covele, Houyang Guo, Huiqian Wang, Morgan Shafer, Auna Louise Moser SOLPS modeling of experiments with the new SAS divertor shows more than a factor of 2 reduction in Te at the strike point compared to a matched open divertor case. This is due to the fact that SAS targets redirect neutrals towards the separatrix, which thereby enhances the neutral buildup by roughly a factor of 4, thus increasing energy losses and facilitating detachment. Moving the strike point from the inboard side to the deep corner of the slot decreases Te and heat flux by a factor of 3 at the strike point but leads to higher values in the far SOL indicating that a too strongly focused neutral distribution may sacrifice far SOL cooling for increased strike point cooling. Increasing the X-point height with a fixed strike point location leads to a decreased molecular density by more than a factor of 2 increasing Te and heat flux. These results, in qualitative agreement with the experimental data, underlie the importance of controlling recycling neutrals for divertor power dissipation. |
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UP11.00021: Comparing Definitions of Detachment on DIII-D Cameron M Samuell, Steven L Allen, Max E Fenstermacher, Aaro E Jarvinen, Charles J Lasnier, Adam Mclean, Joseph L Barton, Jonathan Watkins, Jose A Boedo, Houyang Guo, Anthony W Leonard, Auna L Moser, Dan Thomas, Morgan Shafer, Robert Wilcox, Huiqian Wang DIII-D’s broad range of divertor diagnostics are used to characterize both the onset and trajectory of H-mode detachment for a variety of detachment definitions. Varying definitions of detachment focus on pressure loss, dropping electron temperature, rising neutral pressure, ion flux roll-over, and heat-flux reduction. Clearly defining the state of detachment therefore relies on combining diagnostics so that the relative importance of each physical process can be compared. Identifying the sensitivity of these processes to upstream conditions and radiative power fraction facilitates identifying unstable detachment regimes. On DIII-D, detachment identification is achieved using Langmuir probes, Thompson scatting, EUV/VUV, visible and near-infrared spectroscopy, visible and IR imaging, coherence imaging, neutral pressure gauges, and bolometry. ELM “burn through” in partially detached plasmas is discussed whereby periodic reattachment can occur with associated rises in heat flux and high energy particle transport to the divertor whose magnitude depends on an interplay between degree of detachment and pedestal characteristics. |
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UP11.00022: Parallel Energy Transport in Detached DIII-D Divertor Plasmas Anthony Leonard, Jeremy Lore, Mathias Groth, Aaro Jaervinen, Cameron M Samuell A comparison of experiment and modeling of detached divertor plasmas in DIII-D is examined in the context of parallel energy transport due to electron conduction and plasma convection in order to validate and improve models used for divertor design. Power balance analysis is carried out to determine parallel heat flux and energy dissipation as a function of distance from the divertor target. The relative fractions of conductive and convective heat flux are determined from Thomson scattering measurements of the divertor parallel Te gradient. Modeling with the fluid code SOLPS is found to underestimate experimental measurements of divertor heat flux radiative dissipation due to steeper Te gradients in the region of T ≤ 20 eV resulting in smaller radiating volume. VUV spectroscopy measurements of CIII, CIV and Ly-a reveal a much more distributed radiation pattern from the X-point to the target than produced by modeling. Possible causes of this discrepancy that are examined include increased parallel energy transport through plasma convection and plasma turbulence. |
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UP11.00023: Boundary Fluctuations and Transport During Detachment Jose Armando Boedo, Dmitry Rudakov, Igor Bykov, Eric Matthias Hollmann, Adam Mclean, Charles J Lasnier, Huiqian Wang, Livia Casali, Brent M Covele, Anthony W Leonard Divertor plasma turbulence levels increase when detachment is approached in DIII-D. The root mean square of fluctuation levels of saturation current, floating potential, electron temperature and density measured by a divertor scanning probe increase > 100% of average levels as the divertor plasma starts detaching(Te<10eV)Data from divertor Thomson Scattering and target probes show a scatter of 100% of the mean value, consistent with probe measurements. Increased plasma fluctuation levels often lead to enhanced radial particle and heat transport and to divertor plasma mixing. Fluctuation levels at the midplane feature increased turbulent transport by ~50% during detachment. Modelling by SOLPS5.1 with enhanced transport coefficients are performed and compared to experiments. |
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UP11.00024: Carbon Ion Flow Measurements in the DIII-D Divertor and Scrape Off Layer with Comparisons to UEDGE Modeling withFull Drifts S. L. Allen, C. M. Samuell, A. E. Jaervinen, W. H. Meyer, Max Fenstermacher, Adam Mclean Coherence Imaging Spectroscopy has been used on DIII-D to measure He 1+ and C2+ ion flows; one system views the divertor and a second has a wide angle view of the whole Scrape Off Layer. A tuneable laser provides an unshifted zero velocity reference image which is recorded after each plasma shot. Fourier analysis of the interferogram, followed by tomographic reconstruction, results in images of ion emissivity and flow velocity. Data has been obtained over a wide range of conditions, including divertor detachment. A general feature of the data is that the C2+ flow velocity in the divertor is toward the divertor plate with a velocity in the range of 20-30 km/s, and the sense of the flow flips with the toroidal field, in agreement with UEDGE modelling. Tomographic reconstructions of measured 2D C2+ flow patterns generally show a broader spatial distribution than in the UEDGE modelling with full drifts. |
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UP11.00025: Effects of drifts on divertor particle distribution and detachment near target plate in DIII-D Huiqian Wang, Jonathan Watkins, Joseph L Barton, Houyang Guo, Anthony W Leonard, Dan M Thomas, Mathias Groth, Aaro E Jarvinen For B×∇B drift toward the open divertor in DIII-D LSN (lower single null) plasmas, the particle flux near the inner divertor target exhibits a double-peak structure with low electron temperature. This results from the interplay between strong poloidal Er×B flow from the outer divertor toward the inner divertor and enhanced radial Eθ×B flow at the inner target. Reversing the BT direction reverses both flows, leading to a broad particle flux profile in the outboard SOL exhibiting a similar double-peak structure. In addition, reversing BT facilitates the achievement of detachment near the outer strike point at lower upstream density, presumably due to the E×B drift. However, Jsat, i.e. the particle flux measured by the probe ion saturation current, remains high in the SOL, rendering it difficult to achieve fully detached plasmas. In contrast, divertor detachment with a cold and flat temperature profile and significant Jsat rollover can be achieved at both target plates with the forward Bt. |
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UP11.00026: SAS divertor interpretation using SOLPS model construction and visualization tools in OMFIT David Eldon, Livia Casali, Brent M Covele, Brian A Grierson, Orso Meneghini Sensitivity of detachment to wall recycling coefficient in the DIII-D SAS divertor is studied using SOLPS. The OMFIT framework is used to rapidly construct families of SOLPS models with varying parameters, manage jobs, and visualize results. These improvements in setup and data visualization allow faster access to physics insights from SOLPS. Groups of runs are created from instructions for scanning a particular setting, and key information from the SOLPS manual is presented alongside settings as they are changed. The settings for existing runs can also be read back to clarify how runs differ and to track progress. OMFIT parses SOLPS files so they can be read and modified by python scripts before deploying jobs. Extracting results into arrays allows easier comparisons between SOLPS runs and to other data, greater ability to customize plots, and the ability to interface with other OMFIT modules. In particular, SOLPS is interfaced to the SCOPE module’s database creator to provide visualizations of parameter spaces mapped by families of related SOLPS models. These new tools in OMFIT are used to support a study of sensitivity of detachment to recycling coefficient in the DIII-D SAS divertor. |
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UP11.00027: Automatic UEDGE simulations of a large series of time-slices for tokamak discharges Olivier Izacard, Egemen Kolemen, Orso Meneghini, David Eldon, Maxim V Umansky One challenge of transport edge fluid codes is to extend their computational capability for fast and automatic convergences to be used for analysis of transport on the open field lines. One of the main issues is the missing standardization of an automatic grid generation without requiring manual modifications by the users. Thanks to the development of the grid generator Gingred [1], the automatic generation of UEDGE grids becomes accessible using only the magnetic equilibrium from CAKE [2] at every few ms. With the previous optimization of UEDGE [3], it is now possible to automatically converge a multi-fluid physics model for arbitrary single null shapes. The UEDGE physics model includes deuterium plasma and with DIII-D like transport coefficient profiles. Power and particle fluxes at the core interface are matched accordingly to diagnostics. Time evolutions of the detachment behavior and particles and heat fluxes computed from UEDGE are analyzed via the OMFIT [4] framework for DIII-D experiments and the differences before and after ELMs on the detachment tendencies are shown. |
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UP11.00028: Design for Thomson Scattering measurements in the DIII-D SAS Divertor using in-vessel optics Fenton Glass, Thomas Neil Carlstrom, Detao Du, Adam G McLean, Doug Taussig The Small Angle Slot (SAS) divertor configuration in the DIII-D tokamak presents a challenging geometry for obtaining Thomson scattering measurements of electron temperature and density in this region. Divertor carbon tiles were previously modified to allow the Thomson-scattered light to reach the ex-vessel collection lens. These modifications still restrict the measurement points to no closer than 65mm from the base of the divertor slot. Measurement positions closer to the base are accessed using in-vessel optical components directing Thomson-scattered light from near the slot base to the collection lens. High stray laser light levels, which affect the calibration of the electron density, are mitigated using a Raman-scattering calibration method. This technique collects absolute system response data using a non-monoatomic gas to scatter light to wavelengths further away from the overwhelming intensity at the laser wavelength. |
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UP11.00029: Design of a scanning 2D Divertor Thomson scattering system on DIII-D Detao Du, Thomas Neil Carlstrom, Fenton Glass, Adam G McLean, Doug Taussig, David Su, Rejean L Boivin Measurement of plasma electron temperature and density in the divertor region of high triangularity advanced Tokamak discharges is critical for characterization of boundary performance and for model validation. Previous divertor Thomson scattering (DTS) measurements in DIII-D were restricted to spatial locations along a fixed vertical Nd:YAG laser beam path optimized for low triangularity and without pumping access. With a single chord, the plasma has to be radially swept across the fixed laser path in order to study the full divertor. Recently, DIII-D has demonstrated the capability to redirect the radial measurement location using in-vessel optics. A further expansion is proposed by dynamically scanning a 50 Hz laser beam inside the Tokamak to up to 16 radial chords using an external fast steering mirror. A novel method of synchronizing the collection optics to follow the laser beam has been tested. This expansion will greatly enhance the capability of DTS for study of high performance plasmas and study of core-edge integration. The system will be described and initial prototype results will be presented. |
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UP11.00030: 3D Modeling of impurity ion collection by a probe for application to data from the DIII-D tungsten ring campaign John David Elder, Peter C Stangeby, Zeke Unterberg, Anton L Neff, David Donovan, Jacob H Nichols, Shawn Zamperini During the tungsten ring campaign on DIII-D, a collector probe system with multiple diameter, dual-facing collector rods was inserted into the far scrape off layer (SOL) near the outer midplane to measure the plasma W content. The deposition on these probes has been modeled using 2D OEDGE and the analytic model from [1]; however, it is challenging to experimentally validate the analytic model. In addition, 2D deposition measurements across the poloidal probe face are also available and these cannot be interpreted with the analytic model. An initial 3D Monte Carlo model of ion collection by a probe has been developed by modifying and enhancing the 3D capabilities of the LIM code [2]. Dynamic memory allocation, mirrored and absorbing boundary conditions, and new particle launch options were added. Collector probe simulations indicate greater deposition at the poloidal edges of the probe compared to the center which is similar to the experimental measurements. The simulated deposition on the collector probe is compared to the predictions from the analytic model. [1] P.C. Stangeby, Physics of Fluids 30, 3262 (1987) [2] P.C. Stangeby et al., Nuclear Fusion, 28(11), 1945 (1988) |
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UP11.00031: W deposition patterns on a collector probe in the far SOL provide insight into near SOL phenomena Shawn Zamperini, David Donovan, Zeke Unterberg, David Elder, Peter C Stangeby, Jonah D Duran, Jacob Nichols, Anton L Neff, Dmitry Rudakov, William Raymond Wampler Collector probes inserted into DIII-D during the Metal Rings Campaign in June 2016 collected measurable amounts of tungsten via Rutherford Backscattering Analysis (RBS) and Laser Ablation Mass Spectrometry (LAMS). The operating conditions varied greatly over the campaign, yet, consistent patterns and trends were observed for the probes. Radial decay lengths of W deposits on opposite-facing sides of the probes support a simple model describing transport to each probe face. Comparing the degree of asymmetric W deposition to near SOL parameters indicates that W reached the probe from the direction of the crown of the plasma, even though the only W-coated tiles were in the lower outer divertor with all other tiles being graphite. 2D surface profiles of the isotopic W probe deposits via LAMS show increased W content toward the edges of the probe faces, and development of a 3D collector probe model called 3D LIM is underway to model these effects. The 2D profiles also show random spots of increased W content. These spots are tentatively attributed to W particulates originating from some location in DIII-D where W accumulated over the campaign. |
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UP11.00032: Heat flux measurements based on Surface Eroding Thermocouple in Small Angle Slot divertor target on DIII-D Jun Ren, David C Donovan, Jonathan Watkins, Huiqian Wang, Ezekial Unterberg, Murphy Christopher, Dan Thomas, Rejean Boivin The new Surface Eroding Thermocouples (SETC) in the small angle slot (SAS) divertor target have been used as a fast (sub-10ms) heat flux measurement in DIII-D. In recent experiments, measurable heat flux profiles were obtained by the SETCs when the outer strike point was swept along the SAS target. Those results showed that heat flux decreases on the target as plasma density increases during density scans. During density ramping experiments with the ion B×∇B drift away from the divertor target, a significant reduction in heat flux in the outer Scrape-Off-Layer (SOL) region was observed at the onset of detachment, which was consistent with the occurrence of the rollover of the ion saturation current as density increases. In addition, heat flux on the SAS target also showed a strong relationship with neutral pressure build up in the SAS divertor. Further detailed comparisons will be made with different E×B drift directions and outer strike point locations. |
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UP11.00033: Development of UV Spectroscopy for Improved Measurements of Tungsten Erosion & Re-deposition D.A. Ennis, C.A. Johnson, S.D. Loch, C.J. Favreau, T. Abrams A new high-resolution and high-throughput instrument is being developed to resolve the most promising W I lines observed in the range between 200 to 400 nm during ELMing and ELM free periods. Recently completed spectral surveys in the DIII-D and CTH experiments have identified numerous W I emission lines in the UV region which can be combined with atomic predictions to determine the erosion of plasma facing W surfaces. The importance of metastable level populations on the W spectrum requires that multiple W I emission lines be monitored simultaneously to accurately characterize erosion rates. These design criteria specify a wavelength resolution of 0.2 Å over a window of at least 12 nm with better than 20 ms resolution for measurements using vertically oriented slight lines into the DIII-D divertor. The design of a fiber coupled system located close to the DIII-D device for minimum UV signal attenuation is presented along with expected signal levels for DIII-D plasma conditions. |
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UP11.00034: Current status of atomic physics data for the diagnosis of erosion using low charge states of tungsten and molybdenum Stuart David Loch, David Ennis, Michael Stuart Pindzola, Curtis Johnson, Connor Favreau, Connor Ballance, Ryan Smyth, Michael Turkington Spectroscopic techniques to measure erosion and transport for high-Z plasma facing components require accurate atomic data. The near neutral ion stages are most critical for erosion diagnostics, requiring non-perturbative calculations. The first datasets for W and Mo used a number of approximate methods, perturbative approaches, and selected non-perturbative calculations. In recent years there have been large-scale non-perturbative calculations for low charge states of W and Mo. We present a review of the new data. This includes recent R-matrix excitation calculations for neutral W, W3+, neutral Mo, and Mo+. Ionization calculations include time-dependent close-coupling calculations for neutral W and W+, along with R-matrix calculations for W+, W2+, and a metastable of neutral W. The new data represents a significant improvement over the previously available S/XB, emissivity and effective ionization data. |
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UP11.00035: Evaluation of Small Angle Slot (SAS) divertor with toroidal tungsten-tile rings in DIII-D using OEDGE Xinxing Ma, Tyler W Abrams, John D Elder, Houyang Guo, Jerome Guterl, Peter C Stangeby OEDGE has been used to model the W erosion, transport and deposition during the 2016 experiment that had two toroidal rings of W-tiles in the lower open divertor on DIII-D. The modeling approximately reproduces both the divertor-facing asymmetries and radial decay of the W deposition on an outer-midplane-SOL collector probe. A Small Angle Slot (SAS) with carbon tiles has been installed in the upper divertor of DIII-D. Initial experiments show that SAS divertor can achieve cold divertor plasmas at lower upstream plasma densities with low Te across the target and over a large range of H-mode densities in some operation conditions, which is dependent on strike point locations. The performance of SAS divertor with tungsten rings is evaluated using the OEDGE code with coupled SOLPS plasma background solutions. Results for the effect of the location of the strike point and the location of tungsten sources are presented. Parameter scans of input power, upstream density and cross-field transport coefficient are also reported. This analysis will inform the next stage SAS upgrade with W rings in DIII-D. |
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UP11.00036: WallDYN modeling of tungsten migration in the DIII-D divertor Jacob H Nichols, Tyler W Abrams, David C Donovan, John D Elder, Dmitry Rudakov, Klaus Schmid, Peter C Stangeby, Ezekial Unterberg, William R Wampler The DIVIMP-WallDYN code has been applied to DIII-D for the first time, self-consistently combining impurity production and transport models to dynamically model tungsten (W) re-deposition and re-erosion during the 2016 Metal Rings Campaign. Modeling of the transport of strike point-sourced W to other locations in the divertor during DIII-D L-mode discharges is presented. Collector probes in DIII-D, DiMES and MiMES, have been used to measure W deposition across a range of plasma conditions. However, modeling is needed to link measured W sourcing to measured deposition, and benchmarking with experimental observations from collector probes and W-I spectroscopy can be used as figures of merit for the model. While many aspects of the model are constrained by diagnostics, some key parameters remain ill-constrained, and model sensitivity to these parameters (plasma ion temperature, radial and poloidal electric fields, and mixed-material sputtering) is examined. It is found that both long-range single-step transport and short-range transport due to multiple re-erosion steps are important contributors to W transport in DIII-D. |
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UP11.00037: Spectroscopic Investigation of Tungsten Erosion* Curtis Johnson, David Ennis, Stuart Loch, Connor Favreau, Ryan Smyth, Connor Ballance, Tyler W Abrams, Ezekial Unterberg
Erosion of tungsten at the plasma boundary is spectroscopically diagnosed using improved predictions for atomic coefficients representing the ionizations per photon (S/XB). Previous W I theory estimates do not agree with the experimentally observed S/XB ratio for the 400.9 and 429.4 nm lines but new atomic data is used to calculate a ratio that is consistent with experimental observations. The Erosion of tungsten at the plasma boundary is spectroscopically diagnosed using improved predictions for atomic coefficients representing the ionizations per photon (S/XB). New W I S/XB calculations utilize R-matrix excitation data and classical exchange impact parameter (ECIP) ionization data for recently identified UV emission lines. Theoretical spectral intensities for numerous W I lines in the 200-400 nm region are compared to measurements in DIII-D and the Compact Toroidal Hybrid (CTH). Predicted spectra agree with CTH observations but differ from DIII-D observations by up to a factor of two. The contribution of metastable states to these discrepancies will be discussed. |
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UP11.00038: Metal vs. graphite divertor: effects of material choice on intrinsic fueling source Igor Bykov, Eric Matthias Hollmann, Alexander Pigarov, Jerome Guterl, Mathias Groth, Thomas H Osborne, Ezekial Unterberg The Metal Rings Campaign (MRC) in DIII-D [1] has demonstrated that a relatively small (0.6% of the total wall area) fractional W coverage in the divertor of a full-C tokamak can affect the edge plasma. The density profile in L-mode discharges was 8% higher with outer strike point (OSP) placed on W, which can be attributed to higher divertor fueling flux due to higher flux and energy of reflected neutral D atoms [2]. Simultaneous measurements of D atoms and D2 molecules recycling at OSP enabled us, for the first time, to quantify the relative contribution of D atoms to the total recycling flux at the W and C surface. Better fueling with W than with C is qualitatively similar to what was seen on ASDEX, but upon a complete change from C to W PFCs [3]. To assess the influence of the added W in the divertor, we will also present results of EDGE2D-EIRENE simulation. |
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UP11.00039: Reactor relevant heat flux and displacement damage on ultra-fine grain tungsten Joseph Barton, Dean Alan Buchenauer, Robert D Kolasinski, William Raymond Wampler, Richard Nygren, Ryan Nishimoto, Jeff Chames, Dmitry Rudakov, Zak Fang, Jonathan D Coburn, Charles J Lasnier, Jonathan Watkins, Zeke Unterberg, Houyang Guo Ultra-fine grain tungsten (UFG W) and ITER-grade W plasma-facing samples are exposed to reactor relevant conditions in the DIII-D divertor using DiMES. Effects of displacement damage on the deuterium (D) retention is examined using flat samples pre-damaged with 12 MeV Si ions up to 0.6 dpa. Damaged UFG W releases its trapped D inventory at temperatures < 500 C, but most of the inventory in damaged ITER W is still retained at 500 C. Tritium retention could therefore be mitigated in UFG W with routine low temperature tile baking. Surface damage effects are also examined using protruding angled samples exposed to heat fluxes up to 10 MW/m2 during ELMs. The ITER W shows inter-granular cracks and develops < 1 μm diameter holes on the surface. UFG W develops cracks and vertically displaced grains along with surface features as large as 10s of μm consistent with local melting and droplet migration. These observations contradict e-beam experiments in which transient ELM-like heating causes less damage to the surface of UFG W materials than ITER W. Synergistic effects between heat and plasma particle fluxes may explain the differing results. |
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UP11.00040: SOLPS modeling of neutral fueling effects on pedestal structure in DIII-D Aaron Sontag, Morgan Shafer, Daisuke Shiraki, Florian Martin Laggner, Andrew Nelson Neutral fueling location was varied in a series of DIII-D discharges by comparing plasmas fueled with: 1) core NBI only, 2) core NBI + deuterium pellet injection, 3) minimal NBI + ECH + external gas puff and 4) minimal NBI + ECH. The discharges with core fueling show increased pedestal density and an increase of over 30% in the ratio of the pedestal to separatrix density. Initial interpretive SOLPS modeling of these experimental results shows that for discharges, which are otherwise similar except for the fueling location, very similar anomalous diffusion profiles are required to match the experimental values when modeled with no pellets or gas puff, and drift and pinch effects are off. Pellet fueling is modeled as a constant neutral source in the pedestal region where pellet ablation is observed. The addition of a fueling source in the pedestal due to pellet injection requires a significant change in the anomalous diffusion to match the experimental density profile from the recycling only fueled case. SOLPS-ITER will be used to determine the relative importance of drifts as well as anomalous diffusion and pinch effects on the particle transport in the pedestal. |
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UP11.00041: Effectiveness of wall conditioning by means of boron powder injection in DIII-D plasmas Alessandro Bortolon, Alan W Hyatt, Theresa M Wilks, Jose Armando Boedo, Christopher P. Chrobak, Max Edward Fenstermacher, Rajesh Maingi, Alex Nagy, Jun Ren, Dmitry Rudakov, Cameron M Samuell, Morgan W Shafer, D. Donovan, Erik P Gilson, Robert A. Lunsford, Dennis K. Mansfield, Raffi Nazikian We present DIII-D experiments demonstrating effective wall conditioning by boron (B) powder injection into tokamak plasma discharges. In present-day fusion devices, boronization is commonly used to condition plasma-facing components, to allow operation with reduced wall fueling and impurity sources. Typically gaseous boron in the form of hazardous diborane is introduced in glow discharges. The beneficial effects last until the boron layers are eroded, challenging the scalability of the technique to steady state devices with high power and plasma exhaust rates. In a recent DIII-D experiment, increasing amounts of boron were injected during plasma discharges with identical target parameters. Metallic B powder (size < 0.1 mm) was injected to the upper scrape-off layer by a recently installed impurity powder dropper, for time intervals of 1‑3 s, at rates 5-100 mg/s. B powder injection correlated in a step-wise reduction of plasma density, neutral pressure and radiation from impurity radiation (C, N, O), all signs of improved wall conditions. The effectiveness of this technique for enabling access to low collisionality scenarios such as QH-mode will be discussed. |
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UP11.00042: H-mode pedestal characteristics of SAS divertor discharges on DIII-D Tom Osborne, L. Casali, Houyang Guo, Auna Louise Moser, Morgan W Shafer The Small Angle Slot (SAS) divertor installed on DIII-D combines high closure with small incidence angle to achieve detachment over the entire divertor at low density. SAS discharges with BxgradB drift away from the X-point have improved energy confinement at a given pedestal density, nePED, in comparison to an open divertor. SAS discharges have higher temperature and reduced thermal diffusivity from the top of the H-mode pedestal inwards for a given density. The higher TPED is associated with a wider temperature pedestal with more of an outward shift of the ne pedestal top relative to the Te top. Both the SAS and open divertor configurations lie on the ballooning boundary for the peeling-ballooning mode at a similar normalized pressure gradient, but the pedestal width of the SAS discharge exceeds the EPED1.0 scaling by ~15%. Neon injection into a SAS discharge resulted in increased core Ti and rotation and increased pedestal pressure gradient associated with improved ballooning branch stability due to increased diamagnetic stabilization. |
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UP11.00043: Effect of opacity on the density pedestal structure in DIII-D and Alcator C-Mod Saskia Mordijck, Richard Joseph Groebner, Tom Osborne, Thomas Neil Carlstrom, Fenton Glass, Philip B Snyder, Jerry W Hughes, Aaro E Jarvinen, Antti Salmi, Tuomas Tala, Florian Laggner, George R McKee, Richard A Moyer, Terry L Rhodes, Lei Zeng Analysis of experimental data from DIII-D and C-Mod is used to study the role of opacity on the density pedestal structure. Dα measurements at the midplane on DIII-D indicate that we were able to push the ionization front further out by increasing the separatrix and SOL electron densities. We find that in pressure-record breaking EDA H-modes on C-Mod, adding more gas fueling does not result in an increase of the pedestal density, only an increase in the SOL density. In DIII-D, using gas puff modulations, we observe that when the outer strike point detaches, this strongly reduces the amplitude of the electron density response in the SOL and results in a larger amplitude inside the separatrix. Moreover, we observe the creation of an up-down asymmetry in the electron density with increased gas puff/opacity. Close to the X-point, up to ψN ~ 0.98, the electron density is larger than upstream in DIII-D. SOLPS modeling will be performed to get a better evaluation of the poloidal and radial ionization profiles as well as to better understand the origin of the up-down asymmetry of the electron density in DIII-D. |
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UP11.00044: Pedestal Turbulence Characteristics in Super H-mode Plasmas on DIII-D* Yifan Wu, George R McKee, Zheng Yan, Matthias Knolker, Philip B Snyder, Thomas Osborne, Richard Joseph Groebner, Min Xu, Yi Yu, Minyou Ye Two distinct frequency bands of broadband density turbulence are observed in the pedestal of Super H-mode plasmas at DIII-D with a 2D BES array. One is observed between 60 kHz to 250 kHz, while the other is from 300 kHz to 500 kHz and appears to be unique to Super H-mode pedestals. Significant coherence and cross power are found between poloidally adjacent BES channels at these two distinct frequency bands, even though the amplitude of higher frequency band is only 1/10 of the lower frequency band. The lower frequency band is observed in rho=0.9-1.0 and propagates in the ion-diamagnetic direction, while the higher frequency band is located at rho=0.9-0.94 and propagates in the electron-diamagnetic direction. Comprehensive spatial characteristics of these pedestal instabilities, temporal evolution during the Super-H pedestal development, and parametric dependencies will be presented, along with comparisons to theoretically predicted mode features, such as KBM or MTM, that might exist in the pedestal of Super H-mode plasmas. Also, the understanding of the turbulence characteristics and pedestal physics of Super H-mode is critical to achieve high performance successfully. |
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UP11.00045: Pedestal turbulence characteristics during low Z and medium Z impurity injection on DIII-D Maximillian Major, Morgan Shafer, L. Casali 2D measurements of long wavelength density fluctuation characteristics have been obtained in the pedestal region on DIII-D with low to medium-Z (N2, Ne, Ar) radiative impurity injection and are found to correlate with pedestal height and global confinement changes. Plasma confinement and pedestal pressure respond differently to these impurities, with some impurity accumulation and confinement improvement observed with Ne and Ar seeding. Pedestal density fluctuations are correspondingly slightly reduced and the appearance of a higher frequency band (measured by BES) is observed with Ne seeding. In an experiment with full detachment, fluctuations were observed to increase with N2 seeding, suggesting an increase in pedestal transport; no improvement in global confinement was observed, in contrast to results observed on the AUG tokamak. Changes in characteristics of turbulence, such as amplitude, correlation lengths, turbulence decorrelation rate and turbulence flow velocity with different impurity seeding are investigated. This will provide insights into the mechanisms behind radiative impurity effects on transport in reactor-relevant conditions. |
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UP11.00046: Vertical plasma oscillations as a tool to perturb the pedestal in the DIII-D tokamak Florian M. Laggner, Egemen Kolemen, Ahmed Diallo, Richard Groebner, Kshitish Barada, Andrew Nelson, Thomas H. Osborne, the DIII-D Team Previous experimental approaches used vertical oscillations, also called ‘jogs’ or vertical kicks, for ELM pacing. This contribution presents experiments, where such oscillations were applied to probe inter-ELM pedestal instabilities. Since fast vertical plasma movements induce current mainly at the plasma edge, the applied oscillation can be a strong actuator on pedestal microinstabilities. Such instabilities typically appear as high frequency fluctuations during the last phase of the ELM cycle, when the edge pressure gradient is saturated. In this phase the pedestal is stable but marginal to the stability limit. Therefore, if a perturbation is applied by a vertical plasma oscillation, it becomes highly probable that an ELM crash is triggered. Further, the oscillations modify the detected frequency of the inter-ELM fluctuations, depending on the direction of the induced current. Not all frequency bands are similarly modified, especially the broad band high frequency fluctuations remain unchanged. It is suggested that the frequency changes are related to a shift of the instability location with respect to the rotation profile or a direct modification of the edge rotation. |
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UP11.00047: Investigating DIII-D H-mode pedestal microstability and transport with CGYRO Walter Guttenfelder, Emily A Belli, Jeff Candy, B. A. Grierson, R. J. Groebner The spectral multiscale gyrokinetic code CGYRO is used to calculate theoretical microstability and transport in the edge region of DIII-D H-mode discharges. We focus on two discharges with different divertor geometries in an attempt to clarify the role of transport vs. particle source in setting the pedestal density and temperature profiles. Initial linear simulations predict that ion scale instabilities dominate at the top of the pedestal (ψN=0.9-0.96), where strong rotation shear enhances the growth rates and leads to significant changes in the predicted quasilinear fluxes. In contrast, in the steep gradient region (ψN=0.98), E×B shearing rates are much larger than ion scale instabilities. Instead, the electron temperature profiles appear to closely follow the electron scale ETG instability threshold. Nonlinear simulations are being pursued to predict the energy and particle fluxes in these regions for comparison to experimental analysis. |
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UP11.00048: Effects of medium-Z impurity on H-mode pedestal in DIII-D Heng Lan, Tom Osborne, Brian Grierson, Brian Victor, Richard Joseph Groebner, Kshitish Barada, Terry L Rhodes, George R McKee, Anthony W Leonard, Huiqian Wang, Guosheng Xu The effects of neon level on H-mode pedestal structure at two heating powers in DIII-D lower-single-null discharges are studied. In 3 MW discharges, neon injection resulted in an increase in plasma stored energy and confinement of 30% and 25% respectively. After neon injection, the ELM frequency dropped, the electron pedestal density and pressure increased, and the pedestal temperature slowly decreased. In 6 MW discharges, at the same and even larger neon level, the plasma confinement increased slightly and stored energy gradually increased by up to 15%, while the trend of ELM and pedestal changes was similar to 3 MW discharges. In the neon enhanced confinement phases, the critical pressure gradient along the ballooning boundary of Peeling-Ballooning mode increased. The observed pedestal width will be compared with that predicted by EPED1.0. STRAHL simulations of impurity transport will discussed, as well as the relation between fluctuations and the observed transport changes. |
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UP11.00049: Fueling Efficiency in Pellet Fueled Plasmas on DIII-D, AUG, and LHD and Implications for Larger Burning Plasmas Larry Baylor, Daisuke Shiraki, Peter Lang, Ryuichi Sakamoto Pellet fueling experiments on medium sized fusion experiments, both tokamaks and helical devices, have been examined from the perspective of fueling efficiency, defined as the fraction of the pellet mass that is measured as a plasma particle increase from a single pellet. This fueling efficiency has been determined as a function of pellet penetration depth, injection location and magnetic configuration in these different yet similar plasma volume devices and we find that it has a strong dependence on penetration depth. This adds significant uncertainly in projecting the fueling efficiency of shallow pellet penetration expected in burning plasmas with H-mode pedestals, the triggering of ELMs, and different magnetic topology. We describe these dependencies and point out where more research is needed and what the implications are for divertor operation, pumping, and the fuel cycle operation of future reactors.
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UP11.00050: Integrated Simulations and Applications of Pellet Injection for Tokamak Central Fueling and Management of Transient Events* Lang L Lao, Yueqiang Liu, Brendan C Lyons, Joseph McClenaghan, Orso Meneghini, Paul B Parks, Philip B Snyder, Wen Wu, Charlson C Kim, Vincent S Chan, Jiale Chen, Jiangang Li, Jie Zhang, Michael Lehnen, Alberto Loarte Central fueling and management of transient events are critical tokamak research issues. Scoping studies using a new Pellet Ablation Module (PAM), developed based on an analytic ablation expression, and model density and temperature profiles, show that central fueling can be achieved for L-mode reactor plasmas, but would be challenging for H-mode reactor plasmas, as expected. Peak ablation location depends strongly on pellet injection velocity and size and central electron temperature. A PiC-based Shattered Pellet Injection (SPI) model has been developed to mimic the SPI fragment plume and implemented in the 3D MHD code NIMROD. DIII-D and ITER SPI simulations show that MHD mixing plays an important role in the dynamics of the thermal-quench event. NIMROD simulations with constant and temperature-dependent resistivity and thermal conduction show that the ITER baseline scenario is able to achieve full thermal quench with nearly 100% radiation with pure Ne or mixed Ne/D2 SPI. Detailed DIII-D and CFETR central-fueling simulations with self-consistent OMFIT core-pedestal workflows and DIII-D and ITER NIMROD SPI simulations will be presented.
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UP11.00051: Characterization of inter-ELM Burst Activities in the Pedestal Ahmed Diallo, Julien Dominski, Florian Laggner, Matthias Knolker, Gerrit J Kramer, Kshitish Kumar Barada, Lei Zeng, George R McKee We investigate the inter-ELM stage during which multiple pedestal localized modes have been observed to be correlated with pedestal density and temperature prior to the ELM onset. We report on bursting activities of high frequency modes (~350 kHz) which appear between ELMs. These bursts have also been observed in JET ILW (Bowman et al. NF 58 016021 2018) and in AUG (Hennequin et al. EPS 2017 P1.167). The low frequency modes remain unchanged during these bursts. The high frequency mode's onset is correlated with the pedestal density gradient and localized in the steep gradient region. The bursts are detected using BES, DBS, and magnetic probes. The bursts have an effect on the pedestal temperature evolution, while the pedestal density evolution remains unmodified. This clearly suggests that the bursts provide mainly electron heat transport. Bicoherence analysis shows that the bursts result from nonlinear interactions of pedestal localized modes. Analysis of connection between bursts and an ELM event (hypothesized to be multi-mode interactions) will be discussed. |
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UP11.00052: Utilizing M3D-C1 to understand triggering of ELMs in pellet pacing experiments in DIII-D ITER-like plasmas S. J. Diem, L. R. Baylor, N. M. Ferraro, J. L. Herfindal, B. C. Lyons, D. Shiraki, R. S. Wilcox Large edge localized modes (ELMs) in magnetically confined plasmas can lead to the sudden release of thermal and magnetic stored energy and can potentially cause damage to plasma facing components, especially as stored energy increases in larger devices. ELM pacing via injection of hydrogenic pellets can trigger small ELMs at a rate exceeding the natural ELM frequency and has been shown to be a successful method to mitigate effects of large ELMs. Understanding of the physical mechanisms of ELM triggering and improved modeling are required for confident extrapolation to ITER and beyond. M3D-C1, a code for solving the linear or non-linear extended-MHD equations in toroidal geometry, is currently being used to model pellet ELM triggering in DIII-D ITER-like plasmas. An unstructured triangular mesh provides resolution to capture the sharp gradients present in the pellet deposition layer. Initial linear results utilizing a density perturbation to estimate the effects of the pellet in 2D suggest that the destabilization is a resistive effect. Recent M3D-C1 modeling efforts have focused on 3D nonlinear simulations incorporating a pellet ablation model. |
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UP11.00053: Studies of D2 pellet ELM triggering in ITER-relevant conditions with modeling and experiments in DIII-D Robert Wilcox, Alessandro Bortolon, Larry Robert Baylor, Stephanie J Diem, Daisuke Shiraki Pacing ELMs by injecting pellets in order to reduce peak ELM heat fluxes in ITER is an attractive alternative to RMP ELM suppression in the case that the latter is determined to be unworkable or ineffective. In DIII-D, experiments have been performed using tangentially injected pellets off-midplane to mimic the shallow penetration of pellets in the hot, dense ITER pedestal. Pellets injected with the tangential trajectory did not successfully trigger ELMs, but neither did perpendicular midplane injected pellets in similar plasmas, suggesting that the trajectory was not solely responsible. In separate experiments in low collisionality DIII-D plasmas, pellets injected at the midplane were also unable to trigger ELMs, where this same size pellet had successfully demonstrated ELM triggering in previous discharges with higher collisionality. Initial indications are that pellet particle deposition profiles were not adequate to trigger ELMs in the experimental plasma conditions, and this idea is being tested with modeling of pellet ablation. |
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UP11.00054: Recent investigations of ELM crashes using ECE Imaging on DIII-D Yilun Zhu, Guanying Yu, Jo-Han Yu, Yu Ye, Calvin W Domier, Neville Luhmann Jr., Benjamin J Tobias, Ahmed Diallo, Yang Ren, Gerrit J Kramer, Raffi Nazikian DIII-D Electron Cyclotron Emission Imaging (ECEI) has been fully upgraded with 20 liquid crystal polymer (LCP) based integrated receiver chip modules for 2D electron temperature measurements. The new LCP modules have demonstrated significantly signal-to-noise improvement, which is over 20x better than previous minilens approach. All channels have been mapping in equilibrium and calibrated with the Thomson scattering diagnostic. The calibration coefficients are stable, which allows for the measurement of absolute temperature profiles with high temporal resolution (~ 1 microsecond). The temperature profile crash and recovery between ELMs has been captured. High speed measurements show that the pedestal temperature profile is flattened by an ELM crash. In the calibrated 2D temperature profile, the temperature gradient could be calculated during and after ELM crash. In additional, the higher harmonic electron temperature oscillation has been observed after sawtooth crash. The next generation of ECEI and Microwave Imaging Reflectometer (MIR) is under development for co-located and simultaneously electron temperature and density 2D imaging over a much wider range of plasma configurations. |
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UP11.00055: Profile gradient evolution during the transition from Edge Harmonic Oscillations to Limit Cycle Oscillations in wide-pedestal QH-mode L. Zeng, K. Barada, W. A. Peebles, T. L. Rhodes In recent DIII-D wide-pedestal QH-mode plasmas, a long-lived and predator-prey type Limit Cycle Oscillations (LCO) regime is observed to follow the disappearance of the Edge Harmonic Oscillations (EHO)1. During the fast transition (~3-4 ms) from the EHO termination to LCO, both electron density and temperature gradients significantly increase near the top of pedestal, i.e. 0.8< rho <0.9, while the gradients inside pedestal change slightly. At almost same time, the intermediate-k (k = 4-6 cm-1) density fluctuation magnitude increases at the top of pedestal (rho ~0.85). The possibility that the pressure gradient near the top of pedestal leads to the transition from EHO to LCO regime is investigated. Stability analysis and the evolution of Ti, Er, and ExB rotation shear rate during the transition are also examined. The investigation will help to understand the transition and the mechanism to the LCO dominated regime. 1K. Barada, et al., Phys. Rev. Lettrs. 135002 (2018) |
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UP11.00056: Creation and Sustainment of Quiescent H-mode Plasmas in DIII-D with Zero Net NBI Torque K.H. Burrell, Xi Chen, T.H. Osborne, C. Paz-Soldan, T.M. Wilks Recent experiments have demonstrated both creation and sustainment of wide pedestal QH-mode with zero net neutral beam injected (NBI) torque. QH-mode is an attractive operating regime for future devices since it has excellent energy confinement time and can operate without ELMs at zero net NBI torque. Wide pedestal QH-mode exhibits an increase in the pedestal pressure height and width and increased global energy confinement associated with a bipolar change in the edge ExB shear [1]. Previous low torque QH-mode shots utilized a startup phase with significant NBI torque which was reduced to zero later in the shot. Since future devices will not have appreciable NBI torque, creation and sustainment of QH-mode at zero NBI torque is an important step towards using QH-mode in those devices. These recent shots have plasma conditions relevant for future devices: βN = 1.75, ITER H98y2 = 1.25, ν*ePED = 0.3-0.4 and stationary conditions for at least 20 energy confinement times, limited only by hardware constraints. Unlike the rapid transition to wide pedestal conditions seen previously [1], the wide pedestal state forms gradually over a period of roughly 100 ms in the present cases [1] K.H. Burrell et al, Phys. Plasmas 23, 056103 (2016) |
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UP11.00057: Broadband turbulence characteristics in wide pedestal quiescent H-mode on DIII-D Makoto Ono, George R McKee, Zheng Yan, Keith Burrell, Xi Chen, Yifan Wu ELM-free high confinement operation with a wide pedestal has been achieved in DIII-D, exhibiting a spontaneous transition from the coherent edge harmonic oscillation (EHO) to the broadband MHD turbulence state by a sufficient reduction of input torque. It is thought that ExB rotational shear altered by changes in torque affects the edge turbulence state and associated radial transport, leading to the wider pedestal. Clarifying the characteristics of the turbulence is one of the important issues to understanding the mechanisms of formation of the improved pedestal condition. The broadband turbulence was typically observed in the frequency range of 0-250 kHz with Beam Emission Spectroscopy, and is composed of two components; a lower frequency band propagating in the ion diamagnetic drift direction in the lab frame and a higher frequency band in the electron diamagnetic drift direction near the top of the pedestal. The lower frequency band is found to have a long poloidal correlation length of ~10 cm, which tends to be slightly longer compared to ~9 cm in standard QH-mode phase. Temporal evolution of the broadband turbulence characteristics during the transition from standard QH-mode to wide pedestal QH-mode will be presented. |
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UP11.00058: Progress in two-fluid nonlinear MHD NIMROD modeling of edge harmonic oscillations in DIII-D quiescent H-mode Alexei Y Pankin, Jacob R King, Scott E Kruger, Keith Burrell, Xi Chen, Andrea MV Garofalo, Richard Joseph Groebner, Tariq Rafiq Recent nonlinear extended-MHD modeling [1] of low-torque quiescent H-modes discharges on DIII-D yield several insights: a state with low-n modes is only achieved in the presence of the flow inferred from experiment and the amplitude and phase of the perturbations lead to strong density transport relative to thermal losses. However, discrepancies with experimental measurements remain. In this work we extend the modeling of edge harmonic oscillations (EHO) with the NIMROD code to well-diagnosed DIII-D discharges. Similar to the previous NIMROD [1] and M3D-C1 [2] simulations, the new linear NIMROD results confirm that the flows can destabilize the low-n modes. In this work, the linear NIMROD simulations are continued to nonlinear stages that show the formation of saturated EHO states. The simulations are compared to the electron-cyclotron emission (ECE) measurements. Progress to enable two-fluid modeling is discussed along with a comparison of the nonlinear dynamics of this EHO case relative to the previous NIMROD modeling results which produced a weakly turbulent state.
[1] J.R. King et al. Phys. of Plasmas 24, 055902 (2017). |
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UP11.00059: Internal magnetic fluctuations Measurements in Quiescent and ELMy H-mode DIII-D plasmas using Faraday-effect polarimetry Jie Chen, Weixing Ding, David L Brower Internal magnetic fluctuations in Quiescent and ELMy H-mode plasmas, which play important roles in plasma transport and confinement, and can be used in model validation, have been measured using a multi-channel Faraday-effect polarimeter on DIII-D. In standard Quiescent H-mode (QH) plasmas, Faraday-effect polarimetry shows that the n=2 edge harmonic-oscillation (EHO) has the largest internal radial magnetic fluctuation (~2 Gauss), in contrast to Mirnov coils that show the n=1 EHO has the largest poloidal magnetic fluctuation. In wide-pedestal QH-mode (WPQH) plasmas with almost zero NBI torque, weakly coherent magnetic fluctuations (1.5-2 Gauss) with large toroidal mode numbers (n>6) are observed in the core region. Broadband magnetic fluctuations are also observed from 150 to 450 kHz and from 70 to 150 kHz, respectively, in both QH and WPQH plasmas. In ELMy H mode plasmas, broadband magnetic fluctuations from 150 to 500 kHz are observed between ELMs with maximum amplitude near the mid-plane. Between ELM events, the magnetic fluctuations increase up to ~15 Gauss and saturate when the pedestal gradient saturates, indicating their role in pedestal transport. |
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UP11.00060: Linear Gyrokinetic Analysis of the Grassy-ELM Regime with Edge Resonant Magnetic Perturbations in DIII-D Arash Ashourvan, Brian Grierson, Raffi Nazikian, Walter Guttenfelder, Jeff Candy, Emily A Belli The gyrokinetic code CGYRO is used to study the linear stability of the pedestal during application of resonant magnetic perturbations (RMPs). For the studied discharge, type-I ELMs are suppressed and the pedestal exhibits cyclic pulsations between a grassy-ELM state and an ELM suppressed state. While the supressed state has a more conventional DIII-D H-mode pedestal, the grassy-ELM state has a wider pedestal with a locally flattened region in the density and temperature profiles, in the steep gradient region (SGR). For both states, simulations show that at pedestal top microtearing modes (MT) are dominant for kθρS<1, and ion temperature gradient driven instabilities (ITG) are dominant over the broader ion scales, up to kθρS~2. In the SGR the dominant modes are trapped electron modes (TEM) which transition to electron temperature gradient modes at kθρS~0.3, except in the flattened region in the SGR of the grassy-ELM state. Here, for kθρS<0.1 MT are dominant, whereas for kθρS>0.1 TEM dominate. Although here TEM have higher growth rates than MT, a general mixing length estimate for electron energy flux is larger for MT, suggesting these modes may have a role in the turbulent transport of the grassy-ELM state. |
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UP11.00061: Impact of RMP ELM Suppression On Divertor Heat and Particle Fluxes at ITER-like Condition Dmitriy Orlov, Richard A Moyer, Igor Bykov, Todd E Evans, Brendan C Lyons, Abraham Meles Teklu, Gregorio Luigi Trevisan, Andreas Wingen RMP ELM suppression experiments at ITER-like conditions (shape, collisionality, RMP spectrum) in DIII-D show little splitting of the heat flux to the divertor targets, despite robust splitting in the particle flux. In DIII-D, strike point splitting is routinely observed in the divertor particle flux during RMP operation. The observed splitting is consistent with the toroidal mode number n of the perturbation, but the measured separation of the divertor particle flux lobes exceeds predictions of a vacuum model by factors of 3-5. However, there is little impact of these particle flux lobes on the measured divertor heat flux. The large particle flux lobe separations present a challenge for plasma response modeling, because the predicted response using linear, resistive MHD simulations is dominantly a screening response, which should reduce the divertor lobe splitting below the vacuum model predictions. |
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UP11.00062: Predicting the Toroidal Rotation Profile for ITER Colin Chrystal, Brian A Grierson, Shaun R Haskey, John S Degrassie, Gary M Staebler, Tuomas Tala, Antti Salmi Experiments on DIII-D have increased confidence in a prediction of moderate intrinsic rotation in ITER[1] by investigating the effect of fast-ions and edge neutrals in rotation studies. In a large tokamak like ITER, intrinsic sources of rotation are important because evidence suggests they will be comparable to the neutral beam torque. Measurements of the ρ* dependence of intrinsic rotation in Electron Cyclotron Heated H-modes are consistent with previous measurements of the ρ* scaling of intrinsic torque and momentum confinement in beam heated plasmas with significant fast-ion fractions, showing that fast-ions did not corrupt those results. Also, the small differences in intrinsic rotation of closed and open divertor configurations show that momentum transport due to neutrals in the pedestal is not a significant hidden variable. This result is supported by the similarity of intrinsic rotation before and after the onset of detachment. [1] C. Chrystal et al., Phys. Plasmas 24, 042501 (2017). |
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UP11.00063: Investigating the Role of ELMs in Triggering RMP ELM Suppression Richard A Moyer, Dmitri M Orlov, Todd E Evans, Brendan C. Lyons, Tom H. Osborne, Carlos Alberto Paz-Soldan, Matthias Knölker, Raffi Nazikian H-mode pedestal profiles early in the inter-ELM phase are used to investigate if ELMs trigger RMP ELM suppression in ITER-like DIII-D discharges. Plasma response simulations are usually made at times just before the ELM crash, but recent theory [Callen, UW-CPTC 16-4] suggests that the pedestal modifications due to an ELM crash trigger ELM suppression. Testing this model requires profile and equilibrium analysis just after an ELM crash as input for plasma response simulations. Measurements suggest that this correlation may be fortuitous. Applying the RMP in an ELMing H-mode starts a slow pedestal evolution taking 10s-100s of ms to ELM suppression through a phase with higher frequency, smaller size “mitigated” ELMs. Fast measurements during these mitigated ELMs show that this slow evolution is robust against fast transient pedestal changes (≤5ms) due to the remaining ELMs. ELM suppression has also been obtained by applying the RMP in L-mode, with the discharge transitioning directly to an ELM suppressed H-mode without ever ELMing, suggesting that an ELM crash isn’t a necessary condition for ELM suppression. |
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UP11.00064: Enhancement of Helium Exhaust During ELM suppression by Resonant Magnetic Perturbation Fields at DIII-D E. T. Hinson, H. Frerichs, O. Schmitz, G. Mckee, Z. Yan, C. S Collins, C. Paz-Soldan, M. Wade, T. E. Evans, T. W. Abrams, D. Thomas, B. C. Lyons, B. Grierson, I. Bykov, R. A. Moyer, E. Unterberg, A. Briesemeister, A. G. McLean, J. Watkins, H. Wang Resonant magnetic perturbations (RMPs) used to suppress Edge Localized Modes (ELMs) in ITER-shaped H-mode plasmas led to enhanced global He exhaust, measured by the effective He particle confinement time τp*He, in recent experiments at DIII-D. During ELM suppression, τp*He decreased by 40% vs ELMy cases, and τp*He/τE, where τE is energy confinement time, was reduced by ~15%. These first-time findings are important for ITER, where RMP ELM control is planned, because they suggest RMPs can replace ELM impurity flushing. A multi-reservoir particle balance model is used analyze these data. In the edge reservoir, He density measurements decay faster during ELM suppression, suggesting faster outward transport and/or reduced back-fueling after recycling. Increased He-I and He-II emission in the Scrape-off Layer (SOL) and higher neutral He pressure near the pump show that more He is retained in the SOL and neutral reservoirs, which is important for effective removal of He globally. Initial EMC3-EIRENE modeling suggests magnetic stochastization reduces the thermal force relative to the friction force, enhancing outward transport in the edge. |
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UP11.00065: Distinguishing internal and external sources of measured “3D” magnetic fields in the DIII-D tokamak Edward J. Strait, Ryan M Sweeney The magnetic field on a closed surface can be uniquely decomposed into contributions from currents internal and external to the surface [A.H. Boozer, Nucl. Fusion 55, 025001 (2015)]. This general principle implies that measurements just outside the surface of a magnetically confined plasma can distinguish the plasma’s contribution to the magnetic field from the contribution by external currents. Spatially resolved, normal and 1- or 2-axis tangential measurements are required, but there is no need for a specific model of either the plasma or the external currents. In a tokamak, this technique enables direct measurement of the field of a growing plasma instability, distinct from the current that it induces in the resistive vessel wall, and direct measurement of the stable plasma response to an external magnetic perturbation, distinct from the external perturbation itself. The separation of internally and externally sourced fields also provides a natural framework for determining the electromagnetic torque on the plasma. The DIII-D “3D” magnetic diagnostic system [J.D. King, et al., Rev. Sci. Instrum. 85, 083503 (2014)] is well suited to such measurements, and applications of the technique to DIII-D data will be shown. |
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UP11.00066: Strong nonlinearity of 3D equilibrium responses in DIII-D RMP plasma Yasuhiro Suzuki, Todd E Evans, Morgan Shafer, Dmitri M Orlov, Xiaodi Du, Satoshi Ohdachi A strong nonlinearity in the 3D equilibrium response to an RMP field is found in the DIII-D plasma. In n=1 RMP field phase-flip experiments, differences in the n=1 plasma response are observed in each phase, although the RMP field is periodically symmetric. To understand this plasma response nonlinearity, a 3D equilibrium including the error field, which breaks the field periodicity, is studied. The island width at φRMP=185 is larger than the width at φRMP=5. Because, the error field amplifies an m/n = 1/1 island, the island redistributes the toroidal current density. Thus, the redistributed toroidal current density amplifies the further nonlinear evolution of the m/n = 1/1 island. This is the reason for the strong nonlinearity that appears after the phase flip of the RMP field. |
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UP11.00067: Validating multi-n plasma response simulations Jeremy M Hanson, Jim Bialek, Mitchell Clement, Gerald A Navratil, Francesca Turco, Edward J Strait Frequency-dependent measurements of the magnetic plasma response to n=2 perturbations in DIII-D exhibit apparent multi-mode behavior. Response measurements from the high-field side midplane show a double resonance in the frequency range of -100 to 100 Hz, in discharges near the n=2 ideal MHD no-wall pressure limit. The poloidal structure of the response varies with the perturbation frequency, becoming more peaked on the midplane at low frequency. Thus far, the observations have not been explained by plasma models that simulate a single-n plasma mode. However, the perturbing coils have a significant n=4 sideband, and ideal MHD analysis indicates that the plasma n=4 mode is also near marginal stability. A possible explanation for the observations is that the n=4 sideband couples to the n=2 response via symmetry-breaking wall eddy currents. We will investigate this hypothesis using a newly developed version of the VALEN code that can incorporate multiple n-numbers in response calculations. |
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UP11.00068: Footprint studies of toroidal field misalignments in DIII-D Gregorio L. Trevisan, Todd E. Evans, Lang L. Lao, Wen Wu, Dmitri M. Orlov, Andreas Wingen Reference measurements suggest that the axis of the toroidal field coil set in DIII-D is slightly shifted with respect to the vacuum vessel center by a few millimeters, and slightly tilted with respect to the vertical axis by a few hundreds of a degree. The effects of such misalignment are investigated for various configurations of DIII-D plasmas, including the new Small Angle Slot (SAS) upper divertor, which requires an accurate placement of the strike point in the slot for easier detachment. Detailed three-dimensional models of the B-field shift and tilt, together with efficient footprinting techniques, yield “vacuum” predictions of the footprint size and position on the lower divertor shelves for Lower Single Null plasmas, as well as on the SAS for Upper Single Null plasmas. The resulting footprints are found to be larger than those produced by typical DIII-D error fields and, in the SAS configuration, have a relatively larger effect, comparable to typical Resonant Magnetic Perturbation (RMP) fields. The consequences of the simultaneous presence of both misalignment errors and RMP fields are then studied and discussed. |
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UP11.00069: The high field tokamak path to fusion energy: C-Mod to SPARC to ARC Earl Marmar, Dan Brunner, Martin J Greenwald, Zachary S Hartwig, Amanda Hubbard, James Irby, Brian LaBombard, Joseph Minervini, Robert Mumgaard, Brandon Sorbom, Elizabeth Tolman, Dennis Whyte, Anne White, Stephen Wukitch, MIT PSFC Magnetic Fusion Experiments Team Applying results from the world-wide tokamak experimental databases, we map out a path to realizing fusion energy using the high magnetic field tokamak approach. With conservative assumptions about H-mode confinement, a pre-conceptual design for a net energy gain D-T facility, SPARC, yields Q>2, and possibly much higher, in a medium scale, conventional aspect ratio tokamak with R≈1.6m, BT=12T. The I-mode regime should also be accessible, and could yield similar or higher performance. SPARC will use high-field, high temperature REBCO superconductors to access burning plasma regimes. Extrapolation to a somewhat larger, slightly lower-field design, ARC[1], can produce on the order of 500 MW of fusion power, and would put electricity on the grid. The extrapolation to SPARC relies heavily on the high-field results from Alcator C-Mod. We present details of the SPARC design, and consider physics challenges in operational regimes and limits, heating, fast particle instability drive, and divertor power and particle handling. [1] B.N. Sorbom, et al., Fus. Eng. Des. 100(2015)378. |
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UP11.00070: Diagnostics for a SPARC-like, high-field, compact, net-energy tokamak Anne White, Sean B Ballinger, Alexander J Creely, Samuel Frank, Adam Q Kuang, Bryan Linehan, William McCarthy, Lucio M Milanese, Kevin J Montes, T. Mouratidis, J. F. Picard, Pablo Rodriguez Fernandez, Aaron M Rosenthal, Alexander J Sandberg, Francesco Sciortino, Raspberry Simpson, Roy A Tinguely, Elizabeth A Tolman, Muni Zhou, Brandon Nils Sorbom, Zachary S Hartwig, James Henderson Irby Advances in high temperature superconductor (HTS) technology have opened a path to high field, compact fusion devices, like ARC [Sorbom 2015] and SPARC [Greenwald 2018]. The MQ1 tokamak [Mumgaard 2016] is a conceptual, compact HTS tokamak (R = 1.65 m) with magnetic field on axis B = 12 T and primary mission of achieving net energy. The high magnetic field, high density, and nuclear environment from high power DT operations (Q > 3) present both challenges and opportunities to diagnose MQ1, so diagnostics supporting its mission are explored. A neutronics suite (comprising micro-fission chambers, magnetic proton recoil spectrometers, and neutron cameras) is modeled in MCNP6 with a notional tokamak geometry as well as ne and Te profiles from TSC simulations. Expected neutron signals indicate that measurements of fusion power, ion temperature, deuterium-to-tritium fuel ratio, and self-heating profile are feasible. A combination interferometer-polarimeter, using one set of CO2 and HeNe lasers, is shown to measure line-averaged density for feedback control, poloidal magnetic field for constraining magnetic reconstructions, and fluctuations of both. Additional diagnostics assessed are magnetics, passive radiation detection, x-ray imaging crystal spectroscopy, and Thomson scattering. |
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UP11.00071: Navigational Data Management: a general solution to storage, exploration, and annotation of relationships in scientific data sets Joshua Stillerman, Martin J Greenwald, Jason Stillerman, John Wright The size and complexity of scientific research data sets is growing very rapidly. Many projects are long running collaborations between evolving groups of researchers. Integrating and documenting these large heterogeneous data collections is needed for these data to be exploited, and to retain their value and meaning over time. Most existing projects have application specific solutions for these data management problems. The Navigational Data Management (NDM) project provides a general solution that can be applied irrespective of the science domain. The system represents data and annotations as generalized objects. It stores relationships as labeled property graphs. URIs provide a mechanism to attach external data objects: files, MDSplus records, drawings, etc... NDM stores the metadata and data schemas, and application behaviors as first class objects. These can then be modified, added to, and even annotated as the needs of the research group evolve. |
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UP11.00072: Multi-machine Disruption Databases and Applications of Machine Learning for Fusion Robert Granetz, Abhilash Mathews, Kevin J Montes, Cristina Rea, Francesco Sciortino, R. Alex Tinguely, PSFC MFE Team MIT PSFC researchers are applying Machine Learning to a number of fusion-relevant problems, including real time disruption warning on a number of different tokamaks, automated identification of transport confinement mode in Alcator C-Mod, and determination of impurity transport coefficients in C-Mod. A broad selection of Machine Learning methods are being used: Random Forests, recurrent and feedforward neural networks, Gaussian Bayesian, and logistic regression. All AI methods require extensive databases of relevant information in order to train and test the algorithms. For disruption prediction, we would like to know if a universal algorithm can be derived from current tokamaks, so that ITER, SPARC, and future reactors would not have to first generate their own databases of disruptions. To this end, we have established similar databases of real time disruption-relevant plasma parameters from four very dissimilar tokamaks (C-Mod, DIII-D, EAST, KSTAR). We find that the most useful disruption warning parameters are different on each machine, and that the disruption prediction performance of algorithms trained on each machine's database vary considerably. |
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UP11.00073: Cross-validation of a machine learning disruption predictor on EAST and Alcator C-Mod Kevin Montes, Cristina Rea, Robert S Granetz, Roy Alexander Tinguely A disruption prediction algorithm based on the Random Forests method has been developed using large databases of both disruptive and non-disruptive discharges from EAST and Alcator C-Mod. The machine learning algorithm was trained on flattop data using plasma parameters that can be available in real time; most are dimensionless (e.g. li, βp) or cast in a dimensionless form (e.g. n/nG) to facilitate multi-machine analysis. To make a robust disruption predictor, the algorithm was trained and tested on all disruptions, independently of their cause. A binary classification scheme based on a time sample’s proximity to the disruption achieves F1 scores of 0.71 and 0.50 on EAST and C-Mod, respectively. However, individual time sample predictions must be mapped to a disruption warning alarm using optimized control thresholds, a time window, and a time threshold to distinguish between classes. This poster describes a cross-validation procedure to determine such an optimal mapping for triggering an alarm of an impending disruption. A comparative study of the algorithm’s performance on both tokamaks is shown, and machine learning methods are used to interpret the model predictions and determine the drivers of disruptive behavior. |
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UP11.00074: Confinement mode identification of fusion plasmas utilizing machine learning methods Abhilash Mathews, Jerry W Hughes, Stephen M Wolfe, Amanda E Hubbard, Robert S Granetz, Cristina Rea, Theodore Golfinopoulos, Alcator C-Mod Team Distinguishing features between fusion plasma confinement regimes are explored via machine learning methods to analyze experimental data from the compact, high-field Alcator C-Mod tokamak. Supervised learning techniques with zero-dimensional data and time-independent quantities are employed which increases the generalizability of this approach for instant confinement mode identification and ultimately real-time prediction purposes for fusion devices. Binary classification of L- and H-modes utilizing Gaussian naïve Bayes, logistic regression, multilayer perceptron (i.e. feedforward neural networks), and random forests performed similarly and obtained an average accuracy of 97.2% for L-modes and 86.7% for H-modes using the plasma’s stored energy, volume-averaged density, poloidal beta, ohmic heating power, normalized internal inductance, magnetic axis radial position, and Hα as inputs. Additionally this work investigates I-modes leading to a multi-class classification problem. Development of a new confinement database with over 200 distinct shots consisting of approximately 400 L-, 200 H-, and 100 I-mode periods extends previous databases for large-scale comparative studies. |
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UP11.00075: Comparison of Experimentally Inferred and Simulated Impurity Transport Across Alcator C-Mod Confinement Regimes Francesco Sciortino, Nathan T Howard, Earl Marmar, John C. Wright, Tomáš Odstrčil, Pablo Rodriguez-Fernandez, Norman M. Cao, John E. Rice Understanding and controlling the mechanisms of impurity transport are paramount for both prediction and optimization of burning plasmas. Despite documented challenges in inferring experimental impurity transport coefficients, recent efforts in computational statistics have suggested new pathways [Chilenski 15]. We present the results of inferences employing Bayesian model averaging, nested sampling and Gaussian processes. High performance computing tools have enabled the study of a range of confinement modes (including L-mode, EDA H-mode and I-mode) in Alcator C-Mod discharges. Trace CaF2 impurities are introduced using Laser Blow-Off (LBO), with Ca dynamics diagnosed via spatially resolved X-ray measurements. Experimental transport coefficients are computed via iterations of the STRAHL code [Dux 06]. We present a comparison of experimental transport coefficients with turbulent TGLF [Staebler 07] and neoclassical NEO [Belli 08] simulations, focusing on the sensitivity of the results to subdominant unstable modes, Zeff and velocity shear. Our results offer greater understanding of impurity dynamics in a wide range of confinement regimes and a promising path to gyrokinetic validation via particle transport. |
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UP11.00076: Demonstration of Prototype Mirror Langmuir Probe Control System Using a Red Pitaya Field Programmable Gate Array Board William McCarthy, Charles Vincent, Theodore Golfinopoulos, Brian LaBombard, Adam Q Kuang, James R Harrison, Stephanie Hall, Graham Naylor, Jack Lovell High bandwidth, high spatial resolution measurements of electron temperature, density and plasma potential are valuable for resolving turbulence in the boundary plasma of tokamaks. While Langmuir probes can provide such measurements their temporal and spatial resolution is limited by the sweep rate for obtaining I-V characteristics or by the need to use multiple electrodes, each sampling a single plasma quantity at high bandwidth. The Mirror Langmuir Probe (MLP) bias technique overcomes these limitations by rapidly switching the voltage on a single electrode among three bias states, each dynamically optimized for local plasma conditions. The MLP system on Alcator C-Mod used analog circuitry to perform this function, measuring Te, Vf and Isat at 1.1 MHz. Recently, a new prototype digital MLP controller has been implemented on a Red Pitaya (RP) FPGA board, which reproduces the functionality of the original controller, performs all data acquisition, and is readily customizable at a fraction of the development time and implementation cost. A second RP was used to test the MLP by simulating the current response of a physical probe using C-Mod experimental measurements. |
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UP11.00077: Modeling refraction of LHCD waves by SOL turbulence with GENRAY/CQL3D Bodhisatwa Biswas, Syun'ichi Shiraiwa, Paul Thaddeus Bonoli, Christopher T Holcomb, Gregory Marriner Wallace, Anne Elisabeth White, Stephen James Wukitch Lower hybrid current drive (LHCD) is a promising actuator for driving off-axis current in a tokamak. RF wave and Scrape Off Layer (SOL) turbulence interaction may affect wave propagation, altering core power absorption. A ray-tracing/Fokker-Planck model GENRAY/CQL3D is used to study how the effects of refraction due to SOL turbulence alone impact core LHCD deposition. A model for the turbulent blob field in real space [1] is coupled to the background plasma in a DIII-D geometry using GENRAY. The wave propagation and absorption are calculated to determine the current drive (CD) profile. Initial results show that edge fluctuations lead to lower CD efficiency and radial broadening of the CD profile. This is due to ray refraction, and evolution of N|| through the SOL and core plasma. The CD profile is sensitive to the amplitude and wavenumber of SOL density fluctuations. These results could explain why LHCD efficiency is observed to be lower than what is predicted by standard RF wave theory for conditions where SOL fluctuations are high. The effects of refraction will be compared to those of wave scattering in k-space; L and H-mode SOL models will also be compared. [1] Sierchio RSI 2016. |
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UP11.00078: Design and Implementation of a Gas Puff Imaging System on TCV Woonghee Han, Nicola Offeddu, Theodore Golfinopoulos, Christian Theiler, James Layton Terry, Seung Gyou Baek, Basil Duval, Brian LaBombard, Earl S Marmar Gas Puff Imaging (GPI) diagnostics routinely measure the spatially-resolved edge fluctuations in tokamak plasmas. We plan to install a GPI system on TCV that will address multiple physics topics, such as turbulence characteristics during confinement transitions, and neutral density shadowing effect in GPI. The system’s 120 views comprise a 10x12 array (43x52 mm view), with light detection accomplished by APD arrays with a 1 MHz bandwidth. Parametric scans of the gas puff design parameters - including flow rate of the He or D2 gas puff (FR), spacing between the nozzle and the LCFS (Δ), nozzle design, and gas species - were performed using the DEGAS 2 neutral transport code to model and maximize the expected time-averaged emission within a practical design space. DEGAS 2 results predict that the system will give a brightness above a minimum usable level (2.0 mW/cm2/ster) with He puff and FR=6×1019 atoms/s, up to 2 mm inside the LCFS. The brightness and penetration are increased with higher FR or shorter Δ or collimated puffs, and the penetration will be deeper for D2 puffs. Installation of the system on TCV is planned for summer/fall 2018. |
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UP11.00079: Numerical SOLPS-ITER study of the effect of fueling on ionization and neutral density profiles in EDA H-modes on Alcator C-Mod Richard Reksoatmodjo, Saskia Mordijck, Jerry W Hughes, Jeremy Lore, Xavier P Bonnin The effect of plasma edge opacity to neutrals on density pedestal structure was evaluated in experiments on Alcator C-Mod, in H-mode regimes approaching ITER-like edge opacities. Perturbative gas puffs of varying magnitude were applied to these high-density, high power EDA H-modes at high plasma current along with discharges at reduced current and density and thus opacity. In order to assess the role of fueling versus transport at the plasma edge, we use the SOLPS-ITER code suite to first calculate the radial and poloidal neutral density profiles for both a discharge at high opacity as well as one at low opacity. We match the upstream experimental radial fluxes as well as density and temperature profiles by varying the radial transport coefficients using SOLPS-ITER. We will study the effect of varying the relative ratio of the diffusion and pinch contributions upon the neutral and electron density profiles. By comparing the simulations and experimental observations of two discharges at different opacity we can address the relative role of transport versus fueling in determining the local electron density in the Scrape-Off Layer and pedestal region. |
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UP11.00080: Comparison of GDB edge simulations of Alcator C-Mod IWL L-mode with experimental measurements* Manaure Francisquez, Ben Zhu, Barrett Rogers Four Alcator C-Mod IWL L-mode shots were simulated with the GDB code [1] -- a global 3D drift-reduced Braginskii based electromagnetic edge turbulence model evolves both plasma and flow profiles self-consistently in both SOL and close flux region. In these simulations, magnetic geometry and particle source profile were based on EFIT and KN1D model respectively [2]; plasma profiles were initially set to be close to experiment measurements but allowed to evolve freely afterwards. We found that well beyond turbulence saturated stage (>2 ms), the global plasma and the spontaneous generated E×B flow [3] profiles obtained from GDB simulations agree well with the MLP data. Turbulence statistics analysis [4] and comparison with MLP and GPI data are underway. [1] Zhu et.al, CPC, in press (2018) [2] LaBombard, PSFC/RR-01-3, (2001)[3] Zhu et.al, PoP, 24, 055903, (2017)[4] Francisquez et.al, NF, 57, 116049 (2017)
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UP11.00081: Electron temperature fluctuation measurements of the weakly coherent mode in I-mode at ASDEX Upgrade and Alcator C-Mod Rachel Bielajew, Simon J Freethy, Alexander J Creely, Garrard D Conway, Theodore Golfinopoulos, Tim Happel, Amanda E Hubbard, Anne Elisabeth White, ASDEX Upgrade Team I-mode is a naturally ELM-free improved confinement regime, which exhibits high energy confinement without high particle confinement, and holds promise for future reactors. The weakly coherent mode (WCM) appears in the pedestal region of I-mode plasmas in both ASDEX Upgrade (AUG) and Alcator C-Mod. The precise nature of the mode and its role in particle and heat transport is an open question. In this work, the WCM is studied in I-mode plasmas in AUG and C-Mod through electron temperature fluctuation measurements (CECE) and nT phase angle measurements. In C-Mod the electron temperature fluctuations associated with the WCM are found to be much smaller in magnitude than density fluctuations [1]. In AUG the WCM is observed in I-mode, and a mode at the same radius is also observed in L-mode [2]. Comparison between the two devices and nT phase observations in AUG may provide insight into the effect of the WCM on density and temperature transport. [1] White, Nuclear Fusion 51, 1130005 (2011) [2] Happel 23rd PSI Conference, June 2018 |
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UP11.00082: Performance assessment of tightly-baffled long-leg divertor geometries in the ARC reactor concept Michael Wigram, Brian LaBombard, Maxim V Umansky, Adam Q Kuang, Theodore Golfinopoulos, Dan Brunner, James Layton Terry, Marvin Rensink, Dennis G Whyte, Amanda Hubbard Advanced divertor configurations have been proposed as potential solutions to the divertor heat-load problem, including double-nulls, long-legs and magnetic field flaring with secondary X-points. Modelling of tightly-baffled, long-leg divertor geometries in the divertor test tokamak concept ADX has shown the potential to access passively stable, fully detached regimes over a broad range of parameters. To explore how these advanced divertors may perform in a reactor setting, we have performed numerical simulations in UEDGE of these configurations in the context of the ARC reactor (projected power exhaust of 105 MW). Initial studies employing a Super-X Divertor and 0.5% fixed-fraction neon impurity radiation have shown that a passively stable detached regime exists for power exhaust in the range of 80 to 108 MW. Employing an X-point target geometry, without any impurity seeding, detachment extends up to 90 MW exhaust power, and possibly further, when separations between the flux surfaces of the magnetic X-points are small. Simulations are extended to further study the X-point target divertor in ARC, and to explore solution sensitivity to model parameters. |
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UP11.00083: From Wide Support for High Field Compact Experiments to Popularity, to Damnatio Memoriae Gilberto Faelli, Bruno Coppi, The Ignitor Collaboration By now studies of the characteristics of ignited plasmas and of the requirements of power producing reactors have led to conclude that operating at ignition or near it is necessary for a practically useful fusion reactor. The confinement scaling laws, that were identified originally when the line of high field compact experiments began to be proposed in order in order to investigate igniting plasmas [1], have been rediscovered and confirmed [2]. Both “Damnatio Memoriae” and “Renovatio Memoriae” [2] episodes have occurred in this context as well as in regard to the first introduction of high field superconducting magnet technology [3] in fusion research. The record confinement parameters, beginning to approach the ideal ignition conditions, obtained by the Alcator C Mod machine have validated the perspectives of success of the Ignitor experiment [3]. Relevant developments for the High Field Compact line of experiments, including the investigation of D-D and D-$^{3}$He burning plasmas, are discussed. [1] Coppi, B. AIP, 1721, 1, 020003‐1 (2017). [2] Costley, A.E., et al., Nucl. Fus. 56, 066003 (2016). [3] Coppi, B. et al., Nucl. Fus. 55, 053011 (2015). |
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UP11.00084: HBT-EP Program: MHD Dynamics and Active Control through 3D Fields and Currents Gerald A. Navratil, J. Bialek, John W Brooks, Andrew J Cole, Jeffrey P Levesque, Michael E Mauel, Alex Saperstein, Ian Stewart, Christopher Hansen The HBT-EP active mode control research program aims to: (i) advance understanding of the effects of 3D shaping on advanced tokamak fusion performance, (ii) resolve important MHD issues associated with disruptions, and (iii) measure and mitigate the effects of 3D scrape-off layer (SOL) currents through active and passive control of the plasma edge and conducting boundary structures. SOL current measurements have been used to study SOL dynamics and current-sharing with the vacuum vessel wall during kink-mode growth and disruptions. Asymmetric vessel currents are observed to reach ~5% of the pre-disruption plasma current during the current quench, and have been analyzed in the context of several disruption models. A multi-chord extreme UV/soft X-ray array is being installed to provide detailed internal mode structure information. A GPU-based low latency control system using 96 inputs and 64 outputs to apply magnetic perturbations for active control of kink modes has been extended to control plasma toroidal rotation through feedback on the voltage of a biased electrode in the plasma edge. Applying high voltage and current to the probe induces strong poloidal flow shear and a transition into biased H-mode. |
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UP11.00085: Active-control of mode rotation in HBT-EP using a feedback-driven biased electrode J W Brooks, I G Stewart, J P Levesque, M D Boyer, J Bialek, M E Mauel, G A Navratil Fast rotation of plasmas in tokamaks has been shown to stabilize resistive wall and tearing modes and improve confinement through suppression of edge turbulence. These results are important to both the fusion community and ITER in order to achieve sustained fusion. Edge biasing is one such method shown to induce plasma rotation. In this work, we extend Columbia University’s High Beta Tokamak-Extended Pulse's (HBT-EP) facility [1] and our active GPU feedback system [2] to control plasma rotation through a biased electrode located 3 to 4 cm into the plasma. An inverse relationship between probe current and mode rotation is first characterized, and a proof-of-concept feedforward and feedback active-control system is implemented. The control system shows consistent, initial success in controlling mode rotation, and future work consists of upgraded GPU hardware and a more sophisticated feedback model. 1. J.P. Levesque, et al., Phys Plasmas 22, 056102 (2015). |
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UP11.00086: 3D scrape-off layer currents during tokamak MHD activity and disruptions J. P. Levesque, J. Bialek, J. W. Brooks, C. Hansen, A. Saperstein, I. G. Stewart, M. E. Mauel, G. A. Navratil In tokamaks, currents transfer between the plasma edge and vessel structures by conduction through the scrape-off-layer (SOL). When these currents are not axisymmetric, they can produce large forces capable of damaging wall components. Accurate knowledge of governing principles for these currents is needed to ensure reliability of a future reactor and other large-scale tokamaks. We present measurements of asymmetric SOL and vessel currents in the HBT-EP tokamak, and analyze them in the context of models for asymmetric currents during disruptions. Currents flowing into the low-field-side wall are measured with poloidal resolution in 3 toroidal locations. Tiles can be moved radially along with their associated wall segments between discharges to study shadowing effects and radial structure of SOL currents. Electrically-isolated chamber sections allow measurements of toroidal vessel currents when sections are connected via diagnosed jumpers. Asymmetric vessel currents reach ~5% of the pre-disruption plasma current during the current quench [1]. Plans for installing diagnostic and control tiles on the high-field are also presented. [1] J.P. Levesque et al., Nucl. Fusion 57 086035 (2017) |
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UP11.00087: Biased H-mode and long-distance correlation of edge fluctuations on HBT-EP I. G. Stewart, J. W. Brooks, J. P. Levesque, M. E. Mauel, G. A. Navratil Confinement changes consistent with a biasing induced H-mode have been observed on HBT-EP by using an electrode inserted into the plasma edge to create a local radial electric field well. During the biased H-mode phase, there is a clear reduction in density, temperature, and floating potential fluctuations measured by triple probes, as well as a concomitant rise in the global soft X-ray (SXR) profile, initiating at the edge region. Both positive and negative electrode polarizations were capable of inducing a transition with corresponding signs of the radial electric field. Using a double electrode configuration to confine the biasing region to an area between the two electrodes (both well within the last closed flux surface), an electric field well, SXR increases, and fast MHD rotation could still be achieved out to the edge without bifurcation in the biased electrode current. Additionally, long distance correlation of low frequency oscillations in the floating potentials and ion saturation currents measured by two toroidally separated triple probes during the biased H-mode reveals an n=0 structure with mesoscale radial wavelength that may indicate the presence of zonal flows. |
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UP11.00088: Simulation of Instability Driven Edge Currents with Non-Axisymmetric Resistive Walls in HBT-EP Chris Hansen, Jeffrey P Levesque, Alex Saperstein In experimental magnetized plasmas, currents in the first wall, vacuum vessel, and other conducting structures can have a strong influence on plasma dynamics. These effects are complicated by the 3D nature of device structures, which dictate available current paths. Progress on simulations to study currents flowing in the scrape-off layer during unstable and saturated MHD mode activity in the High Beta Tokamak (HBT-EP) will be presented. The arbitrary geometry, 3D extended MHD code PSI-Tet is applied to study linear and non-linear plasma dynamics in HBT-EP focusing on toroidal asymmetries in the adjustable conducting wall. Comparison of currents and magnetic data from simulations with edge current diagnostics recently installed in HBT-EP will be presented. Comparison between simulations with an experimentally accurate 3D resistive wall, using PSI-Tet, and simulations with an axisymmetric perfectly conducting wall, using NIMROD, will also be presented. Simulations of detailed experimental geometries are enabled by the PSI-Tet code [1], which employs a high order finite element method on unstructured tetrahedral grids generated directly from CAD models. [1] C. Hansen et al. Phys. of Plasmas 22, 042505 (2015) |
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UP11.00089: Removing magnetic pickup from SOL current sensors in HBT-EP Alex R Saperstein, Jeffrey P Levesque, Michael E Mauel, Gerald A Navratil Rogowski coil current sensors have been recently installed on low field side (LFS) diagnostic tiles in three toroidal locations in HBT-EP, for the purpose of determining how scrape-off layer (SOL) currents flow between the plasma and the chamber during discharges. These sensors are attached to the movable walls within the vessel and allow for radial scans of the current. We present numerical methods for removing contributions from external magnetic fields that are picked up by the Rogowski coils due to their imperfect windings. These fields primarily consist of contributions from the equilibrium fields and plasma instabilities, as well as the vertical field, toroidal field, ohmic heating, and non-axisymmetric control coils. The latter can be calibrated using distinct vacuum shots, while the former can be calibrated by shadowing the sensors with other shells and/or limiters during discharges. These calibrations can then be used to compare the amplitudes and directions of the SOL currents relative to the edge magnetic field perturbations. |
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UP11.00090: Observations and Models of Scrape-Off-Layer Current (SOLC) Dynamics During Edge Kink Instabilities in HBT-EP M. E. Mauel, A. J. Cole, J. W. Brooks, A. Saperstein, I. Stewart, J. P. Levesque, G. A. Navratil Scrape-off-layer currents (SOLC) are detected in HBT-EP during kink instabilities, resonant magnetic perturbations (RMPs), and disruptions\footnote{Levesque, \textit{et al.}, \textit{Nuc Fusion}, \textbf{57}, 086035 (2017).} These currents are associated with helical ``filaments'' and ``bubbles'' at the plasma edge\footnote{Angelini, \textit{et al.}, \textit{Plasma Phys Contr Fusion}, \textbf{57}, 045008 (2015).} that become very large during disruptions. SOLC are correlated with the poloidal and toroidal structure of magnetic perturbations. The magnitude of SOLC increase with increasing magnetic perturbations; they can appear with constant amplitude and typically rotate in the electron drift direction. Like ELM filaments, the SOLC in HBT-EP are primarily co-aligned with the plasma current and exhibit temporal distortion, consistent with an elevated electron temperature within the ``filament.'' These SOLC are like large-sized versions of ELM filaments\footnote{Vianello, \textit{et al.}, \textit{Phys Rev Lett}, \textbf{106}, 125002 (2011).} and blobs. This motivates discussion of SOLC models used elsewhere to help on-going investigations of scrape-off layer (SOL) currents during kink instabilities and disruptions in HBT-EP. |
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UP11.00091: Characterizing Laser-Plasma Interaction External to the Laser Entrance Hole of Ignition Scale Hohlraums Nicholas Parrilla, Joseph E Ralph, Benjamin Bachmann, Tilo Doeppner Spatially resolved temperature measurements of plasma flowing from the laser entrance hole (LEH) of NIF hohlraums have allowed correlation of laser energy losses to plasma heating external to the LEH. The resolved temperatures agree with measurements from other diagnostics, including the equatorial hard x-ray imager and static x-ray imager, and were consistent with temperatures calculated using dot spectroscopy data. By asserting that inverse bremsstrahlung is the dominant laser energy loss mechanism, the relationship between the plasma temperature and laser energy absorption was determined. Data show an increase in plasma temperature which scales with laser energy. This analysis validates concern that plasma external to the LEH is contributing to laser energy losses and could be influencing drive symmetry, a critical feature in inertial confinement fusion experiments. The characterization of laser-plasma interaction external to the LEH provides insight for future symmetry tuning experiments, hohlraum structural designs, and refinement of simulation parameters. |
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UP11.00092: Laser Propagation Experiments in a Raman-Amplifier–Relevant Plasma Philip Franke, Jessica Shaw, Avram Milder, Daniel J Haberberger, Dustin H Froula Plasma-based Raman amplification could be the enabling technology for a new generation of ultrahigh-power laser systems. However, experimentally reported gains continue to be significantly lower than those theoretically predicted. We investigated propagation of an unseeded pump pulse through a gas jet at Raman-amplifier–relevant conditions on the Multi-Terawatt Laser at the Laboratory for Laser Energetics. Backscattered energies, backscattered spectra, transmitted pump energies, and transmitted beam profiles were measured over a range of plasma densities and pump-pulse parameters. Filamentation, ionization-induced refraction, and unseeded stimulated Raman scattering appear to scatter a significant amount of the pump energy and inhibit the formation of a uniform plasma channel, which may result in lower gain. Preliminary results will be presented. |
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UP11.00093: Fluid modeling on three dimensional two plasmon decay instability and stimulated Raman scattering using FLAME-MD Shihui Cao, Chuang Ren, Rui Yan, Changwang Lian, Liang Hao Comprehensive simulations of laser plasma instabilities over large spatial and temporal scales in an ICF plasma is important to target design but challenging to perform. Fluid models can be a good candidate for their lower computational cost compared to particle-in-cell models. We continue to improve our 3D fluid code FLAME-MD based on the fluid-like equations. FLAME-MD is designed to simulate all types of LPIs including the stimulated Raman scattering (SRS), the stimulated Brillouin scattering (SBS), and the two-plasmon decay instability (TPD) simultaneously with a self-consistent laser propagation – pump depletion model. Now the SRS and TPD modules have been largely completed and benchmarked with previous codes LTS and Glints. The laser-polarization effect on TPD is simulated for ICF-relevant parameters using FLAME-MD. It is found that TPD with a circular-polarized laser has much lower growth rates than that with a linear-polarized laser given the same Poynting-flux of the two lasers, indicating a likely advantage using CP lasers as the ICF driver. |
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UP11.00094: On the hot-electron generation produced by two-plasmon decay and stimulated Raman scattering in inhomogeneous plasmas On the hot-electron generation produced by two-plasmon decay and stimulated Raman scattering in inhomogeneous plasmas Chengzhuo Xiao, Hongbin Zhuo, Yan Yin, Zhanjun Liu, Chunyang Zheng, Xian Tu He Scalings of hot-electron fraction and temperature are obtained based on two-dimensional particle-in-cell simulations under parameters from direct-drive inertial confinement fusion to shock ignition. The scalings demonstrate a transition from two-plasmon decay (TPD) dominant regime to stimulated Raman scattering (SRS) dominant regime. It is observed that the transition occurs when convective SRS threshold is surpassed, which leads to a decrease in hot-electron fraction and temperature and increase in Raman reflectivity. We propose a new theoretical model for hot-electron generation by coherent Langmuir waves with varying phase velocity, and it is successful in explaining the simulation results in the regime of competing SRS and TPD. These results are coincident with recent experiments on direct-drive scheme and shock ignition, where an increasing importance of SRS is confirmed. |
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UP11.00095: Experiments in pressure boost,plasma smoothing and LPI effect of Hybrid-drive ignition scheme on SG-III laser facility Ji Yan, Ji Wei Li, Xian T He The Hybrid-drive is a novel scheme for the lower compression ratio(~25) and robustly ignition. There are two significant advantages. The one is pressure boost based on “snow plow” effect. it can makes high ablation pressure over 500Mbar in ignition target design; the other one is plasma smoothing effect by long-scale length of ablated plasma. it can increase the uniformly of laser spot over 30 times. Hybrid-drive are studied in experiments that performed on SG-III laser facility.20 laser beams as indirect-drive and 4 laser beams as direct-drive were performed as hybrid-drive. The VISAR collected shock velocity in Quarte.The hybrid-drive makes high shock velocity(84.2km/s) compared with indirect-drive(43.6km/s).the Hybrid-drive proposed 3.5times ablation pressure than indirect-drive. According to the uniformity of shock by VISAR,The hybrid-drive proposed a wider shock signal than direct-drive. Furthermore, the LPI effect also studied in our experiments. the direct-drive laser with a laser intensity of 1.5*1015W/cm2 proposed backscatter light fraction( SRS and SBS are considered) around 8% in direct-drive only shot, 3% in hybrid-drive shot. In hybrid-drive experiments, high pressure, better uniformity and lower LPI effect are proposed. |
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UP11.00096: Particle-in-cell simulations of stimulated Raman scattering for speckled lasers with temporal bandwidth Han Wen, Warren B Mori, Frank Shih-Yu Tsung, Benjamin J Winjum, Archis Joglekar Stimulated Raman scattering (SRS) has been studied using large-scale multi-dimensional particle-in-cell (PIC) simulations with parameters relevant to initial confinement fusion (ICF) experiments. More realistic laser profiles are modeled by including laser speckles without and with temporal beam smoothing such as induced spatial incoherence (ISI) and smoothing by spatial dispersion (SSD), and Spike Train of Uneven Duration and Delay (STUD pulses). Simulation results show that the reflectivity from SRS can be reduced with laser bandwidth comparable to the temporal growth rate of the instability. Kinetic effects such as “inflation” have been found to be important for SRS even with high (6THz) laser bandwidth in two dimensions. Other schemes for mitigating SRS including the use of transverse magnetic fields will be discussed. |
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UP11.00097: LPI Experiments with Long Scale-length Plasmas at the Nike Laser* James L Weaver, Jaechul Oh, David M Kehne, Robert Lehmberg, Andrew J Schmitt, Jason W Bates, Russel Follett, John G. Shaw, Jason F Myatt, Mingsheng Wei, Jarrod Williams, Patrick McKenty, Frank S Tsung, Stephen P Obenschain, Victor Serlin Experiments at the Nike laser are exploring low density (5-25 mg/cm3) CH foam targets for studies of laser plasma instabilities (LPI). This ongoing effort has produced larger volume plasmas with estimated 5-10x longer density and velocity scale-lengths versus solid targets. The platform is being used to examine the effects of wavelength shifting and bandwidth changes on cross beam energy transport (CBET). A first round of experiments has utilized a backlighter beam as a low intensity probe interacting with the tightly focused main beams that create a high intensity pump (~1-2x1015 W/cm2). Nike’s focal zooming of the main beams was exploited to heat a large target volume (~1 mm) at lower intensity (1012-1013 W/cm2) prior to the pump-probe interactions. The new configuration of target diagnostics is presented with example data from the current shot campaign. This poster also discusses LPSE simulations performed for the CBET studies and plans for SRS and TPD studies in these foam targets with the associated laser configurations. |
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UP11.00098: Energy and polarization exchange during simultaneous interactions between many laser beams in plasmas Malcolm Lazarow, Jonathan S Wurtele, Pierre Michel ICF and HED experiments at the National Ignition Facility (NIF) typically involve simultaneous interactions between 96 laser beams overlapping in the same volume of hot plasma, at the entrance of “hohlraum” targets. It has been shown [1] that such interactions can not only lead to energy exchange between beams (depending on their relative frequencies and the presence of Doppler shifts from plasma flows), but also affect the polarizations of the lasers. We investigate new methods to calculate the simultaneous interactions of multiple beams coupled via pair-wise, ponderomotively-driven density modulations (4560 pair-wise couplings for 96 beams). We will present simulation algorithms, conservation laws, and discuss implications for light propagation and laser-plasma interactions in ICF experiments at NIF. [1] P. Michel et al., Phys. Rev. Lett. 113, 205001 (2014). |
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UP11.00099: Isolating the role of ion trapping in the saturation of cross-beam energy transfer David J. Stark, Lin Yin, Brian James Albright, William D Nystrom, Robert F Bird Cross-beam energy transfer (CBET) induces an energy exchange between two crossing laser beams through the mediation of an ion acoustic wave, and this instability has been shown to significantly alter the implosion symmetry in inertial fusion experiments. Our limited understanding of the saturation and evolution of this instability, however, constrains our ability to accurately account for its effects in our implosion simulations. Here we take advantage of the kinetic physics captured in VPIC simulations to examine the role of ion trapping in the saturation of CBET in multi-speckled laser beams. In particular, we vary the overlapping beam intensity to characterize how the trapping effects change as we transition to the higher intensities where we observe the generation of hot electrons from forward stimulated Raman scattering (FSRS). |
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UP11.00100: Cross-beam energy transfer in flowing ICF plasmas with laser speckles and ponderomotive self-focusing Wojciech Rozmus, Stefan Huller, Gaurav Raj, Denis Pesme Cross-BeamEnergy Transfer (CBET) between laser beams remains an active area of research in Inertial Confinement Fusions (ICF) experiments that are related to both, indirect and direct fusion schemes. We have modelled CBET by means of numerical simulations with a wave coupling code [1], taking into account the speckle (hot spot) substructure of “smoothed” laser beams. We have shown that transfer of energy from laser hot spots of one beam to the another beam, via forward stimulated Brillouin scattering, self-focusing in the presence of a plasma flow and beam bending proves to affect considerably the angular distribution and spectra of the laser light behind the region of beam overlap for laser intensities I λ2> 1014W cm-2 μm2. For these reasons the angular distribution and spectra of transmitted light from smoothed laser beams (with speckles) is very different from the angular distribution of beam when the beam speckle structure is disregarded. We have also examined the importance of nonlinear, shock-like structures in ion waves and of plasma-induced smoothing on CBET. [1] G. Raj and S. Hüller, Phys. Rev. Lett. 118, 055002 (2017). |
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UP11.00101: Transverse Beam Structure Formation in Crossed Beam Energy Transfer Holger Schmitz, Raoul M Trines, Robert Bingham Crossed Beam Energy Transfer (CBET) is important in direct drive and indirect drive inertial confinement fusion. Along with the energy transfer between the beams, their profiles are reshaped. While CBET has been at the focus of research for a number of years, the details of the energy transfer and beam reshaping are not yet fully understood. In many situations transverse beam reshaping is interpreted as an effect of the matching conditions for Brillouin or Raman scattering which are met only at certain locations in the non-uniform flowing plasma around a target. This confines the energy exchange to well-defined regions of the plasma and imposes a structure on the beam envelopes. However, this does not explain the beam envelope scalloping or "bursty" behavior that can even be seen in homogeneous plasma, where this argument does not apply. We present 2D numerical simulations of CBET of two perpendicular beams, using a hydrodynamic code coupled to a full Maxwell solver. The simulations exhibit transverse structuring of the outgoing beams caused by pump depletion and subsequent repeated exchange of energy between pump and seed beam. For these conditions beam reshaping can not be explained by local fulfillment of the matching conditions; alternative explanations will be discussed. |
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UP11.00102: Controlling Laser Plasma Instabilities Using Temporal Bandwidths Under Shock Ignition Relevant Conditions Frank S Tsung, James L Weaver, Robert H Lehmberg Using large scale 2D PIC simulations, we are studying the effects of temporal bandwidth on laser plasma interactions under plasma conditions relevant to experiments on the Nike laser with induced spatial incoherence (ISI). With ISI, the instantaneous laser intensity can be 3-4 times larger than the average intensity, leading to the excitation of additional TPD modes and producing electrons with larger angular spread. In our simulations, we observe that although ISI can increase the interaction regions for short bursts of time, time-averaged (over many pico-seconds) laser plasma interactions can be reduced by a factor of 2 in systems with sufficiently large bandwidths (where the inverse bandwidth is comparable with the linear growth time). We will quantify these effects and investigate higher dimensional effects such as laser speckles and the effects of Coulomb collisions. |
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UP11.00103: Impact of laser bandwidth on SRS in ICF plasmas Andrew J Schmitt, John G Shaw, Jason F Myatt, Laurance J Suter In laser driven ICF plasmas, higher pressures are used to drive more hydrodynamically stable implosions, but the pressure (intensity) is limited by laser plasma instabilities (LPI) like stimulated Raman scattering (SRS). Bandwidth is known to stabilize or mitigate LPIs particularly when it is comparable to or greater than the growth-time of the instability1. Because of their faster growth rate, plasmon-based instabilities like SRS need more bandwidth for suppression than ion-acoustic instabilities like SBS or filamentation. However, relatively large bandwidths (Δν > 2 THz) can be impressed on an otherwise narrow-bandwidth laser using stimulated rotatational Raman scattering (SRRS) in gas2. Here we use the LPSE3 code to investigate the impact of SRRS-generated bandwidths on SRS in laser-driven plasmas and report on the results. 1. J.J. Thomson, Nucl.Fusion 15, 237 (1975); S.P. Obenschain and N.C. Luhmann, Appl.Phys.Lett. 30, 452 (1977). 2. J.L. Weaver, et al., Appl.Opt. 56, 8618 (2017). 3. A.V. Maximov et. al, BAPS 62, 75 (2017); J.W. Bates et al., Phys Rev. E 97, 061202 (2018) |
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UP11.00104: A nonlinear geometrical optics model of angular momentum beams in unmagnetized plasma Jason Frank Myatt, Blaine Armstrong In the context of linear optics, optical vortices display a very rich dynamical behavior. Their behavior in nonlinear media (e.g., Kerr media) gives rise to many interesting physical effects, such as the spiraling of vortex solitons1 and may lead to unique practical applications. We describe an attempt to construct a nonlinear geometrical optics model of the interaction of orbital angular momentum (OAM) carrying optical beams with unmagnetized, inhomogeneous plasma. We approximate the wave-fields of ray families representing OAM beams using standard methods; the wavefield is then used to modify the plasma response. The initial ray families are generated using a Poincare sphere approach recently described by Alonso and Dennis2. The potential benefit of using such beams to mitigate various laser-plasma instabilities, such as two-plasmon decay, and cross-beam energy transfer will be discussed.
1 A. S. Desyatnikov in “The Angular Momentum of Light”, D. L. Andrews and M. Babiker (Cambridge University Press, 2013). 2 M. A. Alonso and M. R. Dennis, Optica 4, 476 (2017). |
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UP11.00105: Energy transfer and its effects on laser-plasma instability PIC simulations using variable charge weights C. Ren, A. Sarkar, M. C. Huang, L. Hao, J. Li, Y.-X. Cao Using variable charge weights to represent density variations in Particle-in-Cell (PIC) simulations can maintain good statistics while being computation efficient and is a common practice. But how energy transfer between different charge weights affects simulation results has not been systematically studied. We developed a methodology of quantitatively comparing PIC simulations with variable and constant charge weights, by performing a simulation multiple times to separate random fluctuations from the actual effects. We apply the methodology on a PIC simulation of laser-plasma instabilities (LPI) in shock ignition where the density varies from 0.016 to 0.4 nc (nc the critical density). The variable- and constant-charge-weight runs, their computation loads differing by 6 times, were repeated 20 times and a series of important quantities such as laser reflectivities, plasma wave amplitudes, electron temperatures and hot electron generation were compared. Overall we found no physically significant differences but statistically significant difference did exist. |
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UP11.00106: Some laser plasma considerations for advanced hohlraums with low plasma fill William Kruer, Peter Amendt, Robert Kirkwood, Scott Wilks Some laser plasma considerations for advanced hohlraums with low plasma fill are discussed. It is especially important to control backward stimulated Brillouin scattering, which could cause damage to the laser. Several well-known paths to reducing backward scattering; i.e., by enhancing the ion wave damping and the plasma gradients, are reviewed. An important dependence on angle of incidence for irradiation of high Z wall plasma is explored, and some experiments on density profile modification and potential rippling of the reflecting surface are suggested. Some ways to minimize both enhancement of the laser beam intensity via angular scattering of crossing laser beams (CBET) and further amplification of backscattered light by these beams are outlined. Sensitivity of the gains to the plasma conditions and heat transport is discussed, as well as effects due to speckles. Finally, some measurements to better assess sideward and collective scattering are recommended. |
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UP11.00107: “Rise and Shine”: Exploring a hypothesis that may unify Au sphere and NIF hohlraum modeling * Mordecai D. Rosen We use a liberal electron transport flux limiter, (~ 0.1) to match the high level of x-ray emission (at the end of a 1 ns pulse) from Au spheres driven by the URLLE Omega laser at 1.0 E15 W/sq-cm. But, NIF hohlraums with similar first bounce laser irradiances within it, have capsules imploding later than predicted by such a model. Severely restricted the flux limit (~ 0.02) can reproduce such implosion times. How to reconcile this paradox is a lingering question. Here we explore the following hypothesis. A high leverage part of the NIF hohlraum pulse is on the rise to peak power. A delay of the x-ray emission during that time can significantly delay the implosion time. It turns out that in the rise to peak power of the Au spheres there is also an apparent delay in the x-ray emission. This may be due to LPI processes, or it may be due to early time transport problems, that can be characterized as severely flux limited (~ 0.02) early on. The spheres then relax to a liberal flux limit (~ 0.1) later in the pulse. This “Rise and Shine” model reproduces the late time x-ray emission, while being consistent with implosion time delays in the NIF hohlraum, thus unifying the 2 data sets. |
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UP11.00108: Refinements to the Opacity-on-NIF hohlraum Evan Dodd, Barbara DeVolder, Madison E Martin, Natalia S Krasheninnikova, Richard A London, Ian L Tregillis, Theodore S Perry, Robert F Heeter, Yekaterina P Opachich, Alastair Moore, John L Kline, Heather M Johns, Duane A Liedahl, Tana Cardenas, Bernard Wilde, Todd J Urbatsch, Melissa Douglas The purpose of the Opacity-on-NIF experiments is to make LTE opacity measurements of iron at the same conditions as previous experiments on Sandia’s Z-facility: 156 eV and 190 eV. Iron opacities are important for understanding the structure of the sun, yet there is an ongoing disagreement between opacity theory and data that makes corroborating data highly important. Complex hohlraum geometries are required to achieve the necessary iron plasma conditions and minimize spectrometer background. We have begun taking data using the McFee-Apollo hohlraum. This hohlraum uses separate chambers to isolate the laser-driven gold from the iron sample with less surface area than previous designs, which leads to higher radiation temperatures. Different diameter laser entrance holes (LEH) and a slanted wall were added to block gold emission from the spectrometer’s line-of-sight and reduce background. We discuss additions to the initial Apollo work, including: a scaling of radiation temperature with incident energy and use of liners to better reduce background in the measured spectra. |
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UP11.00109: Evolution of perturbed interfaces subjected to transient accelerations Michael Joseph Wadas, Marc Henry de Frahan, Carolyn C Kuranz, Eric Johnsen In phenomena ranging from the expulsion of stellar core elements during supernova explosions to the degradation of the ignition hot spot during inertial confinement fusion implosions, interfaces separating plasmas of different densities undergo large, transient accelerations. Depending on the respective signs of the acceleration and density gradient, interfacial perturbations may be Rayleigh-Taylor unstable and thus experience significant growth. Although Rayleigh-Taylor analysis is well established for small-amplitude, incompressible, and constant-acceleration situations, predicting perturbation growth when departing from these conditions is more challenging. Our objective is to investigate interfacial perturbation growth driven by transient accelerations with substantial density changes. Analysis based on one-dimensional gas dynamics is used to quantify the acceleration and dilatation experienced by such interfaces, which are incorporated into one-dimensional models to characterize perturbation growth at early and late times. The modeling results are compared to simulations using an in-house, high-order accurate hydrodynamics code. |
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UP11.00110: Alternative fuel fill-tube geometry in relation to the mitigation of hydrodynamic instabilities in ICF implosions Louisa A Pickworth, Bruce A Hammel, Vladimir Smalyuk, Andrew G MacPhee, Edward Marley, Harry Francis Robey, Chris Weber, Michael Stadermann, Steve Johnson, Neal G Rice, Jay Crippen Engineering features on the capsule (gas fill tubes, support structures, etc.) introduce outer surface perturbations that can be ultimately detrimental to the performance of the capsule. Recent experiments have assessed minimal support structures, alternate pulse shapes and their effect in gas filled implosions. In this presentation the effect of the gas fill line in High Density Carbon implosions has been explored with a focus on a “tilted” geometry. This "tilted fill tube" enters the capsule shell at a shallow angle instead of perpendicularly to the shell surface. We present experiments that show the effect of this geometry and the benefit to reduce the overall perturbation caused by the fill tube.
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UP11.00111: Multi-species transport in RT instability during ICF implosion Erik Vold, Lin Yin, Andrei N. Simakov, Grigory A Kagan, Brian James Albright Evidence suggests performance in ICF experiments may be limited by hydrodynamic and transport mixing processes in the deceleration phase driven by Rayleigh-Taylor, RT, instability. This instability has a wavelength dependent maximum growth rate which is strongly determined by plasma viscous and diffusive transport processes. These processes have recently been seen in simulations to be important at early times and may therefore strongly influence the RT instability growth and mixing during deceleration. Based on rigorously derived plasma transport properties during implosions in a typical (Omega-like) implosion of CH shell on DT fuel, we estimate a range of maximum unstable RT wavelengths from a few to a few tens of microns. We present profiles of the viscosity and viscosity-limited maximum RT growth rates across typical mixing profiles between the four dominant ions in the fuel and shell. The AMR radiation-hydro code, xRage, with full multi-species plasma transport, is used in 2D RT instability simulations to examine growth rates for prescribed mode initial conditions and for plasma transport properties varying over a range expected during ICF implosions. We compare the fluid transport results for RT and for Richtmyer-Meshkov instabilities to kinetic simulations where feasible. |
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UP11.00112: Evolution of the Richtmyer-Meshkov process driven by a cylindrically convergent shock David A Yager-Elorriaga, Patrick F Knapp, Matthew R Martin, Forrest W Doss, Dan H Dolan, Kyle R Cochrane, David E Bliss, Thomas R Mattsson, Brent M Jones A platform has been developed to investigate convergent instability growth using the Z machine at Sandia National Laboratories. Cylindrical liners filled with liquid deuterium are imploded with 23 MA, driving a converging shock that interacts with an on-axis beryllium rod. The passage of the shock through the beryllium-deuterium interface initiates the Richtmyer-Meshkov process, seeded by machining single- and multi-mode axial perturbations in the beryllium rod. We present x-ray images capturing the linear and non-linear evolution of the instability, including a complex re-shock phase initiated by the on-axis reflection of the shock. Data are compared to linear theory and simulations. |
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UP11.00113: Studies of Plasma Instabilities using Unstructured Discontinuous Galerkin Method with the Two-Fluid Plasma Model Yang Song, Bhuvana Srinivasan The discontinuous Galerkin (DG) method has the advantage of resolving shocks and sharp gradients that occur in neutral fluids and plasmas. An unstructured DG code has been developed in this work to study plasma instabilities using the two-fluid plasma model. Unstructured meshes are known to produce small and randomized grid errors compared to traditional structured meshes. Positivity and monotonicity limiters that retain high order in the solution are not trivial for the two-fluid plasma model due to the presence of physical plasma dispersions. Some discussion on the limiters will be presented. Richtmyer Meshkov instabilities are studied in an expansion wave test performed in a periodic rectangular domain using MHD model and two-fluid plasma model. Simulations of Kelvin-Helmholtz instabilities using two-fluid plasma model are performed. Computational tests for Rayleigh-Taylor instabilities in radially-converging flows are performed using the MHD model and two-fluid plasma model. Choice of grid geometry is not obvious for simulations of instabilities in these circular configurations. Comparisons of the effects for different grids are made. |
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UP11.00114: New simulation capabilities in HYDRA* M. M. Marinak, G. D. Kerbel, M. V. Patel, J. Koning, C. Schroeder, S. Sepke, B. Chang, J. A. Marozas The ability of HYDRA to simulate indirect drive, direct drive and magnetic drive targets has been enhanced with the addition a several new capabilities. A new multiresolution advection capability which eliminates any need to remap calculations to a new mesh topology during the run. We describe the improved performance obtained with this new method. A new inline model for stimulated Brillouin scattering (SBS) has been implemented. The model integrates the coupled-mode equations for SBS gain and inverse bremsstrahlung absorption along the rays. It can be run in conjunction with the inline SRS model and the model for cross beam energy transfer. A low noise inverse-projection method has been extended to the 3-D laser raytrace package. It reduces ray deposition noise, while allowing direct drive targets to be modelled with only a fraction of the rays of the conventional method. The MHD package can now run effective 2D problems faster with the addition of a new dedicated 2D solver option. The Monte Carlo transport package now includes the effect of magnetic fields on charged particle orbits. We discuss these and other new capabilities. |
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UP11.00115: Modelling electron transport through laser-driven compression of CH plasma with HYDRA* Krish A Bhutwala, John D Moody, Bradley Pollock, Nathan Meezan, Marty Marinak, Joohwan Kim, Scott Wilks, Frank R Graziani, Farhat N Beg The Fast Ignition (FI) scheme of Inertial Confinement Fusion (ICF) involves fusion fuel compression to high densities via long pulse lasers, followed by fuel heating via electron energy deposition. These electrons are produced by irradiating a metal foil with a high intensity short-pulse laser, producing a beam of 1-10 MeV “hot” electrons. Applying an external magnetic field parallel to the electron beam can prolong the confinement of the hot electrons within the dense plasma, increasing the heating efficiency. We use the radiation-hydrodynamics code HYDRA to investigate the laser-driven compression of cylindrical CH foam and determine the macroscopic plasma properties (temperature, density, etc.). We may then input these properties to the hybrid-PIC code LSP to simulate the electron propagation and energy deposition into the plasma. We compare the results with a recent experiment performed at LLE, where Omega-60 was used to compress CH foam and Omega EP was used to produce hot electrons to propagate with and without a magnetic field. LLNL-ABS-XXXXXXXXX |
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UP11.00116: Modeling of LPI and non-local electron transport for radiation hydrodynamic simulation Hideo Nagatomo, Takashi Asahina, Tomoyuki Johzaki, Yasuhiko Sentoku In the last phase of the shock ignition scheme, an intense spike laser pulse drives a strong shock in order to ignite the compressed fuel. The generation of strong shock is in a laser-plasma interaction regime where laser-plasma instabilities are expected. For example, generation of hot electron and its non-local transport cannot be ignored. The kinetic simulations are effective to evaluate these dynamics. However, spatial domain of the implosion simulation is very huge for kinetic simulations, and it is unrealistic. The effect of the LPI, generation of hot electron and laser absorption rate is modeled using PIC simulation and reflected into radiation hydrodynamic simulation [1] where non-local electron conduction model [2] is installed. A typical SI simulation is conducted to evaluate the performance of the implosion using the simulation models. We also consider the effect of external magnetic field for magnetized fast ignition. [1] H. Nagatomo et al., Nucl. Fusion 57, 086009 (2017) [2] Ph. D. Nicolai et al, Phys. Plasmas 13, 032701 (2006)
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UP11.00117: Thermal transport modeling of laser-irradiated low-Z spheres with HYDRA Kevin Hao Ma, Mehul V Patel High-intensity (10^14 - 10^15 W/cm2) laser-irradiated sphere experiments are used to study X-ray conversion efficiencies and electron thermal transport properties in gold and other high-Z materials. In order to highlight the thermal transport modeling, we focus on lower-Z materials in which non-LTE kinetics is easier and has less impact on observables. We assess the effect of the electron thermal transport model (e.g. flux-limited Spitzer-Harm, nonlocal electron multi-group diffusion [1][2]) on laser absorption, coronal electron temperatures and electron densities in the Be-Sphere system, and compare these calculations with recent experiments on OMEGA. In addition, to study the thermal conduction sensitivity variations between elements we extend our analysis to additional cases such as Al and Cu. [1] Schurtz et. al, Phys. Plasmas 7, 4238 (2000) [2] Brodrick et. al, Phys. Plasmas 24, 092309 (2017) |
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UP11.00118: Multiresolution Advection in HYDRA Christopher Schroeder, Michael M Marinak HYDRA simulations play an important role in designing, analyzing, and understanding inertial confinement fusion and high energy density physics experiments at the National Ignition Facility and elsewhere. Many such calculations have been enabled or made dramatically less expensive by the multiresolution Lagrangian evolution capability added to HYDRA in the 1990's. Certain classes of simulations, particularly higher resolution ones, have required users to remap the calculation to a different mesh topology mid-run, which can be problematic for a variety of reasons. This presentation will cover the recent implementation and verification of a new multiresolution advection capability designed to eliminate the need for this remapping along with results from the first calculations to utilize it. |
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UP11.00119: Righi-Leduc Effects in NIF ICF Hohlraums Joseph M Koning, Michael M Marinak The HYDRA 3D MHD package now contains all of the terms in the full Braginskii equations for the magnetic induction, ion thermal and electron thermal equations. One of the recent additions is the Righi-Leduc term for electron thermal conduction perpendicular to the electron thermal gradient and magnetic field directions. Simulations of a representative NIF hohlraum, which includes self-generated fields, will examine the effects of including the Nernst, Ettingshausen and Righi-Leduc terms. LLNL-ABS-753551 |
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UP11.00120: Multi frame synchrotron radiography of pulsed power driven wire explosions David Yanuka, Alexander Rososhek, Savva Theocharous, Simon N Bland, Yakov E Krasik, Margie Olbinado, Alexander Rack We present synchrotron based phase contrast radiography to study pulsed power driven high energy density physics experiments. Over the past decade underwater electrical wire explosions have become of interest due to their ability to efficiently couple stored electrical energy into intense shock waves in the water. These can subsequently be shaped to provide convergent implosions, resulting in very high pressures (1-10 Mbar) being produced on relatively small pulsed power facilities (100s of kA-MA). Multiple experiments have explored how a single wire explodes in water, hoping to understand the underlying physics and better optimise this process; however, diagnostics can be limited. Utilising the phase contrast imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility, we have been able to image both the exploding wire and the shock wave launch in multiple frames. Probing radiation of 20-30 keV radiographed 200 µm tungsten and copper wires, in ~2 cm diameter water cylinders with resolutions of up to 8µm. |
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UP11.00121: Abstract Withdrawn
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UP11.00122: An university-scale pulsed-power system using a parallel plate capacitor bank Po-Yu Chang, Ming-Cheng Jheng, Chih-Ruei Hieh, Mei-Feng Huang, Sheng-Hua Yang A parallel plate capacitor bank is being built for x-ray sources or laboratory astrophysics and space research for university-scale laboratory. The system consists of twenty 1 μF capacitors, two rail-gap switches, two parallel plate transmission lines, and a cylindrical vacuum chamber orientated vertically. Two capacitors are first connected in series forming a stage. Ten stages are connected in parallel as a capacitor bank and charged up to 80 kV. The capacitor bank is separated in two groups, i.e., five stages in each group, located on two opposite sides of the chamber. Parallel plate transmission lines are used to conduct current from each group of capacitor banks to the bottom of the cylindrical vacuum chamber where the high voltage feedthrough locates. The calculated system inductance is in the order of 50 nH. The total capacitance and the stored energy of the system are 5 μF and 16 kJ, respectively. The estimated rise time and the output current are ~800 ns and ~800 kA, respectively. However, the system will be charged to only ~20 kV for testing when it is first built. The experimental discharge characteristics will be given. |
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UP11.00123: Electron temperature and electron number density measurements in laser ablation z-pinch experiments of Al2O3 Eric Dutra, Aaron Covington, Tim Darling, Radu Presura Aluminum Oxide (Al2O3) plasma was generated using Ti: Sapphire/Nd: glass laser and Z-pinch pulse-power generator at University of Nevada, Reno’s Nevada Terawatt Facility. To determine the electron temperature we use the Boltzmann plot method for the transitions (Al III 2D3/2 → 2P1/2) at 451.38 nm, (Al III 2D5/2 → 2P3/2) at 453.06 nm, (Al III 2P3/2 → 2S1/2) at 569.82, and (Al III 2P1/2 → 2S1/2) at 572.43 nm. Simultaneously, Mach-Zehnder interferometry was used to measure the electron number density. We compared simulated spectra from PrismSPECT to both calculated electron number density and electron temperature measurements, and the electron temperature was found to range from 1.8 to 2.8 eV while the electron number density was found to range from 2.3 x 1017 to 6.9 x 1017 cm-3.
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UP11.00124: Millimeter-wave radiometry and coherent Thomson scattering for studies of power balance in COBRA* Maren W Hatch, Mark Gilmore Millimeter-wave radiometer and coherent Thomson scattering diagnostics are being developed in order to characterize radiated power and turbulent density fluctuations in pinch plasmas in the COBRA accelerator at Cornell University. The purpose of these measurements will be to study the overall power balance in COBRA plasmas under various conditions. An initial radiometer channel will operate in the 94 GHz range. It is envisioned that this will be expanded to a number of channels covering the 10 – 300 GHz range in order to characterize emission vs. frequency in the mm-wave band. The coherent Thomson scattering system will operate at λ = 10.6 μm in the Bragg scattering limit, with detection at several scattering angles in order to characterize the evolution of the density fluctuation spectrum in terms of amplitude and wavenumber. Diagnostic system designs and preliminary results will be presented. |
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UP11.00125: DEVELOPING DENSE PLASMA FOCUS CAPABILITIES AT THE UNIVERSITY OF MICHIGAN Akash P Shah, Nicholas M Jordan, Charles E Seyler, Jeff M Woolstrum, Paul C Campbell, Stephanie M Miller, Ryan D McBride University-scale z-pinch experiments can inform high-value experiments conducted at Sandia. A Dense Plasma Focus (DPF) is one such z-pinch, involving a rapid plasma acceleration and compression resulting in a large number of fusion reactions [1]. These reactions are a result of micro-pinch instabilities– regions of extremely high pressure and temperature. Developing a better understanding of micro-pinch behavior could advance the field of high-energy-density physics. As such, we are developing DPF capabilities for integration into the 1-MA, 100-ns MAIZE pulsed power facility. The DPF design is simulated with the Lee model. To modernize the platform, we rewrote the Lee model in Python (the original is written as a Microsoft Excel macro in Visual Basic). Additional simulations were carried out using PERSEUS, an extended magnetohydrodynamics code. This computational work will inform the construction of the DPF. We will report on the progress of the DPF development. [1] M. Krishnan, “The Dense Plasma Focus: A Versatile Dense Pinch for Diverse Applications”, IEEE Trans. Plasma Sci. 40, 3189 (2012). |
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UP11.00126: Magnetization of a Dense Plasma via Laser Beat Waves Kevin Colligan Yates, Scott Chia Hsu, David S Montgomery, John Dunn, Samuel Langendorf, Bradley Pollock, Carsten H Thoma We present results from experiments at the Jupiter Laser Facility (JLF) at LLNL to demonstrate laser beat-wave magnetization of a dense plasma. A beat wave is created between two lasers that will resonantly accelerate thermal electrons which can drive electrical current and embed a magnetic field. The experiment uses Janus 1ω (1053 nm) beam and an Nd:YAG (1064 nm) to drive the beat wave, and the Janus 2ω (526.5 nm) beam to ionize a gas-jet target as well as provide Thomson-scattering (TS) measurements of the target density/temperature and scattered light from the beat wave. TS data captured electron-plasma-wave and ion-acoustic-wave features utilizing N, He, H, and D gas jets. Electron densities range from 1E17 to 1E19 cm-3 with temperatures of tens to hundreds of eV. Interestingly, the formation of a discrete two-density plasma by overlapping the 1ω and 2ω laser beams was observed. This channeling effect forms a plasma density near the desired range for magnetic field generation. We will present results observing a beat wave via TS measurements and compare these results with Chicago PIC simulations. |
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UP11.00127: Enhancing Understanding of Magnetized High Energy Density Plasmas from Solid Liner Implosions Using Fluid Modeling with Kinetic Closures Robert Masti, Bhuvana Srinivasan, Peter Stoltz, Jacob R King, Eric D Held, David B Hansen Recent results from experiments and simulations of magnetically driven pulsed power liners have explored the role of early-time electrothermal instability in the evolution of the MRT (magneto-Rayleigh-Taylor) instability. Understanding the development of these instabilities can lead to potential stabilization mechanisms; thereby providing a significant role in the success of fusion concepts such as MagLIF (Magnetized Liner Inertial Fusion). For MagLIF the MRT instability is the most detrimental instability toward achieving fusion energy production. Experiments of high-energy density plasmas from wire-array implosions have shown the requirement for more advanced physics modeling than that of ideal magnetohydrodynamics. The overall focus of this project is on using a multi-fluid extended-MHD model with kinetic closures for thermal conductivity, resistivity, and viscosity. The extended-MHD model has been updated to include the SESAME equation-of-state tables along with super time stepping of the parabolic PDE's of resistivity and thermal conductivity. Simulations of early time ETI/MRT growth will be presented using tabulated Lee-More based Desjarlais electrical and thermal conductivities. |
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UP11.00128: Study of laser produced plasma in a strong external magnetic field Vladimir V Ivanov, Andrei V Maximov, Riccardo Betti, Luis Leal, Roberto C Mancini, Kyle J Swanson, Hiroshi Sawada, Nicholas Wong, Alexey Astanovitskiy Laser plasma interaction and plasma dynamics in strong magnetic fields was investigated at intensities of 1015-1019 W/cm2. A pulsed power machine generated transverse and longitudinal magnetic fields of 50-300 T. The impact on laser targets due to eddy currents in a fast rising magnetic field, x-ray flux, and plasma from the coil load were studied. The formation of a thin plasma disc around the rod load was observed in the transverse B-field. The plasma disc expanded with a velocity of 250 km/s. Propagation of plasma in the transverse magnetic field after the end of the laser pulse was modelled with 2D MHD simulations. Laser plasma interaction with Si targets in the longitudinal B-field was studied by laser and x-ray spectral diagnostics. The plasma plume collimates in the magnetic field to a narrow jet 0.2-0.3 mm in diameter with a length of 3-4 mm and electron plasma density of (0.2-1.5)x1020 cm-3. Analysis of Si x-ray K-shell spectra shows a trend for the enhanced heating in the magnetic field. |
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UP11.00129: Spectroscopic Measurements of Plasmas in the Power Flow Regions on the Z Machine Mark Johnston, Sonal Patel, Mark Kiefer, Mike Cuneo, R. Doron, Yitzhak Maron Pulsed power devices depend on the ability to deliver high voltages and currents to a variety of complex loads with minimal transmission losses. The Z Machine at Sandia National Laboratories can deliver up to 26MA within a few 10’s of nanoseconds to multiple z-pinch type loads. An effort is underway to measure plasma parameters such as temperatures and densities within the power flow regions on the Z Machine. A proper physics understanding of efficient high current delivery is necessary to ensure that higher current devices such as Z-Next will perform as designed. In the power flow regions, plasmas form on the electrode surfaces and propagate into the vacuum gap, providing a current loss mechanism. These plasmas are measured spectroscopically using a 1m Czerny-Turner spectrometer with a fast (nanosecond) streak camera output. Data is analyzed using detailed, time-dependent, collisional-radiative (CR) and radiation transport modeling. Recent results will be presented.
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UP11.00130: X-ray radiography of gas-gun impact experiments using an X-pinch Peter Allan, Guy C Burdiak, Hannah Poole, Peta Foster, Hugo W Doyle, Simon N Bland, Tim J Ringrose, Jonathan Skidmore, David Chapman, Nicholas Hawker A broadband point-projection X-ray backlighter has been commissioned at First Light Fusion Ltd for radiographing shock driven experiments on a two-stage light gas gun. The X-pinch load consisted of four 7.5 um Tungsten wires driven by 100 kA in ~100 ns rise time. This produced a broadband X-ray source with energies up to 30 keV in a 20 ns pulse with a 200-300 um source size. The imaging setup achieved a spatial resolution of 50-100 um in a 30 mm field of view. An elliptically curved LiF crystal spectrometer was used to measure the time-integrated X-ray emission in the energy range 8.4 to 11.3 keV. The backlighter was used to image density structures within cm-scale length plastic targets driven by a 6 km/s projectile impact. We present radiographs from preliminary experiments along with details of the experimental setup and characterisation of the X-ray source. We demonstrate the ability to image shocks, release waves, projectile deformation and jet density profiles. Results on tungsten L-shell emission will be presented along with plans to perform absorption and/or scattering measurements on both undriven and driven static samples. All these results will be used to inform and validate our in-house modelling capabilities. |
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UP11.00131: Recent Magnetised High Energy Density Experiments on the MAGPIE Pulsed Power Generator Sergey V Lebedev, Thomas Clayson, Samuel Eardley, Jonathan WD Halliday, Jack Davies Hare, George Rowland, Daniel Russell, Roland A Smith, Nicholas Stuart, Francisco Suzuki-Vidal, Lee Suttle, Eleanor Tubman, Vicente Valenzuela-Villaseca, Andrea Ciardi, Nuno F Loureiro We present an overview of recent work on the MAGPIE pulsed power generator at Imperial College London. We use a suite of spatially and temporally resolved laser based diagnostics, including interferometry, Thomson scattering and Faraday rotation imaging to diagnose intrinsically magnetised, high energy density, supersonic plasma flows generated by the ablation of plasma from wire arrays in a range of geometries. We study a variety of astrophysically relevant phenomena such as hypersonic jets, rotating plasmas, magnetic reconnection, magnetised turbulence and the formation of bow shocks around magnetised and unmagnetised obstacles. |
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UP11.00132: Experiments and Simulations of the Recirculating Planar Crossed-Field Amplifier Steven C. Exelby, Geoffrey B. Greening, Nicholas M. Jordan, Drew A. Packard, Yue Ying Lau, Ronald M. Gilgenbach, Brad W. Hoff, David Simon The Recirculating Planar Crossed-Field Amplifier (RPCFA) is a high-power microwave amplifier that has demonstrated amplification exceeding 13 dB and 2 MW of output power. The design is derived from the Recirculating Planar Magnetron [1] which has been a topic of research at the University of Michigan. The RPCFA was designed in simulation using two distinct codes, finite element frequency domain code ANSYS HFSS, and the particle-in-cell code MAGIC. Simulation showed up to 13.5 dB of gain with an output power of 29 MW, zero-drive stability, and a bandwidth of 10%, typical for commercial CFAs. An RPCFA prototype has been fabricated to verify the results of simulation. Greater than 10 dB of amplification is observed in a continuous band from 2.63 to 3.05 GHz for a bandwidth of 15.0%. Zero-drive stability has been confirmed experimentally. Significant (σ = 1.6 dB) shot to shot variation in gain is observed in experiment. Several sources of this inconsistency are considered. Future experiments will attempt to improve reproducibility and measure amplification at MW level RF drive.[1] R.M. Gilgenbach, Y.Y. Lau, D.M. French, B.W. Hoff, J. Luginsland, and M. Franzi, “Crossed field device,” U.S. Patent US 8 841 867B2, Sep. 23, 2014. |
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UP11.00133: A study of disk-jet transitions using pulsed-power generators Hannah Hasson, Pierre-Alexandre Gourdain Astrophysical plasma jets are ubiquitous structures formed by a variety of sources. These jets may range in length from hundreds to millions of AU while maintaining the same overall geometry. Equally intriguing is their radius, which can be thousands of times smaller than their length. Yet such narrow, extended structures are clearly turbulent. Most jets are powered by a gravitational engine, which redirects infalling matter from the accretion disk outward along the axis of rotation of the disk. In this study, we use high energy density plasmas to generate accretion disks and observe how radial flows transition into outward jets. While gravity has been replaced by inward JxB forces, our setup shares similarities with its astrophysical counter-part: flows are supersonic, turbulent, advect magnetic fields in their wake, and radially fall inwards. The rotating accretion disk is generated by running 1MA of current inside thin wires distributed azimuthally around the accretion disk axis. Disk rotation is controlled by forcing the ablated plasma flow inward, and the jet is magnetized by an axial magnetic field. We will look at the impact of this magnetic field on jet formation. |
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UP11.00134: Time- and Space-Resolved studies of X-ray Emission from Hybrid X-pinch Plasmas Ahmed Elshafiey, Ivan Tilikin, Sergey Pikuz, Tatiana Shelkovenko, David A Hammer Time- and space-resolved soft-X-ray spectra produced by Hybrid X-pinches (HXPs) are being obtained with an X-ray streak camera and a time-integrated crystal spectrometer, respectively. The experiments utilize the XP generator at Cornell University in the 200-300 kA peak current, ~100 ns rise time operating mode. The streak camera shows Mo wire HXP continuum X-ray bursts <30 ps and line radiation that lasts ~250 ps. Source size measurements are obtained from high magnification imaging of different objects in which diffraction patterns are observed. The implied source sizes are as small as 1-2 µm. Electrode gap distance optimization was carried out for Ag wire HXPs in order to obtain a single hotspot for imaging purposes. A slit-step wedge camera monitors the source size of the emission in different spectral ranges, and PCDs with different filters are used to determine the radiated energy from the hotspots. Plasma parameters were estimated for Ag, Mo and Al. [1]
[1] Pikuz, S.A., Shelkovenko, T.A. & Hammer, D.A. Plasma Phys. Rep. (2015) 41: 445. https://doi.org/10.1134/S1063780X15060045 |
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UP11.00135: HADES ball switch design Imani West-Abdallah, James R Young, Marissa B Adams, Matthew Evans, Hannah R Hasson, Roman V Shapovalov, Pierre-Alexandre Gourdain, Rick Bernard Spielman Unlike regular spark gap switches, formed by one single current channel, ball switches allow to multiple current channels to form inside one single switch, de facto reducing the overall inductance of the switch. Splitting the current into multiple channels inside the switch also improves its lifetime. To reduce the overall brick inductance of HADES, the High Amperage Driver for Extreme States built at the University of Rochester, we designed and tested a ±100 kV ball switch, that uses pressurized, synthetic air as its insulating medium. The switch can sustain up to 50 kA, for hundreds of nanoseconds. We will show how the charge voltage and trigger pulse impact switch jitter. Several electric field grading methods will also be presented. |
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UP11.00136: A compact 30 T magnetic field platform for magnetized high-energy-density plasma research Gennady Fiksel, R. Backhus, P. McNally, E. Viges, Daniel Barnak, Jonathan Davies, Douglas Jacobs-Perkins, Rick Bernard Spielman, Po-Yu Chang, Riccardo Betti A pulsed high magnetic field device based on inductively coupled coil concept is described. |
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UP11.00137: Nonlinear Instabilities due to Drifting Species and Magnetic Fields in High Energy Density Plasmas Bradley Allan Shadwick, Alexander Stamm, Archis Joglekar, Bedros Afeyan Relative drifts between various species of particles are fundamental driving forces behind many plasma instabilities. Whether it be drifting ions vis a vis electrons or between different populations of electrons such as that caused by a beam in a plasma, excess fields due to charge or current imbalances can drive instabilities such as the Bunemman instability or the bump on tail instability. We study the nonlinear evolution of these processes in the presence of externally imposed transverse magnetic fields. Through the action of v × B drifts and currents, extra electric field components are driven which complicate the response of the high energy density plasma. We aim to suppress undesirable processes and retain advantageous ones by understanding the interplay between these instabilities driven by drifting species. Our results are primarily drawn from simulations using both Vlasov–Maxwell and macro-particle methods. We compare electrostatically driven modes to full electromagnetic treatments. Ion to electron mass ratios of 1, 10 and 100 will be included. |
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UP11.00138: Formation of dense plasma around a small meteoroid: Theory and simulations Yakov S Dimant, Meers M Oppenheim, Glenn Sugar, Robert A Marshall Every second, millions of small meteoroids hit the Earth from space, the vast majority too small to observe visually. Radars easily detect the plasma generated during meteoroid ablation and use this data to characterize the meteoroids and the atmosphere in which they disintegrate. Reflections of radar pulses from this plasma produce a signal called a head echo. This diagnostics requires a detailed quantitative understanding of formation of the meteor plasma. We have developed a first-principle kinetic theory to describe the behavior of meteoric particles ablated from a fast-moving meteoroid and partially ionized through collisions with the atmosphere. This theory produces analytic expressions describing the ion and neutral density and their velocity distributions. Our recent fully kinetic particle-in-cell (PIC) simulations have confirmed the major results of the analytic theory and allowed obtaining better predictions. When used for calculating meteoroid masses, the new meteor plasma model can give meteoroid masses significantly different than those calculated from a spherically symmetric Gaussian distribution, which has been used to calculate the meteoroid masses in the past. |
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UP11.00139: Simulations of Plasmasphere Refilling Observed by the Van Allen Probes Sebastian De Pascuale, Vania Jordanova, Craig Kletzing, William S. Kurth We present four cases of plasmasphere refilling observed by the Van Allen Probes under ideal conditions. Two strong storms during 15 - 19 March 2013 (7- peak Kp-index) and 1 - 6 October 2013 (8- peak Kp-index) produce severe erosion of the plasmasphere in the midnight and dusk MLT sectors respectively. Two moderate storms during 18 - 25 November 2015 (5 peak Kp-index) and 16 - 20 April 2016 (4 peak Kp-index) diminish the extent of the plasmasphere in the noon and dawn MLT sectors respectively. In each case, geomagnetic activity peaks and subsides over the course of 24 hours followed by a long quiet period of several days. Measurements of magnetospheric equatorial electron density by the Van Allen Probes show a two-stage increase in density at apogee near L = 6 for early-times in the first 48 hours after the storm and late-times in the next 48 hours during refilling. A semi-empirical treatment of the contributing rate of ionospheric outflow into the plasmasphere performs best after significant erosion but reduces in efficacy at the end of the recovery period. We simulate the location of the plasmapause with a physics-based model of E x B transport to provide global context for the local changes in density observed by the Van Allen Probes along each spacecraft's trajectory. |
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UP11.00140: Kinetic Scale Magnetosheath Turbulence at Low Electron Beta Christopher Chen, Stanislav A Boldyrev Plasma turbulence has been extensively studied at kinetic scales in the free solar wind at 1 AU where both ion and electron betas are typically order unity. However, this is not the case for other environments, e.g. close to the Sun both betas are much less than one, and in the Earth's magnetosheath the electron beta is often much less than one. Here, we present results from spacecraft observations in the Earth's magnetosheath. We show that a new type of turbulence develops at the electron inertial scale, which we term inertial kinetic Alfven turbulence. This turbulence is characterized by an increased magnetic compressibility, following a mode we term the inertial kinetic Alfvén wave, and a steeper spectrum of magnetic fluctuations, consistent with a spectral index of -11/3 that we obtain from a new set of nonlinear equations. |
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UP11.00141: Laboratory Study of Wave Generation and Particle Energization near Dipolarization Fronts Erik M Tejero, Carl L Enloe, William E Amatucci, Chris E Crabtree, Gurudas Ganguli Experiments conducted in the Space Physics Simulation Chamber at the Naval Research Laboratory (NRL) studying instabilities generated by small-scale plasma flows use plasma equilibrium that replicate those found in dipolarization fronts. It has previously been shown that these small-scale flows can generate waves in the lower hybrid range. Recent experiments at NRL have demonstrated that these flows can also generate electromagnetic waves in the whistler band above and below half the electron cyclotron frequency. These waves are large amplitude, bursty waves that exhibit frequency chirps similar to whistler mode chorus. The waves resonantly interact with electrons in the experiment that lead to particle energization, particle trapping, and other nonlinear wave effects. Recent results from these experiments will be presented. |
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UP11.00142: Characterization of On-orbit VLF Wave Generation in the inner Plasmasphere Kevin A Shipman, Patrick L Colestock, Bruce E Carlsten, Mark Gilmore There has been much interest in the use of whistlers in the inner plasmasphere for interactions with trapped electrons in the Van Allen belts. In preparation for a forthcoming experimental program launching whistlers in the VLF range from a satellite transmitter in this zone, we characterize the radiation from an electric dipole along the projected satellite orbit. We follow a well-established algorithm to determine the antenna far-fields1 and couple these results with a three-dimensional ray tracing analysis of the wave propagation. For frequencies between the lower-hybrid frequency and the electron cyclotron frequency there exists resonance cones which dominate the plasma response. The cold-plasma theory produces a singularity result for the radiated power, and hence the antenna impedance, we regularize this with thermal motion or collisional effects to produce a finite value. However, the matching of the transmitter to the antenna will be very sensitive to the background plasma parameters. This work describes the properties of the expected wave propagation and produces quantitative estimates of the transfer function to on-orbit receivers.
1. T. N. C. Wang and T. F. Bell, J. Geophysical Res., 77, 7, Mar. (1972) |
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UP11.00143: Global Simulations of Interchange Turbulence in a Dipole-confined Plasma Ou Weike, Bo Li, Lipeng Wang, Michael E Mauel Global nonlinear fluid simulations of interchange turbulence are carried out in a point dipole geometry based on the flux-tube averaged ideal MHD model. Consistent with LDX experimental observations, centrally peaked pressure profiles is obtained in nonlinear steady state. The simulations show that the evolution of interchange mode leads to the large scale plasma convection, and the low mode number is dominated in the saturated fluctuation spectra. |
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UP11.00144: Ferrite Based Antennas for Lanching Shear Alfven and Whistler Waves Walter N Gekelman, Patrick Pribyl, Stephen T Vincena, Dennis Papadopoulis Space missions place a premium on efficient low frequency wave antennas. This is especially true for Radiation Belt Remediation (RBR) intended to precipitate “MeV” satellite “killer” electrons injected following High Altitude Nuclear Detonation. Observations supported by analysis place premium on high amplitude whistlers. We have previously suggested1 that inserting a core containing Co-Fe nanoparticles in conventional antennas can increase the radiation resistance by more than 50dB, with negligible hysteresis loss. We conducted proof-of-principle experiments using less efficient ferrite material. Antenna for launching shear Alfvén and whistler waves were tested on the Large Plasma device at UCLA. A shear wave antenna µ=80 launched a wave f/fci = 0.65 with 80 times the amplitude that an identical antenna without a core. Two or more ferrite based antennas have launched shear waves with kperp/kpara = 98. Whistler waves were launched and detected meters away. Different configurations and phasing of the antennas will be presented 1:Papadopoulos et al. COSPAR 2018, Paper C5.1-0017-18 |
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UP11.00145: Impedance measurement and optimization of Ferrite based antennas for launching whistler waves. Patrick Pribyl, Walter N Gekelman, Dennis Papadopoulis, Stephen T Vincena Coupling power from a ferrite antenna in a magnetized plasma depends on the size, number of turns, and geometry of the ferrites. In this implementation we wind 1 to 5 turns on small ferrite cylinders having different aspect ratios. We experimentally investigate a number of different form factors to determine controlling factors for the impedance and radiation resistance. The impedance of the antenna in plasma is measured using a resistor divider network at the tip of the probe, which diminishes the importance of reliance on network parameters such as S11. Parasitic capacitance in and around the tip complicate the interpretation, but can be mitigated by examining the difference in plasma vs. no-plasma measurements. An optimization to match the capabilities of an existing rf driver can then be performed to enable coupling the most power into the radiated wave. |
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UP11.00146: Using Gyrokinetic Simulations to Explore the Acceleration of Auroral Electrons by Alfven Waves Gregory G. Howes, Jennifer L. Verniero, Kristopher G. Klein, James W. R. Schroeder, Frederick N Skiff, Craig A. Kletzing The physics of the aurora is one of the foremost unsolved problems of space physics. The mechanisms responsible for accelerating electrons that precipitate onto the ionosphere are not fully understood. For more than three decades, particle interactions with Alfven waves have been proposed as a possible means for accelerating electrons and generating aurorae. Due to the limitations of spacecraft conjunction studies and other multi-spacecraft approaches, it is unlikely that it will ever be possible, through spacecraft observations alone, to confirm definitively the proposed electron acceleration mechanism. Our team has pursued a long-term research program to confirm the acceleration of electrons by inertial Alfven waves under conditions relevant to the auroral acceleration zone using a series of experiments on the Large Plasma Device (LAPD) at UCLA. Here we present supporting nonlinear gyrokinetic simulations of the acceleration of electrons by an inertial Alfven wave for parameters relevant to the LAPD experiments, providing a crucial framework needed to interpret our experimental measurements. |
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UP11.00147: How local trapping amplifies the effects of heating from magnetic pumping Emily Lichko, Jan Egedal, William S Daughton, Justin C Kasper One of the outstanding problems across a variety of astrophysical phenomena is how power-law distributions with superthermal tails are generated. Most theories of particle energization rely on energy injection at a specific scale. In contrast magnetic pumping, a complementary heating mechanism to the turbulent cascade in which particles are heated by the largest scale magnetic fluctuations, results in power-law distributions like those observed in the solar wind. We have found that the ability of compressional Alfvenic turbulence to magnetically trap superthermal particles renders magnetic pumping an effective Fermi heating process for particles with v >> ω/k. This process can be further amplified by local trapping. Entropy is created when the plasma becomes untrapped, much in the same way that a gas diffusing into a new compartment creates entropy. This allows even a small amount of scattering to play a large role in heating the plasma. Here we will present an analytic description of the effect of local trapping, as well as a comparison of the analytic model with the spacecraft data obtained from the Magnetospheric Multiscale mission. |
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UP11.00148: Observed Diffusive Emission from the Solar Corona and Role of Endogenous Magnetic Reconnection Mahboubeh Asgari-Targhi, Bruno Coppi, Bamandas Basu, Leon Golub Considering the hot solar coronal loops observed by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), the X-ray telescope (XRT) and the Extreme Ultraviolet Imaging Spectrometer (EIS) on Hinode, we propose a possible explanation for the heating of Corona and the generation of high energy particle populations that may be responsible for the observed Diffusive Emission. Diffusive Emission is a form of X-ray coronal emission not confined within observationally distinct coronal loops. The relevant theoretical model is based on an endogenous reconnection process [1] that is sustained by the electron temperature gradient inside the reconnection layer, when the evolving electron temperature fluctuations are not isotropic. The inputs for this model are obtained from the most recent observations of AIA, XRT and EIS. [1] B. Coppi and B. Basu, Phys. Lett. A, 382, 400 (2018).
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UP11.00149: Energy Transfer and Electron Energization in Collisionless Magnetic Reconnection for Different Guide-Field Intensities Fulvia Pucci, Shunsuke Usami, Xuehan Guo, Ritoku Horiuchi, Shoichi Okamura, Hantao Ji, Masaaki Yamada, Jon Jara-Almonte, Jongsoo Yoo, William Fox Electron dynamics and energization are a key component of magnetic field dissipation in collisionless reconnection. In 2D reconnection, the main mechanism that limits the current density and provides an effective dissipation is most probably the electron pressure tensor term, that breaks the frozen-in condition at the x-point. The electron-meandering-orbit scale controls the width of the electron dissipation region, where electron temperature is seen to increase both in recent MMS observations and laboratory experiments (MRX). By means of 2D, full-particle simulations in an open system we investigate how energy conversion and particle energization depend on guide field intensity. We study energy transfer from the electromagnetic field to the plasma, and the threshold guide field separating dominant parallel rather than perpendicular energy transfer. We calculate the energy partition between fields and kinetic and thermal energy of different species, from the electron scales to ion scales, showing there is no significant variation for different guide field configurations. Finally we study electron distribution functions and self consistently evolved particles orbits for high guide field configuration, investigating possible mechanisms for electron perpendicular heating. |
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UP11.00150: New Insights on the Dominant Acceleration Mechanism and Formation of Power-Law Energy Distribution during Anti-parallel Relativistic Magnetic Reconnection Fan Guo, Xiaocan Li, William S Daughton, Hui Li, Yi-Hsin Liu, Dylan Ma We present new insight on the mechanisms for dominant acceleration and formation of power-law particle energy distributions in an anti-parallel magnetic reconnection layer in the magnetically dominated regime $\sigma = B^2/(4 \pi \rho c^2) \gg 1$. Through first principles kinetic simulations for the evolution of a force-free current sheet, we show that the dominant acceleration mechanism is a Fermi acceleration process in the motional electric field induced from plasma motion in the reconnection layer. By adding a test-particle component that does not experience any non-ideal electric field, we show that the test-particle component can still generate a power-law distribution with the spectral index similar to that of the electrons self-consistently evolved in the system. We conclude that the formation of power-law distribution does not rely on non-ideal electric field at the X-points. |
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UP11.00151: Amplification of Magnetic Field Topologies and Depletion of Electron Thermal Energy Alex Fletcher, Bruno Coppi Magnetic reconnection by the known tearing mode in weakly collisional and collisionless plasmas involves characteristic length scales that are unrealistically small for space plasmas. This fact motivates the search for modes producing magnetic reconnection over microscopic scale distances that remain significant when referring to plasma configurations characterized by large macroscopic scale distances. Modes that, depend on the existence of a significant electron temperature gradient can have this desired property [1]. In particular, a neutral sheet configuration is considered as in the case of Ref. [2] where auroral substorms have been proposed, for the first time, to result from magnetic reconnection processes in the Earth’s magnetotail. A new kind of mode that is localized within the region where reconnection takes place is found with an exact analytical solution of the equation describing the reconnected field. |
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UP11.00152: Magnetic Reconnection in the Lower Solar Atmosphere Vyacheslav S Lukin, Lei Ni We report on recent efforts to self-consistently model magnetic reconnection processes in weakly ionized plasmas, with a focus on the solar chromosphere. The solar chromosphere is a complex and dynamic boundary layer of the solar atmosphere where interdependence of the magnetic field evolution, radiation transport, plasma reactivity, and dissipation mechanisms make it a particularly difficult system to model and understand. Past studies have focused on the micro-physics of multi-fluid magnetic reconnection at magnetic nulls[1]. Here, the previous work is extended by considering a range of spatial scales and plasma β values in a configuration with component magnetic reconnection[2]. We show that in all cases the non-equilibrium reactivity of a weakly ionized plasma is important for determining the properties of a reconnection region, explore current sheet stability to secondary instabilities, and speculate as to the possible observables of magnetic reconnection in the lower solar atmosphere. [1] Leake, et al, ApJ 760 (2012); Leake, et al, PoP 20 (2013); Murphy & Lukin, ApJ 805 (2015). [2] L. Ni, et al, ApJ 852 (2018); L. Ni, et al, PoP 25 (2018). |
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UP11.00153: Magnetosheath reconnection before magnetopause reconnection driven by interplanetary tangential discontinuity: A three-dimensional global hybrid simulation with oblique interplanetary magnetic field Zhifang Guo, Yu Lin, Xueyi Wang, Aimin Du Terrestrial dayside dynamics associated with a southward turning interplanetary magnetic field (IMF) carried by an interplanetary tangential discontinuity (TD) is investigated by performing a three-dimensional (3D) global-scale hybrid simulation. Magnetosheath reconnection is found downstream of the bow shock due to interaction of the directional TD with the bow shock and magnetopause. Results of several runs are presented, in which the unperturbed IMF is in the GSE plane, with a cone angle of 150 degree . The incoming solar wind TD possesses various magnetic field rotation angles to and half-widths. Overall, the TD is compressed while being transmitted into the magnetosheath, with different compression processes downstream of the Q-|| and Q- shocks. Various magnetosheath flux ropes are formed through 3D patchy reconnection in the thinned current sheet. When rotation angle decreases to 90 degree, no reconnection flux ropes are found. Magnetopause reconnection is initiated when a southward turning IMF reach the magnetopause, and the magnetopause flux ropes can be mixed with the magnetosheath ones. Our simulation demonstrates that the effects of a southward turning of the IMF may not be a simple field direction change that leads to reconnection only at the magnetopause. |
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UP11.00154: Ion energization in the asymmetric reconnection diffusion region Shan Wang, Li-Jen Chen, Naoki Bessho, Michael Hesse, Jongsoo Yoo, Masaaki Yamada, Yi-Hsin Liu, Daniel Gershman, Barbara Giles, Thomas Moore We investigate ion energization in the ion diffusion region (IDR) during asymmetric reconnection using particle-in-cell simulations and Magnetosphere Multiscale (MMS) mission observations. In particular, how the ion outflow is built up, the heating characteristics, and the relative importance between the reconnection (Er) and in-plane (Ein) electric fields are studied. The simulation result shows that Er and Ein overall have comparable contribution in energizing ions near the time of the peak reconnection rate, while Ein becomes more important later. Er is most important for accelerating ions entering the central electron diffusion region (EDR). Ein becomes important beyond the central EDR and dominates the net build-up of the ion outflow. Heating of demagnetized ions is mainly due to distributions with counter-streaming populations normal to the current layer. Downstream of the peak outflow, cyclotron turning around the reconnected and Hall magnetic fields leads to further gyrotropization and thermalization. Ion distribution features of acceleration by Er and Ein are observed by MMS. |
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UP11.00155: An new MHD/kinetic model for exploring reconnection-driven particle accelerationin macroscale systems James Drake, Harry Arnold, Marc Swisdak, Joel T Dahlin A novel MHD/kinetic model is being developed to explore magnetic reconnection and particle energization in macro-scale systems. The model blends the MHD description with a macro-particle description. The rationale for this model is based on the recent discovery that the most energetic particles produced during magnetic reconnection are driven by Fermi reflection rather than parallel electric fields. Since the former mechanism is not dependent on |
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UP11.00156: The kinetic structure of the electron diffusion region observed by MMS during asymmetric reconnection Jan Egedal, William S Daughton, Ari Le, Blake A Wetherton NASA’s MMS spacecraft mission has recently explored asymmetric reconnection at the Earth’s dayside magnetopause [1]. Supported by a kinetic simulation, we derive an exclusion energy parameter, providing a lower kinetic energy bound for an electron to cross from one inflow region to the other. Only high-energy electrons are permitted to cross the inner reconnection region, setting the electron distribution function observed along the low-density side separatrix during asymmetric reconnection [2]. The analytic model accounts for the two distinct flavors of crescent-shaped electron distributions observed by spacecraft in a thin boundary layer along the low-density separatrix [1]. Consistent with the MMS observations, the fully kinetic simulation also display beams of electrons flowing toward the topological magnetic x-line. Within the ~ d_e electron diffusion region, the beams become oblique to the local magnetic field, providing a unique signature of the electron-diffusion region where the electron frozen-in law is broken [3]. [1] Burch JL, et al., (2016), Science 352, aaf2939. [2] Egedal J, et al., (2016) Phys. Rev. Lett, 117, 185101. [3] Egedal J, et al., (2018) Phys. Rev. Lett, 120, 055101.
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UP11.00157: Modelling reconstruction of typical magnetic geometries by modified RBF Wen-Ming Chen, Xiaogang Wang Three-dimensional complex magnetic geometry, such as magnetic nulls or separators which are possible sites for magnetic reconnection to take place, makes direct and detailed knowledge of them hard to gain through several crafts’ measurement at a time. Here, this paper gives a method which will reconstruct the magnetic geometry from a time series of data collected by space crafts through expanding the magnetic field into a weighted sum of modified RBF (Andreeva and Tsyganenko, 2016) without any prior restrictions on the electric current form. The method is tested by 2.5-dimensional and 3-dimensional reconnection geometry, showing good agreement with the preset ones. Moreover, the errors of the positions of nulls and directions of spines or fans are mostly below 10% even with 10% random error adding to the magnetic field values. Future application in reconstruction of real space crafts’ measurements such as Cluster or MMS mission is discussed. |
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UP11.00158: Two-fluid ten-moment studies of drift instabilities in magnetotail current sheets Jonathan Ng, Ammar Hakim, Amitava Bhattacharjee The integration of kinetic effects in fluid models is an important problem in global simulations of the Earth's magnetosphere and space weather modelling. We introduce a two fluid ten-moment model, which includes the evolution of the pressure tensor. Here we focus on current sheets in the magnetosphere, which can be unstable to drift instabilities such as the drift-kink, or the lower hybrid drift instability. Although there have been previous fluid studies of these instabilities [1], there are some discrepancies between their predictions and those of kinetic models [2]. We show that the ten moment model improves the agreement between the fluid and kinetic models, especially for the kink instability. In addition, we study the ballooning instability and its interaction with reconnection in driven and undriven magnetotail like current sheets. |
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UP11.00159: Resonant Plasma Modes and Produced High Energy Radiation Before the Merger of Collapsed Objects in Binaries Bruno Coppi Binaries involving collapsed objects (e.g. black holes) are considered to be surrounded by characteristic stationary plasma structures imbedded in a magnetic field (assumed vertical for simplicity) in which oscillatory magneto-gravitational modes can be excited [1]. The sustained modes of interest are of the vertical ballooning mode type, correspond to compressional Alfven waves, and can resonate with the orbiting frequency of the two binary components shortly before their final merger. The analyzed ballooning modes are envisioned to be composed of oppositely propagating waves with relativistic phase velocities which can produce high energy electrons by appropriate mode-particle resonances. The predicted [2] observation of radiation precursors [3] shortly before the binary collapse needs to be verified by the detection of new relevant events. An absence radiation emission following the merger can be interpreted as the absence of a coherent plasma structure around the resulting object. [1] B. Coppi, Phys. Lett. A, 382, 400 (2018). Correction and addition under publication. [2] B. Coppi and M. Medvedev, MIT-LNS HEP 17/02 June 2016. [3] F. Verrecchia, M. Tavani, A. Ursi, et al., Ap.J. Letters 84, 2 (2017). |
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UP11.00160: Instability-Induced Cross Field Transport in a Low Temperature Magnetic Nozzle Shadrach T Hepner, Benjamin Jorns Magnetic nozzles consist of a converging-diverging magnetic field that supersonically accelerates a plasma to generate thrust. A central question pertaining to these devices is that of particle detachment from magnetic field lines. As charged particles tend to follow a field line, the plasma may follow the lines as they curve back towards the thruster. In low-power systems, ions tend to be unmagnetized throughout the plume. However, electrons may remain attached and follow the field lines back to the thruster, inciting electric fields that cause ions to diverge or return to the thruster as well. This effect decreases thrust production. To produce thrust, electrons must be able to separate from magnetic field lines. This work focuses on the presence of instabilities in a magnetic nozzle and their influence on electron detachment. We measure wave propagation in three dimensions of both high and low frequencies. We further describe them theoretically and determine the anomalous collision frequency throughout the plume. We measure background plasma potential, number density, and electron temperature and discuss the influence that these waves have on electron detachment. |
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