Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session TP8: Poster Session VII: C-MOD Tokamak; Divertors; Boundary/Edge Physics; Heating and Current Drive; Turbulence, Transport and Astrophysical Plasmas |
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Room: Preservation Hall |
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TP8.00001: C-MOD TOKAMAK; DIVERTORS; BOUNDARY / EDGE PHYSICS; HEATING AND CURRENT DRIVE |
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TP8.00002: ADX: a high field, high power density, Advanced Divertor test eXperiment R. Vieira, B. LaBombard, E. Marmar, J. Irby, S. Shiraiwa, J. Terry, G. Wallace, D.G. Whyte, S. Wolfe, S. Wukitch The MIT PSFC and collaborators are proposing an advanced divertor experiment (ADX) -- a tokamak specifically designed to address critical gaps in the world fusion research program on the pathway to FNSF/DEMO. This high field (6.5 tesla, 1.5 MA), high power density (P/S $\sim$ 1.5 MW/m$^{2})$ facility would utilize Alcator magnet technology to test innovative divertor concepts for next-step DT fusion devices (FNSF, DEMO) at reactor-level boundary plasma pressures and parallel heat flux densities while producing high performance core plasma conditions. The experimental platform would also test advanced lower hybrid current drive (LHCD) and ion-cyclotron range of frequency (ICRF) actuators and wave physics at the plasma densities and magnetic field strengths of a DEMO, with the unique ability to deploy launcher structures both on the low-magnetic-field side and the high-field side -- a location where energetic plasma-material interactions can be controlled and wave physics is most favorable for efficient current drive, heating and flow drive. This innovative experiment would perform plasma science and technology R{\&}D necessary to inform the conceptual development and accelerate the readiness-for-deployment of FNSF/DEMO -- in a timely manner, on a cost-effective research platform. [Preview Abstract] |
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TP8.00003: Optimizing LHCD launcher using poloidal steering on Alcator C-Mod and ADX P. Bonoli, B. LaBombard, R. Parker, S. Shiraiwa, G. Wallace, S. Wukitch, R. Leccacorvi, R. Vieira The poloidal location of the lower hybrid current drive (LHCD) launcher has a strong influence on the trajectory and absorption of the LH wave (poloidal steering). The physics design of an additional off-midplane launcher (LH3) for Alcator C-Mod exploits this characteristic. By shifting the launcher from the mid-plane by 25cm, it is predicted to realize strong (\textgreater 80{\%}) single pass absorption localized at about r/a $=$ 0.7 in conjunction with the mid-plane (LH2) antenna. While LH3 is a proposal to overcome the LH density limit and to provide a unique opportunity to validate LHCD simulation codes under reactor-like conditions, poloidal steering can be used more extensively by launching waves from the high field side (HFS). On ADX, the LHCD launcher is proposed to be located on the HFS. Better accessibility due to higher magnetic field allows for using lower N$_{\mathrm{//}}$, which results in higher current drive efficiency. Also a more quiescent edge plasma may reduce the effect of N$_{\mathrm{//}}$ shifts due to scattering from density fluctuations. LHCD simulations for target plasmas expected on ADX, optimization of poloidal steering, and RF simulation of high field side launcher will be presented. This work supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466. [Preview Abstract] |
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TP8.00004: ICRF Actuator Development for Alcator C-Mod and ADX W.M. Beck, S.J. Wukitch, P. Koert, B. Labombard, Y. Lin, R. Vieira, J. Terry Future fusion reactors will present more severe constraints on ion cyclotron range of frequency (ICRF) actuators than ITER or present day experiments. One challenge to ICRF utilization is its interaction with the edge plasma, particularly impurity contamination and enhanced localized heat loads. Another is maintaining high coupled power through plasma variations, with high power density and antenna materials compatible with a nuclear environment. The RF plasma edge interaction is thought to be linked to RF electric fields parallel to the magnetic field, E\textbar \textbar . Experiments comparing a field aligned (FA), minimized integrated E\textbar \textbar , and a toriodally aligned (TA) antenna have demonstrated the FA antenna has significantly reduced impurity contamination and antenna impurity source compared to the TA antennas. The FA antenna also shows load tolerance we speculate to be a result of reduced slow wave coupling between straps and reduced RF induced heat flux. Latest results and analysis will be presented including further optimization that can be realized by locating the antenna to the high field side due to the inherent impurity screening observed in near double null configuration. [Preview Abstract] |
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TP8.00005: ARC: A compact, high-field, disassemblable fusion nuclear science facility and demonstration power plant Brandon Sorbom, Justin Ball, Timothy Palmer, Franco Mangiarotti, Jennifer Sierchio, Paul Bonoli, Cale Kasten, Derek Sutherland, Harold Barnard, Christian Haakonsen, Jon Goh, Choongki Sung, Dennis Whyte The Affordable, Robust, Compact (ARC) reactor conceptual design aims to reduce the size, cost, and complexity of a combined Fusion Nuclear Science Facility (FNSF) and demonstration fusion pilot power plant. ARC is a 270 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T. ARC has Rare Earth Barium Copper Oxide (REBCO) superconducting toroidal field coils with joints to allow disassembly, allowing for removal and replacement of the vacuum vessel as a single component. Inboard-launched current drive of 25 MW LHRF power and 13.6 MW ICRF power is used to provide a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing Fluorine Lithium Beryllium (FLiBe) molten salt. The liquid blanket acts as a working fluid, coolant, and tritium breeder, and minimizes the solid material that can become activated. The large temperature range over which FLiBe is liquid permits blanket operation at 800-900 K with single phase fluid cooling and allows use of a high-efficiency Brayton cycle for electricity production in the secondary coolant loop. [Preview Abstract] |
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TP8.00006: Upgraded PMI diagnostic capabilities using Accelerator-based In-situ Materials Surveillance (AIMS) on Alcator C-Mod Leigh Kesler, Harold Barnard, Zachary Hartwig, Brandon Sorbom, Richard Lanza, David Terry, Rui Vieira, Dennis Whyte The AIMS diagnostic was developed to rapidly and non-invasively characterize \textit{in-situ} plasma material interactions (PMI) in a tokamak. Recent improvements are described which significantly expand this measurement capability on Alcator C-Mod. The detection time at each wall location is reduced from about 10 min to 30 s, via improved hardware and detection geometry. Detectors are in an augmented re-entrant tube to maximize the solid angle between detectors and diagnostic locations. Spatial range is expanded by using beam dynamics simulation to design upgraded B-field power supplies to provide maximal poloidal access, including a $\sim20^{\circ}$ toroidal range in the divertor. Measurement accuracy is improved with angular and energy resolved cross section measurements obtained using a separate 0.9 MeV deuteron ion accelerator. Future improvements include the installation of recessed scintillator tiles as beam targets for calibration of the diagnostic. Additionally, implanted depth marker tiles will enable AIMS to observe the in-situ erosion and deposition of high-Z plasma-facing materials. This work is supported by U.S. DOE Grant No. DE-FG02-94ER54235 and Cooperative Agreement No. DE-FC02-99ER54512. [Preview Abstract] |
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TP8.00007: The influence of divertor geometry on access to high confinement regimes on the Alcator C-Mod tokamak J.W. Hughes, B. LaBombard, A. Hubbard, E. Marmar, J. Terry, J. Rice, J. Walk, D. Whyte, Y. Ma, I. Cziegler, E. Edlund, C. Theiler The placement of X-point and strike points in a diverted tokamak can have a remarkable impact on properties of the discharge, including thermal and particle confinement. The distinctive divertor of Alcator C-Mod allows us to demonstrate these effects experimentally, as we vary equilibrium shaping to obtain substantial variation of divertor leg length, field line attack angle and divertor baffling. In response to these changes, we observe differences in both L-mode confinement and access to high-confinement regimes (i.e. ELMy H-mode and I-mode). With the ion grad-B drift directed toward the divertor, scanning the strike point can induce $\sim$ 2x reductions in H-mode power threshold, and can produce a window for I-mode operation with H98 \textgreater\ 1. Recent experiments seek to explore these effects using improved diagnostics, and to extend them to the case with ion grad-B drift directed away from the divertor. [Preview Abstract] |
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TP8.00008: Scalings of nonlinear transfer processes in the edge plasma and their connection to Istvan Cziegler, Eric Edlund, Amanda Hubbard, Jerry Hughes, Jim Irby, Jim Terry, Christian Theiler, George Tynan Nonlinear transfer processes between large-scale edge flows and the ambient broadband fluctuations have been shown to play a significant role in the dynamics of edge turbulence, including spreading power from coherent modes and suppressing turbulence at the formation of edge transport barriers. In order to predict thresholds of confinement regimes, both the transition dynamics and the scalings of nonlinear transfer must be studied. Since the expected flow damping terms depend on ion collision rates and local safety factor, recent experiments on Alcator C-Mod explored the nonlinear drive at various values of the plasma current, density and auxiliary heating power. Nonlinear interactions of zonal flows in L-mode and both zonal flows and geodesic-acoustic modes in I-mode are estimated using bispectral as well as time-resolved methods based on gas-puff-imaging. Experiments were run in both H-mode-favorable and unfavorable geometries to compare threshold physics in L-H and L-I-H transitions. [Preview Abstract] |
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TP8.00009: First analysis of I-mode pedestals with the BOUT$++$ code Zixi Liu, Xueqiao Xu, Xiang Gao, A.E. Hubbard, Jerry Hughes, T.Y. Xia, J.R. Walk, C. Theiler, Tao Zhang, J.G. Li Edge turbulence in I-mode is characterized by a strong reduction of mid-frequency turbulence and the appearance of a higher-frequency (about 200 to 400 kHz) fluctuation, dubbed the ``weakly-coherent mode'' (WCM). The WCM is well characterized experimentally, with density and temperature fluctuations visible on multiple diagnostics. First analysis of C-Mod I-mode pedestals with the BOUT$++$ code will be presented. The magnetic equilibrium is generated using the kinetic EFIT with measured pressure profile and the calculated bootstrap current from the Sauter model. The linear simulations are carried out using fits to measured plasma density and electron temperature profiles, assuming that electron and ion temperature are equal Te $=$ Ti. The electric field is determined by the force balance relation assuming no net equilibrium flow. The preliminary simulation results show that there is no peeling-ballooning mode instability, consistent with earlier ELITE analysis. When turning off the parallel electron pressure gradient term in Ohm's law in 6-field two-fluid model, the linear growth rate is small, indicating that the drift-Alfven instability is dominant. The linear and nonlinear simulation results with experimentally measured Er profile will also be presented. [Preview Abstract] |
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TP8.00010: Light impurity transport in I-mode in Alcator C-Mod W.L. Rowan, I.O. Bespamyatnov, D.R. Hatch, W.L. Horton, K.T. Liao The I-mode hallmarks are H-mode-like electron temperature pedestal and energy confinement simultaneous with L-mode-like density pedestal and particle confinement. The I-mode is observed over a wide range of plasma parameters and is robust. As might be expected from the particle confinement observation, accumulation of naturally-occuring impurities is reduced compared to H-mode. Heavy impurity measurements confirm the observation of L-mode-like particle confinement. In the results reported here for light impurities, I-mode impurity profiles are compared with both H- and L- mode profiles for helium and boron with an emphasis on core confinement. We search for the dependence of the impurity density gradient scale lengths on the main ion density scale length and temperature scale length, the Zeff the collisionality, as well as on total radiation loss, stored energy, and global confinement. The results allow comparisons among discharge modes, as well as comparison to other devices, and turbulence predictions. The data analysis is compared with gyrokinetic simulations using the GENE code. [Preview Abstract] |
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TP8.00011: Validation of spectral MSE for Alcator C-Mod and ITER I.O. Bespamyatnov, W.L. Rowan, R.T. Mumgaard, R.S. Granetz, S.D. Scott, F. Levinton, H. Yuh The MSE spectrum was measured on C-Mod with sufficient accuracy to infer the spectral shifts and relative spectral intensities of the MSE full-energy pi and sigma components. The results were successfully used to benchmark new predictions. (I. O. Bespamyatnov, Nucl. Fusion 53 (12), 123010 (2013)). MSE optics, spectrometer, beam timing, plasma/gas density and magnetic field were optimized. Spectral resolution was improved by 50{\%} over 2012 results by decreasing the spectrometer slits and the aperture of the MSE optics. Spectral fitting analysis was developed and optimized for local diagnostic equipment and plasma conditions. The spectral MSE approaches, one based on line ratios and the other based on line shifts, can be compared to MSE polarimetry which provides reliable pitch angle measurement for C-Mod. The results of these experiments are reported here and applicability both to C-Mod and ITER is discussed. Based on these results, additional hardware improvements are proposed. [Preview Abstract] |
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TP8.00012: Design and construction of a multi-spectral MSE system for Alcator C-Mod Robert T. Mumgaard, Steven D. Scott Extensive studies of polarized inside the Alcator C-Mod tokamak have identified the sources of partially polarized light which contaminates the Motional Stark Effect (MSE) measurement. A multi-spectral MSE approach has been developed utilizing a narrow-bandpass, interference-filter-based polychromator which measures the polarization simultaneously at multiple wavelengths on the same viewing sightline. This allows the MSE polarized background to be wavelength interpolated in real time using off-MSE wavelengths. A 10 sightline, 4-wavelength, high throughput imaging polychromator system was designed, based on experience from a successful prototype. This system incorporates advances in high transmission (\textgreater 90{\%}) narrow bandpass filters, filter temperature tuning and avalanche photodiode detectors. In addition to enabling a factor of 5-10x improvement in background estimation using wavelength interpolation, the system allows for simultaneous measurement of the orthogonally polarized sigma and pi MSE emission. The factor $\sim$ 3x increase in signal collection and real-time background subtraction will enable MSE measurements in higher density, higher power plasmas and across transients with improved time resolution without beam modulation. The detection system is designed to be remotely operable and largely device independent. Initially, it will be deployed on C-Mod to enable MSE measurements in Advanced-Tokamak plasmas under development there. This work is supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466. [Preview Abstract] |
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TP8.00013: Comparison of techniques for determining structure velocities in Gas-Puff Imaging data J.M. Sierchio, A.E. White, J.L. Terry, I. Cziegler, S.J. Zweben The Gas Puff Imagining (GPI) diagnostic on Alcator C-Mod has been used previously in numerous studies involving code validation, GAMS and zonal flows, and turbulent blob dynamics. Different methods of analyzing GPI data for turbulent structure velocities are presented, including Fourier analysis, time delay estimation, and pattern tracking. Representative implementations of these methods are explained and their results are compared on the same GPI data to reveal both agreements and discrepancies in measured velocities. We have developed a code for producing synthetic sequences of images that mimic features of the actual GPI images but move the images' structures at known velocities. This allows quantitative tests of the analysis methods and reveals their strengths and weaknesses. We have found that the methods agree when the structures move in the same direction with little dispersion but disagree when there is significant dispersion or structures appearing to move in opposite directions. Comments on the appropriate use of each of method, as well as some important physics involving multi-scale/field dispersion, will be explained. [Preview Abstract] |
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TP8.00014: Understanding the Role of Electron-Scale Turbulence in the Core of Alcator C-Mod Using Multi-Scale Gyrokinetic Simulation N.T. Howard, C. Holland, A.E. White, M. Greenwald, J. Candy First-of-a-kind, nonlinear gyrokinetic simulations that capture both the ion and electron spatio-temporal scales were performed in the core (r/a $=$ 0.6) of Alcator C-Mod, ITG and TEM dominated, L-mode discharges. These multi-scale gyrokinetic simulations demonstrate the coexistence of ion and electron turbulence, an enhancement of ion-scale transport by the electron-scale turbulence, and the resolution of a previously documented discrepancy between ion-scale simulation and experimental electron heat flux. These simulations, performed using the GYRO code, capture ion and electron-scale turbulence up to k$\theta \rho_s = 48.0$ with realistic electron mass ((m$_D$/m$_e$)$^{.5} = $ 60.0), allowing for the first quantitative comparison of multi-scale simulation with experiment. Electron-scale turbulence plays a significant, even dominant, role in the core of a standard ITG and TEM dominated L-mode discharges, driving experimentally-relevant levels of electron heat flux in the form of radially elongated ETG ``streamers'' that coexist, ion-scale turbulent eddies. The implications of these results for transport model validation are discussed. [Preview Abstract] |
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TP8.00015: Investigating electromagnetic effects on core transport in Alcator C-Mod H-mode discharges W. Guttenfelder, N.T. Howard, J. Irby, F.M. Poli, A.E. White, W.F. Bergerson, D.L. Brower, W.X. Ding, C.E. Kessel, C. Sung, S.M. Wolfe, P. Xu Understanding the importance of electromagnetic effects on core turbulence and transport is being pursued at Alcator C-Mod, especially for higher performance H-mode plasmas at increasing beta. Previously reported measurements from a line-integrated polarimeter diagnostic reveal broadband, high frequency fluctuations [1]. The presence of these features, absent in core and edge density fluctuation measurements from phase contrast imaging, suggest they may be related to fluctuations in the magnetic field. Such features were observed in a number of H-mode plasmas over a range of normalized beta ($\beta_{\mathrm{N}}$ $\sim$ 1-2) and Greenwald fraction (f$_{\mathrm{GW}}$ $\sim$ 0.45-0.85). To investigate the possible influence of electromagnetic effects on core transport and turbulence, gyrokinetic simulations are used to predict microinstability of these discharges, the corresponding relative amplitude of the magnetic fluctuations in comparison to density fluctuations, and the sensitivity of these predictions to variations in beta. Results of both linear and nonlinear simulations and their comparison with transport and turbulence measurements will be presented. This work is supported by US DOE contracts DE-AC02-09CH11466 and DE-FC02-99ER54512.\\[4pt] [1] W.F. Bergerson et al., RSI 83, 10E316 (2012); APS-DPP 2012. [Preview Abstract] |
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TP8.00016: Gyrokinetic Simulations of Impurity Seeded C-Mod Ohmic Plasmas Miklos Porkolab, Paul Ennever, John Rice, J. Chris Rost, Evan Davis, Darin Ernst, Catherine Fiore, Amanda Hubbard, Jerry Hughes, Jim Terry, Naoto Tsujii, Jeff Candy, Gary Staebler, Matthew Reinke Ohmic plasmas on C-Mod were seeded with nitrogen to study the impact of dilution in the LOC (linear ohmic) and SOC (saturated ohmic) regimes [1]. The seeding decreased ion diffusivity and caused the rotation to reverse in certain cases. TGLF, TGYRO, and global GYRO simulations were performed on these plasmas, simulating both the transport and the density fluctuations. TGYRO simulations using TGLF showed that the ion temperature profile only needed slight modification to get agreement with the heat flux, and the electron temperature profile needed almost no modification. However, when these TGYRO modified profiles were simulated with global GYRO the ion and electron fluxes were much lower than the experimental measurements and the TGLF simulated fluxes. The average of the TGYRO and experimental profiles gave ion fluxes that agreed with the experimental fluxes, and the density fluctuations agreed with PCI measurements. The electron flux from GYRO is below experimental levels, and since these plasmas have little TEM turbulence ETG simulations are being performed to make up the difference. Results will be presented.\\[4pt] [1] M. Porkolab, et. al. PPCF, 54, 124029 (2012). [Preview Abstract] |
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TP8.00017: ICRF Induced Argon Pumpout in H-D Plasmas at Alcator C-Mod C. Gao, J.E. Rice, M.L. Reinke, Y. Lin, S.J. Wukitch, E.S. Marmar Argon pumpout during ICRF experiments in H-D plasmas is observed at Alcator C-Mod. This pumpout happens only when 1) the H/D ratio is relatively high ($n_H/n_D\approx 35\% - 65\%$), at which level the H-D mode conversion layer is close to the Ar$^{16+}$ $2^{\rm{nd}}$ harmonic resonance layer, and 2) the ICRF power is above 0.4 MW. At Alcator C-Mod the 80 MHz ICRF is launched from the low-field side, different from the TFR tokamak where the pumpout effect was first reported.\footnote{TFR Group, Nucl. Fusion \textbf{22}, 956 (1982).} The directly measured Ar$^{16+}$ and Ar$^{17+}$ emissivity profiles show that the pumpout happens in all plasma regions for several argon charge states. A scan of H/D ratio shows that the pumpout effect is maximized at $n_H/n_D \approx 42\pm 5\%$, at which about $80\%$ of argon is pumped out within 50 ms. It is yet to be understood whether the resonant effect or the peripheral/edge effect dominates the pumpout process. Preliminary simulation shows that the pure edge effect cannot fully reproduce the fast response of the core emissivity in the experiments. More analyses will be performed using the time-evolving impurity transport code STRAHL.\footnote{R. Dux, STRAHL User Manual, Preprint IPP (2006).} [Preview Abstract] |
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TP8.00018: Extending the Capabilities of the Shoelace Antenna on Alcator C-Mod T. Golfinopoulos, B. LaBombard, R.R. Parker, W. Burke, E.M. Davis, R. Granetz, M. Greenwald, J.W. Hughes, J.H. Irby, R. Leccacorvi, E.S. Marmar, W. Parkin, M. Porkolab, J.L. Terry, R.F. Vieira, S.M. Wolfe, S. Wukitch The mission of the Shoelace antenna is to couple to short-wavelength edge fluctuations in order to study their properties, possible open- and closed-loop control, and potential exploitation to actively drive transport. The antenna matches both perpendicular wave number and frequency to two such fluctuations: the Weakly- and Quasi-Coherent modes, which regulate transport across the plasma boundary in high-performance, ELM-free, steady-state regimes. In initial operation, the antenna induced a drift-wave-like edge mode [Golfinopoulos \emph{Phys. Plasmas} '14], but no measurements were available to assess resultant transport. Here, we present two upgrades to the system. The antenna's pitch angle was adjusted such that, when field-aligned, the antenna maps to the Mirror Langmuir Probe [LaBombard \emph{Phys. Plasmas} '14], providing detailed fluctuation, profile, and transport measurements. In addition, antenna power has been quadrupled to $\geq$8~kW, increasing driven mode amplitude and reach up the pedestal. [Preview Abstract] |
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TP8.00019: Locked-mode avoidance and recovery without external momentum input L. Delgado-Aparicio, D.A. Gates, S. Wolfe, J.E. Rice, C. Gao, S. Wukitch, M. Greenwald, J. Hughes, E. Marmar, S. Scott Error-field-induced locked-modes (LMs) have been studied in C-Mod at ITER toroidal fields without NBI fueling and momentum input. The use of ICRH heating in synch with the error-field ramp-up resulted in a successful delay of the mode-onset when $P_{ICRH}> 1$ MW and a transition into H-mode when $P_{ICRH}> 2$ MW. The recovery experiments consisted in applying ICRH power during the LM non-rotating phase successfully unlocking the core plasma. The ``induced'' toroidal rotation was in the counter-current direction, restoring the direction and magnitude of the toroidal flow before the LM formation, but contrary to the expected Rice-scaling in the co-current direction. However, the LM occurs near the LOC/SOC transition where rotation reversals are commonly observed. Once $P_{ICRH}$ is turned off, the core plasma ``locks'' at later times depending on the evolution of $n_{e}$ and $V_{t}$. This work was performed under US DoE contracts including DE-FC02-99ER54512 and others at MIT and DE-AC02-09CH11466 at PPPL. [Preview Abstract] |
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TP8.00020: Amplitude modulation of lower hybrid waves for transport control G.M. Wallace, S.G. Baek, I.C. Faust, T. Golfinopoulos, B.L. LaBombard, R.T. Mumgaard, R.R. Parker, S.D. Scott, S. Shiraiwa, J.L. Terry Steady, high-power lower hybrid (LH) waves have been shown to alter transport characteristics in the edge and pedestal regions of EDA H-modes on Alcator C-Mod [J. Hughes et al, Nuc. Fus., 2010]. The modifications of the pedestal are particularly striking in high-density H-modes [J. Terry, this conference], perhaps through interaction with the transport-regulating edge Quasi-Coherent Mode (QCM), since it is strongly affected by the injection of LH waves. The transport modification effect is present even at high densities for which LH waves are not accessible to the core plasma and current drive effects are negligible. Experiments have been conducted to determine if modulating LH power near the QCM frequency can enhance the beneficial effects of LH waves on the pedestal and the QCM. A new capability was developed to modulate the net LH wave power at frequencies from 1-200 kHz. The presence and character of edge modes was monitored using gas puff imaging, phase contrast imaging, reflectometry, and magnetic pickup loops. [Preview Abstract] |
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TP8.00021: Scaling of Global LHCD Efficiency in Alcator C-Mod S. Scott, P. Bonoli, R. Mumgaard, S. Shiraiwa, G. Wallace, D. Whyte A database of global current-drive efficiency by Lower Hybrid waves has been assembled covering nine years of C-Mod operation. Plasma conditions were averaged over 50-ms time slices during equilibrated current-profile time periods, excluding transient events such as Prad spikes. The database comprises 1800 time slices spanning: $PLH<1.1$ MW, n$_{\mathrm{\vert \vert }}=$1.5-2.3, Ip $=$ 0.3-1.0 MA, nebar $=$ 0.35-1.5e20. Nine percent of the data points are approximately non-inductive ($\Delta $V/V$>$0.9), while 17 percent experience low m,n MHD that degrades the LHCD efficiency. During LHCD, a simple Spitzer model is used to estimate the residual inductively-driven current which scales the pre-LH current by the ratio of the loop voltage to the pre-LH loop voltage, correcting also for the change in conductivity. The current-drive efficiency is defined as $\eta \quad =$ nebar R I$_{\mathrm{LHCD}}$/P$_{\mathrm{LH}}$ [10$^{\mathrm{20}}$ MA/m$^{\mathrm{2}}$ MW], where I$_{\mathrm{LHCD}}$ is the current driven by LH waves and P$_{\mathrm{LH}}$ is the forward-directed LH power. In approximately non-inductive, MHD-free plasmas, the global current drive efficiency shows a striking positive correlation with plasma current, $\eta \quad =$ 0.065 $+$ 0.40 x Ip [MA], reaching a value of $\eta =$0.47 at Ip$=$1.02 MA. A positive but weaker correlation between $\eta $ and Teo does not explain the $\eta $ dependence on Ip. Preliminary GENRAY/CQL3D simulations at Ip$=$1.0 MA predict 900 kA of driven current versus 1000 kA observed. Comparisons of $\eta $ to numerical simulations over a wide parameter range will be discussed. [Preview Abstract] |
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TP8.00022: $\pi $Scope: python based scientific workbench with visualization tool for MDSplus data S. Shiraiwa $\pi $Scope [1] is a python based scientific data analysis and visualization tool constructed on wxPython and Matplotlib. Although it is designed to be a generic tool, the primary motivation for developing the new software is 1) to provide an updated tool to browse MDSplus [2] data, with functionalities beyond dwscope and jScope, and 2) to provide a universal foundation to construct interface tools to perform computer simulation and modeling for Alcator C-Mod. It provides many features to visualize MDSplus data during tokamak experiments including overplotting different signals and discharges, various plot types (line, contour, image, etc.), in-panel data analysis using python scripts, and publication quality graphics generation. Additionally, the logic to produce multi-panel plots is designed to be backward compatible with dwscope, enabling smooth migration for dwscope users. $\pi $Scope uses multi-threading to reduce data transfer latency, and its object-oriented design makes it easy to modify and expand while the open source nature allows portability. A built-in tree data browser allows a user to approach the data structure both from a GUI and a script, enabling relatively complex data analysis workflow to be built quickly. As an example, an IDL-based interface to perform GENRAY/CQL3D simulations was ported on $\pi $Scope, thus allowing LHCD simulation to be run between-shot using C-Mod experimental profiles. This workflow is being used to generate a large database to develop a LHCD actuator model for the plasma control system. [1]http://piscope.psfc.mit.edu [2]http://www.mdsplus.org. Supported by USDoE award DE-FC02-99ER54512 [Preview Abstract] |
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TP8.00023: Documenting scientific workflow: the metadata, provenance and ontology project Martin Greenwald, J. Stillerman, J. Wright, G. Abla, R. Chanthavong, D. Schissel, A. Romosan, A. Shoshani Careful management of data, its creation and transformation (provenance) and associated metadata is a critical part of any scientific enterprise. Traditionally this was the role of the lab notebook, but the digital era has resulted instead in the fragmentation of data, processing and annotation. This paper describes an ongoing multi-institutional project aimed at remedying this problem by developing tools to automate documentation of scientific workflows and associated information. Data and all processes that create or modify that data are represented mathematically as a directed acyclic graph, providing explicit information about the relationships between elements with all elements having globally unique and persistent IDs. The export of data, for publication, presentation or external databases would be recorded, allowing traceability in either direction -- answering the questions ``Where was this data used?'' or ``Where did the data in this figure come from.'' Namespace management is provided through a well structured ``ontology,'' which can be customized for any particular community or application. [Preview Abstract] |
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TP8.00024: Towards a Lithium Radiative / Vapor-Box Divertor Robert Goldston, Marius Constantin, Michael Jaworski, Rachel Myers, Masayuki Ono, Jacob Schwartz, Filippo Scotti, Zhaonan Qu Recent research has indicated that the peak perpendicular heat flux on reactor divertor targets will be hundreds of MW/m$^{\mathrm{2}}$ in the absence of dissipation and/or spatial spreading. Thus we are attracted to both enhanced radiative cooling and continuous vapor shielding. Lithium particle lifetimes $\le $~100 micro-sec enhance radiation efficiency at T \textless 10 eV, while lithium$^{\mathrm{\thinspace }}$charge-exchange with neutral hydrogen may enhance radiative efficiency for T \textgreater 10 eV and n$_{\mathrm{0}}$/n$_{\mathrm{i}}$ \textgreater 0.1. We are examining if the latter mechanism plays a role in the narrowing of the heat-flux footprint in lithiated NSTX discharges. In parallel we are investigating the possibility of immersing a reactor divertor leg in a channel of lithium vapor. If we approximate the vapor channel as in local equilibrium with lithium-wetted walls ranging from 300$^{\mathrm{o}}$C at the entrance point to 950$^{\mathrm{o}}$C 10m downstream in the parallel direction, we find that the vapor can both balance reactor levels of upstream plasma pressure and stop energetic ions and electrons with energies up to at least 25 keV, as might be produced in ELMs. Each 10 l/sec of lithium evaporated deep in the channel and recondensed in cooler regions spreads 100 MW over a much wider area than the original strike point. [Preview Abstract] |
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TP8.00025: Classification of magnetic configurations for the cloverleaf divertor D.D. Ryutov, M.V. Umansky The cloverleaf divertor [D.D. Ryutov, M.V. Umansky, Phys. Plas., 20, 092509, 2013] is based on magnetic configuration with the third-order poloidal field null. If the currents in the divertor coils are somewhat different from those required for the generation of the third-order null, the latter splits into three closely-spaced first-order nulls. One can move these nulls around by changing the currents in the divertor coils. A large variety of configurations can be created. In this study we provide general topological classification of all configurations possible in the case of the coils situated at sufficiently large distance from the nulls. It turns out that these configurations can be identified by a single dimensionless parameter, analogously to what has been done for the snowflake divertor [D.D. Ryutov, M.A. Makowski, M.V. Umansky, PPCF, 52, 105001, 2010]. In addition to this general classification, we evaluate the length scales of the field variation in each of the three nulls, as well as connection lengths and local magnetic shear for a variety of configurations. Work performed for U.S. DOE by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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TP8.00026: Modifying COGENT to Study Snowflake Divertors R.H. Cohen, M. Dorf, M. Dorr, D.D. Ryutov The snowflake divertor concept entails modifying the poloidal field system of a tokamak to produce a 2nd-order null in magnetic-field strength in place of the conventional 1st-order null x point within the equilibrium magnetic-field separatrix. It more effectively spreads the divertor heat load and offers a number of other advantages. We describe plans to modify the COGENT edge kinetic code to study snowflake divertors. COGENT employs mapped multi-block grid technology to handle the geometric complexity of the conventional divertor configuration. To simulate snowflake divertors, the number of grid blocks is increased from 8 to 12, consistent with the modified topology of the exact snowflake configuration. We examine the applicability of the modified structure to study configurations that are not exactly snowflakes, the so-called ``snowflake-plus'' and ``snowflake-minus'' configurations. Initial applications of the modified code will be assessment of collisionless orbit dynamics and neoclassical transport. [Preview Abstract] |
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TP8.00027: Modeling of ion orbit loss and intrinsic toroidal rotation with the COGENT code M. Dorf, M. Dorr, R. Cohen, T. Rognlien, J. Hittinger We discuss recent advances in cross-separatrix neoclassical transport simulations with COGENT, a continuum gyro-kinetic code being developed by the Edge Simulation Laboratory (ESL) collaboration. The COGENT code models the axisymmetric transport properties of edge plasmas including the effects of nonlinear (Fokker-Planck) collisions and a self-consistent electrostatic potential. Our recent work has focused on studies of ion orbit loss and the associated toroidal rotation driven by this mechanism. The results of the COGENT simulations are discussed and analyzed for the parameters of the DIII-D experiment. [Preview Abstract] |
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TP8.00028: PIC Simulation of plasma detachment Seiji Ishiguro, Theerasarn Pianpanit, Hiroki Hasegawa, Ryutaro Kanno The detached plasma, which is caused by gas puffing, has been proposed and it is the most promising way to reduce the heat load to the divertor plate of fusion oriented devices. Dynamical and kinetic behavior of the detached plasma is unresolved. So we are developing particle-in-cell simulation model with atomic processes such as line radiation, ionization, charge-exchange collision and recombination. As a first step, we have performed PIC simulation with Monte Carlo collisions, where spatial and velocity space distributions of charged particles, self-consistent electric field, and atomic processes such as ionization and charge exchange are included. Temperature decrease and density increase in front of the target is observed and electric potential structure along the axis is created. [Preview Abstract] |
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TP8.00029: Seeking Stable Detachment Scenarios of Advanced X-Divertors with SOLPS 5.1 Brent Covele, Prashant Valanju, Mike Kotschenreuther, Swadesh Mahajan, John Canik, Hutch Neilson, Charles Kessel, Brian LaBombard, Stephen Wolfe A broad investigation into new magnetic equilibria for several tokamaks (C-Mod, NSTX-Upgrade, K-DEMO, and a Fusion Nuclear Science Facility) using the CORSICA code has revealed a host of advanced X-Divertors (XDs) feasible on existing and planned PF coil sets. Because of their flaring flux tubes and higher Divertor Index (DI$_{XD}$ \textgreater 1), XDs may open regimes of stable divertor detachment without negatively impacting H-Mode confinement, something which has not been experimentally achievable with a standard divertor (DI$_{SD} \equiv $ 1). To investigate stable X-Divertor detachment, 2D transport modeling is performed using the SOLPS 5.1 code suite. Sophisticated neutral physics modeling in the Eirene 2008 code, including neutral-neutral interactions, is required to accurately model the evolution of detachment. Initial results show steep, steady-state parallel electron temperature gradients near the divertor targets, as well as a reduction in the target heat fluxes. This is indicative of an arrestment of the detachment front near the targets, as predicted by the Divertor Index. [Preview Abstract] |
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TP8.00030: Direct Measurement of the First Wall Recycling Coefficient on RFX mod Roberto Cavazzana, Matteo Agostini, Lorella Carraro, Paolo Innocente, Lionello Marrelli, Paolo Scarin, Gianluca Spizzo, Monica Spolaore, Nicola Vianello, Matteo Zuin A diagnostic for the direct measurement of the Deuterium particle fluxes and the recycling coefficient $R=\Gamma_{in}/\Gamma_{out}$ at the first wall has been recently installed on RFX-mod. The system is composed by a set of combined diagnostics. Two sets of Langmuir probes (LP) are mounted on a on a movable graphite tile; one is configured as a triple probe (measuring $T_e, n_e$), while the other set uses two floating potential for the measurement of the electric field and plasma flow velocity transverse components. The two LP sets can be also combined as a Mach probe for estimation of the parallel flow velocity, allowing the separation between the convective and the turbulent contributions. A set of spectroscopic measurements (calibrated D$_{\alpha}$, C-II and Li-I emissions), a single point thermal infrared sensor and a fast camera in the visible range aimed at the graphite tile, are used to evaluate the particle influx and the response of the graphite sample at different plasma conditions. The intent is to determine the behavior of the asymmetry of the recycling coefficient due to the geometry of the magnetic field. Furthermore it will be possible to establish the role of the particle source in the density accumulation effect induced by magnetic islands present at the edge. [Preview Abstract] |
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TP8.00031: Fundamental study on separation of fuel and impurity particles by using divertor simulator TPD-Sheet IV Takafumi Maekawa, Takaaki Iijima, Takuya Hase, Akira Tonegawa, Kohnosuke Sato, Kazutaka Kawamura The pumping of helium ash has become important for the control in the SOL/divertor plasma because of the helium ash makes dilution of the fuel density and makes decreases the core plasma temperature. The selective removal of helium ash using by ion cyclotron resonance (ICR) method has been studied in a linear divertor simulator, TPD-Sheet IV. We have demonstrated the ICR method of the helium or helium/hydrogen sheet plasma by the RF electrodes of two parallel plates, sandwiching the plasma. Measurements of the ion temperature in the plasma were carried out a fast scanning Faraday cup. In addition, the ion densities in the plasma were measured by an omegatron mass analyzer and the neutral densities of resonant ions were measured by a quadrupole mass analyzer. As a result, the ion densities of heated ion decrease with increasing the RF power. It is found that the selective removal of the helium ions in the sheet plasma is successful by ICR method. [Preview Abstract] |
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TP8.00032: Impact of the Pedestal Plasma Density on ELM Dynamics and Energy Loss Scaling X.Q. Xu, J.F. Ma, G.Q. Li The latest BOUT$++$ studies show an emerging understanding of ELM dynamics and the consistent collisionality scaling of ELM energy losses with ITPA multi-tokamak database. A series of BOUT$++$ simulations are conducted to investigate the scaling characteristics of the ELM energy losses vs collisionality via a density scan, while keeping the plasma cross-sectional shape, total stored energy, total plasma current, pressure profiles fixed. The neoclassical collisionality at peak gradient position increases by a factor of 3262 from 0.0019 to 6.197. The critical trend of linear simulations emerges as a transition from ballooning-dominated states at high collisionality to peeling-dominated states at low collsionality with decreasing density. Nonlinear BOUT$++$ simulations show a two-stage process of ELM crash evolution of (i) initial bursts of pressure blob and void creation and (ii) inward turbulence spreading as void propagation. The inward void propagation stirs the top of pedestal plasma and yields an increasing ELM size with decreasing collisionality after a series of micro-bursts. The pedestal plasma density plays a major role in determining the ELM energy loss through its effect on the edge bootstrap current and ion diamagnetic stabilization. [Preview Abstract] |
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TP8.00033: Studies of impact of plasma shaping on edge localized modes with a nonlinear code BOUT$++$ G.Q. Li, X.Q. Xu, P.B. Snyder, A.D. Turnbull, T.Y. Xia The plasma shaping has important effects on the edge localized modes (ELMs). In this work, with the 3-field BOUT$++$ code, we study the impact of the plasma shaping on the ELMs. Three kinds of typical plasma shapes are studied: circular (cbm), elongated (dbm) and shaped with X-point (meudas). Our calculations show that the shaped plasma and the X-point geometry have stabilizing effect on the ELMs. For linear ideal MHD calculation we benchmark BOUT$++$ results with ELITE and GATO codes. Then we study the role of non-ideal effects such as resistivity on the ELMs for the X-point geometry. Also the nonlinear calculations are carried out to study the impact of plasma shape on the ELM size. [Preview Abstract] |
(Author Not Attending)
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TP8.00034: Development of test particle module for impurity generation and transport in BOUT++ framework Xiaotao Xiao, Xueqiao Xu Developing the test particle module in BOUT++ framework is the first step to enhance its capability to simulate impurity generation and transport in edge plasmas, which potentially can be extended to efficiently simulate both turbulence and neoclassical physics in realistic geometry. The motion of impurity charged particles are governed by guiding-center (GC) equations in the presence of turbulent electromagnetic fields. The GC equations are the well-known Hamiltonian guiding center equation given by Littlejohn, Boozer, White and others. The Fourth-order Runge-Kutta algorithm is used to advance the GC equations in time. In order easily to couple with BOUT++ fluid module, the same field aligned coordinates are used except near the region close to X-point. The bilinear interpolation is used to interpolate 3D fluid turbulent electromagnetic fields from grid points to particle positions. The calculated orbits in equilibrium configuration are checked to conserve constants of motion. The various guiding-center orbits in divertor configuration under BOUT++ framework are demonstrated and benchmarked. Then spatial distribution of impurities in edge plasmas from given sources at the divertor plates and at the protection limiters near RF antennas is obtained in given background plasma. [Preview Abstract] |
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TP8.00035: Simulations of particle and heat fluxes to divertor targets of ELMy H-mode in DIIID and EAST T.Y. Xia, X.Q. Xu, M.E. Fenstermacher, G.Q. Li, H.Y. Guo The BOUT$++$ simulations for the evolution of the particle and heat fluxes during ELM bursts on DIIID and EAST will be presented. The profiles of DIIID H-mode discharge 144382 with fast target heat flux measurements are used as the initial conditions. This ELM is found to be driven by the resistive-ballooning mode. A flux-limited parallel thermal conduction is used with three model values of the flux-limiting coefficient $\alpha _{\mathrm{j}}$. A larger $\alpha_{\mathrm{j}}$ leads to a larger radial heat flux, which can enlarge the total energy loss and also the heat fluxes to targets. The sheath-limit value is the most appropriate one here which shows ELM sizes very close to the measurements. The evolution of the spreading widths and amplitudes of the heat flux profiles on targets due to ELMs is well reproduced. Magnetic flutter combined with parallel thermal conduction can enhance the radial transport and enlarge the total energy loss by 33{\%}, and is able to generate longer and wider lobe structures near the X-point at LFS. The EAST ELMy H-mode driven by ideal peeling-ballooning modes is used to validate the influence of the B$_{\mathrm{T}}$ direction. The results show consistent asymmetric distributions of the particle flux on lower and upper targets. [Preview Abstract] |
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TP8.00036: Mitigating impact of rectified RF sheath potential on the ELMs Bin Gui, Xueqiao Xu, Tianyang Xia Here we report on the BOUT++ simulation results for the mitigating impact of rectified RF sheath potential on the peeling-ballooning modes. The limiter and the RF wave antenna are placed at the outer middle plane in the scrape-off-layer (SOL) in shift-circle geometry. The external shear flow is induced by the limiter and the RF wave. Besides this, the sheath boundary conditions are imposed on the perturbed potential and parallel current. From the three-field simulations [1], it is found that the energy loss is suppressed by the external shear flow in the nonlinear phase. The external shear flow due to the RF wave leads to a broad turbulence spectrum. The wider spectrum leads to a weaker turbulence transport and results in a smaller energy loss. The perturbed electric potential and the parallel current near the sheath region are also suppressed locally due to the sheath boundary condition. Based on this work, this effect of limiter will also be applied in six-field which includes more physics effects [2]. The effect of sheath boundary conditions on the thermal conductivities and heat flux will be studied.\\[4pt] [1] Xu, et al., Nucl. Fusion, 2011.\\[0pt] [2] Xia, et al., Nucl. Fusion, 2013. [Preview Abstract] |
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TP8.00037: Couple an ICRF core spectral solver to and edge FEM code John Wright, Syunichi Shiraiwa The finite element method (FEM) and the spectral approaches to simulation of ion cyclotron (IC) waves in toroidal plasmas each have strengths and weaknesses. For example, the spectral approach (eg TORIC) has a natural algebraic representation of the parallel wavenumber and hence the wave dispersion but does not easily represent complex geometries outside the last closed flux surface, whereas the FEM approach (eg LHEAF) naturally represents arbitrary geometries but does not easily represent thermal corrections to the plasma dispersion. The two domains: thermal core in flux surface and cold edge plasma may be combined in such as way that each approach is used where it works naturally. Among the possible ways of doing this, we demonstrate the method of mode matching. This method provides an easy way of combining the two linear systems without significant modifications to the separate codes. We will present proof of principal cases and initial applications to minority heating. [Preview Abstract] |
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TP8.00038: Iterative addition of perpendicular kinetic effects to finite-difference simulation of radio-frequency heating David Green, Lee Berry In previous work we have demonstrated\footnote{D. L. Green and L. A. Berry, ``Iterative addition of parallel temperature effects to finite-difference simulation of radio-frequency wave propagation in plasmas,'' Comp. Phys. Comm., 185(3), pg. 736-743 (2014); doi:10.1016/j.cpc.2013.10.032} the iterative addition of parallel kinetic effects to finite-difference frequency-domain simulation of radio-frequency (RF) wave propagation in fusion relevant plasmas. Such iterative addition in configuration space bypasses several of the difficulties with traditional spectral methods for kinetic RF simulation when applied to problems that exhibit non-periodic geometries. Furthermore, the direct numerical integration of particle trajectories in real magnetic field geometries removes violations of the stationary phase approximation inherent in the spectral approach.\footnote{D. L. Green and L. A. Berry, ``Investigating stationary phase violations in kinetic RF simulation of real plasmas,'' http://meetings.aps.org/link/BAPS.2013.DPP.BP8.70} Here we extend this approach to include perpendicular kinetics. [Preview Abstract] |
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TP8.00039: Modeling RF-induced Plasma-Surface Interactions with VSim Thomas G. Jenkins, David N. Smithe, Alexei Y. Pankin, Christine M. Roark, Peter H. Stoltz, Sean C.-D. Zhou, Scott E. Kruger An overview of ongoing enhancements to the Plasma Discharge (PD) module of Tech-X's VSim software tool is presented. A sub-grid kinetic sheath model, developed for the accurate computation of sheath potentials near metal and dielectric-coated walls, enables the physical effects of DC and RF sheath dynamics to be included in macroscopic-scale plasma simulations that need not explicitly resolve sheath scale lengths. Sheath potential evolution, together with particle behavior near the sheath (e.g. sputtering), can thus be simulated in complex, experimentally relevant geometries. Simulations of RF sheath-enhanced impurity production near surfaces of the C-Mod field-aligned ICRF antenna are presented to illustrate the model; impurity mitigation techniques are also explored. Model extensions to capture the physics of secondary electron emission and of multispecies plasmas are summarized, together with a discussion of improved tools for plasma chemistry and IEDF/EEDF visualization and modeling. The latter tools are also highly relevant for commercial plasma processing applications. Ultimately, we aim to establish VSimPD as a robust, efficient computational tool for modeling fusion and industrial plasma processes. [Preview Abstract] |
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TP8.00040: Validation Studies of the Finite Orbit Width version of the CQL3D code Yu.V. Petrov, R.W. Harvey The Finite-Orbit-Width (FOW) version of the CQL3D bounce-averaged Fokker-Planck (FP) code [1] has been further developed and tested. The neoclassical radial transport appears naturally in this version by averaging the local collision coefficients along guiding center orbits, with a proper transformation matrix from local (R,Z) coordinates to the midplane computational coordinates, where the FP equation is solved. In a similar way, the local quasilinear rf diffusion terms give rise to additional radial transport of orbits. The main challenge is the internal boundary conditions (IBC) which add many elements into the matrix of coefficients for the solution of FPE on the computational grid, effectively making it a non-banded matrix (but still sparse). Steady state runs have been achieved at NERSC supercomputers in typically 10 time steps. Validation tests are performed for NSTX conditions, but using different scaling factors of equilibrium magnetic field, from 0.5 to 8.0. The bootstrap current calculations for ions show a reasonable agreement of current density profiles with Sauter et al. model equations [2] which are based on 1st order expansion, although the magnitudes of currents may differ by up to 30{\%}. \\[4pt] [1] R.W. Harvey and M. McCoy, ``The CQL3D Fokker Planck Code,'' www.compxco.com/cql3d\\[0pt] [2] O. Sauter, C. Angioni, and Y. R. Lin-Liu, Phys. Plasmas 6 (1999) 2834. [Preview Abstract] |
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TP8.00041: RF sheaths for arbitrary B field angles Daniel D'Ippolito, James Myra RF sheaths occur in tokamaks when ICRF waves encounter conducting boundaries and accelerate electrons out of the plasma. Sheath effects reduce the efficiency of ICRF heating, cause RF-specific impurity influxes from the edge plasma, and increase the plasma-facing component damage. The rf sheath potential is sensitive to the angle between the B field and the wall, the ion mobility and the ion magnetization. [J.R. Myra et al., Nucl. Fusion 30, 845 (1990)]. Here, we obtain a numerical solution of the non-neutral rf sheath and magnetic pre-sheath equations (for arbitrary values of these parameters) and attempt to infer the parametric dependences of the Child-Langmuir law. This extends previous work [D.A. D'Ippolito and J.R. Myra, APS-DPP Meeting, 2013] on the magnetized, immobile ion regime. An important question is how the rf sheath voltage distributes itself between sheath and pre-sheath for various B field angles. This will show how generally previous estimates of the rf sheath voltage and capacitance were reasonable, and to improve the RF sheath BC. [Preview Abstract] |
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TP8.00042: Plasma-Based Electrical Transformers and Electrostatic Current Drive in Tokamaks Richard Nebel, W. Gibson, K. Moser, D.C. Barnes, L.L. Glascock, J.M. Finn, J. Dunn Recent nonlinear simulations shown that it is possible to drive current in tokamaks with app-lied helical electrostatic fields. These electrostatic studies have uncovered a new nonlinear MHD relaxation principle. This new principle states that if helical electrostatic fields are applied to a plasma, it tries to relax to a state where the magnetic field aligns parallel with the electrodes. If an m$=$1, n$=$1 driving electrostatic field is applied at the boundary, the plasma tries to relax to a state where q $\sim$ 1 everywhere even if no loop voltage is applied to the plasma. It is possible to operate a tokamak steady-state without applying a loop voltage. At Tibbar Technologies we are primarily interested in using this new MHD relaxation principle to build DC-DC electrical transformers. This technology is important for High Voltage DC electrical transmission. We have now demonstrated this new physics in a linear device in the laboratory. The plasma tries to align the magnetic fields parallel to the electrodes, as the theory predicts. It also doesn't matter which electrode is positive and which is negative, which is also consistent with the theory. Finally, changing the direction of the magnetic field in the solenoid also changes the direction of the current flow in the secondary of the transformer. Efficiencies of 50{\%}-60{\%} are regularly observed. [Preview Abstract] |
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TP8.00043: Suprathermal electron dynamics and hard X-ray tomography in TCV Josef Kamleitner, Stefano Coda, Joan Decker Theoretically predicted toroidal and poloidal emission asymmetries are observed by energy-resolved hard X-ray tomography (HXRS [1]) on the TCV tokamak. These bremsstrahlung measurements, in conjunction with Fokker-Planck modeling and synthetic diagnostics [2], characterize the suprathermal electron distribution during electron cyclotron resonance heating and current drive (ECRH, ECCD). The dynamics and the transport of suprathermal electrons in real and velocity space are studied, also with respect to quasilinear effects in EC wave absorption. Further new results are presented concerning the interaction of fast electrons with magnetohydrodynamics (MHD) instabilities, especially the m/n=1/1 internal kink, and including runaway electron creation and transport.\\[4pt] [1] S. Gnesin, S. Coda, J. Decker and Y. Peysson, Rev. Sci. Instrum. {\bf 79} (2008) 10\\[4pt] [2] Y. Peysson and J. Decker, Phys. Plasma {\bf 15} (2008) 092509 [Preview Abstract] |
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TP8.00044: Experimental Study of RF Sheaths due to Shear Alfv\'{e}n Waves in the LAPD Michael Martin, Walter Gekelman, Bart Van Compernolle, Patrick Pribyl, Troy Carter Ion cyclotron resonance heating (ICRH) is an important tool in current fusion heating experiments and will be an essential part of heating power in ITER. Radio frequency (RF) sheaths in the near-field (at the antenna) and in the far-field (e.g. the divertor region) form during ICRH and may cause deleterious effects, such as destruction of wall materials and plasma impurity generation. In this study a shear Alfv\'{e}n wave is launched from an antenna in the LAPD bulk plasma (n$_{e}$ $\sim$ 10$^{12}$ cm$^{-3}$, T$_{e}$ $\sim$ 5 eV, B$_{0}$ = 1.8 kG, diameter = 60 cm, length = 18 m) and forms an RF sheath on a limiter plate. Plasma potential rectification is observed with an emissive probe in the bulk plasma only on field lines connected to the limiter. The largest enhancement occurs inside the current channel of the Alfv\'{e}n wave. Plasma potential measurements at various axial distances from the limiter show the rectification decreases with distance. 2-D maps of plasma potential as well as $\bar{E} = -\nabla \Phi$ will be presented. The scaling of sheath potential with wave power and plasma parameters will also be shown. [Preview Abstract] |
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TP8.00045: Scattering of radio frequency waves by density fluctuations A.K. Ram, K. Hizanidis The scattering of radio frequency waves by density fluctuations in magnetized fusion plasmas is studied using two different techniques. For coherent fluctuations, such as blobs in the edge region, we use a full-wave model for which the theory is similar to that for Mie scattering of electromagnetic waves by dielectric objects [1]. The blobs are considered to be either spherical or cylindrical with their axes aligned along the magnetic field. For incoherent planar fluctuations, which can be either in the core of the plasma or in the edge region, we use the Kirchhoff approach in tandem with Huygen's principle. The anisotropy induced by the magnetic field is such that the propagation characteristics and the polarization of the wave modes depend on the polar angle with respect to the direction of the magnetic field. An incident plane wave is not only scattered by the coherent and incoherent fluctuations, but also couples power to a different plasma wave. The scattered spectrum is affected by the size of the fluctuations, the frequency, and the direction of propagation of the incident wave. We present the two theoretical models along with numerical results on the spectral characteristics of the scattered waves.\\[4pt] [1] A. K. Ram {\it et al.}, {\it Phys. Plasmas} {\bf 20}, 056110 (2013). [Preview Abstract] |
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TP8.00046: Effects of lower hybrid waves on temperature gradient driven drift-modes in tokamaks: Momentum and impurity transport Salil Das, Hogun Jhang, Raghvendra Singh, Hans Nordman An important goal in tokamak fusion research is the evaluation of the effects of intrinsic rotation on transport barrier formation, determination of momentum pinch velocity and its theoretical basis, and, the~significant~effect of impurities on tokamak performance by their contribution to radiation losses and plasma dilution resulting in lower fusion power. We use the four-wave parametric process to study these effects invoking a fluid model for ion-temperature-gradient and trapped-electron mode driven turbulence\footnote{Turbulent Particle transport in Magnetized Plasmas: Garbet et al, PRL, 91, 3, 2003} in the presence of radio frequency fields in the lower hybrid (LH) range of frequencies. Explicit expressions for the non-linear growth rate and the associated ion thermal conductivity~and effective~impurity diffusivity are derived. Parametric coupling of the pump and the sidebands exert a ponderomotive force on electrons, modifying the eigenfrequency of the drift waves and influencing the growth rates and the turbulent transport properties. The effects of the rf fields on the momentum and impurity transport coefficients are evaluated for key parameters like rf power, temperature gradients, and magnetic shear. [Preview Abstract] |
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TP8.00047: Parallel heat flux and flow acceleration in open field line plasmas with magnetic trapping Zehua Guo, Xianzhu Tang, Chris Mcdevitt Various simulations and experimental observations have suggested the importance of kinetic effects, such as particle orbital losses, the anisotropy of distribution functions, and the long mean-free-path of superathermal particles, in the tokamak edge region. The magnetic field strength modulation in a tokamak scrape-off layer (SOL) provides both flux expansion next to the divertor plates and magnetic trapping in a large portion of the SOL. In this work, the effects of magnetic trapping and a marginal collisionality on parallel heat flux and parallel flow acceleration are examined. The various transport mechanisms are captured by kinetic simulations in a simple but representative mirror-expander geometry. The observed parallel flow acceleration is interpreted and elucidated with a modified Chew-Goldberger-Low (CGL) model that retains temperature anisotropy and finite collisionality. We will also show that the use of sheath-boundary-condition in modelling tokamak SOL to be problematic since it simply prohibits the flow transition from subsonic to supersonic at the mirror throat far away from the divertor. [Preview Abstract] |
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TP8.00048: Kinetic applications of the ArbiTER eigenvalue code D.A. Baver, J.R. Myra, M.V. Umansky ArbiTER is a flexible eigenvalue code designed for linear fluid or kinetic plasma models is various dimensionalities and topologies. This flexibility derives from the use of specialized equation and topology parsers, which permit run-time specification of a particular linearized physics model, geometry, and grid connectivity, which in turn determine how a particular equation set will be discretized. The resulting matrix form of the problem is then solved using the SLEPc [1] eigensolver package, and can be solved either as a generalized eigenvalue problem, or as a matrix solve in the case of source-driven problems. While the ArbiTER code and its predecessor 2DX have demonstrated significant utility in tokamak edge fluid problems due to their inherent flexibility, the primary aim of its development is to solve kinetic eigenvalue problems. To address this goal, we present first results from implementation of a gyrokinetic model in slab geometry. These results are compared to known solutions for limiting cases. \\[4pt] [1] http://www.grycap.upv.es/slepc/ [Preview Abstract] |
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TP8.00049: Kinetic stability analysis on electromagnetic filamentary structure Wonjae Lee, Sergei Krasheninnikov A coherent radial transport of filamentary structures in SOL region is important for its characteristics that can increase unwanted high fluxes to plasma facing components. In the course of propagation in radial direction, the coherency of the filaments is significantly limited by electrostatic resistive drift instability (Angus \textit{et al.}, 2012). Considering higher plasma pressure, which would have more large impact in heat fluxes, electromagnetic effects will reduce the growth rate of the drift wave instability and increase the instabilities from electron inertial effects. According to a linear stability analysis on equations with fluid approximation, the maximum growth rate of the instability from the electron inertia is higher than that of drift-Alfv\'en wave instability in high beta filaments such as ELMs. However, the analysis on the high beta filaments requires kinetic approach, since the decreased collisionality will make the fluid approximation broken. Therefore, the kinetic analysis will be presented for the electromagnetic effects on the dynamics of filamentary structures. [Preview Abstract] |
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TP8.00050: Particle Simulation of the Blob Propagation in Non-Uniform Plasmas Hiroki Hasegawa, Seiji Ishiguro The kinetic dynamics on blob propagation in non-uniform plasmas have been studied with a three dimensional electrostatic plasma particle simulation code. In our previous studies, we assumed that grad-B is uniform in the toroidal and poloidal directions. In scrape-off layer (SOL) plasmas of real magnetic confinement devices, however, the direction of grad-B is different between the inside and the outside of torus. In this study, we have investigated the blob kinetic dynamics in the system where grad-B is spatially non-uniform. We observe different potential and particle flow structures from those shown in our previous studies. Thus, it is found that propagation properties of blobs in non-uniform grad-B plasmas are also distinct. These properties depend on the initial blob location in the toroidal directions. We will also discuss the application of this study to pellet dynamics. [Preview Abstract] |
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TP8.00051: Turbulent transport regimes and the SOL heat flux width J.R. Myra, D.A. D'Ippolito, D.A. Russell Understanding the responsible mechanisms and resulting scaling of the scrape-off layer (SOL) heat flux width is important for predicting viable operating regimes in future tokamaks, and for seeking possible mitigation schemes. Simulation and theory results using reduced edge/SOL turbulence models have produced SOL widths and scalings in reasonable accord with experiments in many cases. In this work, we attempt to qualitatively and conceptually understand various regimes of edge/SOL turbulence and the role of turbulent transport in establishing the SOL heat flux width. Relevant considerations include the type and spectral characteristics of underlying instabilities, the location of the gradient drive relative to the SOL, the nonlinear saturation mechanism, and the parallel heat transport regime. Recent SOLT turbulence code results are employed to understand the roles of these considerations and to develop analytical scalings. We find a heat flux width scaling with major radius R that is generally positive, consistent with older results reviewed in J. W. Connor et al., Nucl. Fusion 39, 169 (1999). The possible relationship of turbulence mechanisms to the heuristic drift mechanism is considered, together with implications for future experiments. [Preview Abstract] |
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TP8.00052: Modeling of Weakly Collisional Parallel Electron Transport for Edge Plasma Simulations M.V. Umansky, A.M. Dimits, I. Joseph, J.T. Omotani, T.D. Rognlien The parallel electron heat transport in a weakly collisional regime can be represented in the framework of the Landau-fluid (LF) model [1]. Practical implementation of LF-based transport models has become possible due to the recent invention of an efficient non- spectral method for the non-local closure operators [2]. Here the implementation of a LF based model for the parallel plasma transport is described, and the model is tested for different collisionality regimes against a Fokker-Plank code [3]. The new method appears to represent weakly collisional parallel electron transport more accurately than the conventional flux-limiter based models; on the other hand it is computationally efficient enough to be used in tokamak edge plasma simulations. Implementation of an LF-based model for the parallel plasma transport in the UEDGE code is described, and applications to realistic divertor simulations are discussed. \\[4pt] [1] G.W. Hammett and F.W. Perkins, Phys.Rev.Lett., 64, 3019(1990).\\[0pt] [2] A.M. Dimits et al., Phys. Plasmas 21, 055907 (2014).\\[0pt] [3] J.T. Omotani and B.D. Dudson, PPCF 55, 055009 (2013). [Preview Abstract] |
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TP8.00053: Calculation of plasma dielectric response in inhomogeneous magnetic field near electron cyclotron resonance Evstati Evstatiev, Vladimir Svidzinski, Andy Spencer, Sergei Galkin Full wave 3-D modeling of RF fields in hot magnetized nonuniform plasma requires calculation of nonlocal conductivity kernel describing the dielectric response of such plasma to the RF field. In many cases, the conductivity kernel is a localized function near the test point which significantly simplifies numerical solution of the full wave 3-D problem. Preliminary results of feasibility analysis of numerical calculation of the conductivity kernel in a 3-D hot nonuniform magnetized plasma in the electron cyclotron frequency range will be reported. This case is relevant to modeling of ECRH in ITER. The kernel is calculated by integrating the linearized Vlasov equation along the unperturbed particle's orbits. Particle's orbits in the nonuniform equilibrium magnetic field are calculated numerically by one of the Runge--Kutta methods. RF electric field is interpolated on a specified grid on which the conductivity kernel is discretized. The resulting integrals in the particle's initial velocity and time are then calculated numerically. Different optimization approaches of the integration are tested in this feasibility analysis. [Preview Abstract] |
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TP8.00054: Iterative Methods to Solve Linear RF Fields in Hot Plasma Joseph Spencer, Vladimir Svidzinski, Evstati Evstatiev, Sergei Galkin, Jin-Soo Kim Most magnetic plasma confinement devices use radio frequency (RF) waves for current drive and/or heating. Numerical modeling of RF fields is an important part of performance analysis of such devices and a predictive tool aiding design and development of future devices. Prior attempts at this modeling have mostly used direct solvers to solve the formulated linear equations. Full wave modeling of RF fields in hot plasma with 3D nonuniformities is mostly prohibited, with memory demands of a direct solver placing a significant limitation on spatial resolution. Iterative methods can significantly increase spatial resolution. We explore the feasibility of using iterative methods in 3D full wave modeling. The linear wave equation is formulated using two approaches: for cold plasmas the local cold plasma dielectric tensor is used (resolving resonances by particle collisions), while for hot plasmas the conductivity kernel (which includes a nonlocal dielectric response) is calculated by integrating along test particle orbits. The wave equation is discretized using a finite difference approach. The initial guess is important in iterative methods, and we examine different initial guesses including the solution to the cold plasma wave equation. [Preview Abstract] |
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TP8.00055: Fully kinetic particle simulation of radio frequency waves in toroidal geometry Animesh Kuley, Jian Bao, Zhihong Lin RF particle simulation has been developed in this work to provide a first-principles tool for studying the RF nonlinear interactions with plasmas. In this model, ions are considered as fully kinetic particles using the Vlasov equation and electrons are treated as guiding centers using the drift kinetic equation. This model has been implemented in a global gyrokinetic toroidal code GTC with realistic electron-to-ion mass ratio in cylindrical geometry and verified the linear physics of ion plasma oscillation, ion Bernstein wave, lower hybrid wave and its propagation in cylindrical and toroidal geometry. Recently we have verified the linear mode conversion of slow and fast waves in cylindrical geometry. Also we have extended the cyclotron integrator in Boozer coordinates to capture the ion Bernstein, and ion cyclotron modes in toroidal geometry. Our goal is to develop a nonlinear toroidal particle code to study the radio frequency wave heating and current drive in fusion plasmas. [Preview Abstract] |
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TP8.00056: On the nonlinear couplings among ICRF waves observed in GAMMA 10 R. Ikezoe, M. Ichimura, T. Okada, M. Hirata, T. Yokoyama, Y. Iwamoto, S. Sumida, K. Takeyama, S. Jang, T. Oi, M. Yoshikawa, J. Kohagura, Y. Shima Effective ICRF heating creates high ion-temperature plasma of several kiloelectronvolts and the ion-temperature anisotropy exceeds 10 near the midplane of the GAMMA 10 tandem mirror. In such environment, left-hand polarized Alfven wave becomes unstable overcoming ion-cyclotron damping and so-called Alfven-ion-cyclotron (AIC) wave is spontaneously excited. Density fluctuations associated with AIC waves and ICRF waves for heating have been recently measured by using reflectometers on GAMMA 10. The measured fluctuations show fruitful wave-wave couplings more clearly than magnetic fluctuations measured by pick-up coils at the plasma periphery. The signals showing the axially transported energetic-ion flux and the diamagnetism display apparent effects of such nonlinear couplings on the global energy confinement of GAMMA 10. Bispectral analysis is applied to the density fluctuations and the detailed characteristics of the nonlinear couplings occurring among the AIC waves and ICRF waves for heating in GAMMA 10 are presented. [Preview Abstract] |
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TP8.00057: TURBULENCE, TRANSPORT AND ASTROPHYSICAL PLASMAS |
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TP8.00058: Global transition from drift wave dominated regimes to multi-instability plasma dynamics and simultaneous formation of a radial transport barrier in helicon plasma Saikat Chakraborty Thakur, Lang Cui, Jordan Gosselin, Payam Vaezi, Chris Holland, George Tynan Recent studies in CSDX reported a sharp global transition in the plasma dynamics during the route to turbulence [1]. For B \textless 140 mT, the plasma is dominated by density gradient driven drift waves [DW]. For B \textgreater 140 mT, a new global equilibrium is achieved with simultaneous existence of three radially separated plasma instabilities: coherent Rayleigh Taylor [RT] modes at the center, DW at the density gradient and turbulent, shear driven Kelvin-Helmholtz [KH] instabilities at the edge. Only the RT modes rotate in the ion diamagnetic drift direction. The radial particle flux is directed outward for small radii and inward for large radii, forming a radial particle transport barrier leading to stiff profiles and increased core density. Simultaneously the core Ar-II light emission increases (x 10) forming a very bright blue core. The radial extent of the inner RT mode and the blue core coincides with the radial location of the particle transport barrier. This equilibrium with simultaneous RT-DW-KH instabilities shows very rich plasma dynamics including intermittency, blob formation and propagation, inward particle flux against density gradients etc. We report detailed studies of azimuthal momentum balance and time resolved dynamics leading to the transition using Langmuir probes, fast imaging, spectroscopy, laser induced fluorescence etc. \\[4pt] [1] S. Chakraborty Thakur \textit{et. al.}, \textit{Plasma Sources Sci. and Technol.} \textbf{23} (2014) [Preview Abstract] |
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TP8.00059: Heavy Impurity Entrainment in the Parallel Flows of CSDX Jordan Gosselin, Saikat Thakur, George Tynan The lifetime of the plasma facing components (PFCs) in a tokamak, governed primarily by material erosion and redeposition, has been identified as a crucial research topic. While some work has been done that shows evidence of the entrainment of impurities in linear machines and in tokamaks, detailed controlled studies of entrainment in plasma flows are harder to come by. Recently, experiments in CSDX have shown increasing parallel ion velocity positively correlated with increasing magnetic field. In an effort to study the effects of the background flow on impurity transport, a laser blow off apparatus was installed on the Controlled Shear Decorelation eXperiment (a 3m long linear helicon source operated plasma machine). Results are shown for parallel entrainment of Bismuth impurities in a relatively light background Ar plasma (5.2 mass ratio). [Preview Abstract] |
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TP8.00060: Influence of Parallel Dynamics and Electron Temperature Fluctuations on Collisional Drift-Wave Simulations of CSDX Payam Vaezi, Christopher holland, George Tynan, Saikat Chakraborty Thakur, Christian Brandt Previous 2D numerical simulations of collisional drift-wave turbulence in the linear Controlled Shear Decorrelation Experiment (CSDX) device were unable to reproduce experimental observations at magnetic fields above 1.4 kG at either the quantitative or qualitative level. Experimental observations [1] suggest that dynamics of previously neglected ion parallel velocity and associated parallel shear-flow driven instabilities become important at the higher fields. In this poster, we present comparisons of new 3D simulations performed with the BOUT++ framework [2] which include parallel ion velocity dynamics, as well as self-consistent electron temperature fluctuations, to the CSDX observations at multiple magnetic field strengths. We compare the simulated scalings of density and potential fluctuation spectra with magnetic field, as well as radial particle flux and Reynolds stress to 2D results and experimental observations. The comparisons are made using synthetic probe and fast camera diagnostics that incorporate both the electron density and temperature dynamics. \\[4pt] [1] S. Chakraborty Thakur et al., Plasma Sources Science and Technology (2014)\\[0pt] [2] B. D. Dudson, et al., Comp. Phys. Comm. 180 (2009) 1467 [Preview Abstract] |
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TP8.00061: Up-Gradient Particle Flux driven by Nonlinear Flow-to-Fluctuation Energy Transfer Lang Cui, George Tynan, Patrick Diamond, Saikat Thakur, Christian Brandt We report a fluctuation-driven particle flux that transports particles up the mean density gradient when density-gradient driven collisional drift waves generate a sufficiently strong radially sheared azimuthal zonal flow in a cylindrical magnetized plasma. Time-domain and bispectral Fourier domain analysis shows that at the peak of the shear layer, where the particle flux is outward, the turbulent stress acts to nonlinearly reinforce the shear flow. Between the peak of the shear layer and the maximum density gradient, the zonal flow nonlinearly drives fluctuations which give rise to an up-gradient particle flux carried mostly by blobs (holes) that move up (down) the gradient, resulting in a steepening of the mean density gradient. The observations show that spatially separated multiple free-energy sources can drive non-diffusive up-gradient transport that affects global plasma equilibrium. Possible links to toroidal confinement and space plasma systems are discussed. [Preview Abstract] |
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TP8.00062: Waves and instabilities in high $\beta$, warm ion plasmas in LAPD Troy Carter, Seth Dorfman, Giovanni Rossi, Daniel Guice, Walter Gekelman, Kris Klein, Greg Howes The LArge Plasma Device (LAPD) has been upgraded with a second LaB$_6$ cathode plasma source that permits the creation of higher density ($\sim 3\times 10^{13}$cm$^{-3}$), higher temperature ($T_e \sim 12$eV), warm ion ($T_i\sim 6$eV) plasmas. Along with lowered magnetic field, significant increases in plasma $\beta$ can be achieved with this new source (e.g. at $B=100G$, $\beta \sim 1$). These new plasma conditions permit a range of new experimental opportunities on LAPD including: linear and nonlinear studies of Alfv\'{e}n waves in warm ion, high $\beta$ plasmas; pressure-gradient driven instabilities in increased $\beta$ plasmas and electromagnetic modifications to turbulence and transport; instabilities driven by ion temperature anisotropies (e.g. firehose and mirror instabilities). The characteristics of the new plasma will be presented along with a discussion of these new research areas. [Preview Abstract] |
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TP8.00063: Turbulence and transport in high density, increased $\beta$ LAPD plasmas Giovanni Rossi, Troy Carter, Danny Guice A new LaB$_6$ cathode plasma source has recently been deployed on the Large Plasma Device (LAPD), allowing for the production of significantly higher plasma density ($n_e \sim 3 \times 10^{13}$cm$^{-3}$) and temperature ($T_e \sim 12$eV and $T_i \sim 6$eV). This source produces a smaller core plasma ($\sim$20cm diameter) that can be embedded in the lower temperature, lower density standard LAPD plasma (~60cm diameter, $10^{12}$cm$^{-3}$, $T_e \sim 5$eV, $T_i \sim 1$eV). We will present first results from experiments exploring the nature of turbulence and transport produced by this high density core plasma. In contrast to the edge of the standard LAPD plasma, coherent fluctuations are observed in the edge of the high density core plasma. These coherent modes are dominant at low field ($\sim$400G) with a transition to a more broadband spectrum at higher fields ($\sim$1kG). The combination of increased density and temperature with lowered field in LAPD leads to significant increases in plasma $\beta$ (in fact $\beta \sim 1$ can be achieved for $B \sim 100$G). As the field is lowered, the strength of correlated magnetic fluctuations increases substantially. [Preview Abstract] |
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TP8.00064: Scaling of Turbulence and Transport with $\rho^*$ in LAPD Daniel Guice, Troy Carter, Giovanni Rossi The plasma column size of the Large Plasma Device (LAPD) is varied in order to investigate the variation of turbulence and transport with $\rho^* = \rho_s/a$. The data set includes plasmas produced by the standard BaO plasma source (straight field plasma radius $a$~$30cm$) as well as the new higher density, higher temperature LaB6 plasma source (straight field plasma radius $a$~$10cm$). The size of the plasma column is scaled in order to observe a Bohm to Gyro-Bohm diffusion transition. The main plasma column magnetic field is held fixed while the field in the cathode region is changed in order to map the cathode to different plasma column scales in the main chamber. Past experiments in the LAPD have shown a change in the observed diffusion but no transition to Gyro-Bohm diffusion. Results will be presented from an ongoing campaign to push the LAPD into the Gyro-Bohm diffusion regime. [Preview Abstract] |
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TP8.00065: A Procedure to Predict the Subcritical Turbulent Onset Criterion Applied to a Modified Hasegawa-Wakatani Model Brett Friedman, Troy Carter Linear eigenmode analysis is often used to predict whether a plasma or fluid system will be turbulent, but it fails for systems which have highly non-orthogonal linear eigenvectors [1]. In fact, such systems may become turbulent despite having no unstable linear eigenvectors at all (subcritical turbulence). For about a century, researchers have attempted to predict critical parameters that mark the onset of subcritical turbulence with little success. Using recently-developed intuition regarding the role of non-orthogonal linear eigenvectors in subcritical turbulent sustainment, we have developed a method to calculate turbulent growth rates, which can be used to predict the onset of subcritical turbulence. We apply our procedure to 2D and 3D versions of the Hasegawa-Wakatani (HW) model [2], showing good agreement with nonlinear simulation results. We also use a modified version of the 3D HW model [3], which is subject to subcritical turbulence, in order to test our method in predicting the subcritical turbulent onset. \\[4pt] [1] L. N. Trefethen, A. E. Trefethen, S. C. Reddy, and T. A. Driscoll, Science 261, 578 (1993).\\[0pt] [2] A. Hasegawa and M. Wakatani, Phys. Rev. Lett. 50, 682 (1983).\\[0pt] [3] D. Biskamp and A. Zeiler, Phys. Rev. Lett. 74, 706 (1995). [Preview Abstract] |
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TP8.00066: Fast Ion Transport by Magnetic Flux Ropes Adam Preiwisch, William Heidbrink, Heinrich Boehmer, Roger McWilliams, Troy Carter, Walter Gekelman, Shreekrishna Tripathi, Bart Compernolle, Steven Vincena Energetic Lithium test ions (500 $\le $ E$_{\mathrm{fast}}$ / T$_{\mathrm{i}} \quad \le $ 1000) are launched in a Helium plasma in the presence of current-produced magnetic flux ropes at the upgraded Large Plasma Device (LAPD) at UCLA. Perturbing flux ropes are introduced via a hot, biased LaB6 cathode in the main chamber.\footnote{B. Van Compernolle, Phys. of Plasmas. \textbf{19,} 102102 (2012).} Ion beam broadening up to fifty percent above background levels is observed in the radial direction after passing through the flux rope region (T$_{\mathrm{e,max}}=$7eV, B$_{\mathrm{perp}}=$7G, $\Delta $V$=$160V). Density, temperature, and magnetic fluctuation profiles are also obtained. A noise model has been developed to assess the quality of ion signals during the flux rope discharge period. The enhancement to transport may be a result of increased Coulomb scattering, magnetic fluctuations, or electric fields. Further analysis to determine the primary mechanism is ongoing. [Preview Abstract] |
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TP8.00067: MHD turbulence analyses in the plasma wind-tunnel of the Swarthmore Spheromak Experiment D.A. Schaffner, A. Wan, E.R. Hudson, P.J. Weck, M.R. Brown, V.S. Lukin An MHD plasma produced in the wind-tunnel of the Swarthmore Spheromak Experiment (SSX) provides a test bed for studying magnetic turbulence in the laboratory. Results show favorable statistical comparisons to solar wind and magnetosphere turbulence. Analysis of temporal and spatial magnetic fluctuations shows power-law spectra, intermittency and variance anisotropy. Magnetic spectra have indices steeper than Kolmogorov theory and feature a steepening consistent with the onset of dissipation at ion inertial length scales. Comparisons of frequency and wavenumber spectra constructed from multi-channel probes are made to investigate the validity of the Taylor Hypothesis. Intermittency analysis shows increasing kurtosis of PDFs of magnetic field increments with decreasing time scale and increasing magnetic helicity. Taylor microscale is determined through radial correlation length analysis and the magnetic Reynolds number calculation compares well to the value computed using resistivity. Results compare well to Hall-MHD simulations generated using the HiFi framework. Simulations are used to explore the wave mode content through correlations of density and parallel magnetic field. Finally, permutation entropy analysis of SSX is presented. [Preview Abstract] |
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TP8.00068: Overview of Recent Results on Turbulence and Flows Under Biasing in a Magnetized Linear Plasma M. Gilmore, T.R. Desjardins, D. Fisher, J.M. Reyholds Barredo Ongoing experiments on the effects of flow shear on electrostatic turbulence in the presence of electrode biasing are being conducted in helicon plasmas in the linear HelCat (Helicon-Cathode) device. It is found that changes in flow shear, affected by electrode biasing through Er x Bz rotation, can strongly affect fluctuation dynamics, including fully suppressing the fluctuations or inducing chaos. Parameters such as B-field, gas fill pressure, and RF source power also strongly affect fluctuation dynamics. In some cases, multiple modes (resistive drift, rotation-driven interchange and/or Kelvin-Helmholtz) are present, and interact nonlinearly. It is found that neutral particle profiles are hollow, and that neutrals may exert significant Fr x Bz torque on the plasma column through collisions. At high positive electrode bias, a large amplitude, global instability, identified as the potential relaxation instability is observed. Here, an overview of recent experimental results, as well as linear stability analysis using an eigenmode solver, are presented. Additionally, preliminary results from global nonlinear three fluid (electron, ion, neutral) results are discussed. [Preview Abstract] |
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TP8.00069: Observation of the Early Transition from Slab to mixed Slab-Toroidal ETG Turbulence Abed Balbaky, Vladimir Sokolov, Amiya K. Sen Parametric studies of the transition between the slab branch of electron temperature gradient (ETG) mode and the mixed slab-toroidal branch of the ETG mode in CLM are reported. CLM was operated in a mirror machine configuration with a cell length of 50-100 cm, and a mirror ratio of 1-2. For typical CLM parameters and a mode localized at r=2 cm this provides a range for inverse radius of curvature ${R_c}^{-1}$ between 0 and .006 ${cm}^{-1}$. Under normal conditions theory predicts transition between slab and toroidal modes would occur when the parameter ${k_{||}R_c}/{2k_{\perp}{\rho}} \sim 1$ [1]. Recent experiments have obtained an experimental scaling of mode amplitude and frequency as a function of ${R_c}^{-1}$. They indicate that even for much more modest levels of ${k_{||}R_c}/{2k_{\perp}{\rho}} \sim .1$, there are substantial increases in saturated mode, up to 5 times larger than the pure slab mode. Changes in real frequency in the mode are generally small, on the order of $<$ 5\%. \\[4pt] [1] J.Y. Kim and W. Horton, Phys.Fluids B 3, 1167 (1991). [Preview Abstract] |
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TP8.00070: Measurement of Electron Thermal Transport Induced by ETG Modes in the Transition from the Slab to the Toroidal Branch of Mode Vladimir Sokolov, Abed Balbaky, Amiya K. Sen Transition from the slab to the toroidal branch of the electron temperature gradient (ETG) mode has been successfully achieved in a basic experiment in Columbia Linear Machine CLM [1]. The measurement of the radial electron thermal conductivity shows its increase with transition from slab to the toroidal ETG mode. A miniature triple probe was used for these measurements [2] and the value of thermal $\chi_{\bot e}$ conductivity is found to be about $2-12$ m$^2$/s. The corresponding gyrobohm diffusion coefficient $ \chi_{\bot e,GB}\sim 2-4$ m$^2$/s. A similar result of the transport measurement was obtained by using a novel diagnostic system. \\[4pt] [1] A. Balbaky, V. Sokolov and A.K. Sen, Bulletin of 55th APS DPP, PP8.98, 2013.\\[0pt] [2] V.Sokolov and A.K.Sen, Phys.Rev.Lett.,107, 155001 (2011). [Preview Abstract] |
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TP8.00071: On Zonal Flow Formation in Plasmas and Fluids: Probing the Drift-Rossby Analogy M.J. Burin, G.R. Tynan, H. Ji, E. Edlund, E. Gilson, P. Dang, R. Ezeta A well-recognized isomorphism exists between the equations describing drift waves within magnetized plasmas and Rossby waves on rotating planets. Both systems also exhibit large-scale zonal flows that arise due to nonlinear energy~transfer from smaller, turbulent motions. While such energy transfer has been recently characterized in plasmas (via Reynolds' stresses), similar data from geophysically relevant flows has been lacking. We report on a new experimental effort to characterize large-scale flow generation within a laboratory fluid. Results are discussed with the aim of better quantifying the analogy. [Preview Abstract] |
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TP8.00072: Turbulence and transport in a 3D magnetic boundary Matteo Agostini, Lorella Carraro, Giovanni Ciaccio, Gianluca De Masi, Cristina Rea, Paolo Scarin, Gianluca Spizzo, Monica Spolaore, Nicola Vianello In present fusion devices the interaction between 3D magnetic field, edge kinetic properties and turbulence is a crucial issue; not only in intrinsically 3D configurations such as the stellarators, but also in tokamaks, where magnetic perturbations are applied to control ELMs and plasma wall interaction. In the RFX-mod reversed field pinch the spontaneous development at high plasma current of a helical magnetic state displays strong analogies with the aforementioned configurations. At the edge the presence of a stochastic layer and magnetic islands with a well-defined helical symmetry leads to a helical pattern of flow, pressure gradients and turbulent fluctuations: larger fluctuations and shorter correlation lengths are observed near the X-point of the magnetic island, where also a flow slowing-down occurs. Aim of this work is to study the effect of edge turbulence on particle transport in a 3D magnetic boundary, characterizing the properties of the edge blobs along the helical deformation. The magnetic topology also modifies kinetic properties, with higher pressure gradients observed close to the O-point of the island. The measurement of the time evolution of pressure gradient and blob characteristics, can clarify the mutual relation between these two quantities. [Preview Abstract] |
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TP8.00073: Experimental Study on an Intermediate Plasma in the HYPER-II Device Kenichiro Terasaka, Fumiya Kawazu, Kanshi Furuta, Riku Nakano, Shinji Yoshimura, Masayoshi Y. Tanaka Recently, the onset of azimuthal plasma rotation and the saturation of acceleration along the magnetic field line have been observed in an inhomogeneous magnetic field (HYPER-I). These interesting results are considered to be the unique in an intermediate plasma, which consists of magnetized electrons and unmagnetized ions, and are clearly different from that of well-known magnetized plasmas. However, the flow structure formation of intermediate plasma has not been well understood so far, since there are a few experiments. In order to study the flow structure generated in the intermediate plasma, we have started a new experiment using the HYPER-II device at Kyushu Univ, Japan. The HYPER-II device has a large volume chamber, in which an intermediate plasma in a magnetic field intensity of the order of 10 gauss is produced. We have especially interested in the effect of plasma rotation in the magnetized region on the flow structure of intermediate plasma. For this purpose, we have developed a set of coaxial electrodes to control the plasma rotation. In the poster session, we will present the characteristic feature of the HYPER-II device: performance of HYPER-II, diagnostic methods including the preliminary results. [Preview Abstract] |
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TP8.00074: Investigation of the Scattering of Electromagnetic Signals from Low Frequency Turbulence Erik Tejero, Lon Enloe, Vladimir Sotnikov, Chris Crabtree, Eric Gillman, Bill Amatucci, Guru Ganguli The linear Electron-Ion Hybrid (EIH) instability was previously predicted to explain the observation of waves in applications from the plasma sheet boundary layer to laser produced plasmas. PIC simulations have shown that a key feature of the nonlinear evolution of the EIH mode is that it leads to the formation of coherent, closed potential contours in the fluctuating electrostatic potential. We have expanded the theory to include collisional plasmas for applications to the plasma surrounding a hypersonic vehicle. In this collisional plasma, strongly sheared transverse flows can exist due to the relative motion of the vehicle and the surrounding atmosphere. If the scale size of these sheared flows is sufficiently small, they can give rise to the EIH instability. We are study whether the resulting lower hybrid turbulence can impede communication to and from a hypersonic vehicle. Experiments conducted in the Space Physics Simulation Chamber have demonstrated the existence of this instability in the linear phase in a collisional environment and have begun examining the nonlinear evolution of the instability. Results from laboratory experiments and theory on the generation of the EIH instability in a collisional plasma environment and microwave scattering will be presented. [Preview Abstract] |
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TP8.00075: Stability criteria for MHD equilibrium configurations with flows: a Hamiltonian approach P.J. Morrison, T. Andreussi, F. Pegoraro Stability criteria for equilibrium MHD configurations with flows can be obtained by exploiting the Hamiltonian structure of the magnetohydrodynamics (MHD) equations by referring to three different kinds of energy principles. Following up on previous work [Phys.\ Plasmas 19, 052102 (2012); 20, 092104 (2013)] we compare the Lagrangian, Eulerian, and Dynamically Accessible stability criteria of a simple set of MHD equilibria. These criteria differ because of the different constraints that are imposed on the variations of the equilibrium quantities in the stability analyses. We illustrate these constraints along with the corresponding stability criteria for cylindrical magnetized plasma configurations with flows. [Preview Abstract] |
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TP8.00076: General Framework for statistical tracer analysis as a diagnostic for turbulent transport in gyrokinetic codes Jose-Miguel Reynolds-Barredo, Jorge-Alberto Alcuson, Raul Sanchez, Victor Tribaldos, David Newman It has been known for a long time that tracers can be useful tools to characterize the nature of transport. On the other side, several state-of-the art gyrokinetic codes exist (GENE, GYSELA, UCAN...) that offer us a rich variety of turbulent data to analyze, that belongs to regimes of interest for the nuclear fusion program. Advancing tracers in these codes is not easy. It would be best if they could be advanced in parallel with the turbulence. However, gyrokinetic runs are very expensive and one cannot afford to repeat runs to relocate tracer particles in another location, or to repeat their initialization. For that reason, it is better to develop an independent tool that can read the turbulence information from gyrokinetic runs stored in file, and then spline it in time and space as needed to carry out as many tracer studies as desired. In this contribution, we present such a framework. It has been developed in FORTRAN90 and accepts input from all the aforementioned codes, including both the electrostatic potential and the magnetic field configurations. Tracers can be advected considering either of the following effects: ExB drifts, magnetic drifts, parallel motion, etc. Preliminary effects of the use of the tool with several GK codes will be presented. [Preview Abstract] |
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TP8.00077: Coherent Structures and Reconnection in Collisionless Turbulence Vadim Roytershteyn, Homa Karimabadi The sub-proton range of collisionless turbulence has attracted considerable attention in the last decades due to its role in the dissipation of cascading energy and increased availability of high-quality measurements capable of constraining the relevant models. Coherent structures, such as current sheets, have long been considered important sites for the dissipation of energy. However, a self-consistent treatment of their formation and of the relevant collisionless dissipation mechanisms has only become possible recently. Here we discuss several examples from recent kinetic simulations of turbulence focusing on the role of current sheets and magnetic reconnection. In the 3D fully kinetic simulations with initial conditions relevant to solar wind turbulence, current sheets form over a large range of scales and are shown to be sites of increased energy transfer between fluctuating fields and particles. Moreover, depending on the initial conditions and the type of driving, other types of coherent structures are possible, such as magnetic holes. 2D and 3D global hybrid simulations of the interaction between solar wind and planetary magnetospheres demonstrate inherent connection between collisionless shocks, turbulence, and magnetic reconnection. Specifically, the interaction of foreshock turbulence driven by reflected ions with the shock itself leads to a variety of fascinating phenomena in the magnetosheath, seeding both small-scale turbulence and large-scale global perturbations. [Preview Abstract] |
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TP8.00078: Large Eddy Simulations of 2D Lattice Boltzmann MHD Turbulence Christopher Flint, George Vahala, Linda Vahala, Min Soe Dellar's LBM of 2D incompressible MHD introduced both a velocity and magnetic distribution functions. As a result div \textbf{B} $=$ 0 is automatically enforced through the trace of an antisymmetric perturbed tensor. We have extended this algorithm to 3D MHD turbulence, with excellent parallelization to many thousands of cores. In LES of MHD turbulence, only the subgrid modes are modeled for using some ad hoc closure scheme. In the Smagorinsky model, the filtered Reynolds stresses are modeled by mean field gradient terms. Recently, Ansumali et. al. have developed an LES for Navier-Stokes turbulence by filtering the underlying mesoscopic LB. The filtered LB equations are then subjected to the Chapman-Enskog expansion. A Smagorinsky LES is recovered with no ad hoc assumptions other than the subgrid terms contribute at the transport time scales. This forces a relationship between the filter width and the Knudsen number. Here we extend these ideas to MHD turbulence and achieve closures under the simple assumption that the subgrid terms affect the evolution on the transport time scale. These ideas will first be tested on the flow of 2D jets in a magnetic field. The DNS data base is being generated from a multiple relaxation time (MRT) model for both the velocity and magnetic fields. [Preview Abstract] |
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TP8.00079: Characterizing transport with local perturbations and Lagrangian trajectories in two-dimensional plasma turbulence Douglas Ogata, David Newman, Raul Sanchez, Jose-Miguel Reynolds-Barredo Perturbative experiments such as pellet injections, gas puffs, heats pulses have been used to investigate the transport characteristics in hot plasmas where probes are not suitable. However, the addition of too large a perturbation can alter the local transport characteristics making it a poor measure of the underlying transport. This work attempts to evaluate both the impact of different sized local perturbations on transport characteristics and the evolution of that perturbation as a measure of transport within a general non-diffusive transport framework. This is done by comparing the evolution of the local perturbation profile and the advection of a passive scalar with the transport characteristics extracted from the Lagrangian trajectories in a two-dimensional electrostatic plasma fluid turbulence model. This work presents the methodology and preliminary comparisons between the trajectories analysis and the evolution of a profile perturbation in order to find experimentally feasible observables to characterize the transport dynamics. [Preview Abstract] |
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TP8.00080: Wave-Kinetic Simulations of Lower-Hybrid Turbulence driven by Velocity Ring Instabilities Guru Ganguli, Chris Crabtree, Leonid Rudakov, Manish Mithaiwala We develop numerical solutions to the wave-kinetic equation in a periodic box including the effects of nonlinear (NL) scattering of Lower-hybrid waves giving the evolution of the wave-spectra in wavenumber space. Simultaneously we solve the particle diffusion equation of both the background plasma particles and the ring ions, due to both linear and nonlinear Landau resonances. At initial times for cold ring ions, an electrostatic beam mode is excited, while the kinetic mode is stable. As the instability progresses the ring ions heat, the beam mode is stabilized, and the kinetic mode destabilizes. When the amplitude of the waves becomes sufficient the lower-hybrid waves are scattered (by either nearly unmagnetized ions or magnetized electrons) into electromagnetic magnetosonic waves [Ganguli et al 2010]. The effect of NL scattering is to limit the amplitude of the waves, slowing down the quasilinear relaxation time and ultimately allowing more energy from the ring to be liberated into waves [Mithaiwala et al. 2011]. The effects of convection out of the instability region are modeled, additionally limiting the amplitude of the waves, allowing further energy to be liberated from the ring [Scales et al., 2012]. Results are compared to recent 3D PIC simulations [Winske and Duaghton 2012]. [Preview Abstract] |
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TP8.00081: Investigation of magnetic field generation by non-Gaussian, non-Markovian velocity fluctuations using meshless, Lagrangian numerical schemes Raul Sanchez, David Newman Turbulent velocity fields can generate perturbations of the electric current and magnetic field that, under certain conditions, may generate an average, large-scale magnetic field. Such generation is important to understand the behavior of stars, planetary and laboratory plasmas. This generation is traditionally studied by assuming near-Gaussian, random velocity fluctuations. This simplification allows to exprese the effective electromotive force in Faraday's law in terms of a piece proportional to the large-scale magnetic field itself (the $\alpha$-term) and another proportional to its curl (the $\beta$ term) assuming certain symmetry conditions are met. Physically, the $\alpha$-term is a measure of the mean helicity of the flow and drives the dynamo process. In a previous contribution, we examined theoretically what consequences would follow from assuming instead Levy-distributed, Lagrangianly-correlated velocity fields, that have been recently identified as of relevance in regimes of near-marginal turbulence or in the presence of a strong, stable sheared flow. Here, we will discuss and extend these results numerically by implementing the kinematic dynamo equation using a Lagrangian, meshless numerical method inspired by the SPH schemes frequently used in hydrodynamics. [Preview Abstract] |
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TP8.00082: Electromagnetic corrections to the zonal flow residual Istvan Pusztai, Peter J. Catto, Felix I. Parra The axisymmetric zonal flow residual calculation in tokamak plasmas is generalized to include electromagnetic perturbations. Instead of imposing magnetic perturbations externally, we formulate and solve a description retaining the fully self-consistent temporal and spatial perturbations in the electric and magnetic fields. Simple expressions for the electrostatic, shear and compressional magnetic residual responses derived provide a fully electromagnetic test of the zonal flow residual in gyrokinetic codes. We find that at $\beta\sim\mathcal{O}$(1) the most easily testable quantity is the compressional magnetic perturbation generated by the density perturbation corresponding to the zonal flow potential, while at small values of $\beta$, the electrostatic and shear magnetic responses to an initial compressional magnetic perturbation can also be detectable. Without collisions any initial magnetic perturbation remain completely undamped. [Preview Abstract] |
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TP8.00083: Theoretical Issues on the Spontaneous Rotation of Axisymmetric Plasmas* T. Zhou, B. Coppi An extensive series of experiments have confirmed that the observed phenomenon of ``spontaneous rotation'' in axisymmetric plasmas is connected to the excitation of relevant collective modes [1] and, consequently, both to the confinement properties of the concerned plasmas, when referring to electrostatic modes, and to the magnetic reconnection processes associated with the excitation of electromagnetic modes [2]. Internal localized modes [2] can extract angular momentum from the plasma column from which they grow while, the background plasma has to recoil in the direction opposite to that of mode phase velocity. In the case of the excitation of plasma edge modes, the loss of their angular momentum can be connected to the directed particle ejection to the surrounding medium. The recoil angular momentum is then redistributed inside the plasma column mainly by a process that includes [1] the contribution of an effective viscous diffusion and of an inward (pinch) angular momentum flux that is connected, for instance, to ETG or ITG driven modes. *US DOE partly sponsored.\\[4pt] [1] B. Coppi, 18th IAEA Fusion Energy Conf. THP 1/17 (2000). and \textit{Nucl. Fus.} \textbf{42}, 1 (2002).\\[0pt] [2] B. Coppi and T. Zhou, to be published in \textit{Nucl. Fus.} (2014) and M.I.T.-L.N.S. Report 13/06 (2014). [Preview Abstract] |
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TP8.00084: Hamiltonian gyro-averaged area preserving map models of finite Larmor radius effects on ExB chaotic transport Julio Fonseca, Diego del-Castillo-Negrete, Ibere Caldas Area preserving maps have been extensively used to model 2-dimensional chaotic transport in plasmas and fluids. Here we focus on three types of area preserving maps describing ExB chaotic transport in magnetized plasmas with zonal flows perturbed by electrostatic drift waves. We include finite Larmor radius (FLR) effects by gyro-averaging the corresponding Hamiltonians of the maps. The Hamiltonians have frequencies with monotonic and non-monotonic profiles. In the limit of zero Larmor radius, the monotonic frequency map reduces to the standard Chirikov-Taylor map, and, in the case of non-monotonic frequency, the map reduces to the standard nontwist map. We show that FLR leads to chaos suppression, modifies the stability of fixed points, and changes the robustness of transport barriers. FLR effects also modify the phase space topology and give rise to bifurcations of the zonal flow ExB velocity profile. Dynamical systems methods based on recurrence time statistics are used to quantify the dependence on the Larmor radius of the threshold for the destruction of transport barriers. [Preview Abstract] |
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TP8.00085: Partial barriers to heat transport in monotonic-$q$ and reversed shear 3-dimensional chaotic magnetic fields Diego del-Castillo-Negrete, Daniel Blazevski The quantitative understanding of the role of magnetic field stochasticity on transport in 3-D configurations is of paramount importance for the magnetic confinement of fusion plasmas. Problems of interest include the control of ELMs by RMPs and the assessment of heat fluxes at the divertor. In this contribution we present numerical solutions of the time dependent parallel heat transport equation describing transport of heat pulses in 3-D chaotic magnetic fields. To overcome the limitations of standard approaches, we use a Lagrangian-Green's function (LG) method that allows the efficient and accurate integration of the anisotropic heat transport equation with local and non-local parallel heat flux closures in integrable and chaotic B fields. The results provide conclusive evidence that even in the absence of flux surfaces, chaotic magnetic field configurations exhibit partial barriers to heat transport. In particular, high-order islands and remnants of destroyed flux surfaces (Cantori) act as partial ``leaky'' barriers that slow down or even stop the inward propagation of heat pulses. The magnetic field connection length, \textless $l_{B}$\textgreater , exhibits a strong gradient where the partial barriers form, and it reaches a plateau whose value determines the ``porosity'' of the barrier. Heat pulses are shown to slow down considerably in the shear reversal region and, as a result, the time delay of the temperature response in chaotic reversed shear configurations is about an order of magnitude larger than the time delay in monotonic q-profiles. [Preview Abstract] |
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TP8.00086: Simulating MHD/Fluid type electromagnetic modes in the total-f gyrokinetic code XGC1 J. Lang, S.-H. Ku, C.-S. Chang, Y. Chen, S.E. Parker For a more complete description of the MHD/fluid type mode activities including ELMs and neoclassical tearing modes, their interaction with the kinetic neoclassical and microturbulence dynamics needs to be simulated together. Evolution of the background profile should also be captured self-consistently. We report recent development activity of the MHD/fluid modes capability in the total-f gyrokinetic codes in the limit of small delta-B. Verification of the Alfven wave modes, low-n tearing modes, and transition from ITG to KBM modes will be presented. Plan for further development will be discussed. Important implication of the new development to the XGC1 program and fusion physics will also be discussed. [Preview Abstract] |
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TP8.00087: Energy Dissipation in Electromagnetic Microturbulence G.G. Whelan, M.J. Pueschel, P.W. Terry Typically, almost all roots of the gyrokinetic plasma dispersion relation are damped modes. Through nonlinear transfer, often involving coupling with zonal flows, these modes receive energy from unstable modes. This has signifigant consequences and in cases the effects from mode coupling are even the dominant contributions for the saturation physics of plasma turbulence. Using the gyrokinetic code \textsc{Gene}, we track the zonal-flow-enabled energy transfer at a single wave number by making use of both proper orthogonal decomposition and linear eigenmode representation. Expanding on previous, electrostatic work [K.D.~Makwana et al., Phys.~Rev.~Lett.~\textbf{112}, 095002 (2014)], we investigate how finite-beta physics affect zonal flow coupling, as well as the cumulative effects of zonal modes and frequency matching. In particular: how effective zonal flows are in facilitating energy transfer to stable modes, the energy dissipation by stable modes in the drive range and the possible contributions by resonant effects respectively. In this context, consequences for the understanding of electromagnetic stabilization of ion-temperature-gradient-driven turbulence are detailed. [Preview Abstract] |
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TP8.00088: Gyrokinetic Simulation of Microturbulent Saturation at Finite $\beta$ P.W. Terry, M.J. Pueschel, D. Carmody, G.G. Whelan Saturation and zonal flow physics for microturbulence is investigated for tokamaks and the RFP using gyrokinetic computation to understand scalings with respect magnetic shear and $\beta$. Modeling an MST discharge shows that the critical instability gradient for TEM is higher than the tokamak threshold by the aspect ratio $(R/a)$. This factor is rooted in the shorter magnetic field scale length of the RFP. Nonlinear simulations show strong zonal flows and a large Dimits shift exceeding the tokamak shift by a factor of order $(R/a)$. The non zonal transition (NZT), a critical $\beta$ for which zonal flows are disabled by flutter-induced charge loss is also considered. The critical $\beta$ occurs when the radial displacement of a magnetic field line over a half connection length is equal to the radial correlation length. These quantities scale with the connection length and magnetic drift scale lengths entering the instability threshold and quasilinear diffusivities, making the RFP critical $\beta$ for NZT higher than the tokamak value by $(R/a)^{1.5}$ times tokamak $q$. These results are consistent with magnetic shear and $q$ dependence in the kinetic ballooning threshold, indicating that $\beta$ effects will only arise at high $\beta$ relative to typical RFP operation. [Preview Abstract] |
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TP8.00089: Passive advection in a collisionless plasma Anjor Kanekar, Alexander Schekochihin, Greg Hammett, William Dorland, Nuno Loureiro We consider a simple kinetic model for the evolution of the particle distribution function in a magnetized turbulent plasma that includes both phase mixing (Landau damping) and advection by a stochastic velocity field: a ``kinetic passive scalar'' in the Batchelor regime. The advection due to stochastic velocity field allows for a stochastic version of the plasma echo by coupling the ``phase-mixing'' and the ``un-phase-mixing'' components of the free energy. We have developed a new analytical framework to diagnose the efficiency of such coupling. We have also developed a new GPU code named Gandalf that solves this kinetic model numerically. In this poster, we shall present numerical and analytical results related to this work. [Preview Abstract] |
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TP8.00090: Self-Consistent evolution of radial electric field in a low flow ordered drift kinetic system W. Sengupta, A.B. Hassam, T.M. Antonsen We present a closed set of low collisionality drift kinetic equations which are full-f, nonlinear, with arbitrary geometry, electromagnetic with arbitrary beta, with ExB flow of order the diamagnetic flow, and with time variations ordered as second order in gyro-radius expansion. Our equations can be applied to the tokamak edge including evolution of the profiles. As has been pointed out by Parra et.al [1-2], we show that in order to self-consistently evolve the radial electric field, second order terms need to be retained. The complete set involves evolution of f, the magnetic field, and the ExB flow. We shall compare our drift kinetic system with the Gyrokinetic system developed by Calvo-Parra [2]. We shall also discuss the effects of the higher order corrections on the Rosenbluth-Hinton residual Zonal flows.\\[4pt] [1] Parra, F. I., \& Catto, P. J. (2008). {\it{Plasma Physics and Controlled Fusion}}, 50(6), 065014.\newline [2] Calvo, Ivan, and Felix I. Parra. {\it{Plasma Physics and Controlled Fusion}} 54.11 (2012): 115007. [Preview Abstract] |
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TP8.00091: Compression of spinning plasma Vasily Geyko, Nathaniel Fisch Adiabatic compression of a spinning plasma in cylindrical geometry is studied in thermodynamical limit. Compared to spinning neutral gas, additional electrostatic energy of charge separation yields to increased heat capacity for both axial and longitudinal compressions. Radial compression of plasma with external axial magnetic field is also considered. The obtained results can be used as thermodynamical estimations for z-pinch compression. [Preview Abstract] |
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TP8.00092: Modeling magnetospheres of spinning black holes Alex Ford, Brett Keenan, Mikhail Medvedev We numerically model the magnetospheres of spinning (Kerr) black holes (BHs) and the production of relativistic jets in active Galactic Nuclei, quasars and micro-quasars, blazars, etc. There is a lore that Kerr BHs in an external magnetic field form {\em force-free} magnetospheres, whose structure is believed to determine how relativistic jets are launched and how the BH energy is extracted, e.g., via Blandford-Znajek mechanism. The key assumption for the force-free condition is the presence of plasma with the density being above the Goldreigh-Julian density. Unlike NSs which can in principle supply electrons from the surface, plasma around BHs must be generated {\it in situ} via a pair cascade. Here we we present numerical modeling of the ``gap'' region, where the cascade can occur. We explore the conditions of the plasma generation, without which AGN, quasar/blazar and other jets cannot exist. [Preview Abstract] |
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TP8.00093: ``Pheudo-cyclotron'' radiation and transport of non-relativistic particles in inhomogeneous sub-Larmor-scale electro-magnetic fields Brett Keenan, Alex Ford, Mikhail Medvedev Plasma turbulence in some astrophysical objects (e.g., weakly magnetized collisionless shocks in GRBs and SN) has small-scale electro-magnetic field fluctuations. We study spectral characteristics of radiation produced by particles moving in such turbulence and relate it to transport properties (diffusion) of these particles. It was shown earlier that relativistic particles produce jitter radiation, which spectral characteristics are markedly different from synchrotron radiation. Here we study radiation produced by non-relativistic particles. Unlike radiation in homogeneous field, which spectrum consists of a single cyclotron harmonic, radiation in the sub-Larmor-scale turbulence reflects statistical properties of the underlying magnetic field. We present both analytical estimates and results of {\it ab initio} numerical simulations. We also show that particle propagation in such turbulence is diffusive and evaluate the diffusion coefficient. We demonstrate that the diffusion coefficient correlates with some spectral parameters. These results can be very valuable for remote diagnostics of laboratory and astrophysical plasmas. [Preview Abstract] |
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TP8.00094: Shear-Driven Dynamo Waves in Fully Nonlinear Regime Peera Pongkitiwanichakul, Fausto Cattaneo, Giuseppina Nigro Dynamo action is often invoked to explain the origin of magnetic fields in astrophysics. Often, the generated magnetic fields are organized on spatial and temporal scales much larger than that of the underlying turbulence. This process of large-scale dynamo action is well understood when the magnetic Reynolds number is small or moderate, but not as clear when the magnetic Reynolds number becomes large. In this regime, the fluctuations control the operation of the dynamo obscuring the large-scale behavior. Recently, Tobias and Cattaneo (2013) developed a dynamo model involving strongly helical flows and large scale shear that could generate well organized large-scale magnetic fields in the form of a traveling dynamo waves. Their model, however, was only kinematic, and did not include the back reaction of the Lorentz force on the flow. Here, we have undertaken a systematic extension of their work to the fully nonlinear regime. Helical turbulence, and large scale shear are produced self-consistently by prescribing body forces that, in the kinematic regime replicate the original velocity used by Tobias {\&} Cattaneo. I will present results in the nonlinear regime for different magnetic Prandtl numbers and show different cases of large-scale organization. [Preview Abstract] |
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TP8.00095: Towards an Analytical Model of Differential Rotation in the Solar Convection Zone Lee Gunderson, Amitava Bhattacharjee The Solar Convection Zone (SCZ) is a region of turbulent convection in a rotating stratified plasma. Helioseismology and numerical models have provided mean flow profiles of this region, showing characteristic differential zonal rotation and meridianal circulation (Thompson et al. 2003). Numerical simulations have reproduced these profiles, including in the hydrodynamic limit (Fan and Fang 2014). However, the theoretical underpinnings are still being debated. Balbus (2009) proposed the following ansatz: the isentropic and isorotational contours coincide. Indeed, with this assumption, the resulting solutions to the thermal wind equation gave profiles of remarkable similarity to observations. We have developed a Grad-Shafranov treatment of axisymmetric equilibrium in the hydrodynamic case, however the result suggests that entropy should be a function of angular momentum, which give profiles that are not characteristic of the SCZ. We attempt to improve on this formulation by introducing a mean-field model of the Reynolds stress and testing Balbus' ansatz, and will report on the progress towards an analytical model of differential rotation in the SCZ. [Preview Abstract] |
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TP8.00096: Nonmodal growth and the magnetorotational dynamo instability Jonathan Squire, Amitava Bhattacharjee Unravelling the important dynamo processes in magnetized rotating shear flows remains fundamental in understanding turbulent transport in astrophysical disks. We consider the dynamo of the magnetorotational instability (MRI) in its simplest possible form, studying the unstratified shearing box without a mean magnetic field. Despite the lack of spectral instability, sustained turbulence and dynamo is possible in this system, with the non-normality of the linear operator playing an important role. An analysis of the MRI from this non-normal perspective has proved enlightening, illustrating that the fastest growing non-axisymmetric disturbances are very different from the eigenmodes, invariably resembling waves shearing with the background flow (shear waves). With the goal of understanding the core dynamo process, we evolve an statistical ensemble of shear waves in a quasi-linear version of the shearing box system. Among the most interesting ideas resulting from this approach is the existence of a mean field dynamo instability of homogenous background turbulence. The instability saturates at levels consistent with fully nonlinear turbulence simulations and depends strongly on magnetic Prandtl number. [Preview Abstract] |
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TP8.00097: MRI Turbulence at High Reynolds Numbers Justin Walker, Stanislav Boldyrev, Geoffroy Lesur The properties of magnetic turbulence driven by the magnetorotational instability (MRI) are studied at large Reynolds numbers by simulation. The results are compared with previous published results at lower Reynolds number and with forced magnetohydrodynamic (MHD) turbulence. Preliminary results suggest that spectra exhibit a power law within a short inertial range, and similarities and differences with the inertial range in MHD turbulence are established. [Preview Abstract] |
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TP8.00098: Towards kinetic simulation of MRI using GPU T. Tatsuno, W. Dorland, A. Kanekar Graphics processor Units (GPUs) are nowadays widely used in fluid and MHD simulations. Due to their fast computation and memory access, they enable us to make large simulations on a desktop that used to be only accessible by a large supercomputer. So far, however, there are not many attempts to make kinetic simulations on GPU. In this work, we will try to develop a kinetic simulation code on GPU. We plan to apply the developing kinetic simulation code to analyze magnetorotaional instability (MRI). It is shown by the fluid analyses with some kinetic effects included in the model [1, 2] that the pressure anisotropy brings an important effect on the accretion rate. However, the results depend on the free parameters in the terms for kinetic effects. Thus evaluation using kinetic simulation is desired for quantitative estimate. \\[4pt] [1] E. Quataert, W. Dorland, and G. W. Hammett, Astrophys. J. \textbf{577}, 524 (2002).\\[0pt] [2] P. Sharma, G. W. Hammett, E. Quataert, and J. M. Stone, Astrophys. J. \textbf{637}, 952 (2006). [Preview Abstract] |
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TP8.00099: ABSTRACT WITHDRAWN |
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TP8.00100: Particle In Cell Codes on Highly Parallel Architectures Adam Tableman We describe strategies and examples of Particle-In-Cell Codes running on Nvidia GPU and Intel Phi architectures. This includes basic implementations in skeletons codes and full-scale development versions (encompassing 1D, 2D, and 3D codes) in Osiris. Both the similarities and differences between Intel's and Nvidia's hardware will be examined. Work supported by grants NSF ACI 1339893, DOE DE SC 000849, DOE DE SC 0008316, DOE DE NA 0001833, and DOE DE FC02 04ER 54780. [Preview Abstract] |
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TP8.00101: Particle-in-Cell Simulations of Driven Solar Wind Turbulence Jason TenBarge, James Juno Turbulence is a ubiquitous phenomenon in space and astrophysical plasmas and is responsible for mediating the transfer of large scale electromagnetic energy to small scales where the energy is eventually damped onto the particles. Solar wind observations suggest that the turbulence is dominated by low frequency Alfv\'{e}nic fluctuations, which approximately follow the predictions of critical balance. We present results from the first driven, three-dimensional particle-in-cell simulations of Alfv\'{e}nic turbulence spanning inertial to electron kinetic scales. The simulations are driven to a statistically steady state with an oscillating Langevin antenna intended to mimic turbulent energy cascaded from scales larger than the simulation domain. Since a primary focus of the turbulence community is energy dissipation and one of the dominant mechanisms is likely Landau damping, a parameter scan in wavelength, plasma beta, and particle number is presented to determine the computational requirements of particle-in-cell simulations to accurately capture the Landau damping of Alfv\'{e}n waves. [Preview Abstract] |
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TP8.00102: Gyrokinetic Study of Intermittency in Interstellar Turbulence G. Lau, P.W. Terry, M.J. Pueschel The temporal broadening of pulsar signals from scintillation is dependent on the distance to the pulsar, but the exact scaling cannot be recovered from electron density fluctuations that follow Gaussian statistics - L\'{e}vy statistics are required [S. Boldyrev and C. Gwinn, Astrophys. J. \textbf{584}, 791 (2003)].$^{\mathrm{\thinspace }}$We investigate the possibility that interstellar turbulence produces L\'{e}vy statistics in electron density fluctuations, focusing on the intermittency associated with current filaments and sheets in decaying gyrokinetic turbulence at ion gyroradius scales. It has been shown that the proper distributions arise in a 2D fluid model for kinetic Alfv\'{e}n turbulence [P. W. Terry and K. W. Smith, Astrophys. J. \textbf{665}, 402 (2007)]. Using the G\textsc{ene} code, we continue this work by confirming that gyrokinetics returns similar results. We then extend to 3D and show the effects of collisions on the emerging structures. [Preview Abstract] |
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TP8.00103: A new method of driving turbulence in particle-in-cell simulations James Juno, Jason TenBarge, Marc Swisdak, William Dorland We present a novel approach for driving turbulence in particle-in-cell (PIC) simulations with the implementation of an oscillating Langevin antenna which drives ${A}_{\parallel}$ across the domain. The antenna obtains its name from its similarity to the Langevin equation for Brownian motion and allows us to more realistically model the injection of energy from scales larger than the simulation domain so we can simulate a more computationally feasible subrange of the turbulent cascade. Oftentimes, PIC simulations are driven from a single point, or from the edge of the simulation domain; however, the Langevin antenna works like a body force, driving the plasma from all points in space. Furthermore, studies of turbulence with PIC are often decaying, but with the antenna, we can model steady state conditions. Thus, we can create a more physically motivated simulation of the turbulent evolution from large scales to small scales and better understand the dissipation of turbulence in systems such as the solar wind. Though we focus on driving low frequency Alfv\'{e}n waves, the flexibility of the antenna allows for driving any range of frequencies. Comparisons to linear theory and fully non-linear gyrokinetic simulations are presented as validation of our method. [Preview Abstract] |
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TP8.00104: Parametric Antenna Concept for Efficient Very Low Frequency (VLF) Wave Excitation T. Kim, V. Sotnikov, E. Mishin, T. Genoni, D. Rose Concept of a parametric antenna in the ionospheric plasma is analyzed. Such antennas are capable of exciting electromagnetic radiation fields, specifically the creation of whistler waves generated at the very low frequency (VLF) range, which are also capable of propagating large distances away from the source region. The mechanism of whistler wave generation is considered a parametric interaction of quasi-electrostatic low oblique resonance (LOR) oscillations excited by conventional antenna. The transformation of LOR waves on quasi-neutral density perturbations in the near field of an antenna gives rise to whistler waves on combination frequencies. Amplitude of these waves can considerably exceed the amplitude of whistler waves directly excited by a loop. Simulation to demonstrate excitation and spatial structure of VLF waves excited by a loop antenna using a PIC code LSP will be presented as well. [Preview Abstract] |
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TP8.00105: Nonlinear simulation of the whistler wave excited by the energetic particles Ge Wang, Guoyong Fu The energetic particle induced whistler waves are widely observed in the magnetosphere and fusion related devices. We report the nonlinear delta f- PIC simulation of the oblique whistler wave in the homogeneous background magnetic field, where the longitudinal electric field is observed to be important and the whistler wave deviates from the circular polarization. A nonlinear resonance plays an important role on the damping mechanics at the half electron cyclotron frequency, which helps to transfer the whistler wave energy into the thermal energy of background particles. We also explore the possibility of the frequency chirps when the cyclotron resonance and Landau resonance coexist, where the hole and clump generated in the phase space will provide a candidate for the whistler chorus in the magnetosphere. [Preview Abstract] |
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TP8.00106: Effects of Induced Scattering of Lower-Hybrid Waves by Plasma Particles on the Lifetime of Plasma-sheet Turbulence Manish Mithaiwala, Leonid Rudakov, Gurudas Ganguli, Chris Crabtree The portion of broadband electrostatic turbulence in the plasma sheet consisting of lower-hybrid waves is thought to be generated by proton-ring distributions [1]. However it remains a mystery why these ring distributions are so long lived. The possibility that induced non-linear scattering of lower-hybrid waves from plasma electrons to longer wavelengths has been considered as a saturation mechanism for these instabilities, which saturate the wave amplitude at very low levels, allowing the ion-ring distributions to be long-lived. This was demonstrated theoretically, as well as in 3D simulations [2, 3]. A comparison of the electric field fluctuation amplitude at two different values of plasma beta confirms that the saturation amplitude depends on temperature. This is consistent with nonlinear scattering by particles being the dominant nonlinearity rather than three-wave interaction. Though it has been shown previously that it is inappropriate to the treat the nonlinear scattering of lower-hybrid waves only in the electrostatic limit, the suggested electromagnetic generalizations only included changes to the nonlinear charge density. Thus a self-consistent treatment that includes both a nonlinear charge and current density is shown here. [1] Huba, J. D., J. Chen, and R. R. Anderson (1992), J. Geophys. Res., 97(A2), 1533--1540 [2] Winske and Daughton [Phys Plasmas, 19, 072109, 2012 [3] L. Rudakov, C. Crabtree, M. Mithaiwala, and G. Ganguli, Arxiv 2012 [Preview Abstract] |
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TP8.00107: Gyrokinetic simulations of kink modes in astrophysical jets Joseph McClenaghan, Ken Fowler, Hui Li, Zhihong Lin With the prediction that powerful astrophysical jets that are formed around supermassive black holes maintain collimation by presence of a self generated magnetic field, linear MHD theory predicts that these magnetically confined jet plasmas would be unstable to kink modes. Kink modes have been studied extensively in tokamak experiments, theory, and MHD simulations, however, their dynamical evolution can depend on the nonlinear coupling of multiple physical processes. In this work, we have applied Gyrokinetic Toroidal Code (GTC) to study internal kink modes in a proposed jet equilibrium. Linear stability properties suggest that kink modes in astrophysical jets are linearly unstable with reasonable agreement to MHD. Nonlinear saturation amplitude and continued evolution of kink modes are examined to better understand how the jet can remain collimated in the presence of these modes without being disrupted. We also look at the generation of a mean parallel electric field by the nonlinear evolution of internal kink modes and the potential implication of this field on particle acceleration in jets. [Preview Abstract] |
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TP8.00108: Small-Scale Irregularities in Equatorial Spread-F Yakov Dimant, Meers Oppenheim Equatorial Spread-F is a spectacular plasma phenomenon that reshapes the nighttime ionosphere and disrupts GPS navigation and radio communication. Current computer models simulate the evolution of large-scale spread-F phenomena (1000km-to-kilometer), but they do not explain what causes the meter-scale irregularities observed by radars and space-borne instruments. Our recent particle-in-cell (PIC) simulations of weakly collisional plasma have demonstrated that large-scale plasma density gradients and related electric fields may drive local plasma instabilities, although only for a limited set of parameters. Motivated by these PIC simulations, we have revisited the linear theory of this instability, employing a novel and sophisticated eigenmode analysis. This method identified eigenmode wave structures in regions having strong plasma density gradients. These wave structures are not linearly unstable, but are not damped either. This means that small-scale fluctuations provided by an external source (e.g., by a nonlinear spectral cascade from longer-wavelength spread-F turbulence) can be resonantly amplified and may explain radar observations without invoking linear instability. [Preview Abstract] |
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TP8.00109: Development and Validation of a Critical Gradient Energetic Particle Driven Alfv\'en Eigenmode Transport Model for DIII-D Tilted Neutral Beam Experiments R.E. Waltz, E.M. Bass Recent experiments on DIII-D with tilted neutral beam injection (NBI), which significantly vary the beam energetic particle (EP) source profiles, have provided strong evidence that unstable Alfv\'en eigenmodes (AE) drive stiff EP transport at a critical EP density gradient [1]. We hope to identify the critical gradient with the condition that the maximum local AE growth rate falls to the local ion temperature gradient (ITG)/trapped electron mode (TEM) rate at the same low toroidal mode number. This condition was supported by early nonlinear local GYRO simulations [2] and more is more optimistic than stiff EP transport at the AE marginal stability gradient used in a recent ITER projection of AE driven alpha confinement losses [3]. The AE and ITG/TEM growth rates are from GYRO with comparison of Maxwellian and slowing down beam-like EP distributions.\par \vskip6pt \noindent [1] W.W.\ Heidbrink, et al., Nucl.\ Fusion {\bf 53}, 093006 (2013).\par \noindent [2] E.M.\ Bass and R.E.\ Waltz, Phys.\ Plasmas {\bf 17}, 112319 (2010).\par \noindent [3] E.M.\ Bass, Bull.\ Am.\ Phys.\ Soc.\ {\bf 58}, 168 (2013). [Preview Abstract] |
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