Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session TP10: Poster Session VII (MFE: Turbulence and Transport; Astrophysical Plasmas; Scrape-off Layer and Plasma Surface InteractionsPoster
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Room: Exhibit Hall 1 |
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TP10.00001: MFE: TURBULENCE AND TRANSPORT |
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TP10.00002: Cross-phase Modification: A mechanism for the I-mode and other enhanced confinement regimes? David Newman, Paul Terry, Raul Sanchez, A Bustos New confinement regimes such as the I-mode offer good confinement properties with reduced density limit issues and better control. Previously, a number of different mechanisms have been identified for the formation and maintenance of enhanced confinement regimes. However, few if any allow enhanced confinement in one channel but not another as is seen in the I-mode. We propose modifications of cross-phases as a possible mechanism for different transport in different channels. Using simple dynamical models which have been able to capture a remarkable amount of the dynamics of the core and edge transport barriers found in many devices, we add cross phase to investigate the new mechanism. To this basic 7 field transport framework a simple model for phase effects, due to multiple instabilities, between the transported fields such as density and temperature is added with which we can investigate whether the dynamics of more continuous transitions such as the I-mode can be captured and understood. It can. This is backed up by multi-scale simulations on full gyro-kinetic codes. We then look at the question: If this mechanism is valid, what can the model tell us about control knobs for these promising regimes? [Preview Abstract] |
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TP10.00003: Non-diffusive Transport in a Simple Flux-driven Plasma Turbulence System Douglas Ogata, David Newman, Raul Sanchez A simple 2D flux-driven drift-wave type plasma fluid turbulence model has been developed to demonstrate the affect on the overall transport due to the interplay between turbulent driven gradient relaxation and self-generated flows. The mixed overall transport characteristics in this system can then be captured through the non-diffusive transport framework. Super-diffusive transport has been observed as a consequence of the turbulent relaxation process triggered by the combination of a flux-driven background profile and a critical gradient. Sub-diffusive transport can arise from self-generated sheared poloidal flows due to the inhomogeneity in the geometry of the sources. The diffusive character exists in regimes where neither the super-diffusive nor the sub-diffusive element is dominant. With an externally applied sheared poloidal flow, once again the sub-diffusive character dominates. Finally, a relation between Lagrangian trajectories and a passive scalar advection will be shown, which provides a potential bridge between experimental transport measurements and analytical theory. [Preview Abstract] |
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TP10.00004: Control of Internal Transport Barriers in Magnetically Confined Fusion Plasmas Soma Panta, David Newman, Raul Sanchez, Paul Terry In magnetic confinement fusion devices the best performance often involves some sort of transport barriers to reduce the energy and particle flow from core to edge. Those barriers create gradients in the temperature and density profiles. If gradients in the profiles are too steep that can lead to instabilities and the system collapses. Control of these barriers is therefore an important challenge for fusion devices (burning plasmas). In this work we focus on the dynamics of internal transport barriers. Using a simple 7 field transport model, extensively used for barrier dynamics and control studies, we explore the use of RF heating to control the local gradients and therefore the growth rates and shearing rates for barrier initiation and control in self-heated fusion plasmas. Ion channel barriers can be formed in self-heated plasmas with some NBI heating but electron channel barriers are very sensitive. They can be formed in self-heated plasmas with additional auxiliary heating i.e. NBI and radio-frequency(RF). Using RF heating on both electrons and ions at proper locations, electron channel barriers along with ion channel barriers can be formed and removed demonstrating a control technique. Investigating the role of pellet injection in controlling the barriers is our next goal. [Preview Abstract] |
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TP10.00005: Echoes and phase-unmixing in fully developed tokamak turbulence William Dorland, Alex Schekochihin, Noah Mandell, David Hatch, Anjor Kanekar, Jake Bringewatt, Greg Hammett Echoes in quiescent plasma are well understood. Here, we describe the role of echoes (and more generally, "phase-unmixing") in fully developed turbulence. We demonstrate that echoes are ubiquitous, important, and easily understood and modeled. We demonstrate their role in preserving the observed density fluctuations in the solar wind -- density fluctuations which would otherwise be Landau damped. We then explain their importance in tokamak turbulence and demonstrate how they can be modeled, particularly with highly efficient gyrofluid methods. The essential difference between what we present here and what we have published recently is our inclusion of finite Larmor radius physics. We extend our previous analysis and simulations of long wavelength fluctuations to treat fluctuations with wavelengths comparable to or much smaller than an ion gyroradius, as is typical in tokamak microturbulence. [Preview Abstract] |
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TP10.00006: Features of Discontinuous Galerkin Algorithms in Gkeyll, and Exponentially-Weighted Basis Functions G. W. Hammett, A. Hakim, E.L. Shi There are various versions of Discontinuous Galerkin (DG) algorithms that have interesting features that could help with challenging problems of higher-dimensional kinetic problems (such as edge turbulence in tokamaks and stellarators). We are developing the gyrokinetic code Gkeyll based on DG methods. Higher-order methods do more FLOPS to extract more information per byte, thus reducing memory and communication costs (which are a bottleneck for exascale computing). The inner product norm can be chosen to preserve energy conservation with non-polynomial basis functions (such as Maxwellian-weighted bases), which alternatively can be viewed as a Petrov-Galerkin method. This allows a full-$F$ code to benefit from similar Gaussian quadrature employed in popular $\delta f$ continuum gyrokinetic codes. We show some tests for a 1D Spitzer-H\"arm heat flux problem, which requires good resolution for the tail. For two velocity dimensions, this approach could lead to a factor of $\sim 10$ or more speedup. [Preview Abstract] |
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TP10.00007: Continuum Gyrokinetic Simulations of Turbulence in Open-Field-Line Plasmas E.L. Shi, G.W. Hammett, T. Stoltzfus-Dueck, A. Hakim We have performed our first 3D2V gyrokinetic simulations of electrostatic plasma turbulence in open-field-line geometries using the full-F discontinuous-Galerkin code Gkeyll. These simulations include the basic elements of a scrape-off layer: localized sources to model plasma outflow from the core, cross-field turbulent transport, parallel flow along magnetic field lines, and parallel losses at the limiter or divertor with sheath boundary conditions. The set of boundary conditions used in our model allows currents to flow through the walls and satisfies energy conservation. In addition to details of our numerical approach, we will present results from flux-tube simulations of devices containing straight-field lines (such as LAPD) and helical-field-lines (such as the TORPEX simple magnetized torus). Preliminary results show turbulent fluctuation levels similar to fluid simulations, which are comparable to the observed fluctuation level in LAPD but somewhat smaller than observed in TORPEX. [Preview Abstract] |
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TP10.00008: Sparse Grid Methods in Discontinuous Galerkin Algorithms, and Initial Continuum Gyrokinetic Simulations of NSTX-like SOL Turbulence Ammar Hakim, Greg Hammett, Eric Shi We have developed a hybrid discontinuous/continuous Galerkin scheme for gyrokinetic equations. Our scheme solves the equations in the Poisson bracket formulation and, with a careful choice of basis functions, conserves energy exactly. We use a sparse grid representation to reduce cost. We have developed a novel form of sheath boundary conditions, going beyond logical-sheath BCs, that allows current flow into the boundaries, yet retains overall charge continuity as well as conserves energy. First applications to a simplified model of the NSTX-like scrape-off-layer (SOL) are presented. We treat the SOL in simplified geometry, retaining magnetic curvature effects, and study the turbulent spreading of particle and heat flux. Extensions to include magnetic fluctuations and collisions are discussed. [Preview Abstract] |
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TP10.00009: Using 3-D shaping to manipulate ITG turbulence saturation in stellarators C. C. Hegna, P. W. Terry A frontier research area for stellarator design is to develop methods to alter turbulent transport. In this work, efforts are developed to understand how 3-D shaping can be used to affect turbulent transport saturation physics. To accomplish this goal, we utilize a paradigm for turbulent saturation that relies on zonal flow mediated transfer of energy from linear instability to damped eigenmodes. A simplified 3-field fluid model for ion temperature gradient turbulence is developed that allows for the presence of general 3-D geometry. The crucial nonlinear physics is associated with the triplet interaction of a linear instability, a zonal flow and a damped mode. The most vigorous interaction occurs when the three-wave frequency mismatch of these three modes is minimized, connoting a large nonlinear interaction time with saturated turbulence levels proportional to the three-wave frequency mismatch. Initial studies will be geared toward how 3-D geometry can be used to minimize this frequency mismatch. [Preview Abstract] |
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TP10.00010: Energy Transfers Saturating ITG Microturbulence Garth Whelan, MJ Peuschel, Paul Terry In plasma microturbulence, stable modes occur at the same wavenumbers as unstable modes, creating a dissipation range that overlaps with the driving range. We track nonlinear energy transfer in gyrokinetics through linked sequences of energy-conserving wavenumber triplets, seperating stable and unstable modes. For ITG turbulence, most of the transferred energy involves zonal flows. Low-wavenumber interactions transfer energy into the flows while higher-wavenumber ones transfer energy out. Increasing plasma beta increases energy transfer to damped modes, consistent with the nonlinearly enhanced beta stabilization of ITG turbulence. Furthermore, we focus on the Dimits regime, of importance in predicting turbulent onset in fusion devices. Here, energy transfer differs from the fully turbulent case in the balance between nonlinear energy transfer into and out of the zonal flows. At the linear ITG threshold, transfer into the flow is mostly dissipated linearly, while nonlinear energy transfer out increases with temperature gradient until the nonlinear critical threshold is reached. In contrast, above the nonlinear critical gradient, nonlinear energy transfer in to and out of the flows are approximately in balance, independent of the temperature gradient drive. [Preview Abstract] |
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TP10.00011: Transport and Zonal Flows at Ion and Electron scales in the MST Reversed-Field Pinch Zachary Williams, James Duff, M.J. Pueschel, Paul Terry Reversed-field pinches (RFPs) operating in an improved-confinement regime, Pulsed Poloidal Current Drive (PPCD), exhibit microturbulence that contributes to heat and particle transport. Gyrokinetic simulations are used to characterize various PPCD discharges with differing values of $\eta\equiv d\mathrm{ln}T/d\mathrm{ln}n$. Zonal flows play an important role in regulating ITG, TEM, and ETG transport in these discharges. Residual magnetic fluctuations from tearing modes in RFPs degrade zonal flows, setting flux levels and critical gradients. In the absence of such fluctuations, RFPs can generate very large zonal flow amplitudes, resulting in negligibly small fluxes. Potential mechanisms for this strong zonal flow generation are addressed here, including zonal flow residuals, secondary instability, and strong density gradients. Beyond fluxes, comparisons between simulation and the experiment are sought out through the study of carbon impurities. Additionally, longer-term plans for experimental comparison are introduced here through the investigation of fast ion dynamics, with a focus on particle diffusivity to assess NBI heating for RFPs. [Preview Abstract] |
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TP10.00012: Self-consistent modeling of multiscale gyrokinetics and transport Jeffrey Parker, Lynda LoDestro, Daniel Told, Frank Jenko In the core of tokamak plasmas, a separation of timescales between turbulence and transport makes direct simulation of both processes computationally expensive. A workable, practical method to exploit the separation of timescales will be a key component in enabling the self-consistent solution of macroscopic profiles of density and temperature. We report on progress to implement the LoDestro scheme [Shestakov et al., J.~Comp.~Phys.~185 (2003) 399] coupled with the gyrokinetic code GENE to perform for the first time coupled turbulence and transport simulations using a global gyrokinetic code. One of the advantages of the LoDestro scheme, which is essentially a method of solving an implicitly advanced nonlinear transport problem, is that it does not use Newton iteration and hence avoids difficulties that arise from calculating Jacobians or Jacobian-vector products in the presence of noisy fluxes. Instead, the implicit timestep equation is solved with an iteration scheme by representing the turbulent flux as the sum of diffusive and convective pieces, after which Picard iteration is used to converge to the self-consistent solution. Preliminary results will be presented. [Preview Abstract] |
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TP10.00013: Density and magnetic fluctuations at JET: experimental observation and numerical characterization Gianluca De Masi, Italo Predebon, Silvia Spagnolo, Ivan Lupelli, Jon Hillesheim, Luis Meneses, Costanza Maggi, Ephrem Delabie Density and magnetic fluctuations have been experimentally observed on JET in the inter ELM phases in low beta discharges.They have been characterized in terms of typical frequency range (60-80 kHz), wavenumber (0.01 $\le $ky $\rho $i$\le $0.1), radial localization (pedestal top) and correlation with the relevant kinetic quantities [1].A linear simulation with gyrokinetic code GENE [2], matching the experimental edge condition has been performed to gain insight on their possible physical interpretation. ITG modes turn out to be the most unstable modes for 0$\le $ky $\rho $i$\le $1, while microtearing modes (MTMs) are the dominant instabilities for ky $\rho $i $\le $ 0.1.A typical oscillation frequency of about 50-100 kHz is associated to both unstable modes, with opposite propagation direction.Different considerations suggest an interpretation in terms of MTMs for the observed magnetic fluctuations, while density fluctuations appear to be dominated by ITG instabilities.[1]G. De Masi et al., 57th DPP APS Conference, 2015, Savannah (GA) [2]http://www.genecode.org [Preview Abstract] |
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TP10.00014: Studies of L-H Transition Thresholds in Electron Heated Regimes Mikhail Malkov, Patrick Diamond, Pei-Chun Hsu We discuss transitions in collisionless, electron heated regimes where the electron-ion coupling is $ anomalous $, due to the fluctuation of $\left\langle \mathbf{E}\cdot\mathbf{J}\right\rangle$ work and explore new transition scenarios, characterized by the sensitivity of transition evolution to pre-existing L-mode profiles $ where\; turbulence\; driven\; shear\; flows\; are\; absent$. We have developed a reduced model that evolves the collisionally coupled electron and ion temperatures, density, turbulence intensity, and flow profiles. The power threshold minimum in density is revealed $ as\;a\;combined\;effect\;of\;the\; density\;dependence\;of\;collisionless\;electron-ion\;coupling\;and\;e-i\;heating\;mix$. To treat collisionless regimes, we have included anomalous power coupling between electrons and ions, using a recent theory of minimum enstrophy relation to model flow damping. Our preliminary results suggest that $ L\to H $ transition occurs as the endstate of an $electron-ion\;thermal\;coupling\;front$. Upon arriving at the edge, it impulsively raises $ T_i $ there, thus strengthening the diamagnetic electric field shear. This study highlights the importance of collisionless energy transfer process to transitions of regimes of ITER relevance. [Preview Abstract] |
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TP10.00015: Construction of reduced transport model by gyro-kinetic simulation with kinetic electrons in helical plasmas S. Toda, M. Nakata, M. Nunami, A. Ishizawa, T. -H. Watanabe, H. Sugama A reduced model of the turbulent ion heat diffusivity is proposed by the gyrokinetic simulation code (GKV-X) with the adiabatic electrons for the high-$T_{i}$ Large Helical Device discharge. The plasma parameter region of the short poloidal wavelength is studied, where the ion temperature gradient mode becomes unstable. The ion heat diffusivity by the nonlinear simulation with the kinetic electrons is found to be several times larger than the simulation results using the adiabatic electrons in the radial region $0.46\le r/a \le 0.80$. The electromagnetic contribution is about a several percent in the ion energy flux. The model of the turbulent diffusivity is derived as the function of the squared electrostatic potential fluctuation and the squared zonal flow potential. Next, the squared electrostatic potential fluctuation is approximated with the mixing length estimate. The squared zonal flow potential fluctuation is shown as the linear zonal flow response function. The reduced model of the turbulent diffusivity is derived as the function of the physical parameters by the linear GKV-X simulation with the kinetic electrons. This reduced model is applied to the transport code with the same procedure as [1]. [1] S. Toda et al., J. Phys.: Conf. Ser. Vol. 561 012020 (2014) [Preview Abstract] |
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TP10.00016: A Model of the Saturation of Multi-scale Turbulence by Zonal Flow Mixing G.M. Staebler, J. Candy, C. Holland, N. Howard Analysis of the spectrum of the saturated electric potential fluctuations from multi-scale (both ion and electron scales) gyrokinetic turbulence simulations, in tokamak geometry, reveals that fluctuating zonal (axisymmetric) ExB flows couple the ion and electron scales. The zonal flows are driven by the ion-scale instabilities but strongly regulate the amplitude of the electron-scale turbulence. The electron-scale turbulence can grow to large amplitude when the linear growth rate of the ETG modes exceeds the zonal flow mixing rate due to advection of the ETG modes. The model of the zonal flow mixing is shown to capture the suppression of electron-scale turbulence by ion-scale turbulence and the threshold for the increase in electron-scale turbulence when the ion-scale turbulence is reduced. The nonlinear upshift of the effective critical ion temperature gradient (Dimits shift) is also captured by the new model. Prediction of the core plasma fusion performance of ITER with TGLF using the new saturation model yields a 19$\%$ increase in fusion power for hybrid regime operation. [Preview Abstract] |
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TP10.00017: Stability of Microtearing Modes and the Resulting Electron Thermal Transport in Tokamak Discharges T. Rafiq, J. Weiland, L. Luo, A. Kritz, A. Pankin Microtearing modes (MTMs) have been identified as a source of significant electron thermal transport in tokamak discharges. In order to understand how MTMs affect transport, and, consequently, the evolution of electron temperature in tokamak discharges, a reduced transport model for MTMs was developed for use in integrated predictive modeling studies [T Rafiq, \textit{et al.,} Phys. Plasmas \textbf{23}, 062507 (2016)]. A unified fluid/kinetic approach was used to derive the nonlinear dispersion relation in order to advance the kinetic description and to include the nonlinear effects due to magnetic fluctuations. The dependence of the MTM real frequency and growth rate on radial and poloidal mode numbers (k$_{\mathrm{y}})$, electron beta, collisionality, safety factor, magnetic shear, density gradient, temperature gradient, and curvature is examined in a numerical study. The magnetic fluctuation amplitude saturation level is computed for each flux surface using the nonlinear MTMs envelope equation. This level depends upon the most unstable eigenvalue as well as on the sidebands in the k$_{\mathrm{y}}$ spectrum. The magnetic fluctuation levels are then used to compute electron thermal transport that is due to the presence of the unstable microtearing modes. [Preview Abstract] |
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TP10.00018: Sub-Alfv\'enic reduced full-f Kinetic MHD equations to study flute like instabilities W Sengupta, A Hassam, T. M Antonsen Jr. We develop a set of reduced sub-Alfv\'enic fluid as well as kinetic MHD equations which are suitable for studying flute like instabilities in MHD ordering. The full-f kinetic equations are obtained by reducing Kulsrud's complete set of kinetic MHD system and includes trapped ion dynamics in a toroidal geometry. The nonlinear equations show the presence of Mercier modes, electromagnetic effects, GAMs and Rosenbluth-Hinton zero frequency zonal flows. Linear stability based on our equations shall be compared to the well known Kruskal-Oberman Kinetic MHD stability criteria. In the supersonic limit, for large q, our system can be shown to be equivalent to CGL double adiabatic theory. In the marginal stability limit, we shall discuss trapped particle stabilization of interchange modes. Comparison will also be made to the sub-Alfv\'enic reduced MHD fluid equations in a large aspect ratio tokamak. We shall show that the trapped particle effects in kinetic theory can be treated as a boundary layer of width the square root of the inverse aspect ratio in phase space. [Preview Abstract] |
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TP10.00019: GTC Simulation of Static Magnetic Island Effect on Micro-turbulence Hongying Feng, Wenlu Zhang In magnetic confinement fusion physics, tearing mode is dangerous by changing the magnetic field topology and breaking the confinement. Micro-turbulences are also of great significance since the anomalous transport is believed to play a key role in the energy confinement. By introducing a static magnetic island, the relaxing process is investigated to construct the new steady profiles that evolve from the prior equilibrium profiles. The consequent pressure flattening within the magnetic island may pose significant impacts on properties and evolutions of micro-turbulence, which is also~investigated through large scale particle simulations in this work. [Preview Abstract] |
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TP10.00020: ABSTRACT WITHDRAWN |
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TP10.00021: Accuracy of two points correlation length measurement and its applications in H-1NF heliac. Jaewook Kim, C.A. Michael, Y.U. Nam, M. Lampert, Y. C. Ghim Anomalous transport observed in fusion-grade plasmas is widely accepted to be correlated with spatial and temporal correlation characteristics of the turbulent eddies. While temporal and 2D spatial (radial and poloidal) correlation characteristics have been studied in detail, the lack of such information in the parallel direction, with respect to the background magnetic field, of hot core plasmas precludes us from full understanding and controlling plasma turbulence. KSTAR is equipped with a couple of 2D diagnostic systems measuring ion-scale density fluctuations, namely the BES and MIR systems, at two different toroidal locations. These systems provide a possibility to measure a parallel correlation length. As it is necessary to identify how reliably one can measure correlation length with only two spatial positions, there has been such a study [Jaewook Kim et al., Nucl. Fusion accepted] recently. Based on this recent study, we experimentally obtained 3D correlation functions from H-1NF heliac using the data from a set of Langmuir probes. One probe is spatially fixed, while the second one is scanned radially and poloidally at a different toroidal location. H1-NF heliac plasmas are highly reproducible, therefore we construct the 3D correlation functions with multi-discharges. [Preview Abstract] |
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TP10.00022: Emergence of Macroscopic Transport Barriers from Staircase Structures Arash Ashourvan, Patrick H. Diamond A theory is presented for the formation and evolution of coupled density staircases (SC) and zonal shear profiles in a simple model of drift-wave turbulence. Density, vorticity and fluctuation potential enstrophy are the fields evolved for this system. Formation of SC structures is due to inhomogeneous mixing of generalized potential vorticity (PV), resulting in the sharpening of density and vorticity gradients in some regions and weakening them in others. The positive feedback which drives SC formation is implemented via a Rhines scale dependent mixing length. When PV gradients steepen, the density SC structure develops into a lattice of mesoscale 'jumps', and 'steps', which are respectively, regions of local gradient steepening and flattening. The jumps merge and migrate in radius, leading to the development of macroscale profile structures from mesoscale elements. Furthermore, depending on the sources and boundary conditions, either a region of enhanced confinement, or a region with strong turbulence can form at the edge. We present extensive studies of bifurcation physics of the \emph{global} state, including results on the flux-gradient landscapes. This model is the first to demonstrate how mesoscale condensation of SCs leads to global states of enhanced confinement. [Preview Abstract] |
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TP10.00023: Hybrid Gyrofluid/Gyrokinetic Modeling of Tokamak Turbulence with GryfX Noah Mandell, Bill Dorland, Edmund Highcock, Greg Hammett Gyrofluid models are more efficient than gyrokinetic models, but have a disadvantage in their potential lack of physics fidelity. Here we present three major improvements to the physics fidelity and speed of gyrofluid models, which we encapsulate in the GryfX gyrofluid turbulence code. First, we implement a new nonlinear closure to model the cascade of free energy simultaneously in $k_\perp$ and $v_\perp$ via nonlinear phase-mixing (NLPM). Second, we use a hybrid algorithm that improves zonal flow physics by simulating zonal flow modes with a fully gyrokinetic model. These two improvements bring heat flux predictions from nonlinear GryfX simulations into agreement with the gyrokinetic code GS2. Third, we implement the equations on modern heterogeneous computing platforms, both as a standalone simulation tool that exploits the power of GPUs and as a component of TRINITY (a transport modeling code for tokamaks). GryfX has a roughly 1,200 times performance advantage over GS2 due to the combination of GPU acceleration and the reduction of hundreds of velocity space grid points to six gyrofluid moments. This makes GryfX ideal for large parameter scans, and enables the use of the TRINITY-GryfX system for efficient multi-scale analysis of tokamak turbulence on transport time scales. [Preview Abstract] |
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TP10.00024: The effect of energetic particle induced geodesic acoustic modes on microturbulence Mirjam Schneller, GuoYong Fu, WeiXing Wang, Ilija Chavdarovski, Philipp Lauber The control of turbulent transport reveals essential to achieve a successful fusion reactor. Together with turbulence, energetic particles are ubiquitous in present and future tokamaks due to heating systems and fusion reactions. Anisotropy in the distribution function of the energetic particle population is able to excite oscillations from the continuous spectrum of geodesic acoustic modes, which cannot be driven by plasma pressure gradients due to their toroidally and nearly poloidally symmetric structures. These oscillations are known as energetic particle-induced geodesic acoustic modes (EGAMs) [G.Y.Fu'08] and have been observed in recent experiments [R.Nazikian'08]. EGAMs are particularly attractive in the framework of turbulence regulation, since they lead to an oscillatory radial electric shear which can potentially saturate the turbulence. In recent years, numerical simulations have shown however, that turbulent transport could also be enhanced in the presence of EGAMs [D.Zarzoso'13]. For the presented work, the nonlinear gyrokinetic, electrostatic, particle-in-cell code GTS [W.X.Wang'06] has been extended to include an energetic particle population. With this new tool, the interaction of EGAMs with microturbulence is investigated in more detail. [Preview Abstract] |
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TP10.00025: Role of magnetic shear and equilibrium flow on stability of collisionless mixed parity and microtearing modes Aditya Krishna Swamy, Deepak Verma, Rajaraman Ganesh, Stephan Brunner, Laurent Villard Turbulent transport of energy, particles and momentum is one of the important limiting factors for long time plasma confinement. Modern study using gyrokinetic formalism and simulation has progressed to identify several microinstabilities that cause ion and electron thermal transport. Typically, these have been ballooning parity modes ( $\tilde{\varphi}$ is even and $\tilde{A}_\parallel$ is odd) such as Ion Temperature Gradient mode (ITG), Kinetic Ballooning Mode (KBM) and Electron Temperature Gradient mode (ETG) which cause transport through fluctuations or tearing parity modes ( $\tilde{\varphi}$ is odd and $\tilde{A}_\parallel$ is even) such as Microtearing modes (MTM) which change the local magnetic topology and cause transport through stochastization of the magnetic field. Here, the role of global safety factor profile variation on the MTM instability and global mode structure is studied in large aspect ratio tokamaks. Multiple subdominant branches of MTM are linearly unstable in several shear profiles. At lower shear, linearly unstable Mixed Parity Modes are found to exist. The growth rate spectrum, $\beta$-scaling in reverse shear profiles and the role of equilibrium flow on the stability and global mode structures of these modes will be presented. [Preview Abstract] |
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TP10.00026: On extended analytic theory of 2D ballooning modes in tokamak plasmas Peshwaz Abdoul, David Dickinson, Colin Roach, Howard Wilson We have extended the leading order ballooning theory which typically yields more unstable isolated mode (IM) that usually sit on the outboard mid-plane, to higher order where less unstable general mode (GM) sits at a different poloidal location. Our analytic theory has revealed that any poloidal shift of the mode with respect to the outboard mid-plane $-$ arising from the effect of profile variations, for example $-$ is always accompanied by an asymmetry of the radial eigenmode structure. Hence, GMs have radial asymmetry. Our theory can have important consequences, especially for calculations that invoke quasilinear theory to model intrinsic rotation arising from Reynolds stress. This is very important in ITER for which external torques are small. In such theories it is the radial asymmetry in the global GM mode which can generate a Reynolds stress that could in principle contribute to the poloidal flow during the low to high (L-H) mode transition in tokamaks. [Preview Abstract] |
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TP10.00027: Gyrokinetic simulations of the ITER pedestal Mike Kotschenreuther, D. Hatch, S. Mahajan, P. Valanju, L. Zheng, X. Liu The projected ITER pedestal is several times more gyroradii wide than in existing experiments. Hence, the ExB shearing rate is much lower, compared to instability rates, and turbulent suppression is weaker. We perform a scan of normalized gyroradius, using GENE, going from existing experiments to ITER. The transport scales close to gyroBohm, over most of the range of present experiments. However, for JET-ILW parameters, turbulent suppression is weakened, giving higher transport, and this is much stronger for ITER. Numerous trends for JET-ILW are reproduced, including, enhanced transport from lack of low Z impurities, and higher separatrix density (from gas puffing). Projected transport for ITER is unacceptably high. However, it is possible to produce lower transport in several ways, including a) optimizing the pedestal density profile b) operation at higher poloidal beta (the so-called hybrid mode). With sufficiently strong optimization, the pedestal transport might even be considerably lower than conventional expectations, leading to confinement significantly better than H-mode. Here we describe the unfavorable trends, their physical origin, and the possible favorable effects, including whether they can be realized in practice. [Preview Abstract] |
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TP10.00028: Gyrokinetic revelation of transport bifurcation events in tokamak edge plasmas using XGC1 C.S. Chang, S. Ku, R. Hager, R.M. Churchill, I. Cziegler, G. Tynan, J. Highes, A. Hubbard, M. Greenwald Even though the edge transport bifurcation phenomenon was discovered in a tokamak experiment over three decades ago, and is critical to the success of the magnetic fusion program, there has been no kinetic level theoretical reproduction of such a phenomenon in realistic geometry. We report the first kinetic simulation of edge transport bifurcation events in realistic single X-point magnetic field geometry using XGC1, with the ion magnetic drift oriented both toward and away from the X-point. We observe that the edge turbulence and transport bifurcation is triggered when the shearing rate of the turbulence-driven edge ExB-flow exceeds a critical level, and that the actual bifurcation may be completed in conjunction with the neoclassical orbit loss phenomenon. It is also found that there is a significant difference in the behaviors of geodesic acoustic modes depending upon the direction of the magnetic drift. [Preview Abstract] |
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TP10.00029: Gyrokinetic simulation of I-mode C-Mod pedestal using GENE Xing Liu, H David, M Kotschenreuther, S Mahajan, J Huges, A Hubbard, P Valanju Naturally stable to ELMs, and with widths larger than EPED predictions, the I-modes are an excellent laboratory for investigating the role of drift microinstabilities in pedestal formation since I-mode pedestal are not ``limited'' by MHD instabilities---Peeling Ballooning or the Kinetic Ballooning. Because the Weakly Coherent Mode (WCM) is shown to be correlated, primarily, to particle transport, the pedestal heat transport, in some sense, must be controlled by drift-type modes. We present here a study based on gyrokinetic simulations (using GENE) to model heat transport in the I-mode pedestals in C-Mod. Nonlinear ETG simulations, found to be streamer-dominated, can match experimental heat flux with profile adjustment well within experimental error bars. The ETG simulations reveal very notable fine-scale structure (in the parallel direction) of the eigenfunctions in both linear and nonlinear simulations. Simulations, varying impurity level (Zeff) and temperature and density profiles (within experimental error bars), are used to probe the sensitivity of ETG heat transport to the most important input parameters. Efforts to identify an instability corresponding to the WCM will also be discussed. [Preview Abstract] |
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TP10.00030: Statistical analysis of edge turbulence and blob structures simulated in the full-f gyrokinetic code XGC1 Randy Churchill, CS Chang, Seung-Hoe Ku, Robert Hager Turbulence in the edge (pedestal + SOL) is difficult to probe both experimentally and computationally, yet it may play a large role in the feasibility of a tokamak fusion reactor. XGC1 is a full-f gyrokinetic code aimed at including all of the important physics necessary to faithfully simulate transport in the edge region. Here we present statistical analyses of turbulent fluctuations of XGC1 outputs, including density, potential, and temperature. The skewness and kurtosis are investigated as indicators of the underlying distribution, and are shown to follow closely the skewness/kurtosis relation for gamma distributions. Spatial cross-correlation and cross-phase analysis will be presented to identify poloidal wavenumber regions of strongest particle and energy transport, helping to identify dominant turbulence modes in the pedestal and SOL. These comparisons will be made using both flux-surface averaged (important for total transport) and low-field side localized data (what experiments usually measure). Distributions of size and speed of edge turbulent eddies and blobs, and the connected physics associated with their radial transport will be explored. [Preview Abstract] |
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TP10.00031: A Gyrokinetic Perspective on the JET-ILW Pedestal David Hatch, Mike Kotschenreuther, Swadesh Mahajan, Prashant Valanju, Xing Liu Simulations using the GENE code based on JET-ILW profiles and equilibria quantitatively capture experimental transport levels for a representative experimental discharge and qualitatively recover the major experimental trends. Microtearing turbulence is a major transport mechanism for the low-temperature pedestals characteristic of unseeded JET-ILW discharges. At higher temperatures, we identify electrostatic ITG-like transport of a type that is strongly shear-suppressed on smaller machines. Consistent with observations, this transport mechanism is substantially reduced by the presence of a low-Z impurity (e.g., carbon or nitrogen at the level of Z-effective\textasciitilde 2). Multiple transport mechanisms, including ITG, ETG, microtearing modes, and neoclassical transport are found to play important roles depending on the pedestal parameters. The picture that emerges involves several parameters---notably, rho*, Z-effective, pedestal top temperature, and separatrix density---mediating the relative roles of these transport mechanisms. This study maps out important regions of this parameter space, providing insights that may point to optimal physical regimes that can enable the recovery of high temperatures on JET. [Preview Abstract] |
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TP10.00032: Benchmarking finite- $\beta$ ITG gyrokinetic simulations A.M. Dimits, W.M. Nevins, J. Candy, C. Holland, N. Howard We report the results of an electromagnetic gyrokinetic-simulation benchmarking study based on a well-diagnosed ion-temperature-gradient (ITG)-turbulence dominated experimental plasma. We compare the 4x3 matrix of transport/transfer quantities for each plasma species; namely the (a) particle flux, $\Gamma_{a}$, (b) momentum flux, $\Pi_{a}$, (c) energy flux, $Q_{a}$, and (d) anomalous heat exchange, $S_{a}$, with each transport coefficient broken down into: (1) electrostatic ($\delta \varphi )$ (2) transverse electromagnetic ($\delta A_{\parallel })$, and (3) compressional electromagnetic, ($\delta B_{\parallel })$ contributions. We compare realization-independent quantities (correlation functions, spectral densities, etc.), which characterize the fluctuating fields from various gyrokinetic simulation codes. [Preview Abstract] |
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TP10.00033: Understanding and Controlling Turbulent Mixing in a Laboratory Magnetosphere M.E. Mauel In a laboratory magnetosphere, plasma is confined by a strong dipole magnet, and complex nonlinear processes can be studied and controlled in near steady-state conditions. Because a dipole's magnetic field resemble the inner regions of planetary magnetospheres, these laboratory observations are linked to space plasma physics. Unlike many other other toroidal configurations, interchange and entropy modes dominate plasma dynamics, and turbulence causes self-organization and centrally-peaked profiles as the plasma approaches a state of minimum entropy production. We report progress in understanding and controlling turbulent mixing through a combination of laboratory investigation, modeling, and simulation. Topics discussed: (i) extending the global extent of local regulation of the interchange and entropy mode turbulence through current injection, (ii) measurement and interpretation of the statistical properties of stationary turbulence, and (iii) advancements in the nonlinear simulation of turbulence control in a dipole plasma torus. [Preview Abstract] |
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TP10.00034: Fluid Simulation of plasma interchange turbulence in a Dipole Configuration Ou Weike The plasma interchange turbulence and convective transport in the dipole and Z-pinch systems are explored with two-fluid global simulations. The nonlinear evolution of interchange modes produces global convective cells, which leads to the marginally stable pressure profiles. The effects of magnetic field line curvature are particularly strong in a magnetic dipole field and the short curvature radius leads to a steep pressure gradient for the marginal stability. As a result, the plasma pressure profiles in a nonlinear saturated state are strongly peaked in a dipole compared to those in a Z-pinch. Preliminary results with gyro-fluid model, which includes both interchange and entropy modes, will also be discussed. [Preview Abstract] |
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TP10.00035: Investigations of Particle Transport in the Texas Helimak. E.I. Taylor, W.L. Rowan, K.W. Gentle, H. Huang, C.B. Williams The correlation between electrostatic turbulence and particle flux is investigated in a simple magnetic torus, the Helimak. The Helimak is an experimental realization of a sheared cylindrical slab that generates and heats a plasma with microwaves at 2.45 GHz and confines it in a helical magnetic field. Although it is MHD stable, the plasma is always in a nonlinearly saturated state of microturbulence. The causes of this turbulence are diverse and it is thought that it is either due to drift wave instabilities or interchange instabilites. The local particle flux is estimated over most of the plasma cross section by measuring the particle source using filtered cameras. Plasma flow along the field lines is physically similar to SOL flows in tokamaks. It is significant and can be measured directly as well as inferred from asymmetries in the electron density. The cross field transport due to electrostatic turbulence is measured as the cross correlation of radial electric field fluctuations with electron density fluctuations with the data acquired using Langmuir probes. [Preview Abstract] |
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TP10.00036: Characterization of Instabilities in a Simple Magnetized Torus C.B. Williams, K.W. Gentle The Texas Helimak is an approximation to the cylindrical slab with large physical size compared to the correlation lengths of its instabilities and an open magnetic field line configuration. As such, it functions effectively as a test-bed for the physics of the SOL at low densities and temperatures. This allows for the use of Langmuir probe diagnostics. Much of the research performed on the device has focused on its high turbulent amplitudes. It was initially believed, both experimentally and theoretically, that the turbulence is dominated by a fluid drift wave. However, more recent evidence suggests that the identification of the Helimak instabilities is not so straightforward, but may vary with the connection length of the magnetic field lines through both drift wave and interchange instability regimes. In this work we document efforts to characterize the turbulence based on measurements of both parallel and perpendicular wavenumbers and other Langmuir probe data. We use a variety of statistical methods to differentiate instabilities. We also conduct an analysis of possible blobs in the plasma. Finally, we apply a radial electric field to the plasma to determine its effect on the wavenumbers of the instabilities. [Preview Abstract] |
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TP10.00037: Measurements of Turbulent Transport in Simple Interchange Turbulence Kenneth Gentle, William Rowan, Chad Williams, Mark Koepke, Sam Nogami The Helimak is an approximation to the infinite cylindrical slab with a size large compared with turbulence transverse scale lengths, but with open field lines of finite length. It is also an excellent simplified model of a tokamak SOL. Interchange modes are the dominant instability, characterized by a very high level of nonlinearly saturated fluctuations, $\sim$50\%. Flow velocity profiles and shear can be greatly modified by the application of radial electric fields through external biasing of the flux surfaces -- cylindrical shells. Using newly-developed novel baffled probes that can directly measure the actual plasma potential, a multi-tip configuration optimized for local inference of the electrostatic particle flux has been built for the Helimak. The first radial profiles of the particle flux and the changes with bias will be presented. The contrast between the baffled probe measurements of plasma potential and conventional probe measurements of floating potential will be shown. Work supported by the Department of Energy OFES DE-FG02-04ER54766. [Preview Abstract] |
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TP10.00038: Magnetically Insulated Baffled Probe Measurement of Unfiltered Fluctuating Space Potential in the Texas Helimak M.E. Koepke, S.H. Nogami, V.I. Demidov, C.B. Williams, K. Gentle Success is reported in employing magnetically insulated baffled (MIB) probes [1] for the measurement of fluctuating space potential in the Texas Helimak [2]. The combination of the MIB probe and an unbaffled probe provides the necessary ingredients for determining cross-field transport without contamination between fluctuating space potential and electron temperature. The performance of the MIB probe is quantified by its ability to produce a probe characteristic with partially reduced magnitude of electron saturation current. The baffled probe employed in the 2016 experiments performed optimally (i.e., the magnitude of the electron saturation current is equal to the magnitude of the ion saturation current), meaning there is no difference between the probe floating potential and the space potential. The performance of the baffled probe is compared to the performance of the plug probe, tested in 2015 on the Texas Helimak [1]. Recent radial scans at the plasma edge of unfiltered fluctuating space potential are presented. [1] http://meetings.aps.org/link/BAPS.2015.DPP.PP12.101 [2] PoP 21, 092302 (2014) [Preview Abstract] |
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TP10.00039: Parallel closures in an inhomogeneous magnetic field Hankyu Lee, Jeong-Young Ji We solve a reduced drift kinetic equation with a Krook-type model collision operator to obtain parallel closures. Grid points in the velocity space are chosen for Gauss-Laguerre quadrature to take closure moments. For trapped and passing regimes, analytical solutions are expressed as kernel-weighted integrals of thermodynamic drives. The analytical sloutions are compared to numerical solutions obtained from a finite difference method. Inverting the free streaming operator near a bouncing point is investigated to improve accuracy of solutions. [Preview Abstract] |
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TP10.00040: Intrinsic toroidal rotation drive by electromagnetic turbulence in high pressure pedestal plasmas Shuitao Peng, Lu Wang Intrinsic rotation has long been of interest to researchers in magnetic fusion plasmas field since it may be helpful to suppress micro-turbulence and to stabilize RWMs in ITER, where neutral beam injection (NBI) is of low efficiency and high cost. Kinetic ballooning mode (KBM) can be the dominant electromagnetic instability in pedestal region from GTC simulation [1]. In DIII-D, the high frequency coherent (HFC) modes observed in high pedestal pressure Quiescent H-mode (QH-mode) plasmas may be relevant to KBM as well [2]. We analytically derive the toroidal rotation velocity equation by using electromagnetic gyrokinetic theory in toroidal geometry. Intrinsic rotation drive includes the usual residual stress, cross Maxwell stress, kinetic stress [3] and turbulent acceleration [4, 5]. Quasilinear estimates for these terms in KBM turbulence are presented, and electromagnetic effects on intrinsic toroidal rotation drive are also discussed. [1] I. Holod, D. Fulton and Z. Lin, Nucl. Fusion 55, 093020 (2015). [2] Z. Yan \textit{et al}., Phys. Rev. Lett. \textbf{107}, 055004 (2011). [3] W. X. Ding \textit{et al.}, Phys. Rev. Lett.\textbf{ 110}, 065008 (2013). [4] Lu Wang and P. H. Diamond , Phys. Rev. Lett. \textbf{110}, 265006 (2013). [5] Lu Wang, Shuitao Peng and P. H. Diamond, Phys. Plasmas \textbf{23}, 042309 (2016). [Preview Abstract] |
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TP10.00041: Analytic investigation of a long wavelength coherent structure in stellarator TEM turbulence Benjamin Faber, Chris Hegna Recent nonlinear gyrokinetic simulations of HSX plasmas have shown the presence of a nonlinear coherent structure that significantly enhances thermal transport due to density gradient driven TEM turbulence. Projecting the nonlinear state onto linear eigenmodes shows the coherent structure is strongly linked to a stable ion mode comprised of passing particles that is excited on two scales: an inner toroidal connection length-like scale and an outer envelope believed to be influenced by the low average magnetic shear in HSX. To investigate the properties of these modes, a fluid theory has been applied with cold ions and adiabatic electrons. The resulting eigenmode equation along the field line is cast in terms of local 3D MHD equilibrium in order to elucidate the role of the integrated magnetic shear and magnetic curvature and is solved using a two-scale WKB treatment. This treatment hopes to help shed light on the complex mode interactions in stellarators and could provide insight into proxy-based turbulent transport optimization methods. [Preview Abstract] |
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TP10.00042: ASTROPHYSICAL PLASMAS |
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TP10.00043: Kinetic turbulence and non-thermal particle acceleration in relativistically hot plasma Vladimir Zhdankin, Gregory Werner, Dmitri Uzdensky We describe particle-in-cell numerical simulations of driven turbulence in collisionless, relativistically hot pair plasma. We initialize each simulation as a thermal bath, which is disrupted by the driving to develop turbulent fluctuations across a broad range of scales. We measure the energy spectra at fluid scales and at sub-Larmor scales, showing them to be consistent with a magnetohydrodynamic cascade and phase-space cascade, respectively. We demonstrate that a non-thermal particle distribution develops across a broad range of energies, with a late-time power-law index that decreases with increasing magnetization (decreasing plasma beta), much like in similar studies of magnetic reconnection. We suggest that turbulence may a ubiquitous and versatile mechanism of non-thermal particle acceleration in high energy astrophysical systems such as pulsar wind nebulae. [Preview Abstract] |
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TP10.00044: Proton and Helium Injection Into First Order Fermi Acceleration at Shocks: Hybrid Simulation and Analysis Galina Dudnikova, Mikhail Malkov, Roald Sagdeev, Tatjana Liseykina, Adrian Hanusch Elemental composition of galactic cosmic rays (CR) probably holds the key to their origin. Most likely, they are accelerated at collisionless shocks in supernova remnants, but the acceleration mechanism is not entirely understood. One complicated problem is “injection”, a process whereby the shock selects a tiny fraction of particles to keep on crossing its front and gain more energy. Comparing the injection rates of particles with different mass to charge ratio is a powerful tool for studying this process. Recent advances in measurements of CR He/p ratio have provided particularly important new clues. We performed a series of hybrid simulations and analyzed a joint injection of protons and Helium, in conjunction with upstream waves they generate. The emphasis of this work is on the bootstrap aspects of injection manifested in particle confinement to the shock and, therefore, their continuing acceleration by the self-driven waves. The waves are initially generated by He and protons in separate spectral regions, and their interaction plays a crucial role in particle acceleration. The work is ongoing and new results will be reported along with their analysis and comparison with the latest data from the AMS-02 space-based spectrometer. [Preview Abstract] |
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TP10.00045: Positron Anomaly in Galactic Cosmic Rays: Constraining Dark Matter Contribution Roald Sagdeev, Mikhail Malkov, Patrick Diamond An explanation of the unexpected rise in the positron fraction of the cosmic ray (CR) leptonic energy spectrum, is proposed. It is argued that the $e^\pm$ spectra are different because they are accelerated by a charge-sign selective mechanism. This premise was hinted at by a recent result from the AMS-02 spectrometer that revealed a difference between $e^+$ and antiproton spectra, which both are secondary CRs but of the opposite charges. The new mechanism extends the diffusive shock acceleration (DSA) to make it charge-sign selective. The DSA, operating in Galactic supernova remnant (SNR) shocks, is held responsible for the production of the bulk of the CRs. The new mechanism was found to account for the positron data with an excellent agreement, except in a limited energy range between 100-300 GeV. In this range, the data exceed the theoretical prediction systematically, thus opening a window for a contribution from dark matter decay or annihilation as well as nearby pulsars. The charge-sign selectivity of the DSA arises from an electric field induced by the CR protons illuminating the neutral gas clumps in the SNR surroundings. The electric field expels positrons from the clump but traps electrons and secondary antiprotons, thus suppressing their acceleration in such SNRs. [Preview Abstract] |
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TP10.00046: Electron-ion instability coupling in Alfv\'en waves propagating in high beta plasmas James Juno, Jason TenBarge, Ammar Hakim, William Dorland Recently, it was demonstrated there is a stringent amplitude limit on Alfv\'en waves in high beta plasmas due to the Alfv\'en wave destabilizing itself via the proton parallel firehose instability [1]. Here, we present extensions to this work by performing simulations of Alfv\'en waves in high beta plasmas with more robust models: a ten moment two fluid model, and a multi-species Vlasov-Maxwell solver. The presence of an electron model which can also develop anisotropy and agyrotropy permits the electron species to assist in destabilizing the Alfv\'en wave, in this case, via the electron parallel firehose instability. The presence of the electron parallel firehose provides an interesting nonlocal coupling between electron and proton scales. Since fluctuations from the electron parallel firehose instability are left handed and have real frequency comparable to the ion cyclotron frequency if the proton to electron temperature is order 1, the electron parallel firehose instability also heats the protons in our system. We also demonstrate how this phenomenon is affected by the presence of other instabilities by performing similar simulations in 2D, where oblique firehose, mirror, and whistler anisotropy instabilities can manifest. \\ [1] Squire, J. et al. arXiv:1605.02759 (2016) [Preview Abstract] |
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TP10.00047: Hybrid-Kinetic Simulations of the Collisionless Turbulent Dynamo. D. A. St-Onge, M. W. Kunz Dynamically important magnetic fields are ubiquitous in many astrophysical plasmas. Their existence is generally assumed to be a result of the turbulent dynamo, by which a shearing velocity field amplifies a magnetic field to near-equipartition energies. While the collisional dynamo is well understood within the framework of magnetohydrodynamics, the collisionless regime is complicated by pressure anisotropies generated adiabatically by changes in magnetic-field strength. In high-beta plasmas, these anisotropies almost immediately trigger instabilities at the (ion) Larmor scales (e.g. firehose and mirror). These microscale instabilities feed back on the macroscale magnetic field, affecting its growth, structure, and saturation in completely unknown ways. To address this problem, we perform high-resolution, hybrid-kinetic, particle-in-cell simulations of the turbulent dynamo in a collisionless plasma. Our chosen parameters guarantee a healthy scale separation between the forcing scale, the initial ion Larmor radius, and the grid scale. Results will be presented, with a particular focus on growth rates, power spectra, the structure and intermittency of the magnetic field, wave-particle interactions, and the interplay between micro- and macroscales. [Preview Abstract] |
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TP10.00048: Radiative transfer dynamo effect Vadim Munirov, Nathaniel Fisch Radiating slabs of plasma, in relative motion to each other, can create a dynamo effect as radiation from one slab is absorbed by the second slab. One instance of this dynamo effect might occur in differentially rotating and radiating plasma atmospheres in a number of astrophysical contexts. The efficiency of current drive, and with it the associated dynamo effect, is considered in a number of limits. [Preview Abstract] |
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TP10.00049: Magnetorotational stability in a self-consistent three dimensional axisymmetric magnetized warm plasma equilibrium with a gravitational field Peter Catto, Sergei Krasheninnikov Magnetorotational stability is revisited for self-consistent three-dimensional magnetized hot plasma equilibria in a gravitational field. The eikonal analysis presented finds that magnetorotational stability analysis must be performed with some care to retain compressibility and density gradient effects, and departures from strict Keplerian motion. Indeed, retaining these effects highlights differences between the magnetorotational instability found in the absence of gravity [Velikhov 1959 \textit{Sov. Phys. JETP} \textbf{36}, 995-998] and that found the presence of gravity [Balbus and Hawley 1991 \textit{Astrophys. J.} \textbf{376}, 214-222]. In the non-gravitational case, compressibility and density variation alter the stability condition, while these effects only enter for departures from strict Keplerian motion in a gravitational field. The conditions for instability are made more precise by employing recent magnetized equilibrium results [Catto, Pusztai and Krasheninnikov 2015 \textit{J. Plasma Phys.} \textbf{81}, 515810603], rather than employing a hydrodynamic equilibrium. We focus on the stability of the $\beta $ \textgreater 1 limit for which equilibria were found in the absence of a toroidal magnetic field, where $\beta \quad =$ plasma/magnetic pressure. [Preview Abstract] |
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TP10.00050: PIC simulations of the MagnetoRotational instability in electron-positron plasmas Giannandrea Inchingolo, Thomas Grismayer, Nuno F. Loureiro, Ricardo A. Fonseca, Luis O. Silva The magnetorotational instability (MRI) is a crucial mechanism of angular momentum transport in a variety of astrophysical scenarios, as e-e+ plasmas accretion disks nearness neutron stars and black holes. The MRI has been widely studied using MHD models and simulations, in order to understand the behavior of astrophysical fluids in a state of differential rotation. When the timescale for electron and ion collisions is longer than the inflow time in the disk, the plasma is macroscopically collisionless and MHD breaks down. This is the case of the limit of weak magnetic field, i.e., as the ratio of the ion cyclotron frequency to orbital frequency becomes small. Leveraging on the recent addition of the shearing co-rotating frames equations of motion and Maxwell's equations modules in our PIC code OSIRIS 3.0, we intend to present our recent results of the analysis of MRI in electron-positron plasma in the limit of weak magnetic field. We will recall the theoretical 1D linear model of Krolik et Zweibel that describes the behavior of MRI in the limit of weak magnetic field and use it to support our results. Moving to 2D simulations, the analysis of MRI via PIC code permits to investigate also how MRI will act in comparison with other Kinetic instabilities, like mirror instability. [Preview Abstract] |
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TP10.00051: Large scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability Pallavi Bhat, Fatima Ebrahimi, Eric G. Blackman We study dynamo generation (exponential growth) of large scale (planar averaged) fields in the in shearing box simulations of magnetorotational instability (MRI). By computing space-time planar averaged fields and power spectra, we find large scale dynamo action in early MRI growth phase, a previously unidentified feature. Non-axisymmetric linear MRI modes with low horizontal wavenumbers and vertical wavenumbers near that of expected maximal growth, amplify the large scale fields exponentially before turbulence and high wavenumber fluctuations arise. Thus the large scale dynamo requires only linear fluctuations but not nonlinear turbulence (or mode-mode coupling). In contrast to previous studies restricted to horizontal ($x$-$y$) averaging, we also show the presence of large scale fields when vertical ($y$-$z$) averaging is employed instead. We compute the terms in the mean field equations to identify the contributions to large scale field growth in both types of averaging. The large scale fields obtained from vertical averaging are found to match well with global simulations and quasilinear analytical analysis from a previous study by Ebrahimi \& Blackman. We discuss implications of our new results for understanding large scale MRI dynamo saturation and turbulence. [Preview Abstract] |
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TP10.00052: A generalized model for small scale dynamo at finite correlation times Kandaswamy Subramanian, Pallavi Bhat Fluctuation (or small scale) dynamos are generic in astrophysical plasmas. The Kazantsev model of the fluctuation dynamo assumes a delta-correlated in time velocity field, which is unrealistic. Using renewing flows with finite time correlation, $\tau$, we derive a generalized model of the fluctuation dynamo. We recover the standard Kazantsev equation for the evolution of longitudinal magnetic correlation, $M_L$, for $\tau \to 0$. The generalized equation involves third and fourth spatial derivatives of $M_L$ indicating nonlocality. We solve the equation in the large $k$ limit and by using Landau-Lifschitz approach, we recast the equation to one which involves at most second derivatives of $M_L$. Remarkably, we then find that the magnetic power spectrum, remains the Kazantsev spectrum of $M(k) \propto k^{3/2}$, in the large $k$ limit, independent of $\tau$. [Preview Abstract] |
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TP10.00053: Integrated mechanism that both removes accretion disk angular momentum and drives astrophysical jets Paul Bellan Using concepts from laboratory experiments, Hamiltonian mechanics, Hall MHD, and weakly ionized plasmas, I propose a mechanism [1] that simultaneously drives astrophysical jets and removes accretion disk angular momentum. The mechanism depends on the extreme stratification of ionization between the weakly ionized accretion disk and the highly ionized exterior region. In the exterior region, axisymmetric Hamiltonian mechanics constrain charged particles to move on nested poloidal flux surfaces. In contrast, fluid elements in the weakly ionized, highly collisional accretion disk behave like \textit{collisionless meta-particles} with effective $q/m$ reduced from than that of an ion by the nominal disk 10$^{\mathrm{-15}}$ -- 10$^{\mathrm{-8\thinspace }}$fractional ionization; this means that the meta-particle effective cyclotron frequency $\omega _{\mathrm{c}}$ can be of order of the Kepler frequency $\omega _{\mathrm{K}}=$ (\textit{MG/r}$^{3})^{\mathrm{1/2}}$. Meta-particles with $\omega _{\mathrm{c}} \quad = \quad -$2$\omega_{\mathrm{K}}$ have zero canonical angular momentum, experience no centrifugal force and spiral in towards the central body. Because these inward spiraling meta-particles are positive, their accumulation near the central body produces radially and axially outward electric fields. The axial outward electric field drives an out-of-plane poloidal electric current along poloidal flux surfaces in the external region. As in lab experiments, this current and its associated toroidal magnetic field drive astrophysical jets flowing normal to and away from the disk. [1] Bellan, P.M., Monthly Notices Royal Astronomical Society \textbf{458}, 4400 (2016) [Preview Abstract] |
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TP10.00054: Instrumental Implementation of an Experiment to Demonstrate $\alpha\omega$-dynamos in Accretion Disks Jiahe Si, Richard Sonnenfeld, Art Colgate, Hui Li, Mark Nornberg The New Mexico Liquid Metal $\alpha\omega$-dynamo experiment is aimed to demonstrate a galactic dynamo. Our goal is to generate the $\omega$-effect and $\alpha$-effect by two semi-coherent flows in laboratory. Two coaxial cylinders are used to generate Taylor-Couette flows to simulate the differential rotation of accretion disks. Plumes induced by jets injected into the Couette flows are expected to produce helicities necessary for the $\alpha$-effect. We have demonstrated an 8-fold poloidal-to-toroidal flux amplification from differential rotation (the $\omega$-effect) by minimizing turbulence in our apparatus. To demonstrate the $\alpha$-effect, the experimental apparatus is undergoing significant upgrade. We have constructed a helicity injection facility, and are also designing and testing a new data acquisition system capable of transmitting data in a high speed rotating frame. Additional magnetic field diagnostics will also be included. The upgrade is intended to answer the question of whether a self-sustaining $\alpha\omega$-dynamo can be constructed with a realistic fluid flow field, as well as to obtain more details to understand dynamo action in highly turbulent Couette flow. [Preview Abstract] |
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TP10.00055: Exploring the effect of conducting endcaps on the Princeton MRI Experiment Kyle Caspary, Erik Gilson, Jeremy Goodman, Hantao Ji, Peter Sloboda The magnetorotational instability (MRI) is believed to be the primary mechanism which generates the turbulence required to explain the rapid accretion rates observed in some magnetized accretion disks. The Princeton MRI experiment is a modified Taylor-Couette device which uses GaInSn eutectic as a working fluid to study rotating MHD Flows. Diagnostics include Ultrasound Doppler Velocimetry (UDV) for flow profile measurements and an array of magnetic Hall sensors located on the inner and outer cylinders. Results are presented from experiments with conducting endcaps which were installed in order to increase the saturation amplitude of the MRI signal and the angular momentum coupling to the fluid*. The effect of conducting endcaps on Shercliff layer instabilities** is examined with a comparison to previous results with insulating endcaps. With sufficient velocity shear and magnetic field strength, the fluid exerts a torque on the conducting endcaps due to a coupling via the magnetic field. The onset criterion of this torque is currently under investigation. Motivated by results from the spectral/finite-element code SFEMaNS, the inner ring speed was varied in order to minimize the contribution to the radial magnetic field measurements from non-MRI sources such as Ekman flows. * Wei et al. submitted to PRE (2016) ** Roach et al. PRL 2012 [Preview Abstract] |
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TP10.00056: Numerical Prediction of the Onset of the Magnetorotational Instability in the Princeton MRI Apparatus Erik Gilson, Kyle Caspary, Fatima Ebrahimi, Jeremy Goodman, Hantao Ji, Tahiri Nu\~{n}ez, Xing Wei The liquid metal magnetorotational instability experiment at PPPL is designed to search for the MRI in a controlled laboratory setup. MRI is thought to be the primary mechanism behind turbulence in accretion disks, leading to an enhanced effective viscosity that can explain observed fast accretion rates. The apparatus has several key differences from an accretion disk. The top and bottom surfaces of the vessel exert stresses on the surfaces of the working fluid. There are no surface stresses on an accretion disk, but rather a free-surface. To interpret experimental results, the Spectral Finite Element Maxwell and Navier Stokes (SFEMaNS) code (Guermond et al., 2009) has been used to simulate experiments in the MRI apparatus and study MRI onset in the presence of residual flows induced by the boundaries. These Ekman flows lead to the generation of radial magnetic fields that can obfuscate the MRI signal. Simulation results are presented that show the full spatial distribution of the velocity field and the magnetic field over a range of experimental operating parameters, including both above and below the expected MRI threshold. Both the residual flow and the radial magnetic field at the location of the diagnostics are computed for comparisons with experimental results. [Preview Abstract] |
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TP10.00057: Tethered mass-spring experiment in a quasi-Keplerian Taylor-Couette device Derek Man Hon Hung, Steven Stemmley, Kyle Caspary, Erik Gilson, Peter Sloboda, Hantao Ji, Eric Blackman Angular momentum transport in astrophysical accretion disks is primarily attributed to the magnetorotational instability (MRI). High electrical conductivity in these disks causes magnetic field lines to be “frozen-into” matter, and spring-like magnetic tension between neighboring fluid elements arises. The tethered mass-spring model is commonly used to describe this system. We attempt to demonstrate this analog in the laboratory. The behavior of such a model is explored in a quasi-Keplerian Taylor-Couette device with neutrally buoyant test masses, metal springs, and water as the medium. Masses, spring strengths, and flow profiles are varied to investigate stability conditions. Motion capture and video analysis are utilized to examine the trajectories of test masses. Results obtained are presented and compared to predictions from MRI theory. Complications from hydrodynamic turbulence, secondary flows, and finite-size effects are discussed. Corresponding mitigation efforts are also proposed. [Preview Abstract] |
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TP10.00058: Non-thermal Plasmas Around Black Holes, New Configurations, Magnetic Field Generation and Relevant Collective Modes* M. Asgari-Targhi, B. Coppi The radiation emission from Shining Black Holes is most frequently observed to have non-thermal features. It is therefore appropriate to consider relevant collective processes of plasmas surrounding black holes [1] that contain high energy particles with non-thermal distributions in momentum space. For simplicity we use a fluid description considering the case where significant temperature anisotropies are present. These anisotropies are shown to have a critical influence on: a) the existence and characteristics of stationary plasma and field configurations; b) the excitation of magneto-gravitational modes driven by temperature anisotropies and differential rotation; c) the generation of magnetic fields over macroscopic scale distances; d) the outward transport of angular momentum. *Sponsored in part by the U.S. D.O.E. [1] B. Coppi, Phys. of Plasmas, 18, 032901 (2011). [Preview Abstract] |
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TP10.00059: Modeling production of $e^\pm$-pair plasma in AGNs Alex Ford, Mikhail V. Medvedev Processes around spinning supermassive black holes in active galactic nuclei (AGN) are believed to determine how relativistic jets are launched and how the black hole energy is extracted. The key question in these processes is the origin of plasma in black hole magnetospheres. The only reasonable mechanism is believed to be the electron-position cascade -- the multistage process involving seed photons from an accretion disk, which are Compton up-scattered by charges accelerated in a gap region of a force-free magnetosphere with subsequent photon-photon pair production. In order to explore the process of the $e^\pm$ plasma production, we developed a numerical code which models the dynamics of the cascade along magnetic field lines. We demonstrate that plasma production is sensitive to the spectrum of the ambient photon and magnetic fields, the black hole mass and spin, and other parameters. We discuss the results and observational predictions. [Preview Abstract] |
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TP10.00060: Simulations of Rayleigh Taylor Instabilities in the presence of a Strong Radiative shock Matthew Trantham, Carolyn Kuranz, Dov Shvarts, R. P. Drake Recent Supernova Rayleigh Taylor experiments on the National Ignition Facility (NIF) are relevant to the evolution of core-collapse supernovae in which red supergiant stars explode. Here we report simulations of these experiments using the CRASH code. The CRASH code, developed at the University of Michigan to design and analyze high-energy-density experiments, is an Eulerian code with block-adaptive mesh refinement, multigroup diffusive radiation transport, and electron heat conduction. We explore two cases, one in which the shock is strongly radiative, and another with negligible radiation. The experiments in all cases produced structures at embedded interfaces by the Rayleigh Taylor instability. The weaker shocked environment is cooler and the instability grows classically. The strongly radiative shock produces a warm environment near the instability, ablates the interface, and alters the growth. We compare the simulated results with the experimental data and attempt to explain the differences. [Preview Abstract] |
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TP10.00061: Probing particle acceleration in lower hybrid turbulence via synthetic diagnostics produced by PIC simulations F. Cruz, R.A. Fonseca, L.O. Silva, A. Rigby, G. Gregori, R.A. Bamford, R. Bingham, M. Koenig Efficient particle acceleration in astrophysical shocks can only be achieved in the presence of initial high energy particles. A candidate mechanism to provide an initial seed of energetic particles is lower hybrid turbulence (LHT). This type of turbulence is commonly excited in regions where space and astrophysical plasmas interact with large obstacles. Due to the nature of LH waves, energy can be resonantly transferred from ions (travelling perpendicular to the magnetic field) to electrons (travelling parallel to it) and the consequent motion of the latter in turbulent shock electromagnetic fields is believed to be responsible for the observed x-ray fluxes from non-thermal electrons produced in astrophysical shocks. Here we present PIC simulations of plasma flows colliding with magnetized obstacles showing the formation of a bow shock and the consequent development of LHT. The plasma and obstacle parameters are chosen in order to reproduce the results obtained in a recent experiment conducted at the LULI laser facility at Ecole Polytechnique (France) to study accelerated electrons via LHT. The wave and particle spectra are studied and used to produce synthetic diagnostics that show good qualitative agreement with experimental results. [Preview Abstract] |
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TP10.00062: QED-PIC simulations of electromagnetic cascades at the surface of pulsar's polar cap. Thomas Grismayer, Marija Vranic, Ricardo Fonseca, Luis Silva The recent implementation of the QED module in the OSIRIS 3.0 framework has enabled to simulate various scenarios where pair production or gamma-rays emission can be produced with ultra-intense lasers and/or relativistic particles beams [1,2]. In this study we leverage on these numerical tools to study extreme astrophysical scenarios where self-consistent produced electron-positron pair plasmas are of relevance such as in pulsar magnetospheres. The dynamics of pulsar's polar cap cascade, based on the Ruderman-Sutherland model, has been investigated for the first time numerically in one dimension by Timokhin [3]. Including quantum synchrotron radiation additionally to curvature photon radiation for the possible processes responsible for photon emission, we present the results of one and two dimensional QED-PIC simulations of the development of electromagnetic cascades at the surface of the polar cap and the subsequent plasma discharges that are accompanied by strong electrostatic waves. [1] M. Vranic et al., NJP, in press (2016) [2] T. Grismayer et al., Physics of Plasmas 23, 056706 (2016) [3] A. N. Timokhin, Mon. Not. R. Astron. Soc. 408, 20922114 (2010) [Preview Abstract] |
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TP10.00063: Estakhr's Proper-Time Averaged of Material-Geodesic Equations (an umberella term equation for Relativistic Astrophysics, Relativistic Jets, Gamma-Ray Burst, Big Bang Hydrodynamics, Supernova Hydrodynamics) Ahmad Reza Estakhr ${\frac{D\overline{J}^{\mu}}{D\tau}=\overbrace{\overline{J}^{\nu}\partial_{\nu}\overline{U}^{\mu}+\partial_{\nu}\overline{T}^{\mu\nu}+\Gamma^{\mu}_{\alpha\beta}\overline{J}^{\alpha}\overline{U}^{\beta}}^{\text{Steady Component}}+\overbrace{\partial_{\nu}R^{\mu\nu}+\Gamma^{\mu}_{\alpha\beta}R^{\alpha\beta}}^{Perturbations}}$ EAMG equations are proper time-averaged equations of relativistic motion for fluid flow and used to describe Relativistic Turbulent Flows. [Preview Abstract] |
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TP10.00064: SCARE-OFF LAYERS |
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TP10.00065: Fusion Ash Separation in the Princeton Field-Reversed Configuration Reactor Joseph Abbate, Meagan Yeh, Nick McGreivy, Samuel Cohen The Princeton Field-Reversed Configuration (PFRC) concept relies on low-neutron production by D-3He fusion to enable small, safe nuclear-fusion reactors to be built, an approach requiring rapid and efficient extraction of fusion ash and energy produced by D-3He fusion reactions. The ash exhaust stream would contain energetic (0.1-1 MeV) protons, T, 3He, and 4He ions and nearly 1e5 cooler (ca. 100 eV) D ions. The T extracted from the reactor would be a valuable fusion product in that it decays into 3He, which could be used as fuel. If the T were not extracted it would be troublesome because of neutron production by the D-T reaction. This paper discusses methods to separate the various species in a PFRC reactor’s exhaust stream. First, we discuss the use of curved magnetic fields to separate the energetic from the cool components. Then we discuss exploiting material properties, specifically reflection, sputtering threshold, and permeability, to allow separation of the hydrogen from the helium isotopes. [Preview Abstract] |
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TP10.00066: What sets the minimum tokamak scrape-off layer width? Ilon Joseph The heat flux width of the tokamak scrape-off layer is on the order of the poloidal ion gyroradius, but the “heuristic drift” physics model [1] is still not completely understood. In the absence of anomalous transport, neoclassical transport sets the minimum width. For plateau collisionality, the ion temperature width is set by $q\rho_i$, while the electron temperature width scales as the geometric mean $q(\rho_e\rho_i)^{1/2}$ and is close to $q\rho_i$ in magnitude. The width is enhanced because electrons are confined by the sheath potential and have a much longer time to radially diffuse before escaping to the wall. In the Pfirsch-Schluter regime, collisional diffusion increases the width by the factor $(qR/\lambda)^{1/2}$ where $qR$ is the connection length and $\lambda$ is the mean free path. This qualitatively agrees with the observed transition in the scaling law for detached plasmas [2]. The radial width of the SOL electric field is determined by Spitzer parallel and “neoclassical” radial electric conductivity and has a similar scaling to that for thermal transport. [1] R. J. Goldston, Nucl. Fusion 52, 013009 (2012). [2] H. J. Sun, et al., Plasma Phys. Control. Fusion 57, 125011 (2015). [Preview Abstract] |
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TP10.00067: Experimental test of the mass dependence of the heuristic drift model M.A. Makowski, T.H. Osborne, A.W. Leonard, A. Bortolon, R. Nazikian An empirical scaling for the heat flux scrape-off-layer (SOL) width has been developed [1], but the physics setting the heat flux width has yet to be established. However, a heuristic model [2] for the width scaling has been developed which is consistent with currently available data. Recent experiments in helium discharges and in which the pedestal and SOL had ${> 75}\%$ impurity (He, Li, C, N, or Ne) content, provide a means of testing the mass scaling of the heuristic model. Preliminary results indicate either no or a weakly negative scaling of the measured heat flux width with mass. This result lies between the predicted approximate $\sqrt{\bar{A}/\bar{Z}}$ (electron drift dominated) and $\sqrt{\bar{A}/\bar{Z^3}$ (ion drift dominated) dependencies predicted by the model, indicating a complicated interplay between the electrons and multiple ion species present in the SOL. These and other results related to the heuristic model will be presented.\par \vskip3pt \noindent [1] T. Eich, et al., Phys. Rev. Lett. 107 (2011) 215001.\par \noindent [2] R.J. Goldston, Nuc. Fusion 52 (2012) 013009. [Preview Abstract] |
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TP10.00068: Progress in snowflake divertor research in DIII-D, NSTX and NSTX-U V A Soukhanovskii, S Allen, M Fenstermacher, O Izacard, C Lasnier, M Makowski, A McLean, W Myer, D Ryutov, F Scotti, D Eldon, E Kolemen, P Vail, G Canal, R Groebner, A Hyatt, A Leonard, T Osborne, R Bell, A Diallo, S Gerhardt, S Kaye, B LeBlanc, J Menard, M Podesta Recent snowflake (SF) divertor DIII-D experiments focused on divertor heat transport under attached and radiative divertor conditions, incl 1-understanding of increased scrape-off layer width in SF-plus configuration at lower densities; 2-particle, heat and radiation distribution in the SF divertor with CD$_4$ seeding. NSTX data was analyzed to understand the link between SF divertor and ELM (de)stabilization with and without CD$_4$ seeding and lithium conditioning. Prep for SF divertor experiments in NSTX-U include 1-equilibria modeling with ISOLVER code using various sets of divertor coils and L- and H-mode plasma scenarios; 2-transport and impurity radiation modeling with UEDGE code; 3-new diagnostics (ie-a 100-200 kHz camera for null-region mode observations). [Preview Abstract] |
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TP10.00069: Finding the magnetic field structure of a snowflake divertor from a limited set of the input parameters D.D. Ryutov, V.A. Soukhanovskii One of the steps in developing the control systems for magnetic divertors is a quick reconstruction of their magnetic topology from a limited set of the input parameters. Normally used as these input parameters are the poloidal field (PF) components at several spatial points. These PF data can be obtained from the magnetic equilibrium code output or (conceptually) from the direct PF measurements. In the case of a snowflake (SF) divertor with its two nearby nulls, the field structure is characterized by the presence of multiple separatrices that complicates the reconstruction process. We describe a technique that makes possible an unambiguous identification of the configuration, in particular, distinguishing between SF-plus vs. SF-minus configurations. The technique is based on the consistent use of a complex representation of the poloidal field, and its description in terms of a flux function and scalar potential. The disambiguation of topological features of the magnetic configuration is based on the information on the direction of the plasma current. Several examples of the application of this technique to specific configurations of the SF family are presented. Work performed for U.S. DOE by LLNL under Contract DE-AC52-07NA27344; supported by the U.S. DOE OFES.. [Preview Abstract] |
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TP10.00070: Resilient non-resonant divertors for stellarators A. Bader, A. H. Boozer, C.C. Hegna, S.A. Lazerson In this work, we investigate whether resilient non-resonant divertor solutions exist for optimized stellarators. Resiliency is measured by the consistency of performance over a broad range of operational states, such as through bootstrap current and modified plasma pressures. A non-resonant configuration is one where the crucial topological feature is the existence and sharpness of ridges along the last closed flux surface. We develop a modified field-line following method for testing the resiliency of stellarator divertors and apply it to altered HSX configurations generated by varying external coil currents, wall positioning, and internal plasma currents. We compare a magnetic diffusion calculation with a ``zero-diffusion'' calculation that endeavors to measure the first escaping flux tubes. The results from these calculations are corroborated with a more complete edge simulation with EMC3-EIRENE. The EMC3-EIRENE simulations show resilient helical stripes that are consistent with the simpler field line following methods. The goal of the study is to find a metric for edge/divertor optimization of stellarators, a crucial piece that is missing from current optimization schemes. [Preview Abstract] |
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TP10.00071: Local temperature effects in the helical scrape-off layer of startup plasmas at Wendelstein 7-X due to N seeding Tullio Barbui, F. Effenberg, O. Schmitz, M. Krychowiak, R. K\"onig, S. Klose, D. Zhang, J. M. Mu\~noz Burgos, P. Drews, Y. Liang, S. Liu, O. Neubauer, A. Terra, B. Schweer, B.D. Blackwell N seeding discharges have been performed at Wendelstein 7-X during its startup limiter campaign. In this study, the cooling effects on the local electron temperature Te measured by three diagnostic systems are discussed, which have a defined alignment in the helical SOL topology during the W7-X limiter phase. Radial Te profiles obtained from a thermal helium beam and Te measurements from a reciprocating Langmuir probe system, both located inside the same flux tube as the N injection system, are compared to Te from Langmuir probes installed on a limiter tile, which is not directly connected magnetically to the injection flux channel. This setup enables to study the N cooling effect in the flux tube geometry as an important test which impacts the 3-D topology in a low shear stellarator and the edge cooling symmetry. A clear reduction of Te in the entire SOL has been measured as a reaction to the N seeding and the relative variation in the temperature, however, depends on the actual placing of the diagnostic in the flux tube geometry and on the distances from the N injection point. A direct comparison of the measurements to EMC3-EIRENE modeling will be presented. [Preview Abstract] |
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TP10.00072: 3-D plasma boundary and plasma wall interaction research at UW-Madison Oliver Schmitz, Adrian Akerson, Aaron Bader, Tullio Barbui, Florian Effenberg, Kurt Flesch, Heinke Frerichs, Jonathan Green, Edward Hinson, Thierry Kremeyer, Ryan Norval, Laurie Stephey, Ian Waters, Victoria Winters The necessity of considering 3-D effects on the plasma boundary and plasma wall interaction (PWI) in tokamaks, stellarators and reversed field pinches has been highlighted by abundant experimental and numerical results in the recent past. Prominent examples with 3-D boundary situations are numerous: ELM controlled H-modes by RMP fields in tokamaks, research on boundary plasmas and PWI in stellarators in general, quasi-helical states in RFPs, asymmetric fueling situations, and structural and wall elements which are not aligned with the magnetic guiding fields. A systematic approach is being taken at UW-Madison to establish a targeted experimental basis for identifying the most significant effects for plasma edge transport and resulting PWI in such 3-D plasma boundary situations. We deploy advanced 3-D modeling using the EMC3-EIRENE, ERO and MCI codes in combination with laboratory experiments at UW-Madison to investigate the relevance of 3-D effects in large scale devices with a concerted approach on DIII-D, NSTX-U, and Wendelstein 7-X. Highlights of experimental results from the on-site laboratory activities at UW-Madison and the large scale facilities are presented and interlinks will be discussed. - This work was supported by US DOE DE-SC0013911, DE-SC00012315 and DE-SC00014210. [Preview Abstract] |
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TP10.00073: Measurement of limiter particle fluxes and carbon erosion in the helical scrape-off layer of startup plasmas at W7-X V. Winters, C. Biedermann, S. Brezinsek, F. Effenberg, H. Frerichs, J. Harris, O. Schmitz, L. Stephey, E. Unterberg, G. Wurden Measurement of the 2D recycling flux and calculations of the carbon erosion from the limiter in startup plasmas of W7-X provides a first insight into neutral particle release and impurity inflow into the helical scrape-off layer. H-alpha, C-II (514.5nm) and C-III (465.1nm) line emissions were collected with filter-scopes and a visible camera aimed at limiter 3 of W7-X. Local plasma parameters are considered to estimate physical and chemical sputtering contributions. The analytical model for chemical sputtering by Roth is used to convert the measured particle flux into a chemically eroded C flux. The particle flux as well as the extracted C erosion pattern deviates from the measured heat flux distribution and also from the predicted particle flux distribution from EMC3-EIRENE. Candidates to resolve this discrepancy are measurement uncertainties and physics related (e.g. asymmetry in the last closed flux surface position). Post-mortem analysis of the limiter will be taken into account and compared to these in-situ measurements to gather first detailed insight on the net C erosion distribution and the impurity sourcing into the helical scrape-off layer. [Preview Abstract] |
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TP10.00074: Synthetic reconstruction of recycling on the limiter during startup phase of W7-X based on EMC3-EIRENE simulations Heinke Frerichs, Florian Effenberg, Oliver Schmitz, Laurie Stephey Interpretation of spectroscopic measurements in the edge region of high-temperature plasmas can be a challenge due to line of sight integration effects. The EMC3-EIRENE code - a 3D fluid edge plasma and kinetic neutral gas transport code - is a suitable tool for full 3D reconstruction of such signals. A versatile synthetic diagnostic module has been developed recently which allows the realistic three dimensional setup of various plasma edge diagnostics to be captured. We present an analysis of recycling on the inboard limiter of W7-X during its startup phase in terms of a synthetic camera for $H_\alpha$ light observations and reconstruct the particle flux from these synthetic images based on ionization per photon coefficients (S/XB). We find that line of sight integration effects can lead to misinterpretation of data (redistribution of particle flux due to neutral gas diffusion), and that local plasma effects are important for the correct treatment of photon emissions. [Preview Abstract] |
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TP10.00075: High-density operation of the Proto-MPEX High Intensity Plasma Source J.B.O. Caughman, R.H. Goulding, T.M. Biewer, T.S. Bigelow, I.H. Campbell, J. Caneses, S.J. Diem, E.H. Martin, C.M. Parish, J. Rapp, H.B. Ray, G.C. Shaw, M.A. Showers, D. Donovan, P.A. Piotrowicz, D.C. Martin The Prototype Materials Plasma Experiment (Proto-MPEX) is a linear high-intensity RF plasma source that combines a high-density helicon plasma generator with ion and electron heating sections. It is being used to study the physics of heating over-dense plasmas in a linear configuration with the goal of delivering a plasma heat flux of \textasciitilde 10 MW/m$^{\mathrm{2}}$ at a target. The helicon plasma is produced by coupling 13.56 MHz RF power at levels of \textgreater 100 kW. A \textasciitilde 30 kW ion cyclotron antenna has recently been installed, and microwaves at 28 GHz (\textasciitilde 150 kW) are coupled to the electrons in the over-dense helicon plasma via Electron Bernstein Waves (EBW). High plasma densities near the target have been produced in D (\textasciitilde 5 x10$^{\mathrm{19}}$/m$^{\mathrm{3}})$, and electron temperatures range from 3 to \textgreater 10 eV, depending on the source parameters. IR camera images of the target plate indicate plasma heat depositions \textgreater 10 MW/m$^{\mathrm{2}}$ for some operating conditions. Details of the experimental results of the operational domain with respect to T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ as well as results from initial plasma material interaction tests will be presented. [Preview Abstract] |
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TP10.00076: Helicon wave field measurements in Proto-MPEX Juan Francisco Caneses, Pawel Piotrowicz, Richard Goulding, John Caughman, Missy Showers, Nischal Kafle, Juergen Rapp, Ian Campbell A high density Deuterium discharge (ne \textasciitilde 5e19 m-3, Te \textasciitilde 4 eV) has been recently observed in ProtoMPEX (Prototype Material Plasma Exposure eXperiment). The discharge (100 kW, 13.56 MHz, D2, 700 G at the source, 1e4 G at the Target) begins with a low density plasma with hollow Te profile and transitions in about 100 ms to a high density mode with flat Te profile. It is believed that the transition to the high density mode is produced by a ``helicon resonance'' as evidenced by the centrally-peaked power deposition profile observed with IR imaging on a 2 mm thick metallic target plate. In this work, we present b-dot probe measurements of the radial helicon wavefields 30 cm downstream of the antenna during both the low and high density modes. In addition, we compare the experimental results with full wave simulations. [Preview Abstract] |
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TP10.00077: ECH/EBW Plasma Coupling and Heating Experiments on the Proto-MPEX Tim Bigelow, John Caughman, Juan Caneses, Stephanie Diem, Richard Goulding, Nischal Kafle, Juergen Rapp ECH and EBW have been under development on the Proto-Material Plasma Exposure eXperiment device (Proto-MPEX) to provide additional plasma electron heating. Proto-MPEX$^{\mathrm{1}}$ has a linear magnetic field configuration and a helicon plasma source that forms a high-density medium-temperature central core plasma of typically 0.08m diameter$^{\mathrm{2}}$. A plasma density of up to 6x10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$ is generated which is \textgreater 6 times over-dense for 28 GHz microwave power available from the experiment's gyrotron system. Modeling using Genray-C code$^{\mathrm{3}}$ has indicated that some heating of the plasma core should be possible at this frequency using the optimum O-X-EBW coupling scheme. Several improvements to the waveguide system have been made to increase the reliable operating power level and launch beam quality. To improve the plasma heating efficiency, work is underway to optimize the beam launch by adding a remotely adjustable launch angle, adding a polarization rotating miter bend, moving the launch point closer to the plasma edge and providing some beam focusing. Preliminary heating experiments have indicated some over-dense heating has been achieved. A launch power of \textasciitilde 75 kW has been achieved out of a possible 150 kW. [1] Rapp, J [2] Caughman, J.B, [3] Diem, S. (see posters this session) [Preview Abstract] |
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TP10.00078: EBW Propagation and Damping in Proto-MPEX Steffi Diem, T. Bigelow, J.F. Caneses, J.B.O. Caughman, R.H. Goulding, D.L. Green, R.W. Harvey, Yu.V. Petrov, J. Rapp Linear plasma devices, such as Proto-MPEX, are an economic method to study plasma-material interactions under high heat and particle fluxes. The Proto-MPEX device at ORNL is a high-density helicon plasma generator with additional resonant electron heating to study plasma-material interactions in ITER-like conditions. Heating of the overdense plasma is being studied using a microwave-based system. Power levels up to 100 kW at 28 GHz are injected into low T$_{e}$, high n$_{e}$ plasma. A modified version of the GENRAY ray-tracing code, GENRAY-C, has been used to determine EBW and ECH wave accessibility for these overdense plasmas. Modeling has shown that greater than 80\% mode conversion to EBW is possible near the 28 GHz fundamental and second harmonic resonance locations. Comparisons of experimental and modeling results will be presented for a variety of plasma configurations. Calculations show a strong dependence on the electron density gradient and injection angle. [Preview Abstract] |
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TP10.00079: Ion Cyclotron Heating on Proto-MPEX R. H. Goulding, J. B. O. Caughman, J. Rapp, T. M. Biewer, I. H. Campbell, J. F. Caneses, N. Kafle, H. B. Ray, M. A. Showers, P. A. Piotrowicz Ion cyclotron heating will be used on Proto-MPEX (Prototype Material Plasma Exposure eXperiment) to increase heat flux to the target, to produce varying ion energies without substrate biasing, and to vary the extent of the magnetic pre-sheath for the case of a tilted target. A 25 cm long, 9 cm diameter dual half-turn helical ion cyclotron antenna has been installed in the device located at the magnetic field maximum. It couples power to ions via single pass damping of the slow wave at the fundamental resonance, and operates with $\omega \sim 0.8\ \omega_{ci}$ at the antenna location. It is designed to operate at power levels up to 30 kW, with a later 200 kW upgrade planned. Near term experiments include measuring RF loading at low power as a function of frequency and antenna gap. The plasma is generated by a helicon plasma source that has achieved $n_e > 5 \times 10^{19} m^{-3}$ operating with deuterium, as measured downstream from the ion cyclotron antenna location. Measurements will be compared with 1-D and 2-D models of RF coupling. The latest results will be presented. [Preview Abstract] |
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TP10.00080: Characterization of ion fluxes and heat fluxes for PMI relevant conditions on Proto-MPEX*~ Clyde Beers, Guinevere Shaw, Theodore Biewer, Juergen Rapp Plasma characterization, in particular, particle flux and electron and ion temperature distributions nearest to an exposed target, are critical to quantifying Plasma Surface Interaction (PSI). In the Proto-Material Plasma Exposure eXperiment (Proto-MPEX), the ion fluxes and heat fluxes are derived from double Langmuir Probes (DLP) and Thomson Scattering in front of the target assuming Bohm conditions at the sheath entrance. Power fluxes derived from ne and Te measurements are compared to heat fluxes measured with IR thermography. The comparison will allow conclusions on the sheath heat transmission coefficient to be made experimentally. Different experimental conditions (low and high density plasmas (0.5 - 6 x 10$^{\mathrm{19}}$ m$^{\mathrm{-3}})$ with different magnetic configuration are compared. [Preview Abstract] |
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TP10.00081: Particle and heat flux estimates in Proto-MPEX in Helicon Mode with IR imaging* M.A. Showers, T.M. Biewer, J.B.O Caughman, D.C. Donovan, R.H. Goulding, J. Rapp The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at Oak Ridge National Laboratory (ORNL) is a linear plasma device developing the plasma source concept for the Material Plasma Exposure eXperiment (MPEX), which will address plasma material interaction (PMI) science for future fusion reactors. To better understand how and where energy is being lost from the Proto-MPEX plasma during ``helicon mode'' operations, particle and heat fluxes are quantified at multiple locations along the machine length. Relevant diagnostics include infrared (IR) cameras, four double Langmuir probes (LPs), and in-vessel thermocouples (TCs). The IR cameras provide temperature measurements of Proto-MPEX's plasma-facing dump and target plates, located on either end of the machine. The change in surface temperature is measured over the duration of the plasma shot to determine the heat flux hitting the plates. The IR cameras additionally provide 2-D thermal load distribution images of these plates, highlighting Proto-MPEX plasma behaviors, such as hot spots. The LPs and TCs provide additional plasma measurements required to determine particle and heat fluxes. Quantifying axial variations in fluxes will help identify machine operating parameters that will improve Proto-MPEX's performance, increasing its PMI research capabilities. *This work was supported by the U.S. D.O.E. contract DE-AC05-00OR22725. [Preview Abstract] |
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TP10.00082: Spatial and Temporal characterization of plasma properties via emission spectroscopy in fusion materials testing device Proto-MPEX* Casey Morean, Theodore Biewer, Guinevere Shaw, Josh Beers, Holly Ray The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma source, and is intended to study plasma-material interactions (PMI) in conditions similar to those found in future fusion reactors. A high-resolution McPherson Czerny-Turner visible range spectrometer has been utilized to study the behavior of ions in the plasma. Analysis of the spectral lines, D\textunderscore beta, D\textunderscore gamma, and D\textunderscore delta yields valuable information regarding the temperature and density of plasma ions at various locations along Proto-MPEX. Spectroscopic temperature and density measurements are compared to double Langmuir probe measurements to determine plasma behavior as a function of radius. Temporal and spatial measurements along the length of Proto-MPEX are constructed and compared to a photomultiplier tube based diagnostic manufactured at ORNL to determine the plasma's axial behavior along Proto-MPEX. Relative emission of beta, gamma, and delta lines are used to assess recycling effects in the device. [Preview Abstract] |
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TP10.00083: Development of a Digital Holography Diagnostic for Surface Characterization at ORNL T.M. Biewer, C.E. Thomas The Fusion and Materials for Nuclear Systems Division (FMNSD) at Oak Ridge National Laboratory (ORNL), in collaboration with Third Dimension Technologies (TDT), proposes to develop a digital holography (DH) surface erosion/deposition diagnostic for imaging 3D plasma facing component (PFC) surfaces in real time. Digital holography is a technique that utilizes IR lasers reflected from a material surface to form a holographic interferogram, which carries information about the topology of the surface when reconstructed. The interrogated surface (at a distance of ~ 3 m) is a region of ~ 2.3 cm x 2.3 cm, and the surface feature resolution is ~ 10 micron or better in depth, around 1 mm transverse to the beam. This is being accomplished in a multi-staged research program at ORNL: 1) establishment of a single-laser DH system “on the bench,” 2) establishment of a dual-laser DH system “on the bench,” and 3) implementation of the dual-laser DH system on the Proto-MPEX device. The status of the diagnostic development effort will be presented. [Preview Abstract] |
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TP10.00084: Comparison of 3D ion velocity distribution measurements and models in the vicinity of an absorbing boundary oriented obliquely to a magnetic field Miguel F. Henriquez, Derek S. Thompson, Shane Kenily, Rinat Khaziev, Timothy N. Good, Julianne McIlvain, M. Umair Siddiqui, Davide Curreli, Earl E. Scime Understanding particle distributions in plasma boundary regions is critical to predicting plasma-surface interactions. Ions in the presheath exhibit complex behavior because of collisions and due to the presence of boundary-localized electric fields. Complete understanding of particle dynamics is necessary for understanding the critical problems of tokamak wall loading and Hall thruster channel wall erosion. We report measurements of 3D argon ion velocity distribution functions (IVDFs) in the vicinity of an absorbing boundary oriented obliquely to a background magnetic field. Measurements were obtained via argon ion laser induced fluorescence throughout a spatial volume upstream of the boundary. These distribution functions reveal kinetic details that provide a point-to-point check on particle-in-cell and 1D3V Boltzmann simulations. We present the results of this comparison and discuss some implications for plasma boundary interaction physics. [Preview Abstract] |
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TP10.00085: Boltzmann-BCA Analysis on the Role of Charge Exchange in Microscopic Erosion of Fusion-Relevant Plasma Facing Components Shane Keniley, Davide Curreli Charge-exchange is expected to play an important role in microscopic erosion of plasma facing components under fusion-relevant conditions. In this work we present a set of detailed Boltzmann simulations of the near-wall plasma including surface response, with the goal of highlighting the relative role of charge exchange against ion-induced erosion. The simulations reveal that the charge-exchange processes occurring in the collisional presheath release energetic neutrals toward the wall with angular distributions ranging from grazing to normal incidence; the ions accelerated across the collisional and magnetic presheath acts as a dominant factor in affecting the initial phase of the neutral population reaching the wall, and ultimately its energy-angle distribution at the surface. The effect on erosion rates, plasma sheath/presheath structure, and moments of the distributions are highlighted. The study has been made possible thanks to a newly-developed dynamically-coupled Boltzmann-BCA model retaining the effects of both the plasma and the material erosion. [Preview Abstract] |
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TP10.00086: Experimental Analysis of Nano-structures on Anode Metal Surfaces in Atmospheric Pressure Yao Kovach, John Foster Nano-structures were observed on the metal (tungsten, molybdenum) surface with helium plasma under fusion relevant plasma condition. It could bring serious problems for fusion reactors such as material erosion, dust formation and divertor lifetime etc. However, In order to solve these problems, further studies on topic of finding more unknown conditions of Nano-structure formation will be indispensable. This work focuses on the investigations of Nano-structures with its formation factors in atmospheric pressure. An electron microscopy is used to assess anode metal surface morphological changes. In particular, various Nano-structures are observed on both tungsten and stainless steel anode surfaces by the exposure to helium plasma. The characteristics of Nano-structures are documented in terms of type and size. Furthermore, material composition spectrum and mapping are used to define the status of extra growth and local area on anode metal surface with and without helium plasma effect. [Preview Abstract] |
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TP10.00087: Erosion behavior of lithium coated tungsten fuzz samples under D and He ion irradiation Anton Neff, Eric Lang, Jean Paul Allain As the primary candidate for the ITER divertor, tungsten (W) should be tailored to produce a more radiation tolerant plasma facing component (PFC). This alteration must overcome the surface microstructure changes such as bubbles, pores, fuzz, etc. that form under D and He ion irradiation in order to reduce tungsten erosion from the ITER divertor. Studies have shown that adding low Z impurities (C and Be) to a mixed D-He plasma can inhibit the growth of fuzz [1]. In contrast, previous studies have shown that low Z lithium (Li) does not inhibit fuzz production but does appear to persist on the surface among the fuzz. To further investigate this, we exposed $\sim$1000 nm Li coatings on a fuzz coated W to D and He ion bombardment. The erosion yield was measured with a quartz crystal microbalance and surface chemical changes were measured in operando with our HP-XPS system IGNIS (Ion-Gas-Neutral Interactions with Surfaces) at UIUC. Helium and D ion fluxes were $\sim$10$^{18}$ m$^{-2}$s$^{-1}$ at room temperature. After irradiation, the surfaces of the samples were characterized with scanning electron microscopy (SEM). These results will be presented along with SIMS results investigating the concentration depth profiles. [1] M.J. Baldwin et al., J. Nucl. Mater. 390–391 (2009) 886–890 [Preview Abstract] |
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TP10.00088: An Analytical/Computational Approach to the Effect of Roughness on Erosion: Global and Local Angles A. Lasa, J.M. Canik, E.A. Unterberg, J. Rapp, C. Chrobak, P.C. Stangeby Plasma-material interactions lead to erosion of plasma facing surfaces, limiting component lifetime, and leading to impurity production and plasma contamination. Surface erosion depends, among other parameters, on the impact angle, which is determined both by local plasma conditions and surface morphology. For rough surfaces ($O(\mu $m)), the ``local'' particle impact angle can differ significantly from the ``global'' impact angle defined by the average surface contour. So far, studies targeted at bridging these local and global angles have been of interpretative focus, aiming to model and understand erosion of naturally occurring surfaces following their exposure to plasma. Here, a more general study of how surface morphology impacts erosion is undertaken by deriving impact angle and density distributions for analytically described surfaces, while systematically varying the ``global angle'' and degree of roughness. These distributions are used to derive spatially resolved erosion yields, as well as estimating the impact of roughness on the total erosion. Surfaces of interest also include ones intentionally sculpted to control material surface migration. [Preview Abstract] |
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TP10.00089: Designs of LiMIT as a Limiter in the EAST Tokamak Matthew Szott, Michael Christenson, Kishor Kalathiparambil, David Ruzic Liquid metal plasma facing components (PFCs) provide a constantly refreshing, self-healing surface that can reduce erosion and thermal stress damage to prolong device lifetime, and additionally decrease edge recycling, reduce impurities, and enhance plasma performance. The Liquid Metal Infused Trench (LiMIT) system, developed at UIUC, has demonstrated thermoelectric magnetohydrodynamic (TEMHD) driven flow of liquid lithium through series of solid trenches. This TEMHD effect drives liquid lithium in fusion systems using the plasma heat flux and the toroidal magnetic field, and the surface tension of the liquid lithium maintains a fresh surface on top of the solid trenches. LiMIT has been successfully tested at UIUC as well as HT-7 and Magnum PSI at heat fluxes up to 3 MW/m$^{\mathrm{2}}$. The next step is demonstrating system viability in full-scale fusion-relevant conditions. In collaboration with a team in Hefei, design and testing has begun for a large scale LiMIT system that will act as a limiter in EAST. The designs improve upon previous versions of LiMIT tested at Illinois and incorporate lessons learned from earlier tests of liquid metal PFCs at EAST. Existing infrastructure is used to load and supply lithium to the system, and the LiMIT trenches will help maintain a smooth, fresh surface as well as aid in propelling the lithium out of direct plasma flux to improve heat transfer. [Preview Abstract] |
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TP10.00090: Simulating W Impurity Transport in Tokamaks Timothy R. Younkin, David L. Green, Ane Lasa, John M. Canik, Brian D. Wirth The extreme heat and charged particle flux to plasma facing materials in magnetically confined fusion devices has motivated Tungsten experiments such as the ``W-Ring'' experiment on the DIII-D tokamak to investigate W divertor viability. In this domain, the transport of W impurities from their tile locations to other first-wall tiles is highly relevant to material lifetimes and tokamak operation. Here we present initial results from a simulation of this W transport. Given that sputtered impurities may experience prompt redeposition near the divertor strikepoint, or migrate far from its origin to the midplane, there is a need to track the global, 3-D, impurity redistribution. This is done by directly integrating the 6-D Lorentz equation of motion (plus thermal gradient terms and relevant Monte-Carlo operators) for the impurity ions and neutrals under background plasma parameters determined by the SOLPS edge plasma code. The geometric details of the plasma facing components are represented to a fidelity sufficient to examine the global impurity migration trends with initial work also presented on advanced surface meshing capabilities targeting high fidelity simulation. [Preview Abstract] |
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TP10.00091: Modeling of transient dust events in fusion edge plasmas with DUSTT-UEDGE code R.D. Smirnov, S.I. Krasheninnikov, A.Yu. Pigarov, T.D. Rognlien It is well known that dust can be produced in fusion devices due to various processes involving structural damage of plasma exposed materials. Recent computational and experimental studies have demonstrated that dust production and associated with it plasma contamination can present serious challenges in achieving sustained fusion reaction in future fusion devices, such as ITER. To analyze the impact, which dust can have on performance of fusion plasmas, modeling of coupled dust and plasma transport with DUSTT-UEDGE code is used by the authors. In past, only steady-state computational studies, presuming continuous source of dust influx, were performed due to iterative nature of DUSTT-UEDGE code coupling. However, experimental observations demonstrate that intermittent injection of large quantities of dust, often associated with transient plasma events, may severely impact fusion plasma conditions and even lead to discharge termination. In this work we report on progress in coupling of DUSTT-UEDGE codes in time-dependent regime, which allows modeling of transient dust-plasma transport processes. The methodology and details of the time-dependent code coupling, as well as examples of simulations of transient dust-plasma transport phenomena will be presented. These include time-dependent modeling of impact of short out-bursts of different quantities of tungsten dust in ITER divertor on the edge plasma parameters. The plasma response to the out-bursts with various duration, location, and ejected dust sizes will be analyzed. [Preview Abstract] |
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TP10.00092: Time evolution of the particle and heat flux of the detached plasma Theerasarn Pianpanit, Seiji Ishiguro, Hiroki Hasegawa The detached plasma is a regime when the particle and heat flux of the plasma are largely reduced before reaching the divertor target. Linear devices experiment data show that when the neutral gas pressure in front of the target increases the heat flux to the target largely decreases. The 1D-3V particle simulation with Monte Carlo collision and cumulative scattering angle Coulomb collision has been developed to study the kinetic effect of the detached plasma\footnote{T. Pianpanit, S. Ishiguro, and H. Hasegawa, Plasma Fusion Res. \textbf{11}, 2403040 (2016)}. The simulation was performed with the constant temperature and pressure of neutral gas in front of the target. A large decrease in the electron temperature from 5eV to below 1 eV follows a large decrease in the ion temperature inside the neutral gas area in the case with high neutral gas pressure in front of the target. The energy flux at the target decreases in the process of attaining the detached state. [Preview Abstract] |
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TP10.00093: A Sensitivity Analysis of SOLPS Plasma Detachment D.L. Green, J.M. Canik, D. Eldon, O. Meneghini Predicting the scrape off layer plasma conditions required for the ITER plasma to achieve detachment is an important issue when considering divertor heat load management options that are compatible with desired core plasma operational scenarios. Given the complexity of the scrape off layer, such predictions often rely on an integrated model of plasma transport with many free parameters. However, the sensitivity of any given prediction to the choices made by the modeler is often overlooked due to the logistical difficulties in completing such a study. Here we utilize an OMFIT [1] workflow to enable a sensitivity analysis of the midplane density at which detachment occurs within the SOLPS [2] model. The workflow leverages the \underline {TaskFarmer} technology developed at NERSC to launch many instances of the SOLPS integrated model in parallel to probe the high dimensional parameter space of SOLPS inputs. We examine both predictive and interpretive models where the plasma diffusion coefficients are chosen to match an empirical scaling for divertor heat flux width or experimental profiles respectively. [1] O. Meneghini et al., Nucl. Fusion 55, 083008 (2015) [2] R. Schneider et al., Contrib. Plasma Phys. 46: 3-191 (2006) [Preview Abstract] |
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TP10.00094: An atomic and molecular fluid model for efficient edge-plasma transport simulations at high densities. Thomas Rognlien, Marvin Rensink Transport simulations for the edge plasma of tokamaks and other magnetic fusion devices requires the coupling of plasma and recycling or injected neutral gas. There are various neutral models used for this purpose, e.g., atomic fluid model, a Monte Carlo particle models, transition/escape probability methods, and semi-analytic models. While the Monte Carlo method is generally viewed as the most accurate, it is time consuming, which becomes even more demanding for device simulations of high densities and size typical of fusion power plants because the neutral collisional mean-free path becomes very small. Here we examine the behavior of an extended fluid neutral model for hydrogen that includes both atoms and molecules, which easily includes nonlinear neutral-neutral collision effects. In addition to the strong charge-exchange between hydrogen atoms and ions, elastic scattering is included among all species. Comparisons are made with the DEGAS 2 Monte Carlo code. [Preview Abstract] |
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TP10.00095: Interaction of neutral atoms and plasma turbulence in the tokamak edge region Christoph Wersal, Paolo Ricci, Rogerio Jorge, Jorge Morales, Paola Paruta, Fabio Riva A novel first-principles self-consistent model that couples plasma and neutral atom physics suitable for the simulation of turbulent plasma behaviour in the tokamak edge region has been developed~[1] and implemented in the GBS code~[2]. While the plasma is modelled by the drift-reduced two fluid Braginskii equations, a kinetic model is used for the neutrals, valid in short and in long mean free path scenarios. The model includes ionization, charge-exchange, recombination, and elastic collisional processes. The model was used to study the transition form the sheath to the conduction limited regime, to include gas puffs in the simulations, and to investigate the interplay between neutral atoms and plasma turbulence. \\ C. Wersal and P. Ricci 2015 {\it Nucl. Fusion} {\bf 55} 123014\newline P Ricci et al 2012 {\it Plasma Phys. Control. Fusion} {\bf 54} 124047 [Preview Abstract] |
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TP10.00096: Implementation of a plasma-neutral model in NIMROD S. Taheri, U. Shumlak, J. R. King Interaction between plasma fluid and neutral species is of great importance in the edge region of magnetically confined fusion plasmas. The presence of neutrals can have beneficial effects such as fueling burning plasmas and quenching the disruptions in tokamaks, as well as deleterious effects like depositing high energy particles on the vessel wall. The behavior of edge plasmas in magnetically confined systems has been investigated using computational approaches that utilize the fluid description for the plasma and Monte Carlo transport for neutrals. In this research a reacting plasma-neutral model is implemented in NIMROD to study the interaction between plasma and neutral fluids. This model, developed by E. T. Meier and U. Shumlak, combines a single-fluid magnetohydrodynamic (MHD) plasma model with a gas dynamic neutral fluid model which accounts for electron-impact ionization, radiative recombination, and resonant charge exchange. Incorporating this model into NIMROD allows the study of the interaction between neutrals and plasma in a variety of plasma science problems. An accelerated plasma moving through a neutral gas background in a coaxial electrode configuration is modeled, and the results are compared with previous calculations from the HiFi code. [Preview Abstract] |
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TP10.00097: Filamentary structures in the scrape-off layer using ArbiTER D. A. Baver, J. R. Myra ArbiTER is a flexible eigenvalue code designed for plasma physics applications. In this presentation, we will be using it to examine the role of spherical torus geometry on quantities of interest for scrape-off layer (SOL) turbulence. This will include assessment of instability growth rates and three-dimensional eigenmode structure under various plasma conditions with particular attention to the penetration of modes radially and along field lines into the divertor region. Preliminary work will focus on developing and demonstrating routines which can be applied to NSTX-U in future work. *Work supported by the U.S. Department of Energy Office of Science, Office of Fusion Energy Sciences, under Award Number DE-FG02-02ER54678. [Preview Abstract] |
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TP10.00098: Edge-relevant plasma simulations with the continuum code COGENT M. Dorf, M. Dorr, D. Ghosh, J. Hittinger, T. Rognlien, R. Cohen, W. Lee, P. Schwartz We describe recent advances in cross-separatrix and other edge-relevant plasma simulations with COGENT, a continuum gyro-kinetic code being developed by the Edge Simulation Laboratory (ESL) collaboration. The distinguishing feature of the COGENT code is its high-order finite-volume discretization methods, which employ arbitrary mapped multiblock grid technology (nearly field-aligned on blocks) to handle the complexity of tokamak divertor geometry with high accuracy. This paper discusses the 4D (axisymmetric) electrostatic version of the code, and the presented topics include: (a) initial simulations with kinetic electrons and development of reduced fluid models; (b) development and application of implicit-explicit (IMEX) time integration schemes; and (c) conservative modeling of drift-waves and the universal instability. [Preview Abstract] |
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TP10.00099: Effects of divertor's topology on stochastic particle transport Y. Nishimura, J.C. Lyu Particle behavior and resultant transport are studied in realistic divertor geometries incorporating topologies of both the closed and the open magnetic field lines. The magnetic stochasticity allows particles to move from the closed field line region to open field line regions. When the closed field lines are connected to the open field lines, particle and heat can move along the field lines to the divertor plate and immediately disappear. The transport process then is no longer diffusive. Computational model employing Duffing equation\footnote{S. Abdullaev, {\it Magnetic Stochasticity in Magnetically Confined Fusion Plasmas}, Springer (2014).} and the method of transformation to magnetic flux coordinate are presented. [Preview Abstract] |
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TP10.00100: Blob-Filament characteristics in XGC1 simulations and implications for the SOL width Ioannis Keramidas Charidakos, James Myra, Scott Parker, Seung-Hoe Ku, Jugal Chowdhury, Randy Churchill, Robert Hager, Choong-Seock Chang Blob-filament structures, formed due to plasma stratification, caused by strong turbulence near the separatrix, have been believed to be responsible for the convective transport at the SOL. Detachment of those coherent structures from the bulk can account for the intermittent nature of edge transport and their dynamics impact the heat flux width. The SOL width is a parameter of paramount importance in modern tokamaks as it controls the amount of power deposited at the divertor plates, directly affecting thus the viability of fusion. So far, studies of blobs have been confined to reduced fluid models and simplified geometries, leaving out important pieces of physics. Here, we analyze the results of simulations performed with the full-f, gyrokinetic code XGC1 which includes both turbulence and kinetic neoclassical effects in realistic divertor geometry. The blob contribution to the SOL width is estimated from examining the radial blob velocity and the parallel confinement time. [Preview Abstract] |
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TP10.00101: Formation of Non-Monotonic Potential Structure in the Detached Plasma Seiji Ishiguro, Theerasarn Pianpanit, Hiroki Hasegawa Plasma detachment has been investigated by means of PIC simulation which includes plasma-neutral collision and Coulomb collision. In our previous study, we have shown that a strong gradient in temperature appears in front of the target plate in the case that high density and low temperature neutral gas is introduced.\footnote{T. Pianpanit et al. Plasma and Fusion Res. \textbf{11}, 2403040 (2016).} It is observed that a potential hill is created in the neutral gas region where ions lose energy due to the elastic and charge exchange collision and, as a result, the ion density increases. This potential structure traps the low energy electrons and may play a role in the development of plasma detachment state. [Preview Abstract] |
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TP10.00102: Configuration of Self-consistent Flows in a Hole Structure Hiroki Hasegawa, Seiji Ishiguro Self-consistent particle flows in a hole structure have been studied with a three dimensional electrostatic plasma particle simulation code. In our previous study, we investigated kinetic effects on plasma blob dynamics with the particle simulation code\footnote{H. Hasegawa and S. Ishiguro, Phys. Plasmas \textbf{22}, 102113 (2015).}$^{,}$\footnote{H. Hasegawa and S. Ishiguro, 57th APS-DPP meeting, TP12.147 (2015).}. In this study, we have improved the code in order to investigate the hole propagation dynamics. Here, the hole is the intermittent filamentary structure along the magnetic field line in peripheral plasmas of fusion magnetic confinement devices and the plasma density in the hole is lower than that of background plasma. In the simulation, a hole structure is initially set as a cylindrical form elongated between both end plates and propagates in the grad-B direction. The simulation confirms that a spiral current system is formed in a hole structure. Further, the investigation into the effect of impurities on the flow configuration will be reported. [Preview Abstract] |
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TP10.00103: Recent Progress in BOUT$++$ simulations X.Q. Xu BOUT$++$ has been applied for a range of problems, including edge-localized mode (ELM) simulations, flux-driven simulations of an edge transport barrier formation, pedestal MHD turbulence, and validating the magnitude and scaling of the divertor heat load width for C-Mod, DIII-D, NSTX, and EAST. BOUT$++$-PIC simulations supporting RF antenna design show impurity migration pattern from RF sputtering. The latest 3-field 2-fluid BOUT$++$ simulation results demonstrated the linear and nonlinear characteristics of ELMs at different collisionality {\&} electric fields E$_{\mathrm{r}}$ shear via a density scan. The BOUT$++$ simulation results show an emerging understanding of dynamics of ELM crashes and the consistent collisionality scaling of ELM energy losses with the world multi-tokamak database. The impact of radial electric field E$_{\mathrm{r}}$ shear on low-n peeling and high-n ballooning modes is different. The increase E$_{\mathrm{r}}$ shear significantly enhances the linear growth rate of low-n peeling modes at low density, but only weakly impacts on nonlinear saturation amplitudes. In contrast, the increasing E$_{\mathrm{r\thinspace }}$shear leads to large suppression of nonlinear peeling-ballooning saturation amplitudes at high density, but only weakly impacts on their linear growth rates. [Preview Abstract] |
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TP10.00104: Towards understanding the role of turbulence on scaling of divertor heat flux profile widths on C-Mod B. Chen, X.Q. Xu, T.Y. Xia, B. LaBombard, M.Y. Ye The BOUT$++$ code has been exploited in order to improve the understanding the role of edge turbulent transport on scaling of divertor heat load widths. For the C-Mod EDA H-mode discharges, BOUT$++$ six-field two-fluid nonlinear simulations show a reasonable agreement of upstream turbulence and divertor target heat flux behavior: a) Distinguished quasi-coherent modes (QCMs) is observed with frequency around 120 kHz, and k$_{\mathrm{\theta }}$ around 1.5 rad/cm, which are consistent with experimental measurements; b) For the divertor heat flux, it displays a qualitatively similar shape in the near SOL region, while showing a sharp fall-off in the private flux zone; c) Heat flux width estimated by Eich fitting formula based on plasma current scan is corresponding to experimental scaling law (which are averaged over different time windows to eliminate the uncertainty in the nonlinear initial-value simulations). In order to do consistent scrape-off-layer transport calculations, the 2D fluid code SOLPS has been externally coupled to BOUT$++$ by using time-averaged turbulent heat flow and transport coefficients measured from BOUT$++$, detailed description of main simulation results will be shown. [Preview Abstract] |
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TP10.00105: Towards understanding the role of turbulence on scaling of divertor heat flux profile widths on DIII-D T.F. Tang, X.Q. Xu, T.Y. Xia, M. Makowski, C. Lasnier, T. Leonard, T.H. Osborne, J.Z. Sun, D.Z. Wang Recent multi-machines scaling law of parallel heat flux width proportional to the inverse of plasma current. Theoretical models, such as Critical gradient models can match the experiment at certain points, however, the role of the neoclassical transport and the turbulence transport, on the heat flux remain uncertain. We have used BOUT$++$ 6-field 2-fluid module to analyze the nonlinear turbulent transport of three H-mode discharges from DIII-D. With the evolution of the plasma profiles, the heat flux amplitude is greater and the heat flux width is broader, comparing to the experimental results. The curvature drift and neoclassical transport contribution on the heat flux is estimated, which shows the heat flux is mainly carried by the turbulence transport. The simulation also shows the electron heat flux, saturated at the divertor target, is much larger than the ion heat flux. While the cases without evolution inside the separatrix gives a smaller heat flux amplitude and narrower width, comparable to the experimental results. To eliminate the uncertainty of the simulation, time averaged simulation results of the heat flux at different time windows are also given. [Preview Abstract] |
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TP10.00106: Impurity migration with RF sheath and ELMs perturbed electric field in tokamak Bin Gui, Xiaotao Xiao, Tengfei Tang, Xueqiao Xu In radio frequency (RF) experiments, impurity generation and transport are important due to the phenomenon of RF enhanced impurity generation. In BOUT$++$ framework, the equilibrium radial electric field with RF sheath boundary condition on the limiter or the divertor surface is self-consistently calculated by using a two-field model. Based on this self-consistent calculation, it is found the positive radial electric field forms in the SOL region which qualitatively agrees with the experimental on the TEXTOR. The test particle module is developed in BOUT$++$ framework to simulate both turbulence and neoclassical physics in realistic geometry. Firstly, the drift orbit is calculated in cylinder coordinates due to singularity of x-point in flux coordinate. The turbulence transport of impurity generated from hot spot of RF limiter is simulated by random walk model. The numerical results show that less impurities will migrate into core and divertor region, more impurities migrate into nearby SOL boundary when turbulence transport enhanced. Then the effect of RF sheath potential on impurity migration will be simulated. Using the perturbed electric field from our BOUT$++$ nonlinear ELMs simulation, the transport of the impurities in different phase of ELMs are also discussed. [Preview Abstract] |
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TP10.00107: Numerical investigation on lithium transport in the edge plasma of EAST real-time- Li-injection experiments in the frame of BOUT$++$ N.M. Li, J.Z. Sun, Z.H. Wang, X.Q. Xu, Z. Sun, L. Wang, J.S. Hu, D.Z. Wang Experimental observations on applications of Lithium (Li) have indicated that Li could benefit plasma performance. But all these call for further investigation on lithium transport. A simple model has been developed by reducing Braginskii's equations with assumed quasi-neutral condition for transport of Li species in the edge plasma in the EAST experiments of real-time Li aerosol injection and implemented in the frame of BOUT$++$. The simulation results show that Li atoms propagate inwards continuously during the Li injection, and the propagating depth of Li atoms depends on both the local plasma conditions along its path and the Li injection velocity. It is also found that Li ions accumulate rapidly in the edge, and only a small fraction of Li species can transport cross the magnetic field into the core. In the poloidal direction, Li ions drift swiftly downwards along the field lines, and transport much faster at the high field side than at the low field side. The strong interaction between background plasma and Li ions plays a critical role in determining the edge plasma profile. It is found that real-time Li injection raises the plasma density in the pedestal region and reduces the plasma temperature, just as has been observed experimentally [Preview Abstract] |
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TP10.00108: Stellarator Microinstability and Turbulence Simulations Using Gyrofluid (GryfX) and Gyrokinetic (GS2) Codes Mike Martin, Matt Landreman, Noah Mandell, William Dorland GryfX is a delta-f code that evolves the gyrofluid set of equations using sophisticated nonlinear closures, with the option to evolve zonal flows (ky$=$0) kinetically. Since fluid models require less memory to store than a kinetic model, GryfX is ideally suited and thus written to run on a Graphics Processing Unit (GPU), yielding about a 1,200 times performance advantage over GS2. Here we present the first stellarator simulations using GryfX. Results compare linear growth rates of the Ion Temperature Gradient (ITG) mode between GryfX and the gyrokinetic code, GS2, using stellarator geometries from the National Compact Stellarator Experiment (NCSX) and Wendelstein 7-X (W7X). Strong agreement of \textless 10{\%} for maximum growth rates is observed between GS2 and GryfX for temperature gradients away from marginal stability for both NCSX and W7X geometries. Nonlinear stellarator results using GS2/GryfX are also presented. [Preview Abstract] |
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TP10.00109: Graphene as a Coating for Plasma Facing Components Marcos Navarro, Richard Rojas, Gerald Kulcisnki, Max Lagally, John Santarius Graphene has been a source of interest for multiple applications because of its unusual electronic and mechanical properties. A number of experimental studies have established that defect-free graphene is an excellent chemical-barrier material, but there have been no reports of graphene proposed as a protective coating against ion and/or neutral interactions with material surfaces. In the presence of such irradiation, plasma facing components (PFC's) tend to develop ``fuzz/grass'' structures that lead to the sputtering of wall material, diminishing the lifetime of the PFC's and plasma performance. We have shown that graphene can reduce or eliminate changes on surface morphology due to energetic helium. In the case of graphene-covered tungsten, our results show that, compared to the uncovered W, graphene suppresses these morphologies that form on the surface of hot W. Using Raman spectroscopy as a diagnostic, the graphene coating shows little sign of damage after being irradiated, indicating that there is little to no sputtering of carbon impurities from the surface. We have also determined that the mass losses in W have been reduced significantly. Both decreases in impurities can lead to an improved plasma performance and longer lifetimes for PFC's. [Preview Abstract] |
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