Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session NO6: Transport and Edge Physics in Magnetically Confined Plasmas |
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Chair: Felix Parra, Massachusetts Institute of Technology Room: Governor's Square 11 |
Wednesday, November 13, 2013 9:30AM - 9:42AM |
NO6.00001: Resonances in drift-kinetic particle motions in perturbed tokamaks and their impact on neoclassical toroidal viscosity transport Kimin Kim, Jong-Kyu Park, Allen Boozer Neoclassical toroidal viscosity (NTV) transport in perturbed tokamaks is driven by the net radial drift of particles across flux surfaces due to the nonaxisymmetric magnetic perturbations. The complicated interactions between drift-kinetic orbits and magnetic perturbations can be elucidated by accurately tracking the guiding-center particle motions [1]. This presentation will show that the perpendicular motion by ExB precession can largely suppress radial particle drifts by phase-mixing, but the phase-mixing can disappear if the ExB increases further such that it resonates with the parallel motion, which is called the bounce-harmonic resonance [2]. The bounce-harmonic resonance can significantly change the NTV in the presence of ExB, and thereby is the main driving mechanism of NTV transport. It will be discussed that the drift-kinetic orbit resonance should be adequately considered to calculate the anisotropic pressure tensor for self-consistent perturbed equilibrium. This work was supported by DOE Contract DE-AC02-09CH11466.\\[4pt] [1] K. Kim et al., Phys. Plasmas 19, 082503 (2012)\\[0pt] [2] K. Kim et al., Phys. Rev. Lett. 110, 185004 (2013) [Preview Abstract] |
Wednesday, November 13, 2013 9:42AM - 9:54AM |
NO6.00002: Theory verification and numerical benchmarking on neoclassical toroidal viscosity Z.R. Wang, J.-K. Park, Y.Q. Liu, N.C. Logan, J.E. Menard Systematic verification and numerical benchmarking has been successfully carried out among three different approaches of neoclassical toroidal viscosity (NTV) theory and the corresponding codes: IPEC-PENT is developed based on the combined NTV theory but without geometric simplifications [1]; MARS-K originally calculating the kinetic energy is upgraded to calculate the NTV torque based on the equivalence between kinetic energy and NTV torque [2]; MARS-Q includes smoothly connected NTV formula [3]. The derivation and numerical results both indicate that the imaginary part of kinetic energy calculated by MARS-K is equivalent to the NTV torque in IPEC-PENT. In the benchmark of precession resonance between MARS-Q and MARS-K/IPEC-PENT, it is first time to show the agreement and the correlation between the connected NTV formula and the combined NTV theory in different collisional region. Additionally, both IPEC-PENT and MARS-K indicates the importance of the bounce harmonic resonance which could greatly enhance the NTV torque when E cross B drift frequency reaches the bounce resonance condition. Since MARS-K also has the capability to calculate the plasma response including the kinetic effect self-consistently, the self-consistent NTV torque calculations have also been tested. [1] J.-K. Park et al Phys. Rev. Lett 065002 (2009) [2] J.-K. Park, Phys. Plasmas 18 110702 (2011) [3] K.C. Shaing et al Nucl. Fusion 50 025022 (2010) [Preview Abstract] |
Wednesday, November 13, 2013 9:54AM - 10:06AM |
NO6.00003: Nonambipolar Transport and Torque in Perturbed Equilibria N.C. Logan, J.-K. Park, Z.R. Wang, J.W. Berkery, K. Kim, J.E. Menard A new Perturbed Equilibrium Nonambipolar Transport (PENT) code has been developed to calculate the neoclassical toroidal torque from radial current composed of both passing and trapped particles in perturbed equilibria. This presentation outlines the physics approach used in the development of the PENT code, with emphasis on the effects of retaining general aspect-ratio geometric effects. First, nonambipolar transport coefficients and corresponding neoclassical toroidal viscous (NTV) torque in perturbed equilibria are re-derived from the first order gyro-drift-kinetic equation in the ``combined-NTV'' PENT formalism. The equivalence of NTV torque and change in potential energy due to kinetic effects [J-K. Park, Phys. Plas., 2011] is then used to showcase computational challenges shared between PENT and stability codes MISK and MARS-K. Extensive comparisons to a reduced model, which makes numerous large aspect ratio approximations, are used throughout to emphasize geometry dependent physics such as pitch angle resonances. These applications make extensive use of the PENT code's native interfacing with the Ideal Perturbed Equilibrium Code (IPEC), and the combination of these codes is a key step towards an iterative solver for self-consistent perturbed equilibrium torque. [Preview Abstract] |
Wednesday, November 13, 2013 10:06AM - 10:18AM |
NO6.00004: Clarification of the new XGC0-based bootstrap current formula Robert Hager, Choong-Seock Chang Additional proof for the validity of a new bootstrap current formula based on the results of the neoclassical particle-in-cell code XGC0 [Koh et al., Phys. Plasmas \textbf{19}, 072505 (2012)] is presented. In the collisional regime in the pedestal region of tight aspect ratio tokamaks, this formula predicts a higher bootstrap current than the popular formula by Sauter [O. Sauter et al., Phys. Plasmas \textbf{6}, 2834 (1999)]. Otherwise, bootstrap currents from XGC0 agree approximately to Sauter's formula. Koh et al. argue that collisions in conjunction with the almost toroidal field lines at the high field side of a spherical tokamak enable some of the trapped particles to contribute to the bootstract current. To corroborate this rationale, corresponding s-$\alpha$, concentric-circle, and Grad-Shafranov configurations are examined. Since the field line pitch is uniform on a flux surface in s-$\alpha$ configuration, the bootstrap current in s-$\alpha$ geometry is found to agree approximately with Sauter's formula. With the field line pitch variation becoming more pronounced in the concentric-circle and Grad-Shafranov configurations, the current increases continuously to the level predicted by Koh's formula. [Preview Abstract] |
Wednesday, November 13, 2013 10:18AM - 10:30AM |
NO6.00005: Coherent and turbulent density fluctuation measurement with a microwave imaging reflectometry on KSTAR W. Lee, J. Leem, J.A. Lee, Y.B. Nam, G.S. Yun, H.K. Park, H. Park, K.W. Kim, C.W. Domier, N.C. Luhmann, Jr., J.H. Jeong, M. Jeong, Y.S. Bae, W.H. Ko, S.G. Lee Both coherent and turbulent electron density fluctuations in KSTAR plasmas were measured by a semi two-dimensional microwave imaging reflectometry (MIR) system, which utilizes a detector array with 16 poloidal channels and two frequency X-mode probe beams (two radial positions). With a refractive imaging optics, the system measures density fluctuations with the poloidal wavenumber up to 2.5 cm$^{-1}$. The MIR system was tested with a known coherent density fluctuation during the pre-cursor phase of the sawtooth oscillation. The simultaneously measured phase oscillations from two cutoff layers were consistent with those of the electron temperatures at same radial locations. Furthermore, poloidal rotation velocities ($V_{pol}$) of the electon density turbulence were obtained by correlation analysis in an on-axis ECH heated ohmic plasma ($V_{pol} \sim$ -2 km/s in the electron diamagnetic direction) and a neutral beam heated L-mode plasma ($V_{pol} \sim$ 9 km/s in the ion diamagnetic direction), and implications are under study [Preview Abstract] |
Wednesday, November 13, 2013 10:30AM - 10:42AM |
NO6.00006: Nonlinear stabilization of tokamak microturbulence by fast ions J. Citrin, G.M.D. Hogeweij, C. Bourdelle, J. Garcia, J.W. Haverkort, F. Jenko, D. Told, T. Johnson, P. Mantica, M.J. Pueschel Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to significantly reduce ion-temperature-gradient microturbulence in tokamaks. This effect augments the electromagnetic stabilization due to thermal pressure. The degree of nonlinear electromagnetic stabilization is considerably greater than the linear stabilization. Based on a comprehensive investigation with the nonlinear gyrokinetic code GENE, this effect can explain the ion heat flux and stiffness reduction observed in ion heat transport experiments on the JET tokamak, described by Mantica \textit{et al.}~[Phys. Rev. Lett. \textbf{107} 135004 (2011)], which until now was not reproduced by gyrokinetic simulations. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes, which are predicted to have a significant suprathermal pressure fraction. [Preview Abstract] |
Wednesday, November 13, 2013 10:42AM - 10:54AM |
NO6.00007: Gyrokinetic particle simulation of linear instabilities in DIII-D pedestal plasmas Daniel Fulton, Zhihong Lin, Ihor Holod, Yong Xiao Understanding the physics in the edge pedestal region of toroidal plasmas is critical to obtaining confinement with high core temperatures. The pedestal region is characterized by large gradients in pressure, temperature, and density profiles, providing a source of free energy to drive instabilities, such as ion and electron temperature gradient modes (ITG/ETG), kinetic ballooning mode (KBM), and trapped electron modes (TEM). Studying these instabilities can provide information on the limits of allowable gradients in the pedestal. In this study, we explore linear instabilities, in the pedestal region of the DIII-D discharge 131997 using the gyrokinetic toroidal code (GTC). Results using parameters from a region at the top of the pedestal show dominant mode to be an ion temperature gradient instability. In the peak gradient region, a qualitatively different mode structure, peaking at $\theta = \pm\pi/2$ is observed. Mode structure with this property have been predicted in theoretical papers. Additionally, we present a $\beta_e$ scan of linear frequency and growth rate demonstrating the transition from ITG to TEM to KBM instability, as a verification of GTCs electromagnetic capability. [Preview Abstract] |
Wednesday, November 13, 2013 10:54AM - 11:06AM |
NO6.00008: Chaotic Neoclassical Separatrix Dissipation in Parametric Drift-Wave Decay C.F. Driscoll, A.A. Kabantsev, D.H.E. Dubin, Yu.A. Tsidulko Experiments and theory characterize a parametric decay instability between plasma drift waves when the standard nonlinear mode coupling is modified by chaotic dissipation on a separatrix. Experimentally, we utilize pure electron plasma columns with a central electrostatic ``squeeze'' barrier. We launch a large-amplitude $m_\theta=2$, $k_z=0$ dioctron mode, and observe it decay into an exponentially growing $m_\theta=1$, $z$-anti-symmetric ``Trapped Particle Diocotron Mode.'' Measurements of the growth rates $\Gamma_1$ and relative mode phases $\Delta \Theta_{12}$ during exponentiation accurately characterize both the standard nonlinear coupling term and the enhanced dissipation due to chaotic neoclassical transport. Here, the $m=2$ pump wave dynamically ``ruffles'' the separatrix, causing chaotic separatrix crossings. Similar enhancements are predicted and observed when the ruffle is static and the plasma drifts along the separatrix.\footnote{Dubin, Kabantsev, Driscoll, Phys Plas 19, 056102 (2012).} This novel chaotic dissipation is essentially {\it independent} of collisionality, and may dominate in the low-collisionality regimes of toroidal fusion plasmas where trapped particles and separatrices are endemic. [Preview Abstract] |
Wednesday, November 13, 2013 11:06AM - 11:18AM |
NO6.00009: Nitrogen-induced complete divertor detachment during stable H-Mode operation in ASDEX Upgrade Felix Reimold, Matthias Bernert, David Coster, Rainer Fischer, Arne Kallenbach, Rachael McDermott, Steffen Potzel, Ulrich Stroth, Eleonora Viezzer, Marco Wischmeier Future fusion devices like DEMO will likely require to be operated with a detached divertor to meet power exhaust requirements and material limits alike. Detachment of the inner divertor target is regularly observed in H-mode operation in various tokamaks. Despite some efforts complete divertor detachment in H-mode at both divertor targets simultaneously had been an inaccessible operation regime in the all-tungsten ASDEX Upgrade so far. Recently, however, stable H-mode operation with completely detached divertor targets has been achieved. The complete detachment of the outer target was induced by simultaneous, strong nitrogen and deuterium puffing into the private-flux region, i.e. the divertor. An appropriate seeding scheme avoids central accumulation of tungsten and leads to almost complete mitigation of tungsten sputtering at the divertor targets. The newly found detachment regime in H-mode is compared to recent findings in L-mode and the impact of strong nitrogen seeding on the (divertor) plasma is assessed. In order to understand the experimental results and gain further insight into the H-mode detachment process accompanying numerical modeling efforts with SOLPS are carried out. [Preview Abstract] |
Wednesday, November 13, 2013 11:18AM - 11:30AM |
NO6.00010: Computational Study of Poloidal Angular Momentum Transport in DIII-D Alexei Pankin, Scott Kruger, Arnold Kritz, Tariq Rafiq, Jan Weiland The new Multi-Mode Model, MMM8.1, includes the capability to predict the anomalous poloidal momentum diffusivity~[T. Rafiq, {\em al.}, Phys. Plasmas 20, 032506 (2013)]. It is important to consider the effect of this diffusivity on the poloidal rotation of tokamak plasmas since some experimental observations suggest that neoclassical effects are not always sufficient to explain the observed poloidal rotation~[B.A. Grierson {\em et al.}, Phys. Plasmas 19, 056107 (2012)]. One of the objectives of this research is to determine if the anomalous contribution to the poloidal rotation can be significant in the regions of internal transport barriers (ITBs). In this study, the MMM8.1 model is used to compute the poloidal momentum diffusivity for a range of plasma parameters that correspond to the parameters that occur in DIII-D discharges. The parameters that are considered include the temperature and density gradients, and magnetic shear. The role of anomalous poloidal transport in the possible poloidal spin up in the ITB regions is discussed. Progress in the implementation of poloidal transport equations in the ASTRA transport code is reported and initial predictive simulation results for the poloidal rotation profiles are presented. [Preview Abstract] |
Wednesday, November 13, 2013 11:30AM - 11:42AM |
NO6.00011: Physical interpretation of the nonlocal transport and dynamic equilibrium computation with coupling to the transport Linjin Zheng The so-called nonlocal transport was observed in various tokamaks. A sudden cooling at plasma edge is found to lead to the temperature rise at plasma center. It has been shown that the Ohmic heating power redistribution is insufficient to explain the center temperature increase and the typical local transport models are unable to explain this phenomenon. We point out that the Ohmic current redistribution can also cause the center plasma compression. The compression can provide an additional heating mechanism for center plasma. To demonstrate this mechanism we develop the dynamic transport-equilium code (DynTEQ) to compute the dynamic evolution process. In difference from the existing coupled transport-equilibrium codes which couple directly the transport and the MHD equilibrium packages through iterations, the numerical scheme in DynTEQ code includes the convective effects into the transport process with MHD force balance constraint imposed consistently. Both physics mechanism and numerical scheme will be explained, with cylinder results detailed. It is also pointed out that the plasma compression can also lead to the formation of the steep pressure gradient region (i.e., the so-called transport barrier) as typically observed during the off-axis heating process. [Preview Abstract] |
Wednesday, November 13, 2013 11:42AM - 11:54AM |
NO6.00012: Drift waves and vortices: a dynamical point vortex model Xavier Leoncini, Alberto Verga Interactions of localized vortices with drift waves are investigated using a model of point vortices in the presence of a transverse or longitudinal wave. This simple model shows a rich dynamical behavior including oscillations of a dipole, splitting and merging of two like-circulation vortices, and chaos. The analytical and numerical results of this model have been found to predict under certain conditions, the behavior of more complex systems, such as the vortices of the Charney-Hasegawa-Mima equation, where the presence of waves strongly affects the evolution of large coherent structures. [Preview Abstract] |
Wednesday, November 13, 2013 11:54AM - 12:06PM |
NO6.00013: Boundary Condition Effects and the Role of Helicity Conservation in Dynamos in Taylor-Couette Flows Krista Martocci I will present three-dimensional simulations of magnetized, Taylor-Couette flows in the centrifugally stable regime. In this regime the motions are initialized by the magno-rotational instability (MRI). Three different initial magnetic field configurations and boundary conditions are considered. The purpose of this work is to investigate the role magnetic helicity conservation in the operation and stability of the dynamo. We show evidence of large-scale dynamo action when boundary conditions are applied that allow a flow of magnetic helicity out of the domain. Simulations seeded with an existing dynamo solution having vertical, periodic boundaries are shown to be unstable. A system conserving helicity by only allowing horizontal fields at the boundaries can only sustain fields and turbulence when external currents are applied. This supports the need for a helicity outflow to generate a successful large-scale dynamo. [Preview Abstract] |
Wednesday, November 13, 2013 12:06PM - 12:18PM |
NO6.00014: Asymmetric diffusion in turbulent plasmas Mikhail Medvedev Particle transport in plasmas with turbulent magnetic fields in the presence of a gradient of the mean magnetic field and weak pitch-angle diffusion is analyzed. We demonstrate that such transport is described by asymmetric diffusion: the generalization of the conventional diffusion process to the case of random walk with unequal probabilities. Using a Markov chain analysis of a toy model, we demonstrate that the particle density distribution becomes exponential in distance, instead of linear as is the case for the standard diffusion process. Some implications of our results are discussed. [Preview Abstract] |
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