Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session PO7: Advances in MFE Theory and Computation |
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Chair: Greg Hammett, Princeton Plasma Physics Laboratory Room: 556AB |
Wednesday, October 31, 2012 2:00PM - 2:12PM |
PO7.00001: Intrinsic rotation of toroidally confined magnetohydrodynamics Jorge Morales, Wouter Bos, Kai Schneider, David Montgomery Time-dependent 3D toroidal visco-resistive MHD computations are performed, using the recently developed penalization method for enforcing the boundary conditions. An imposed toroidal magnetic field is present and the current is driven by an imposed toroidal electric field. Both poloidal and toroidal rotation result, and depend strongly on the shape of the toroidal cross section and the value of the Hartmann number. Net toroidal rotation results from a departure from up/down symmetry in the cross-sectional boundary shape. By increasing the Hartmann number at unit magnetic Prandtl number, the plasma seeks out a characteristic configuration in which the velocity aligns increasingly with the magnetic field lines. The resulting flow is characterized by both toroidal and poloidal rotation, starting from initial conditions in which such flows are absent. Ideal MHD equilibrium considerations appear not to play an important role. [Preview Abstract] |
Wednesday, October 31, 2012 2:12PM - 2:24PM |
PO7.00002: Constructing the spectral web of rotating plasmas Hans Goedbloed Rotating plasmas are ubiquitous in nature. The theory of MHD stability of such plasmas, initiated a long time ago, has severely suffered from the wide spread misunderstanding that it necessarily involves non-self-adjoint operators. It has been shown (J.P. Goedbloed, PPCF 16, 074001, 2011; Goedbloed, Keppens and Poedts, Advanced Magnetohydrodynamics, Cambridge, 2010) that, on the contrary, spectral theory of moving plasmas can be constructed entirely on the basis of energy conservation and self-adjointness of the occurring operators. The spectral web is a further development along this line. It involves the construction of a network of curves in the complex omega-plane associated with the complex complementary energy, which is the energy needed to maintain harmonic time dependence in an open system. Vanishing of that energy, at the intersections of the mentioned curves, yields the eigenvalues of the closed system. This permits to consider the enormous diversity of MHD instabilities of rotating tokamaks, accretion disks about compact objects, and jets emitted from those objects, from a single view point. This will be illustrated with results obtained with a new spectral code (ROC). [Preview Abstract] |
Wednesday, October 31, 2012 2:24PM - 2:36PM |
PO7.00003: Free boundary ballooning mode representation and its application Linjin Zheng Free boundary ballooning mode representation is developed in this paper. This representation allows the two dimensional problem of peeling ballooning modes to be treated in one dimensional formalism. In contrast to the conventional ballooning representation, which requires the translational invariance of the Fourier components of the perturbations, the new invariance reflects that the independent solutions of the high n mode equations are translationally invariant from one radial interval surrounding a single singular surface to the other intervals. The conventional ballooning mode invariance breaks down at the vicinity of plasma edge, since the Fourier components with rational surfaces in vacuum region are completely different from those with rational surfaces in plasma region. But, the new type of invariance remains valid. This overcomes the limitation of the conventional ballooning mode representation for studying free boundary modes. Since the current formalism is adaptable to the cases with various sharp boundary changes, it therefore can be also generalized to study the piece-wise equilibrium cases, for example, the H-mode pedestal and X-point physics. [Preview Abstract] |
Wednesday, October 31, 2012 2:36PM - 2:48PM |
PO7.00004: Local and global gyrokinetic studies of turbulence in transport barriers Daniel Told, Andreas Burckhart, Tobias Goerler, Frank Jenko, Elisabeth Wolfrum, Stephan Brunner, Olivier Sauter In the present work, the gyrokinetic turbulence code GENE is employed to study plasma microturbulence in both core and edge transport barriers. Physically comprehensive simulations, making use of both the local and global versions of GENE, are performed. The global version of GENE is applied to discharges of the TCV tokamak, which exhibit an electron internal transport barrier. In discharges with weaker barriers, TEM-type turbulence is found to be the dominant source of electron heat flux, confirming earlier findings. For steeper barriers, a detailed sensitivity study shows enhanced transport due to small-scale ETG turbulence which, for the simulations in closest agreement with experiment, becomes comparable to the large-scale TEM transport. Using, on the other hand, the local approximation, the GENE code is applied to study small-scale ETG turbulence under edge transport barrier conditions as found in the ASDEX Upgrade tokamak, taking into account several different parameter sets. It is found that ETG turbulence can under certain conditions carry a large fraction of the electron heat flux. For these parameters, the absence of radially elongated streamers within the pedestal makes ETG turbulence particularly robust with respect to shear flows. [Preview Abstract] |
Wednesday, October 31, 2012 2:48PM - 3:00PM |
PO7.00005: Six-field two-fluid simulations on edge localized modes with BOUT++ Tianyang Xia, Xueqiao Xu, Bin Gui We develop the six-field two-fluid model based on Braginskii equations to simulate peeling-ballooning (P-B) modes for evolution of edge ion density, ion temperature, electron temperature, parallel ion velocity, magnetic flux and vorticity in tokamak. The effects of energy flux, energy exchange and viscosity are included in our model. The flux-limited parallel thermal conductivity model is also used. Compared with previous three-field P-B model, for the same pressure profile, the density gradient length scale can increase the normalized linear growth rate by 6.2{\%}. The growth rate can be reduced up to 33.6{\%} by the parallel thermal conductivities, most of which are contributed by electrons. Therefore, the saturated electron temperature fluctuation amplitude is only around 1/4 of ions after ELM crashes, and the ELM size is decreased by more than 50.0{\%}. The gyro-viscosity plays the role of stabilizing effects and decreases the growth rate by 16.0{\%}. The sheath boundary conditions have been implemented to study the behaviors of particle and heat flux towards divertor plates. The nonlinear simulations on EAST geometry will be presented. [Preview Abstract] |
Wednesday, October 31, 2012 3:00PM - 3:12PM |
PO7.00006: Gyrokinetic studies of the outer core region in DIII-D and ASDEX Upgrade discharges Tobias Goerler, Daniel Told, Anne White, Clemente Angioni, Emiliano Fable, Greg Hammett, Frank Jenko, Eleonora Viezzer In order to study the outer core region in DIII-D and ASDEX Upgrade discharges, radially local and non-local gyrokinetic simulations with the GENE code are carried out. Using actual plasma parameters and MHD equilibria and employing as much physics as available, particular focus is placed on the degree to which turbulent features can be validated against the experiments. In the recent years, careful and systematic comparisons have largely demonstrated very good agreement with experiment--except for L-mode discharges where a shortfall of almost one order of magnitude has been reported in the outer core ion heat transport, e.g. in [C.~Holland et al., Phys.~Plasmas 16, 052301 (2009)]. Therefore, special emphasis is given to confirm or extend these transport underpredictions and explore possible solutions as, e.g., effects of the highly nonlinear nature of the neighbouring edge turbulence [B.D.~Scott, Phys.~Plasmas 12, 062314 (2005)] or contributions from neighbouring scales (low-k microtearing, short wavelength ITG/TEM/ETG). Comparisons with measured cross phases [A.~White et al., Phys.~Plasmas 17, 056103 (2010)] will help to attribute a possible shortfall either to a corresponding drop in the fluctuation amplitudes or to differing turbulence types in simulations and experiments. [Preview Abstract] |
Wednesday, October 31, 2012 3:12PM - 3:24PM |
PO7.00007: New analytic formula for edge bootstrap current C.S. Chang, S. Koh, J. Menard, H. Weitzner, W. Choe The edge bootstrap current plays a critical role in the equilibrium and stability of the steep edge pedestal plasma. The pedestal plasma has an unconventional and difficult neoclassical property, as compared with the core plasma. A drift-kinetic particle code XGC0, equipped with a mass-momentum-energy conserving collision operator, is used to study the edge bootstrap current in a realistic diverted magnetic field geometry with a self-consistent radial electric field. When the edge electrons are in the low collisionality banana regime, surprisingly, the present kinetic simulation confirms that the existing analytic expressions (represented by O. Sauter, et. al., Phys. Plasmas 6, 1999) are still valid in this unconventional region, except in a thin radial layer in contact with the magnetic separatrix. However, when the pedestal electrons are in plateau-collisional regime, there is a significant deviation of numerical results from the existing analytic formulas. The deviation occurs in different ways between a conventional aspect ratio tokamak and a tight aspect ratio tokamak. A new analytic fitting formula, as a simple modification to the Sauter formula, is obtained to bring the analytic expression to a better agreement with the edge kinetic simulation results. [Preview Abstract] |
Wednesday, October 31, 2012 3:24PM - 3:36PM |
PO7.00008: Zonal flows and ITG saturation in the presence of magnetic perturbations M.J. Pueschel, P.W. Terry, F. Jenko, D.R. Hatch, W.M. Nevins, T. Gorler, D. Told At large $\beta$ but below the KBM threshold, gyrokinetic simulations of ITG turbulence may fail to saturate through zonal flows. It is shown that, if background gradients are sufficiently strong, a threshold exists for a transition to a non-zonal ITG regime with extremely large transport levels, an effect sometimes referred to as ``runaway.'' Resonant (i.e., tearing-parity) radial magnetic fluctuations $B_x$ are usually seen as the primary source of stochasticity, but non-resonant $B_x$ may also become stochastic once the field line displacement after half a poloidal turn exceeds the correlation length. This rapidly shorts out the zonal flows, and they are no longer able to saturate the ITG mode. To examine how flux-surface-breaking magnetic fluctuations affect zonal flows, the Rosenbluth-Hinton residual flow scenario is studied: In the presence of a superimposed, constant resonant $B_x$, initial flows do not decay to the theoretical residual, but vary quadradically in time, reaching zero potential in the time $t_{\Phi=0} \propto q_0 (1 - \epsilon_{\mathrm{t}}) B_x^{-1}$. The flow evolution is calculated analytically from the Laplace-transformed, bounce-averaged ion gyrokinetic equation with a $B_x$ of the above type, and compared with the simulation results. [Preview Abstract] |
Wednesday, October 31, 2012 3:36PM - 3:48PM |
PO7.00009: Flux-driven 3D global fluid simulations of plasma edge turbulence Bo Li, D.R. Ernst We have developed a new 3D electromagnetic, profile evolving fluid edge turbulence code based on drift-ordered Braginskii equations. In this flux-driven code, fixed background profiles are not used. Instead, plasma density and temperature profiles are evolved self-consistently in response to heat and particle source profiles, subject to the transport produced self-consistently by plasma turbulence, and plasma losses at the sheath resulting from sonic parallel flows in the scrape-off layer. To allow large fluctuations such as blobs, no separation is made between perturbations and equilibrium. In contrast to a local simulation with fixed, radially constant equilibrium gradients and periodic boundary conditions, the global code allows radial variations of the equilibrium profile gradients with non-periodic radial boundary conditions. Large radial structures do indeed develop, comparable in size to equilibrium radial scale lengths. For tokamak magnetic geometry, a strong curvature-driven instability is observed around the outboard mid-plane and the self-consistent pressure profiles are monotonically decreasing. The ballooning structure of turbulence along the field line is evident. [Preview Abstract] |
Wednesday, October 31, 2012 3:48PM - 4:00PM |
PO7.00010: On the relevance of uncorrelated sequencies of Lorenzian pulses for the interpretation of turbulent fluctuation data at the edge of magnetically confined toroidal plasmas Raul Sanchez, Bowdewijn van Milligen, Carlos Hidalgo Recently, it has been proposed that the turbulent fluctuations measured in a linear plasma device could be described as a superposition of uncorrelated Lorentzian pulses with a narrow distribution of durations, which would provide an explanation for the reported quasi-exponential power spectra. Here, we study the applicability of this proposal to edge fluctuations in toroidal magnetic confinement fusion plasmas. For the purpose of this analysis, we introduce a novel wavelet-based pulse detection technique that offers important advantages over existing techniques. It allows extracting the properties of individual pulses from the experimental time series, and quantifying the distribution of pulse duration and energy, as well as temporal correlations. We apply the wavelet technique to edge turbulent fluctuation data from the W7-AS stellarator and the JET tokamak, and find that the pulses detected in the data do not have a narrow distribution of durations and are not uncorrelated. Instead, the distributions are of the power law type, exhibiting temporal correlations over scales much longer than the typical pulse duration. These results cast doubt on the proposed ubiquity of exponential power spectra in this context. [Preview Abstract] |
Wednesday, October 31, 2012 4:00PM - 4:12PM |
PO7.00011: Benefits and drawbacks of low magnetic shears on the confinement in magnetic fusion toroidal devices Marie-Christine Firpo, Dana Constantinescu The issue of confinement in magnetic fusion devices is addressed within a purely magnetic approach. As it is well known, the magnetic field being divergence-free, the equations of its field lines can be cast in Hamiltonian form. Using then some Hamiltonian models for the magnetic field lines, the dual impact of low magnetic shear is demonstrated. Away from resonances, it induces a drastic enhancement of magnetic confinement that favors robust internal transport barriers (ITBs) and turbulence reduction. However, when low-shear occurs for values of the winding of the magnetic field lines close to low-order rationals, the amplitude thresholds of the resonant modes that break internal transport barriers by allowing a radial stochastic transport of the magnetic field lines may be much lower than the ones obtained for strong shear profiles. The approach can be applied to assess the robustness versus magnetic perturbations of general almost-integrable magnetic steady states, including non-axisymmetric ones such as the important single helicity steady states. This analysis puts a constraint on the tolerable mode amplitudes compatible with ITBs and may be proposed as a possible explanation of diverse experimental and numerical signatures of their collapses. [Preview Abstract] |
Wednesday, October 31, 2012 4:12PM - 4:24PM |
PO7.00012: Time-spectral solution of initial-value problems Jan Scheffel A time-spectral method for solutions of initial-value partial differential equations has recently been developed [1]. The purpose of the method is to avoid inefficient time stepping for problems in plasma physics with widely separated time scales. Temporal, spatial and parameter domains are all treated using an ansatz in the form of a sum of Chebyshev polynomials. The coefficients of the ansatz is determined using a generalized weighted residual method. A new, efficient solver for the resulting algebraic systems of coefficient equations has been developed [2]. In addition, subdomain methods for the temporal and spatial domains are employed [3]. The question is now: to what extent are time-spectral methods really more attractive than finite difference methods? We will report on results concerning accuracy and efficiency for several linear and nonlinear model partial differential equations.\\[4pt] [1] Scheffel J, Partial Differential Equations: Theory, Analysis and Applications (Nova Science Publishers) 2011, p 1-49.\\[0pt] [2] Scheffel J and Hakansson C, Appl. Numer. Math. 59(2009)2430.\\[0pt] [3] Scheffel J and Mirza A, Am. J. of Comp. Math. 2(2012)72. [Preview Abstract] |
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