Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session TO3: Magnetic Confinement Theory |
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Chair: Paul Terry, University of Wisconsin Room: Rosen Centre Hotel Salon 9/10 |
Thursday, November 15, 2007 9:30AM - 9:42AM |
TO3.00001: Decoupling of ion and electron heat transport via scale separation Frank Jenko, Tobias Goerler, Terry Rhodes, Stan Kaye Traditionally, turbulent transport is thought to be carried mainly by long-wavelength modes with $k_\perp\rho_i\sim 0.2$. While this seems to be generally true for the ion heat channel, there is experimental evidence that the electron heat fluxes behave differently -- both in transport barriers and beyond. Here, we will examine the spectral properties of heat transport in nonlinear gyrokinetic simulations with the {\sc Gene} code, focussing on contributions from shorter wavelengths than the ones mentioned above. Some simulations treat both ion and electron space-time scales fully self-consistently and are therefore very challenging from a computational point of view, requiring of the order of 100,000 CPU-hours or more. In pure ITG and TEM turbulence, the heat fluxes exhibit clear peaks around $k_\perp\rho_i\sim 0.2$, and fall off quickly for higher wavenumbers. However, in cases for which TE/ITG modes and ETG modes coexist, the spectral properties may change completely. Now, a wide range of wavenumbers, from ion scales all the way to electron scales, can contribute to the overall electron heat transport. This implies that the latter is dominated by high-wavenumber TEMs and ETG modes, while the direct contribution from ITG modes is relatively small. Applications to recent experiments at both DIII-D and NSTX will be discussed. Here, the concept of scale separation turns out to be crucial for the interpretation of experimental data. [Preview Abstract] |
Thursday, November 15, 2007 9:42AM - 9:54AM |
TO3.00002: Gyrokinetic Turbulence Simulations for Stellarators F. Merz, P. Xanthopoulos, T. Gorler, F. Jenko, D. Mikkelsen While there is an abundance of publications on plasma microturbulence in tokamaks, not much is presently known about its character in nonaxisymmetric devices. The present work constitutes the first attempt to investigate turbulent transport in modern stellarators, using the gyrokinetic turbulence code {\sc Gene} and realistic magnetic equilibria. First, linear and nonlinear gyrokinetic simulations of ion-temperature-gradient (ITG) and trapped electron modes are presented for the optimized stellarator Wendelstein 7-X which is currently under construction at Greifswald, Germany. The newly developed code {\sc Tracer} -- based on field line tracing -- is employed to extract the required geometric information from the MHD equilibria [Phys. Plasmas {\bf 13}, 092301 (2006)]. Extensive linear studies reveal substantial differences with respect to axisymmetric geometry [Phys. Plasmas {\bf 14}, 042501 (2007)]. Nonlinear ITG simulations are also presented [Phys. Rev. Lett., in print]. Several fundamental features are discussed, including the role of zonal flows for turbulence saturation, the resulting flux-gradient relationship and the co-existence of ITG modes with trapped ion modes in the saturated state. Similar studies will be presented for the stellarator experiment NCSX at PPPL with the aim to comprehend the effects of quasi-axisymmetric geometry on the properties - both linear and nonlinear - of various microinstabilities. [Preview Abstract] |
Thursday, November 15, 2007 9:54AM - 10:06AM |
TO3.00003: The zonal flow back-reaction on ion-temperature-gradient mode turbulence Johan Anderson, Eun-jin Kim, Jiquan Li, Yasuaki Kishimoto Anomalous transport remains one of the main concerns in magnetically confined plasmas. The anomalous transport in the core is commonly attributed to Ion-Temperature-Gradient (ITG) mode turbulence. There is strong evidence indicating zonal flow suppression of the Ion-Temperature-Gradient (ITG) mode turbulence, specifically close to the linear $\eta_i= L_n/L_{Ti} $ threshold. A critical concept is the transport regulation and transport barrier formation by zonal flows. The present study reports on the effects of zonal flow suppression of the ITG turbulence, suggesting an increase in the effective linear ITG threshold. This is also known as the Dimit's shift. While, the zonal flows are generated from ITG background turbulence by the coherent mode coupling, moreover the zonal flow back-reacts on the ITG mode turbulence resulting in a modified linear ITG mode threshold. It is shown that the short wave length zonal flow suppression of drift wave turbulence is significant, particularly close to the threshold ($\eta_{ith}$). [Preview Abstract] |
Thursday, November 15, 2007 10:06AM - 10:18AM |
TO3.00004: Role of magnetic shear in flow shear suppression Eun-jin Kim Flow shear and magnetic shear are thought to be crucial in controlling anomalous transport in laboratory plasmas. In particular, turbulence quenching due to flow shear is believed to be indispensable for the formation of transport barrier and thus plasma confinement. Here, we investigate how magnetic shear interacts with flow shear, affecting turbulence regulation by flow shear in 3D RMHD turbulence [1]. Specifically, we show analytically that near the resonance surface, transport quenching by flow shear is weakened by magnetic shear as the latter interferes with shearing process. Anomalous particle transport thus becomes more efficient in the regime with stronger magnetic shear for a given flow shear while self-regulation of zonal flows becomes less effective. The results suggest that weak magnetic shear could be favorable for the formation of transport barrier. \newline \newline [1] E. Kim, Phys. Plasmas, in press (2007) [Preview Abstract] |
Thursday, November 15, 2007 10:18AM - 10:30AM |
TO3.00005: Trajectory trapping and structure generation in turbulent magnetized plasmas Madalina Olimpia Vlad, Florin Spineanu The ExB drift determines a trapping effect or eddy motion in turbulence with slow time variation. We have shown using a semi-analytical approach that this nonlinear process generates non-standard statistical behavior of the trajectories: memory effects and non-Gaussian probability. The trapped trajectories have quasi-coherent behavior and they form structures similar to fluid vortices. We analyze here the effects of this non-standard statistics of trajectories on the evolution of the drift turbulence in Vlasov description. We consider test modes on turbulent plasma with given statistical characteristics and show that trajectory structures determine the evolution of the drift turbulence toward large scales (inverse cascade). The initial value problem of the drift turbulence is studied. The conclusion is that the main mechanism determining the non-linear evolution of the turbulence toward ordered flow is the trapping and dragging of a part of ions by the potential that moves with the diamagnetic velocity accompanied by the opposite direction average motion of the other ions. [Preview Abstract] |
Thursday, November 15, 2007 10:30AM - 10:42AM |
TO3.00006: A stepped pressure profile model for internal transport barriers Matthew Hole, Stuart Hudson, Robert Dewar \newcommand {\bB} {\mathbf{B}} \newcommand {\curl} {\nabla \times} \newcommand {\etal} {\textit{et al} } We develop a multiple interface variational model, comprising multiple Taylor-relaxed plasma regions separated by ideal MHD barriers. The magnetic field in each region is Beltrami, $\curl \bB = \mu \bB$, and the pressure constant. Between these regions the pressure, field strength, and rotational transform $\iota$ may have step changes at the ideal barrier. A principle motivation is the development of a mathematically rigorous ideal MHD model to describe intrinsically 3D equilibria, with nonzero internal pressure, using robust KAM surfaces as the barriers. As each region is locally relaxed however, such a model may also yield reasons for existence of internal transport barriers (ITBs). Focusing on the latter, we build on Hole \etal Nuc. Fus. 47, pp746-753, 2007, which recently studied the stability of a two-interface periodic-cylinder configuration. In this work, we perform a stability scan over pressure and $\iota$ for a two-interface configuration with no jump in $\iota$, and compare the characteristics of stable equilibria to those of ITB's. [Preview Abstract] |
Thursday, November 15, 2007 10:42AM - 10:54AM |
TO3.00007: Self-consistent Modeling of the Pedestal in Tokamak \mbox{H-mode} Plasmas A.Y. Pankin, G. Bateman, A.H. Kritz, C.S. Chang, S. Ku, G. Park, J. Cummings, C. Sovinec, P. Snyder, L.D. Pearlstein, N. Podhorszki, S. Klasky Simulations of H-mode pedestal growth and ELM crashes in DIII-D discharges are carried out using a combination of four computer codes --- XGC gyro-kinetic, TEQ equilibrium, ELITE linear ideal MHD stability, and NIMROD extended MHD. The pedestal modeling uses computational tools that are most appropriate for the different physical effects and processes. The approach results from a multi-institutional effort to build a robust suite of codes for self-consistent modeling of the H-mode pedestal in tokamak plasmas. The XGC code is used for modeling of neoclassical effects that lead to the H-mode pedestal formation. The XGC code handles charged particle and neutral collisions using a Monte Carlo approach and a model for the source of neutrals at the wall. The TEQ free-boundary equilibrium code is used to follow the equilibrium changes as the plasma profiles evolve in XGC. The triggering of an ELM crash is indicated by the ELITE code, which computes the linear growth rate of selected modes and takes into account the stabilizing diamagnetic effect. Once an ELM crash is triggered, the NIMROD code is used to follow the nonlinear evolution of the ELM crash. The comparison of the numerical simulations and DIII-D data is discussed. [Preview Abstract] |
Thursday, November 15, 2007 10:54AM - 11:06AM |
TO3.00008: Linear theory of n=0 geodesic acoustic mode T. Zhou, H.V. Wong, H.L. Berk The n=0 geodesic acoustic mode (GAM), observed in JET and D- III D is frequently accompanied by fast frequency chirping. A numerical investigation on CASTOR revealed that a global GAM mode arises if the continuum geodesic frequency vs. radius has a local maximum. The global GAM properties are characterized by: a small upward frequency shift from the continuum, radially localized electrostatic components with poloidal numbers m=0,1 and magnetic coupling to a nonlocalized to m=2 component. Here we develop an analytic MHD theory of this n=0 global GAM in a toroidal plasma with r/R and beta small. We choose to start from the MHD quadratic form (with inertial terms). We find the GAM eigenmode is characterized by a radially localized density perturbation and a one order smaller magnetic perturbation that extends throughout the plasma. Its verified that mode existence requires that the continuum GAM profile has a maximum as a function of radius. An asymptotic matching technique shows that the eigenmode frequency shifts from the maximum continuum proportional to the square of the local beta.The asymptotic method agrees precisely with the numerical results. [Preview Abstract] |
Thursday, November 15, 2007 11:06AM - 11:18AM |
TO3.00009: The ideal stability of unity beta tokamaks P.-A. Gourdain, S.C. Cowley The theoretical stability of unity beta equilibria was left unresolved for many years. Using modern computational tools, unity beta configurations stable to all ideal MHD criteria (Mercier, high-n ideal ballooning, fixed and free boundary modes) have been discovered. They are based on a double equilibrium solution of the Grad-Shafranov equation. The interior solution is highly diamagnetic (reaching unity beta on axis) but usually unstable to free boundary modes. The outer part of the equilibrium is paramagnetic and acts as a perfectly conducting wall surrounding the diamagnetic solution, in effect stabilizing the free boundary modes. [Preview Abstract] |
Thursday, November 15, 2007 11:18AM - 11:30AM |
TO3.00010: Dust dynamics and diagnostic applications in quasi-neutral plasmas and magnetic fusion Zhehui Wang, Catalin M. Ticos, Jiahe Si, Gian Luca Delzanno, Gianni Lapenta, Glen Wurden Little is known about dust dynamics in highly ionized quasi-neutral plasmas with ca. 1.0 e+20 per cubic meter density and ion temperature at a few eV and above, including in magnetic fusion. For example, dust motion in fusion, better known as UFO's, has been observed since 1980's but not explained. Solid understanding of dust dynamics is also important to International Thermonuclear Experimental Reactor (ITER) because of concerns about safety and dust contamination of fusion core. Compared with well studied strongly-coupled dusty plasma regime, new physics may arise in the higher density quasi-neutral plasma regime because of at least four orders of magnitude higher density and two orders of magnitude hotter ion temperature. Our recent laboratory experiments showed that plasma-flow drag force dominates over other forces in a quasi-neutral flowing plasma. In contrast, delicate balance among different forces in dusty plasma has led to many unique phenomena, in particular, the formation of dust crystal. Based on our experiments, we argue that 1) dust crystal will not form in the highly ionized plasmas with flows; 2) the UFO's are moving dust dragged by plasma flows; 3) dust can be used to measure plasma flow. Two diagnostic applications using dust for laboratory quasi-neutral plasmas and magnetic fusion will also be presented. [Preview Abstract] |
Thursday, November 15, 2007 11:30AM - 11:42AM |
TO3.00011: Dust in Fusion Plasmas: Modeling Approach R.D. Smirnov, A.Yu. Pigarov, S.I. Krasheninnikov, M. Rosenberg, D.A. Mendis The confinement of enormous energy in the plasma of future fusion devices presents numerous challenges associated with unavoidable interactions of the plasma with bounding walls. The interactions both continuous and intermittent lead to destruction of the walls with the release of a significant amount of wall material in the form of atoms, clusters and dust into the plasma. It appears that the dust is not directly affected by the magnetic field and may have a shorter path toward the plasma core, delivering impurities to it. Part of the impurities re-deposits on the plasma facing surfaces as thin films, stimulating further dust production. While in the plasma, the dust particles driven by plasma flows acquire large speeds, this leads to dust spreading over the volume and the surfaces of a fusion device. Taking into account the high reactivity of the dust and its ability to retain tritium, dust also presents important safety issue in the next-step fusion devices. In this work we report on recent progress in numerical simulation efforts with the DUSTT code toward understanding the dust behavior in fusion plasmas. Dust dynamics, transport, statistics, and impact on the plasma are considered. [Preview Abstract] |
Thursday, November 15, 2007 11:42AM - 11:54AM |
TO3.00012: Modeling of Dust-Wall Collisional Characteristics in Tokamaks David Benson, Roman Smirnov Interaction of plasma with plasma facing components in fusion devices leads to dust production, which can enhance impurity inflow toward the core plasma. It is known that the dust particles are accelerated by plasma flows in tokamaks to high speeds ($\sim $1km/s) that under effect of centrifugal acceleration leads to intensive dust-wall collisions. The collisions may lead as to dust fracturing and destruction as well as to avalanche-like mechanism of dust production [1] depending on properties of the dust and the target surfaces. Parameters of the collisions including the coefficient of restitution, the rebound angle distribution, the loss of dust mass as well as the probabilities of dust sticking to the wall and secondary dust production are not well known for fusion related materials and the speed range of interest. In this work the dust-wall collisions are simulated using the LS-DYNA code to explore their effect on the dust transport in tokamaks. Comparison with available experimental data is presented. \newline [1] S.I. Krasheninnikov et al., IAEA-CN-149/TH/P6-18 [Preview Abstract] |
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