Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session BO3: Basic MHD, Turbulence, and Transport |
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Chair: William Dorland, University of Maryland Room: Reunion A |
Monday, November 17, 2008 9:45AM - 9:57AM |
BO3.00001: Finite frequency zonal flows in multi-scale MHD and ITG turbulence Jiquan Li, Yasuaki Kishimoto, Zheng-Xiong Wang, Miho Janvier Different scale fluctuations, such as the macro-scale MHD and micro-scale drift wave turbulence, may directly interplay each other or indirectly through a zonal flow, which has been widely recognized as a stationary coherent structure. The spatio-temporal nature of the zonal flow is important in the plasma transport. For example, the geodesic acoustic mode (GAM) with a finite frequency leads to a reduction of the suppression role of the zonal flow in transport in a toroidal plasma. In this work, we show an oscillating zonal flow with finite frequency in a multi-scale turbulence simulation in a slab plasma based on a 5-field gyrofluid model in which both ion temperature gradient (ITG) and resistive kink-tearing mode (RKTM) fluctuations can coexist through adjusting the ion temperature gradient and the resistivity. The finite frequency is identified to originate from the nonlinear interaction between MHD and ITG fluctuations in a specific sheared magnetic field geometry, not a linear eigen-mode like the GAM. As a result, the ion heat transport is not efficiently suppressed even if the zonal flows have a large amplitude. This implies that MHD fluctuations may deteriorate the favorable role of ITG zonal flows in fusion plasmas. [Preview Abstract] |
Monday, November 17, 2008 9:57AM - 10:09AM |
BO3.00002: Global zonal flow eigen-mode with spatial localization due to the finite band width Yasuaki Kishimoto, Ken Uzawa, Jiquan Li The zonal flow is characterized by a complicate radial structure in plasma experiments and turbulence simulations, which is represented by a radial spectrum. However, in the conventional theoretic analyses on the zonal flow generation such as the coherent mode coupling or wave kinetic equation method, a single zonal mode is usually sampled to interact with the ambient turbulence through a modulation process so that the zonal flow growth rate versus the radial wave-number is deterministically obtained.[1,2] In this work, we show based on Hasegawa-Mima turbulence that the finite band with effect of zonal flow changes the conventional characteristics of the zonal low generation, leading to a spatially localized wave packet with enhanced growth rate which is the same for all zonal components. We refer to it as a global zonal flow eigen mode, which originates from the successive cross coupling among spectral components of zonal flows and turbulent pumps with the production of sidebands. Most interestingly, the enhanced global growth rate is probabilistically determined by the drift wave structure with given spectral distribution and pump energy for different initial phase factors. A simplified theoretic model is advanced to clarify these new features. 1 A. I. Smolyakov, P. H. Diamond, and M. Malkov Phys. Rev. Letts 84, 491(2000) 2 L. Chen, Z. Lin and R. White Phys. Plasma 7, 3129(2000) [Preview Abstract] |
Monday, November 17, 2008 10:09AM - 10:21AM |
BO3.00003: Self-Organized Zonal Flows and Large Scale Magnetic Field Structures in Flute-Mode Turbulence of High-Beta Plasma J. Kindel, V.I. Sotnikov, O.G. Onishchenko, E. Yasin, J.N. Leboeuf Generation of zonal flows with spatial scales of the order of the ion Larmor radius by flute mode turbulence of plasmas with large ratio of the thermal plasma energy to the magnetic field energy is investigated. Nonlinear equations describing the interaction of small-scale flute turbulence with large scale zonal structures and the evolution of the zonal structures are derived. These nonlinear equations are applicable for arbitrary ratio of spatial scales to the ion Larmor radius in the presence of effective gravitational field. It is shown that such a turbulent plasma is the result of a modulation instability of flute waves self-organized into zonal structures. The growth rate of the modulation instability and its dependence on the spatial scales of the pump wave and zonal structures are analyzed when the Rayleigh-Taylor instability (RTI) is suppressed by the effects of finite ion Larmor radius. [Preview Abstract] |
Monday, November 17, 2008 10:21AM - 10:33AM |
BO3.00004: Coherent vortex extraction in drift wave turbulence using orthogonal wavelets Marie Farge, Wouter Bos, Shinpei Futatani, Sadrudin Benkadda, Kai Schneider A wavelet based technique for extracting coherent vortices, called coherent vortex extraction, is applied to simulations of drift wave turbulence. We show that the coherent vorticiy, represented by few degrees of freedom, is responsible for the dynamics and transport. The radial density flux is carried by these coherent vorticy modes. The quasi-hydrodynamic limit shows a local depletion of nonlinearity and can be quantitatively distinguished from the quasi-adiabatic case by the skewness of the probability distribution function of the Weiss-field. [Preview Abstract] |
Monday, November 17, 2008 10:33AM - 10:45AM |
BO3.00005: Nonlinear gyrokinetic simulation of CTEM turbulence and transport dynamics Yong Xiao, Zhihong Lin The collisionless trapped electron turbulence (CTEM) is regarded as one of the main candidates for the electron anomalous transport observed in tokamaks. The electron transport can be more significant in the burning plasma due to the heating of electrons by fusion products. The gyrokinetic particle simulation is applied to study the nonlinear physics and transport properties of CTEM turbulence using the Gyrokinetic Toroidal Code (GTC). The simulation observed a transition from Bohm to GyroBohm scaling with the increase of the system size, with the ITER-scale devices fall in the range of GyroBohm scaling. The transport dynamics study indicates the possibilities of non-diffusive feature of the electron heat transport, which will be further studied. Relevant physical time and spatial scales are analyzed to reveal the transport mechanism. [Preview Abstract] |
Monday, November 17, 2008 10:45AM - 10:57AM |
BO3.00006: Gyrokinetic simulations predict anomalous poloidal rotation in tokamak plasmas Guilhem Dif-Pradalier, Virginie Grandgirard, Yanick Sarazin, Xavier Garbet, Phillippe Ghendrih, Paolo Angelino First-principle based collisionless gyrokinetic theory consensually provides today's deepest insight on turbulence-related problems in plasma physics. Conversely, neoclassical theory describes the effects of binary Coulomb collisions in a toroidal and inhomogeneous magnetic geometry and its consequences on particle trapping. The interplay between turbulence and collisions is a subject of great current focus for first-principle modeling since recent evidences have started to emphasise its relevance for the onset and the control of enhanced confinement regimes in the next-generation devices like Iter. A finite differences Fokker-Planck ion-ion collision operator is implemented in the \emph{full-$f$} and {\it global} GYSELA code and has been thoroughly benchmarked in neoclassical regimes. Two types of simulations are compared, either purely neoclassical or turbulent including neoclassical effects. In each case, three different values of collisionality in the banana regime are investigated. Preliminary results show an enhancement of about $30\%$ of the poloidal rotation of the main ions (Z=1) in the turbulent regime as compared to its neoclassical value. In all cases the radial force balance equation is satisfied within a few percent. Most of this increase comes from the radial electric field. [Preview Abstract] |
Monday, November 17, 2008 10:57AM - 11:09AM |
BO3.00007: Evidence of a Second Order Phase Transformation in Turbulent Fusion Plasmas J.A. Johnson, III, J.B. Titus, C.T. Raynor, E.-D. Mezonlin, J.M. Moller, E.B. Hooper, H.S. McLean, B. Hudson, C.A. Romero-Talamas, R.D. Wood Turbulence physics characterized as a Ginzburg-Landau phase transformation with the tools from BCS Theory predicts a new universal constant for all turbulent systems. At SSPX, with diagnostics for: ion temperature, Ti, from a Compact Neutral Particle Analyzer; electron temperature, Te, from Profile Thomson Scattering; and electron density, ne, from CO2 laser interferometry, we can now test these predictions using the influence of variations in the SSPX helicity injection during a single shot on turbulent magnetic field fluctuations. We will report on these successful tests and their implication for the broad class of isolated turbulent fusion plasma regimes. [Preview Abstract] |
Monday, November 17, 2008 11:09AM - 11:21AM |
BO3.00008: The decay of MHD turbulence in a bounded domain Kai Schneider, Salah Neffaa, Wouter Bos The effect of non periodic boundary conditions on decaying two-dimensional magnetohydrodynamic turbulence is investigated. We consider a circular domain with no-slip boundary conditions for the velocity and where the normal component of the magnetic field vanishes at the wall. Different flow regimes are obtained by starting from random initial velocity and magnetic fields with varying integral quantities. These regimes, equivalent to the ones observed by Ting, Matthaeus and Montgomery [Phys. Fluids {\bf 29}, 3261, (1986)] in periodic domains, are found to subsist in confined domains. We examine the effect of solid boundaries on the energy decay and alignment properties. The final states are characterized by functional relationships between velocity and magnetic field. [Preview Abstract] |
Monday, November 17, 2008 11:21AM - 11:33AM |
BO3.00009: Experimental Study of the Magnetic Rayleigh-Taylor Instability R. Presura, V.I. Sotnikov, C. Plechaty, S. Wright, S. Neff, D. Martinez Dynamic plasma -- magnetic field interfaces are sites of interchange instabilities in a variety of instances, from the pinch effect to supernova explosions in the interstellar magnetic field. In experiments at the Nevada Terawatt Facility, the development of the Rayleigh-Taylor instability (RTI) was investigated at the front of plasma flows decelerated by external magnetic field. The plasma flow was produced by laser ablation and the magnetic field was produced independently, using a pulsed power generator. Varying the energy and the duration of the laser pulse led to the variation of the plasma parameters and consequently of the mechanism of RTI growth. The usual magneto-RTI ($k\rho _i <<1)$ and the large-Larmor-radius-RTI ($k\rho _i >>1)$ were actuated in experiments ($k$ is the wavenumber and $\rho _i $ is the ion Larmor radius). In principle, this approach allows access to the finite Larmor radius region, predicted theoretically to have a stabilizing effect upon the RTI. [Preview Abstract] |
Monday, November 17, 2008 11:33AM - 11:45AM |
BO3.00010: New approach to magnetohydrodynamic shocks: duality under time reversal Hans Goedbloed The traditional theory of MHD shocks starts from jump conditions obtained by integrating the nonlinear evolution equations through the discontinuity, then eliminates jumps that do not correspond to entropy increase across the shock, and finally eliminates the intermediate shocks because of supposed non- evolutionarity. Since intermediate shocks do occur in nature and computations (the only dispute is on the time-scale of disintegration) and entropy-decreasing discontinuities may be considered as reverse shocks (with upstream and downstream parameters interchanged), these restrictions are not necessary and hamper understanding of the nonlinear structure of the shock conditions. When the restrictions are dropped, the MHD shock conditions may be reduced to their most concise, three- parameter, distilled form by consistent use of the de Hoffmann- Teller transformation (1950) and of scale independence of the MHD equations (Goedbloed \& Poedts, 2004). They then exhibit distinct time reversal duality between entropy-allowed shocks and entropy-forbidden jumps. This yields a new classification of MHD shocks using monotonicity with respect to Alfv\'en Mach numbers, it exhibits the central role of intermediate discontinuities, and permits straightforward construction of all relevant dimensionless quantities of the shocks. The new conditions may easily be incorporated into large-scale nonlinear computations. [Preview Abstract] |
Monday, November 17, 2008 11:45AM - 11:57AM |
BO3.00011: Turbulence Variations in Plasmas driven by Transient Magnetic Fields Chavis T. Raynor, Alonzo B. Alexander, Charlemagne C. Akpovo, Joseph A. Johnson, III When the plasma in a D.C. glow discharge tube is suddenly exposed to an axial magnetic field of substantial strength, the plasma column undergoes a rotation about its axis of symmetry. Given that the plasma is already turbulent, we report our studies of the evolution of turbulence under the influence of this transient magnetic perturbation, focusing on changes in the transport parameters and the underlying turbulent ``force.'' [Preview Abstract] |
Monday, November 17, 2008 11:57AM - 12:09PM |
BO3.00012: Rapid generation of angular momentum in bounded magnetized plasma Wouter Bos, Salah Neffaa, Kai Schneider Direct numerical simulations of two-dimensional decaying MHD turbulence in bounded domains show the rapid generation of angular momentum in non-axisymmetric geometries. It is found that magnetic fluctuations enhance this mechanism. The subsequent generation of a magnetic angular momentum or angular field is due to the relaxation of the flow towards an aligned state. For axi-symmetric geometries the generation of angular momentum is absent, nevertheless a weak angular field can be observed. The derived evolution equations for both angular momentum and angular field yield possible explanations for the observed behaviour. [Preview Abstract] |
Monday, November 17, 2008 12:09PM - 12:21PM |
BO3.00013: Turbulent transport of energetic particles: Mechanisms and scalings Thilo Hauff, Tilman Dannert, Frank Jenko The interaction of energetic particles (like alpha particles or beam ions) with the background tokamak microturbulence is investigated. We study the multitude of different decorrelation mechanisms and obtain a validity condition for `orbit averaging', which is shown to be a very crucial issue for the level of fast particle transport and directly related to to the magnetic shear. Furthermore, resonance mechanisms between the perpendicular particle drifts and the diamagnetic drifts of the bulk plasma are observed. We will show that for beam ions the transport may stay at a significant level for particle energies up to about 10 times the thermal energy of the background plasma, and then falls off like $E^{-1}$, which is much slower than orbit averaging would suggest. Our general studies are confirmed by means of nonlinear gyrokinetic simulations with the GENE code. Comparing the latter with quasilinear simulations, one finds that it is indeed the turbulent nature of the advecting field which is responsible for the slow decay of the particle transport with increasing energy. Finally, we discuss turbulent transport of energetic particles as a candidate for explaining recent experimental results on ASDEX Upgrade. [Preview Abstract] |
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