Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session UO3: Basic Plasma: Turbulence, Transport and Flows |
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Chair: Troy Carter, University of California, Los Angeles Room: Adam's Mark Hotel Governor's Square 15 |
Friday, October 28, 2005 9:30AM - 9:42AM |
UO3.00001: L-H transition physics and energetics of zonal flows in the H-1 heliac Michael Shats, Hua Xia Finite frequency zonal flows, or geodesic acoustic modes observed in the H-1 heliac [1], play an important role in plasma dynamics during spontaneous L-H transitions [2]. Here we describe detailed dynamics of the turbulence spectra as plasma approaches the bifurcation from L to H mode. The inverse energy cascade [3] leads to the accumulation of the turbulence energy in the largest vortex seen as the mean ExB flow, similar to spectral condensation in 2D fluid turbulence [4], until geodesic effects due to the compressibility of the flow lead to the generation of the side-band modes, as shown in numerical simulation [5]. The number of these modes gradually increases towards the L-H transitions. It is speculated that the shear decorrelation mechanism eventually forbids the geodesic conversion of the Er energy into the energy of the side-band modes which reinforces spectral condensation and leads to L-H transition.\newline [1] M. G. Shats and W. M. Solomon, Phys. Rev. Lett.\textbf{ 88, }045001 (2002).\newline [2] H. Punzmann, M. G. Shats, Phys. Rev. Lett. \textbf{93}, 125003 (2004).\newline [4] H. Xia and M. G. Shats, Phys. Plasmas \textbf{11,} 561 (2004).\newline [3] M. Shats et al, Phys. Rev. E \textbf{71}, 046409 (2005) \newline [5] B.D. Scott, New J. Physics \textbf{7}, 92 (2005) [Preview Abstract] |
Friday, October 28, 2005 9:42AM - 9:54AM |
UO3.00002: SOL-dynamamics in the TJ-K torsatron Franko Greiner, Tim Happel, Navid Mahdizadeh, Mirko Ramisch, Ulrich Stroth In the dimensionally similar low temperature plasma of torsatron TJ-K, Langmuir probe measurements with high spatio-temporal resolution are possible over the whole plasma cross section [1]. In the region of closed fieldlines (bulk plasma) the dynamic of the plasma is dominated by drift waves. In the scrape off layer (SOL) dynamic is observed to be curvature driven. Inserting a massive poloidal limiter into the device gives the unique opportunity to enlarge the SOL and to study the plasma in detail. Even when the plasma diameter is reduced to half, the drift dynamic in the bulk plasma remain unchanged. The velocity of the drift structures in the bulk plasma is mainly diamagnetic, structures in the SOL are advected by ExB-velocity. The transition from the drift dynamic of the bulk plasma to the curvature driven dynamics in the SOL is investigated by means of cross-correlation and conditional averaging technique. [1] U. Stroth et al, Phys. Plasmas 11, 2558 (2004) [Preview Abstract] |
Friday, October 28, 2005 9:54AM - 10:06AM |
UO3.00003: Coherent drift wave modes and turbulent structures in a high density helicon plasma Olaf Grulke, Christiane Schroeder, Thomas Windisch, Thomas Klinger Helicon plasma sources are characterized by high plasma densities at low electron temperature. This results in a highly collisional plasma with collision frequencies orders of magnitude higher than characteristic plasma drift frequencies such as E$\times$B and diamagnetic frequencies. In the helicon plasma of the cylindrical linear device, low-frequency coherent fluctuations with mode numbers ranging from $m=2\ldots 8$ are excited using the ambient magnetic field as a control parameter. Based on detailed investigation of fluctuation degrees and phase shifts between plasma density and plasma potential fluctuations, these modes are identified as coherent drift modes. Most striking is the measured finite wavelength of fluctuations parallel to the magnetic field, which unambiguously distinguishes them from flute modes. In contrast to low collisional plasmas the spatial azimuthal mode structure is of spiral-like shape. Numerical solution of the linear drift mode eigenvalue equation reveals that the spiral structure is a result of radial gradients in the collision frequencies. At high magnetic field the drift wave develops to a weakly developed turbulent state. Here, no coherent modes are found but fluctuations develop coherent turbulent structures. The propagation properties of these structures are investigated in detail with special attention to deviations from pure azimuthal propagation and the associated radial transport. [Preview Abstract] |
Friday, October 28, 2005 10:06AM - 10:18AM |
UO3.00004: Fractal ideas and percolation scalings for turbulent transport Oleg Bakunin The essential deviation of transport processes in turbulent fluids and plasma from the classical behavior leads to the necessity of search for new approaches and scaling laws [1]. A variety of turbulence forms requires not only special description methods, but also an analysis of general mechanisms for different turbulence types. One such mechanism is the percolation transport [1,2]. Its description is based on the idea of long-range correlations, borrowed from theory of phase transitions and critical phenomena. The present paper considers the influence of zonal flow and time-dependence effects on the passive scalar behavior in the framework of the percolation approach. It is suggested to modify the renormalization condition of the small parameter of percolation model in accordance with the additional external influences superimposed on the system [3-4]. This approach makes it possible to consider simultaneously both parameters: the characteristic drift velocity $U_{d}$ and the characteristic perturbation frequency $w.$ The effective diffusion coefficient D$_{\rm eff}$$\sim$ $w^{7/10}$ satisfactory describes the low-frequency region $w$ in which the long-range correlation effects play a significant role. This scaling agrees well with the analogous expressions that describe low frequency regimes of transport [1,2]. [1] Isichenko M B 1992 \textit{Rev. Mod. Phys}. 64 961 [2] Bakunin O G 2004 \textit{Reports on Progress in Physics} \textbf{67} 965 [3] Bakunin O G 2005 \textit{Physica }A \textbf{345} 1 [4] Bakunin O G 2005 \textit{J. Plasma Physics } \textbf{71} 756. [Preview Abstract] |
Friday, October 28, 2005 10:18AM - 10:30AM |
UO3.00005: Modeling coherence decay in broad band triplet interaction Felipe Rizzato, Renato Pakter, Marcos Frichembruder In the present analysis we study the transition from coherent to incoherent dynamics in a nonlinear triplet of broad band combs of waves. We first reduce the original set of equations into a set where all submodes within a comb interact democratically with all submodes in the remaining combs. We then develop a spectral formalism that in a self-contained way enables: (i) to determine the point of the transition; (ii) to obtain a convenient set of low dimensional equations modeling the full dynamics. [Preview Abstract] |
Friday, October 28, 2005 10:30AM - 10:42AM |
UO3.00006: Turbulence Onset as a Continuous Phase Transition in Nanoscale Plasmas Joseph Johnson, Ephrem Mezonlin, Stephen Roberson Using a standard set of parameters, turbulent signatures are observed in a roughly 150 nanosecond duration laser--induced plasma from a 4 nanosecond Nd-Yag 450 mJ/pulse laser. Transport parameters for such plasmas show a lambda-like dependence on turbulent fluctuation energy. In this context, the spectral index is defined as a transport parameter when it is determined as the fractional change of turbulent fluctuation energy with respect to the fractional change in frequency. When we postulate a simple dependence of turbulent energy on the ion temperature, the phase space diagram at the onset of turbulence can be derived based on the pressure and temperature in the turbulent plasma along with the measured values of critical spectral index. When we use the critical characteristic fluctuation frequency as a measure of the turbulence inducing constraint, the effective turbulent force is seen to decrease, at fixed temperatures, with increasing pressure. [Preview Abstract] |
Friday, October 28, 2005 10:42AM - 10:54AM |
UO3.00007: Correlation of Electron Temperature and Electron Density to Standard Turbulent Parameters in a Laser Induced Plasma S.D. Roberson, C. Akpovo, E.D. Mezolin, J.A. Johnson III Single pulse plasmas of several nanosecond duration are created at various pressures by a Nd-Yag laser focused on an air target inside a sealed chamber. The evolutions in time of various neutral and singly ionized lines were recorded at sampling rates between 5 and 10 GS/s. Standard calculations of turbulent parameters such as the characteristic frequency, chaotic dimensions, spectral index and turbulent fluctuation energy are performed for various time periods in the evolution of the plasma. A critical turbulent energy is determined at various densities in the sealed chamber. The electron temperature and electron density of the plasma are determined at various densities of the target gas. The possible relationships between electron density, electron temperature, and standard turbulent parameters are explored. These relationships are also qualitatively explained using a second-order phase transformation approach to the transition from a non-turbulent to a turbulent state. [Preview Abstract] |
Friday, October 28, 2005 10:54AM - 11:06AM |
UO3.00008: Fokker-Planck Simulations of Transport in Magnetized Hohlraum Gas-Fill Plasma Robert Kingham, Michael J. Edwards Conditions in the heated `gas-fill' plasma in ICF hohlraums mean that the collisional mean-free-path is a significant fraction of the important scale-lengths. This suggests that non-local transport effects are prevalent. However, significant levels of $\nabla $n$\times \nabla $T magnetic fields have been observed in Nova scale hohlraums. Conceivably, strong magnetization could mitigate non-locality. We report on Fokker-Planck simulations showing the effect of magnetic fields on thermal transport. They were carried out using IMPACT [Kingham {\&} Bell, J. Comput. Phys. \textbf{194}, 1 (2004)] and focus in on a representative region of plasma underneath a typical heater beam. The results suggest that the usual figure of merit for non-local effects in the absence of magnetic fields, k$\lambda _{c}$ (where $\lambda _{c}=\surd {\rm g}\lambda _{ei}{\rm g}\lambda _{ee})$, can be extended in an appropriate way to magnetized plasma. This work complements recent analytical treatments of non-local heat flow in magnetized plasma [Brantov \textit{et al.} Phy. Plasmas \textbf{10}, 4633 (2003); Frolov \textit{et al.} Submitted to Phys. Plasmas] and additionally includes phenomena (e.g. Nernst advection) not present in those models. [Preview Abstract] |
Friday, October 28, 2005 11:06AM - 11:18AM |
UO3.00009: Magnetic-field generation in helical turbulence Stanislav Boldyrev, Fausto Cattaneo, Robert Rosner We investigate analytically the amplification of a weak magnetic field in a homogeneous and isotropic turbulent flow lacking reflectional symmetry (helical turbulence). We propose that spectral distributions of magnetic energy and magnetic helicity can be found as eigenmodes of the self-adjoint, Schr\"odinger-type system of evolution equations. This implies that large-scale and small-scale magnetic fluctuations cannot be effectively separated, and that the conventional $\alpha$-model is, in general, not an adequate description of the large-scale magnetic-dynamo mechanism. As a consequence, the correct numerical modeling of such processes should resolve magnetic fluctuations down to very small, resistive scales. [Preview Abstract] |
Friday, October 28, 2005 11:18AM - 11:30AM |
UO3.00010: Shell to shell energy transfer in turbulent MHD flows Alexandros Alexakis, Pablo Mininni, Annick Pouquet We investigate the transfer of energy from large scales to small scales in fully developed forced three-dimensional MHD-turbulence by analyzing the results of direct numerical simulations in the absence of an externally imposed uniform magnetic field. Our results show that the transfer of kinetic energy from the large scales to kinetic energy at smaller scales, and the transfer of magnetic energy from the large scales to magnetic energy at smaller scales, are local, as is also found in the case of neutral fluids, and in a way that is compatible with Kolmogorov (1941) theory of turbulence. However, the transfer of energy from the velocity field to the magnetic field is a highly non-local process in Fourier space. Energy from the velocity field at large scales can be transfered directly into small scale magnetic fields without the participation of intermediate scales. Some implications of our results to MHD turbulence modeling are also discussed. [Preview Abstract] |
Friday, October 28, 2005 11:30AM - 11:42AM |
UO3.00011: Axisymmetric equilibria with anisotropic resistivity and toroidal flow George Poulipoulis, George Throumoulopoulos, Henri Tasso The equilibrium of an axisymmetric magnetically confined plasma with anisotropic resistivity and toroidal flow is investigated in the framework of MHD. The stationary states are determined by an elliptic differential equation for the poloidal magnetic flux function, a Bernoulli equation for the pressure and two relations for the resistivity components parallel and perpendicular to the magnetic field. The flow can affect the equilibrium properties solely in the presence of toroidicity because in the limit of infinite aspect ratio the axial velocity does not appear in the equilibrium equations. The equilibrium characteristics of a tokamak with rectangular cross-section are studied by means of eigenfunctions in connection with exact solutions for the cases of ``compressible" flows with constant temperature but varying density on magnetic surfaces and incompressible ones. The impact of the flow and the aspect ratio on the resistivity components, electric field and toroidal current density is evaluated. [Preview Abstract] |
Friday, October 28, 2005 11:42AM - 11:54AM |
UO3.00012: Improved energy principle for a linear stability of moving plasma Victor Ilgisonis, Ivan Khalzov, Andrei Smolyakov The way of how to improve the energy principle for linear MHD stability of three-dimensional, inhomogeneous, compressible, moving plasma is suggested. The key point of the analysis is the accounting for a new set of integrals inherent in the linearized plasma dynamics that was not earlier discussed in the literature. Sufficient stability condition for flowing plasma is obtained. The condition appears to be softer (easier to be satisfied) than all previously known variational stability conditions. Analytical example is presented to demonstrate the use of the method for usual fluid rotating around a gravity center (accretion disc). In this case, the obtained stability condition is shown to be both necessary and sufficient that confirms the fruitfulness of the suggested approach. [Preview Abstract] |
Friday, October 28, 2005 11:54AM - 12:06PM |
UO3.00013: Transverse imaging of high velocity plasma flow in the target chamber of CTIX Stephen Howard, David Hwang, Robert Horton, Russell Evans, Samuel Brockington The Compact Toroid Injection eXperiment injects high velocity (200 km/s) magnetized plasma into a target chamber with large windows (0.5 m by 0.2 m) that allow transverse imaging of a large volume of plasma with fast digital cameras at sub-microsecond exposures. Neutral gas puffing in the target region greatly enhances plasma brightness, and spatial variation of image brightness can be used to infer the spatial fluctuations of the plasma electron flux density. These fluctuations have been observed with fast intensified and non-intensified cameras, with and without the use of narrow bandpass optical filters. Some coherent density waves have been measured, and we are working to resolve the structure of shocks and turbulence that are expected to occur in the interaction region. By varying the CT injection velocity, the magnetic Reynolds number of the plasma flow can be controlled, and a large dynamical operating space can be explored. This work is supported by DOE grant $\#$ DEFG0203ER54732. [Preview Abstract] |
Friday, October 28, 2005 12:06PM - 12:18PM |
UO3.00014: The role of boundary conditions in the study of the turbulent diffusion of magnetic fields Lara Silvers In this paper we discuss the role of the choice of the boundary condition in numerical simulations to examine the form of the quenching of the turbulent magnetic diffusivity. We shall present the results of two different models. In the first models we show that if there is no net flux of A$^{2}$ through the boundary of the domain then the turbulent magnetic diffusivity will be catastrophically quenched. In the second model we show that it is an advective flux of A$^{2}$ through the boundary that could alleviate such a catastrophic quench. [Preview Abstract] |
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