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 YI2: Transport and Zonal Flows |
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Chair: Greg Hammett, Princeton Plasma Physics Laboratory Room: Landmark B |
Friday, November 21, 2008 9:45AM - 10:15AM |
YI2.00001: On the nature of transport across sheared zonal flows in electrostatic ion-temperature-gradient turbulence gyrokinetic simulations Invited Speaker: In this contribution, we argue that the usual picture for the suppression of turbulent transport across a stable sheared flow based on a reduction of diffusive transport coefficients is, by itself, incomplete. By means of gyrokinetic simulations of electrostatic, collisionless ion-temperature-gradient turbulence in toroidal geometry with DIII-D-like parameters using the $\delta$f, PIC gyrokinetic UCAN code, we show that the nature of the radial transport across poloidal and toroidal zonal flows is altered fundamentally, and changes from diffusive to correlated and subdiffusive. In the case in which the zonal flows are self-consistently driven by the turbulence, the radial transport gains an additional non-Gaussian character. The analysis has been done using several techniques imported from the theory of stochastic transport processes. The results obtained suggest that modeling transport across these flows via reduced diffusivities or conductivities may be inadequate. They also point out the need for a reexamination of the current understanding of how the dynamics of transport across sheared flows are set. Some plausible mechanisms to fill in this role will be discussed. [Preview Abstract] |
Friday, November 21, 2008 10:15AM - 10:45AM |
YI2.00002: Turbulent Transport Regulation by Zonal Fows in Helical Systems with Radial Electric Fields Invited Speaker: Zonal flows are now well known to play a critical role in regulation of turbulent transport in plasmas. Therefore, for the purpose of improving plasma confinement, it is very important to investigate effects of magnetic configuration on zonal flows generated by turbulence [1-3]. Furthermore, in helical systems such as heliotrons and stellarators, the neoclassically-driven ExB rotaion, which is distinguished from the microscopic sheared ExB zonal flows, is expected to strongly influence not only neoclassical transport but also turbulent transport through enhancing zonal-flow generation [4,5]. In this work, gyrokinetic theory and simulation results are presented to show how the helical geometry and the ExB rotaion affect zonal flows and ion temperature gradient (ITG) turbulent transport. Larger zonal-flow generation and turbulent transport reduction are found by the gyrokinetic ITG turbulence simulation for the neoclassically optimized helical configuration, in which radial drift velocities of ripple-trapped particles decrease [6]. Further zonal-flow enhancement by the ExB rotaion can occur effectively with neoclassical optimization due to the reduction of radial displacements of ripple-trapped particles. These findings are consistent with the confinement improvement observed in the inward-shifted configuration of the Large Helical Device. Also, it is expected that this ExB effect on zonal flows causes the ion mass dependence of the ITG turbulent transport to differ from the conventional gyro-Bohm scaling in a favorable way because the zonal-flow generation increases with increasing the ratio of the ExB velocity to the ion thermal velocity. [1] H. Sugama and T.-H.Watanabe, Phys. Rev. Lett. 94, 115001 (2005). [2] H. Sugama and T.-H. Watanabe, Phys. Plasmas 13, 012501 (2006). [3] T.-H. Watanabe, H. Sugama, and S. Ferrando-Margalet, Nucl. Fusion 47, 1383 (2007). [4] H. Sugama, T.-H. Watanabe, and S. Ferrando-Margalet, Joint Conference of 17th International Toki Conference on Physics of Flows and Turbulence in Plasmas and 16th International Stellarator/Heliotron Workshop 2007 (Toki, Japan, 2007), PI-08. [5] H. E. Mynick and A. H. Boozer, Phys. Plasmas 14, 072507 (2007). [6] T.-H. Watanabe, H. Sugama, and S. Ferrando-Margalet, Phys. Rev. Lett. 100, 195002 (2008). [Preview Abstract] |
Friday, November 21, 2008 10:45AM - 11:15AM |
YI2.00003: Role of Zonal Flows in Trapped Electron Mode Turbulence through Nonlinear Gyrokinetic Particle and Continuum Simulation Invited Speaker: \def\vereq#1#2{\lower3pt\vbox{\baselineskip1.5pt \lineskip1.5pt\ialign{$#1\hfill##\hfil$\crcr#2\crcr\sim\crcr}}} \def\gtrsim{\mathrel{\mathpalette\vereq>}} Trapped electron mode (TEM) turbulence exhibits rich zonal flow dynamics, which depends strongly on plasma parameters. The role zonal flows in TEM turbulence is explored through a series of linear and nonlinear gyrokinetic simulations using both PIC (the GEM code) and continuum (the GS2 code) methods. A new nonlinear upshift $[1,2]$ in the TEM critical density gradient (associated with zonal flow dominated states near threshold) increases strongly with collisionality, for density gradient driven cases. In contrast, zonal flows have little effect on TEM turbulence with strong electron temperature gradients and $T_e = 3T_i$ $[3]$. This apparent contradiction has been resolved in parameteric studies showing that zonal flows are weaker as the electron temperature gradient and $T_e/T_i$ increase $[4]$. The parametric variation of zonal flows is consistent with linear stability properties and nonlinear instability theory. A new stability diagram based on 2,000 GS2 simulations clarifies the roles of resonant and non-resonant TEM, ``ubiquitous,'' and electron temperature gradient (ETG) driven modes. Larger electron temperature gradients couple TEM and ETG modes, resulting in short wavelengths $k_{\alpha}\rho_s>1$. Accordingly, a sudden onset of nonlinear fine scale structure is seen for $\eta_e\equiv d\ln T_e/d\ln n_e \gtrsim 1$. For short wavelengths, the ions are more adiabatic, the zonal flow potential $\langle\phi\rangle \sim \langle n \rangle /k_r^2\rho_s^2$ is weaker, and secondary instability growth rates [5] are reduced. \par $[1]$ D. R. Ernst {\em et al.} Phys. Plasmas {\bf 11} (2004) 2637.\\ $[2]$ D. R. Ernst {\em et al.}, in Proc. 21st IAEA Fusion Energy Conference, Chengdu, China, 2006, paper IAEA-CN-149/TH/1-3.\\ $[3]$ T. Dannert and F. Jenko, Phys. Plasmas {\bf 12} 072309 (2005).\\ $[4]$ J. Lang, Y. Chen, and S. Parker, Phys. Plasmas {\bf 14}, 082315 (2007); also M. Hoffman and D. R. Ernst, BAPS (2007).\\ $[5]$ B. N. Rogers, W. Dorland, M. Kotschenreuther, Phys. Rev. Lett. {\bf 85}(25) 5336 (2000). [Preview Abstract] |
Friday, November 21, 2008 11:15AM - 11:45AM |
YI2.00004: Reduction of TEM/ETG-scale Density Fluctuations in the Core and Edge of H-mode DIII-D Plasmas Invited Speaker: Improved confinement during H-mode has been linked to $E\times B$ shear suppression of large-scale ($k_\theta \rho_s \leq 0.3$) turbulence within an edge transport barrier. While larger scale eddies are preferentially suppressed by increased shear flow in this paradigm, the effects on smaller scale (TEM/ETG-scale) turbulence are less certain. Recent results from DIII-D provide the first experimental evidence that intermediate-scale turbulence ($1 < k_\theta \rho_s \leq 3$) together with larger-scale electron temperature fluctuations [1] are also reduced promptly at the L-H transition. These reductions are not confined to the edge region. Intermediate-scale density fluctuations obtained via Doppler backscattering, are significantly reduced (30\%-50\%) over a range of normalized radii ($0.5 \leq r/a \leq 0.85$) within a few ms of the L-H transition. A larger reduction ($\geq$75\%) is observed at the top of the pedestal ($r/a \sim 0.9$) within 0.2~ms. In addition, low-k electron temperature fluctuations ($k_\theta\rho_s \leq 0.3$, from correlation ECE) are strongly reduced ($>$75\%) at the L-H mode transition and during QH-mode ($r/a \sim 0.7$). Gyrokinetic simulation results [2] predict that $\tilde T_e$ fluctuations contribute significantly to L-mode electron heat transport, hence, the observed reduction is likely an important factor in the observed improved H-mode electron heat confinement ($\chi_e^{QH}/\chi_3^L < 0.25$). Doppler backscattering is also utilized to probe time-dependent shear flows (i.e. zonal flows). The results clearly indicate that zonal flow levels are anti-correlated with the amplitude of intermediate-scale density turbulence in L-mode, suggesting that zonal flows play an important role in turbulence/transport regulation.\par \vskip3pt \noindent [1] L.~Schmitz et al., Phys.\ Rev.\ Lett.\ {\bf 100}, 035002 (2008).\hfil\break [2] A.E.\ White et al., Phys.\ Plasmas {\bf 15}, 056116 (2008).\par [Preview Abstract] |
Friday, November 21, 2008 11:45AM - 12:15PM |
YI2.00005: Electron gyro-scale fluctuations in NSTX plasmas Invited Speaker: The mechanisms responsible for anomalous electron thermal transport are not well understood, but recent gyrokinetic simulations suggest electron temperature gradient (ETG) turbulence on the electron gyro-scale (k$_{\bot }\rho _{e}<$1) may be culpable. The National Spherical Torus Experiment (NSTX) is well-suited to investigate the connection between ETG turbulence and electron thermal transport because the ETG mode is linearly unstable across large regions of NSTX plasmas and electron thermal transport is anomalous in all NSTX confinement regimes. In contrast, ion thermal transport is generally neoclassical in NSTX H-mode plasmas. To investigate the connection between ETG turbulence and electron thermal transport, a collective scattering system has been installed to measure electron gyro-scale density fluctuations with spatial and k-space localization. The system measures up to five distinct fluctuation wave-vectors with k$_{\bot }\rho _{e}<$0.6 and k$_{\bot }<$20 cm$^{-1}$, and measured wave-vectors are primarily radial. Measurements show rich turbulent dynamics on the electron gyro-scale in central and outer regions of NSTX plasmas. Both L-mode and H-mode discharges show enhanced fluctuations when the electron temperature gradient exceeds the ETG critical gradient, thus supporting the conjecture that ETG turbulence exists in NSTX plasmas. In H-mode discharges, fluctuations decrease at higher toroidal field values and when the equilibrium E$\times $B flow shear rate is comparable to GS2 linear growth rates. For a limited set of core measurements in H-mode plasmas, transport analysis with TRANSP shows electron thermal transport in the central region of the plasma decreases when fluctuations increase, thus suggesting the observed fluctuations have limited impact upon transport in this region in the discharges studied. [Preview Abstract] |
Friday, November 21, 2008 12:15PM - 12:45PM |
YI2.00006: Gyrokinetic Turbulence Driven Toroidal Momentum Transport and Comparison to Experimental Observations Invited Speaker: Global gyrokinetic simulations using the GTS code [1] have found that a large inward flux of toroidal momentum is driven robustly in the post saturation phase of ion temperature gradient (ITG) turbulence. As a consequence, core plasma rotation spins up resulting in $\Delta u_{\parallel}$ a few percent of $v_{th}$ in the case with no momentum source at the edge. The underlying physics for the inward flux is identified to be the generation of residual stress due to the $k_ {\parallel}$ symmetry breaking [2] induced by self-generated zonal flow shear which is quasi-stationary in global simulations. The elatively low level momentum flux in the long- time steady state appears to be approximately diffusive, with effective $\chi_{\phi}/\chi_i$ on the order of unity, in broad agreement with experimental observations and theory predictions for ITG turbulence [3]. Neoclassical simulations using the GTC- NEO code [4] also show that the ion temperature gradient can drive a significant inward nondiffusive momentum flux. However, the overall neoclassical contribution to the momentum transport is negligibly small compared to experimental levels for NSTX and DIII-D plasmas. It is also found that finite residual turbulence can survive strong mean ExB shear flow induced damping. This residual turbulence in the presence of strong $\bf{E}\times\bf{B}$ shear may drive an insignificant ion heat flux reasonably close to the neoclassical value, and a finite momentum flux significantly higher than the neoclassical level. Moreover, the equilibrium $\bf{E}\times\bf{B}$ flow shear is found to reduce the turbulence driven transport for energy more efficiently than for momentum. These findings may offer an explanation for rather peculiar observations of near neoclassical ion heat and anomalous momentum transport in experiments, which has been often observed in various machines, but with little theoretical understanding. [1] W.X. Wang et al., Phys. Plasmas 14, 072306 (2007). [2] O.D. Gurcan et al., Phys. Plasmas 14, 042306 (2007). [3] N. Mattor and P.H. Diamond, Phys. Fluids 31, 1180 (1988). [4] W.X. Wang et al., Phys. Plasmas 13, 082501 (2006). In collaboration with: T.S. Hahm, S. Ethier, S.M. Kaye, W.W. Lee, G. Rewoldt, W.M. Tang (PPPL), P.H. Diamond (UCSD), M. Adams (Columbia U.). [Preview Abstract] |
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