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 YI3: Edge Turbulence |
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Chair: Walter Guttenfelder, Princeton Plasma Physics Laboratory Room: Ballroom BC |
Friday, November 2, 2012 9:30AM - 10:00AM |
YI3.00001: Intermittent fluctuations in the Alcator C-Mod scrape-off layer Invited Speaker: Odd Erik Garcia Cross-field transport of particles and heat in the scrape-off layer (SOL) of magnetically confined plasmas is dominated by radial motion of blob-like structures. The average radial particle and heat fluxes caused by such filaments depend on their amplitude distribution and frequency of occurrence. The statistical properties of plasma fluctuations in the SOL are thus crucial for development of a first-principles description of transport and main-chamber interactions. Plasma fluctuations in the Alcator C-Mod SOL have been investigated by analysis of long data time series from Langmuir probe measurements and gas puff imaging at the outboard mid-plane region in Ohmically heated discharges. This reveals frequent occurrence of large amplitude bursts with a fast rise and slow decay. The waiting times between burst events and the burst amplitudes are both found to be exponentially distributed. This implies that large amplitude bursts occur randomly at a constant average rate in the far SOL and are uncorrelated. Based on these properties, a novel stochastic model for the intermittent SOL plasma fluctuations has been constructed. Its input parameters are the burst duration and the waiting time and amplitude distributions. The role of these quantities for large SOL plasma densities and fluctuation levels is elucidated. As a direct consequence of this simple model, the mean plasma density is shown to be proportional to the average burst amplitude and the ratio of the burst duration and average waiting time. An additional consequence is that there must be a parabolic relation between the skewness and kurtosis moments. For exponentially distributed burst waiting times and amplitudes, the stochastic model reveals that the probability density function (PDF) for the plasma fluctuations is a Gamma distribution. This distribution can be rewritten solely in terms of the mean and rms values of the plasma density. Accordingly, it does not involve any fit parameters when compared to experimental measurements. The PDF changes from a normal distribution for small relative fluctuation levels, typical for the near SOL, to an exponential distribution for relative fluctuations of order unity, typical for the far SOL. These predictions of the stochastic model are shown to compare well with the experimental measurements. [Preview Abstract] |
Friday, November 2, 2012 10:00AM - 10:30AM |
YI3.00002: Edge Sheared Flows and Blob Dynamics Invited Speaker: J.R. Myra The dynamics of blob-filaments [S. I. Krasheninnikov, et al. J. Plasma Phys. 74, 679 (2008); D. A. D'Ippolito, et al., Phys. Plasmas 18, 060501 (2011)] in the strongly radially inhomogeneous edge and scrape-off-layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and interaction. The work is motivated by the potential importance of edge sheared flows for turbulence regulation, (e.g. the L-H transition), and the influence of flows on the character of emitted blob-filament structures which ultimately contact plasma-facing components. To study the dynamics of blobs and sheared flows, we employ both numerical simulations and experimental data analysis. The simulations use the fluid-based 2D curvature-interchange model embedded in the SOLT (SOL turbulence) code [D. A. Russell, et al, Phys. Plasmas 16, 122304 (2009)]. A blob-tracking algorithm has also been developed and applied to NSTX and Alcator C-Mod data. The algorithm is based on 2D time-resolved images from the gas puff imaging (GPI) diagnostic [S. J. Zweben, et al. Phys. Plasmas 9, 1981 (2002)]. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. Seeded blob simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale and direction of perpendicular (approximately poloidal) flows and the inferred Reynolds forces, poloidal reversal of blob tracks, and blob trapping and/or ejection. Simulation and experimental data comparisons permit the inference of dynamical mechanisms associated with blob motion and sheared flow generation in these shots, and their relation to previous theoretical work. [Preview Abstract] |
Friday, November 2, 2012 10:30AM - 11:00AM |
YI3.00003: BOUT Simulations of Drift Resistive Ballooning L-mode Turbulence in the Edge of the DIII-D Tokamak Invited Speaker: Bruce Cohen Progress is reported on simulations of electromagnetic drift-resistive ballooning turbulence in realistic single-null tokamak geometry using the BOUT three-dimensional fluid code [1] that solves Braginskii-based fluid equations [2]. The simulation domain models the actual magnetic geometry of the DIII-D tokamak. The simulations follow unstable drift resistive ballooning turbulence in the edge region to saturation. Fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes are compared to experimental probe and beam-emission-spectroscopy data for a well-characterized L-mode discharges in DIII-D. Post-processing of the simulation data using synthetic diagnostics facilitates the comparisons. The simulations are comprised of a suite of runs in which the physics model is extended to include more fluid fields and physics terms. The relative agreement of the simulation results with the experimental data improves as more physics is included in the model. The simulations yield results for fluctuation amplitudes, correlation lengths, particle and energy fluxes and diffusivities in reasonable agreement with measurements near the outer midplane of the discharge. The effects of sheared ExB poloidal rotation are included, and a density scan is presented.\\[4pt] [1] X. Q. Xu, and R. H. Cohen, Contrib. Plasma Phys. 36 (1998) 158.\\[0pt] [2] S. Braginskii, ``Transport Processes in a Plasma,'' in Reviews of Plasma Physics, Vol. 1, ed. M. A. Leontovich (Consultants Bureau, New York, 1965), p. 205. [Preview Abstract] |
Friday, November 2, 2012 11:00AM - 11:30AM |
YI3.00004: Assessing low wavenumber pedestal turbulence in NSTX with measurements and simulations Invited Speaker: David Smith Global confinement predictions depend upon the validation of pedestal turbulence models. Spherical torus (ST) edge turbulence simulations are among the most challenging turbulence simulations, so here we present an assessment of low-k pedestal turbulence (k$_{\theta }\rho _{i}<$1.5, 0.8$<$r/a$<$0.95) with multi-point beam emission spectroscopy (BES) measurements during ELM-free, MHD quiescent H-mode phases in NSTX. BES measurements show broadband turbulence up to 100 kHz with poloidal wavenumber k$_{\theta }\rho _{i}\sim $ 0.2, poloidal correlation length L$_{p}$/$\rho _{i}\sim $ 10, and decorrelation time $\tau _{d}$/(a/c$_{s})\sim $ 5. The dimensionless turbulence parameters are largely consistent with drift-wave turbulence models and previous measurements of tokamak L-mode core turbulence. Parametric dependencies among turbulence quantities and transport-relevant plasma parameters (density and temperature gradients, collisionality, etc) are most consistent with transport driven by trapped-electron mode, kinetic ballooning mode, and microtearing mode turbulence, and least consistent with ion temperature gradient turbulence. Also, the parametric dependencies are consistent with turbulence regulation by equilibrium flow shear and collisional damping of zonal flows. Finally, the dependencies indicate a connection between taller, wider pedestals and larger turbulent structures. The measurements and parametric dependencies broadly characterize low-k pedestal turbulence in high-performance spherical torus plasmas and establish validation benchmarks for pedestal and edge simulations. Additional results cover edge turbulence measurements and parametric dependencies before and after the LH transition, and initial gyrokinetic and fluid simulations of NSTX pedestal turbulence. [Preview Abstract] |
Friday, November 2, 2012 11:30AM - 12:00PM |
YI3.00005: Fluctuating Zonal Flows in I-mode in Alcator C-Mod Invited Speaker: Istvan Cziegler Velocity fields and density fluctuations of edge turbulence have been studied in I-mode [1] plasmas of Alcator C-Mod, which are characterized by a strong thermal transport barrier in the edge while providing little or no barrier to the transport of both bulk and impurity particles. This allows access to steady state, high performance discharges without explosive edge relaxations or impurity accumulation. The key feature in the I-mode edge seems to be a weakly coherent mode (WCM) at $100-300$~kHz, with $\Delta f \approx 150$~kHz and a poloidal wavenumber $k\approx 1.5\,\mathrm{cm}^{-1}$. Although previous work showed no clear geodesic-acoustic modes (GAM) on C-Mod, using a newly implemented, gas-puff-imaging (GPI) based time-delay-estimate (TDE) velocity inference algorithm, GAM are now shown to be ubiquitous in all I-mode discharges, with the time histories of the GAM and the WCM closely following each other through the entire duration of the regime. The central frequency of the WCM is shown to scale with $\mathrm{H_{ITER,98}}$, which itself scales with the depth of the radial electric field well in the edge [2]. Thus, the I-mode presents an example of a plasma state in which quasi-static zonal flows (ZF) and GAM continuously coexist. Using both single- (density) and two-field (density-velocity) bispectral methods, the GAM are shown to be coupled to the WCM and to be responsible for its broad frequency structure. Since the WCM activity is strongly correlated to the I-mode behavior [3], and due to the known dependence of the GAM damping on collisionality [4], the decrease in GAM amplitude, and with it WCM activity, at higher densities offers an explanation for the density limit for I-mode access [3].\\[4pt] [1] F. Ryter et al, Plasma Phys. Control. Fusion 40 725 (1998)\\[0pt] [2] R. McDermott et al, Phys. Plasmas 16 056103 (2009)\\[0pt] [3] D. Whyte et al, Nucl. Fus. 50 105005 (2010)\\[0pt] [4] S. Novakovskii et al, Phys. Plasmas 4 4272 (1997) [Preview Abstract] |
Friday, November 2, 2012 12:00PM - 12:30PM |
YI3.00006: Global Gyrokinetic Simulations of the Dominant High-n and Intermediate-n Instabilities in the H-Mode Tokamak Edge Pedestal Invited Speaker: Scott Parker Global electromagnetic gyrokinetic simulations show the existence of near threshold conditions, for both a high-n Kinetic Ballooning Mode (KBM) and an intermediate-n kinetic version of Peeling-Ballooning Mode (PBM). The KBM and the PBM have been used to constrain the EPED model [1]. Global gyrokinetic simulations show that the H-mode pedestal, just prior to the onset of the Edge Localized Mode (ELM), is very near the KBM threshold. Two DIII-D experimental discharges are studied, one reporting KBM features in fluctuation measurements [2]. Simulations find that in addition to the high-n KBM, an intermediate-n electromagnetic mode is unstable. This kinetic version of the PBM has phase velocity in the electron diamagnetic direction, but otherwise has features similar to the MHD PBM. When the magnetic shear is reduced in a narrow region near the steep pressure gradient, the intermediate-n ``kinetic PBM'' is stabilized, while the high-n KBM becomes the most unstable mode. Global simulation results of the KBM compare favorably with flux tube simulations. The KBM transitions to an unstable electrostatic ion mode as the plasma beta is reduced. The intermediate-n ``kinetic peeling ballooning mode'' is sensitive to the q-profile and only seen in global electromagnetic simulations. Collisions increase the KBM critical beta and growth rate. These results indicate that an improved pedestal model should include, in detail, any corrections to the bootstrap current, and any other equilibrium effects that might reduce the local magnetic shear. It is known that the bootstrap current may flatten the q-profile in the steep gradient region [3]. Simulations are carried out using the global electromagnetic GEM code, including kinetic electrons, electron-ion collisions and the effects of realistic magnetic geometry. In addition to global linear analysis, nonlinear simulations will be reported showing that, while the equilibrium radial electric field has a weak effect on the linear growth rate, it has a larger stabilizing effect nonlinearly.\\[4pt] [1] P. Snyder, et al., Phys. Plasmas 16 056118 (2009).\\[0pt] [2] Z. Yan, et al., Phys. Plasmas 18 056117 (2011).\\[0pt] [3] J. Callen, et al. Nucl. Fusion 50 064004 (2010). [Preview Abstract] |
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