Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session TM9: Mini-Conference: Validations of Two-fluid and Gyro-fluid Models on Turbulence and ELMs |
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Chair: Tony Leonard, General Atomics Room: Governor's Square 16 |
Thursday, November 14, 2013 9:30AM - 9:50AM |
TM9.00001: Simulations of Tokamak Edge Turbulence Including Self-Consistent Zonal Flows* Bruce Cohen, Maxim Umansky Progress on simulations of electromagnetic drift-resistive ballooning turbulence in the tokamak edge is summarized in this mini-conference talk. A more detailed report on this work is presented in a poster at this conference. This work extends our previous work [1] to include self-consistent zonal flows and their effects. The previous work [1] addressed the simulation of L-mode tokamak edge turbulence using the turbulence code BOUT. The calculations used realistic single-null geometry and plasma parameters of the DIII-D tokamak and produced fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes that compare favorably to experimental data. In [1] the effect of sheared ExB poloidal rotation is included with an imposed static radial electric field fitted to experimental data. In the new work here we include the radial electric field self-consistently driven by the microturbulence, which contributes to the sheared ExB poloidal rotation (zonal flow generation). We present simulations with/without zonal flows for both cylindrical geometry, as in the UCLA Large Plasma Device, and for the DIII-D tokamak L-mode cases in [1] to quantify the influence of self-consistent zonal flows on the microturbulence and the concomitant transport. *This work was performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344 at the Lawrence Livermore National Laboratory. \\[4pt] [1] B. I. Cohen et al., Phys. Plasmas \textbf{20}, 055906 (2013) [Preview Abstract] |
Thursday, November 14, 2013 9:50AM - 10:10AM |
TM9.00002: Six-field two-fluid simulations of ELM power depositions on divertor target in real tokamak geometry using BOUT$++$ code T.Y. Xia, X.Q. Xu, M.E. Fenstermacher The six-field two-fluid model based on the Braginskii equations in BOUT$++$ simulation framework is used to study the edge localized modes (ELMs) in realistic tokamak discharges of DIII-D and EAST with the experimentally measured profiles of density, radial electric field, electron and ion temperatures as the initial conditions. The simulations with two different resolutions on the lower single-null geometry are done to describe the evolutions of pedestal energy loss, density profile and heat flux on divertor through the ELM event. The simulation for high resolution shows much faster energy loss than the low resolution one, and leads to the twice of the amplitude for ion heat flux. Our high simulations show that the total energy loss for the small ELM with high frequency is well consistent with the measurement. The amplitudes of heat flux on divertor target are comparable with the early time evolutions of the IR heat flux measurement. Plasma sheath boundary conditions (SBC) are implemented at the divertor plate and they can effectively broaden the heat flux distribution at the outer plate compared to the Dirichlet boundary conditions. The poloidal structures of the heat flux on divertor target will be reported in this paper. [Preview Abstract] |
Thursday, November 14, 2013 10:10AM - 10:30AM |
TM9.00003: BOUT++ Simulations of Edge Turbulence in Alcator C-Mod's EDA H-Mode E.M. Davis, M. Porkolab, J.W. Hughes, B. LaBombard, P.B. Snyder, X.Q. Xu Energy confinement in tokamaks is believed to be strongly controlled by plasma transport in the pedestal. The pedestal of Alcator C-Mod's Enhanced $D_{\alpha}$ (EDA) H-mode ($\nu^* > 1$) is regulated by a quasi-coherent mode (QCM), an edge fluctuation believed to reduce particle confinement and allow steady-state H-mode operation. \texttt{ELITE} calculations indicate that EDA H-modes sit well below the ideal peeling-ballooning instability threshold, in contrast with ELMy H-modes. Here, we use a 3-field reduced MHD model in \texttt{BOUT++} to study the effects of nonideal and nonlinear physics on EDA H-modes. In particular, incorporation of realistic pedestal resistivity is found to drive resistive ballooning modes (RBMs) and increase linear growth rates above the corresponding ideal rates. These RBMs may ultimately be responsible for constraining the EDA pedestal gradient. However, recent high-fidelity mirror Langmuir probe measurements indicate that the QCM is an electron drift-Alfv\'{e}n wave - not a RBM. Inclusion of the parallel pressure gradient term in the 3-field reduced MHD Ohm's law and various higher field fluid models are implemented in an effort to capture this drift wave-like response. [Preview Abstract] |
Thursday, November 14, 2013 10:30AM - 10:50AM |
TM9.00004: Validation of BOUT++ ELM simulations for the EAST Tokamak discharges Zixi Liu, Xueqiao Xu, Xiang Gao, Shaocheng Liu, Tianyang Xia, Guosheng Xu, Jianggang Li EAST ELM experiments validate BOUT++ predictions that low-n modes become dominant at high plasma current, and the bright stripes from visible camera on EAST match ELM filamentary structures of BOUT++ simulations. Four phases of the ELM dynamics including linear growth, nonlinear saturation, pedestal crash, and L-mode-like post-ELM state have been observed in BOUT++ simulations. The simulated radial velocity of ELM explosive event is consistent with the experimental data by Gas Puffing Image (GPI). Energy loss is about 2 percent; more particle and power fluxes are deposited on the outer divertor plate. The small ELMs on EAST are resistive ballooning modes, and higher plasma current and the pressure result in higher growth rate for the lower toroidal numbers. Effect of the diamagnetic drift is stronger than the ballooning instability drive when the pressure gradient increases and the ELM crashes start at the outer mid-plane. [Preview Abstract] |
Thursday, November 14, 2013 10:50AM - 11:10AM |
TM9.00005: Validations of BOUT$++$ transport simulations with HL-2A experiments using SMBI Z.H. Wang, X.Q. Xu, D.L. Yu, A.P. Sun, J.Q. Dong, L.H. Yao In BOUT$++$ code framework, a new trans-neut module has been developed to deal with neutrals and plasmas transport during fueling of super-sonic molecule beam injection (SMBI) or gas puffing (GP) [1]. It modifies BOUT$++$ code of boundary plasma turbulence to study dynamics of neutrals transport and interactions with plasma during fueling. The model couples plasma density, heat and momentum transport equations with neutrals density and momentum transport equations for atoms and molecules. Particle interactions of dissociation, ionization, recombination and charge-exchange have been included. Particle recycling is also considered at both wall and divertor plates. A local molecule flux boundary condition is applied to model SMBI. It is found that neutrals can penetrate deeply across the separatrix. Simulations are done in a realistic HL-2A tokamak geometry. The initial profiles are specified same as the experiment and they are kept stable via radial dependent diffusion coefficients. The simulations of penetration depth and mean profiles during SMBI will be validated with HL-2A experiments. \\[4pt] [1] Z H Wang, X Q Xu, et al, submitted to Nuclear Fusion, 2013. [Preview Abstract] |
Thursday, November 14, 2013 11:10AM - 11:30AM |
TM9.00006: Validation of BOUT$++$ ELM simulation by Comparison with ECEI Measurements in the KSTAR tokamak Minwoo Kim, Jaehyun Lee, Minjun Choi, Gunsu Yun, X.Q. Xu, Woochang Lee, Hyeon Park, C.W. Domier, N.C. Luhmann, Jr. Details of ELM dynamics has been measured in 2D using an electron cyclotron emission imaging (ECEI) diagnostic in the KSTAR tokamak. The observed ELM dynamics show complex evolution stages including linear growth, saturation, changes in mode number and rotation velocity, and localized crash [1]. We studied the mode structure of the observed ELMs in the linear growth phase using 3-field BOUT$++$ simulations [2]. The toroidal mode number (n) of ELMs, which was experimentally determined by an array of toroidal Mirnov coils, was fixed in the simulation. On the other hand, the pressure profile was adjusted to make the linear growth rate finite at the given n number. For direct comparison with the observed images, the simulation results were converted to synthetic ECEI images by taking into account instrumental broadening, intrinsic ECE broadening in the pedestal region, and system noises. The synthetic images were qualitatively well matched with the observations. As a next step, a simulation study in linear phase is planned for a self-consistent equilibrium including bootstrap current. *Work supported by NRF Korea under contract no. 2013035905 and US DoE under contract no. DE-FG-02-99ER54531.\\[4pt] [1] G. S. Yun, et al., PRL 107 (2011)\\[0pt] [2] X. Q. Xu, et al., NF 51 (2011) [Preview Abstract] |
Thursday, November 14, 2013 11:30AM - 11:50AM |
TM9.00007: Linear non-normality as the cause of nonlinear instability in LAPD Brett Friedman, Troy Carter, Maxim Umansky A BOUT$++$ simulation using a Braginskii fluid model reproduces drift-wave turbulence in LAPD with high qualitative and quantitative agreement. The turbulent fluctuations in the simulation sustain themselves through a nonlinear instability mechanism that injects energy into k$_{\mathrm{\vert \vert }}=$0 fluctuations despite the fact that all of the linear eigenmodes at k$_{\mathrm{\vert \vert }}=$0 are stable [1]. The reason for this is the high non-orthogonality of the eigenmodes caused by the non-normality of the linear operator, which is common in fluid and plasma models that contain equilibrium gradients [2]. While individual stable eigenmodes must decay when acted upon by their linear operator, the sum of the eigenmodes may grow transiently with initial algebraic time dependence. This transient growth can inject energy into the system, and the nonlinearities can remix the eigenmode amplitudes to self-sustain the growth. Such a mechanism also acts in subcritical neutral fluid turbulence, and the self-sustainment process is quite similar [3], indicating the universality of this nonlinear instability. \\[4pt] [1] Friedman et al., Phys. Plasmas, 19, 2012.\\[0pt] [2] Camargo et al., Phys. Rev. E, 58 (1998).\\[0pt] [3] Trefethen et al., Science, 261 (1993). [Preview Abstract] |
Thursday, November 14, 2013 11:50AM - 12:10PM |
TM9.00008: The experiment and simulation of the divertor power asymmetry in EAST Shaocheng Liu, Xueqiao Xu, Tianyang Xia, Houyang Guo, Zixi Liu, Liang Wang, Huiqian Wang, Guosheng Xu, Kaifu Gan Divertor asymmetry and scrape-off layer (SOL) flow have been systematically investigated in the Experimental Advanced Superconducting Tokamak (EAST), with respect to toroidal field direction, divertor configuration, power injection methods and heating power. The in-out asymmetry ratio of q$_{t,out}$/q$_{t,total}$ increases with the power across the separatrix P$_{loss}$. The characteristics of the measured SOL parallel flow under various discharge conditions are consistent with the Pfirsch-Schl\"uter (PS) flow with the parallel Mach number M$_{\vert \vert}$ decreasing with the line averaged density but increasing with P$_{loss}$, in the same direction as the PS flow. The mechanisms of divertor asymmetry are investigated by the BOUT$++$ simulation. [Preview Abstract] |
Thursday, November 14, 2013 12:10PM - 12:30PM |
TM9.00009: Comparison of Collisional Drift-Wave Simulation with CSDX Experimental Results Payam Vaezi, Christopher Holland, George Tynan, Saikat Thakur, Christian Brandt, Benjamin Dudson, Brett Friedman, Troy Carter Recent upgrades to the linear Controlled Shear Decorrelation Experiment (CSDX) [Burin et al, PoP 2005] at UCSD (maximum Bz from 1kG to 2.4 kG, increase of helicon source diameter from 10 cm to 15 cm) have revealed a rich array of turbulence dynamics at previously inaccessible conditions. We report initial comparisons of linear and nonlinear collisional drift-wave physics made using analytic theory and the BOUT$++$ code [Dudson et al, CPC 2009] against these observations, focusing upon the transition from nonlinearly coupled but distinct eigenmodes at 0.9 kG to fully developed broadband turbulence at 2.4 kG. Comparisons of predicted linear eigenmode structures, frequencies, and density-potential cross-phases to measurement are presented, as well as predictions for nonlinear frequency power spectra and saturated fluctuation levels. We also report progress on the development and implementation of synthetic Langmuir probe and fast framing camera diagnostics for improving the fidelity of our model-experiment comparisons. [Preview Abstract] |
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