Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session CO7: MF: Gyrokinetics and Edge Physics |
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Chair: MJ Pueschel, IFS Room: Grand F |
Monday, October 21, 2019 2:00PM - 2:12PM |
CO7.00001: Magneto-thermal Reconnection Processes and Tridimensional Ignition D. Grasso, B. Coppi, R. Gatto A new kind of magnetic reconnection process that is associated with the presence of finite electron temperature [1] gradients on rational magnetic surfaces of an axisymmetric confinement configuration, is presented. This is relevant to regimes where the electron thermal conductivity is relatively large and the reconnection layer is smaller than the ``thermal'' layer where the transverse thermal conductivity plays a key role. When referring to fusion burning plasmas the excitation of the considered modes is associated with the nuclear heating of the electron population. This ``thermonuclear instability'' [2] can then develop more easily around closed magnetic field lines, than on non-rational magnetic surfaces.\\ $[1]$ B. Coppi, B. Basu and A. Fletcher, Nucl. Fus., 57, 7 (2017).\\ $[2]$ B. Coppi and the Ignitor Program Members, Nucl. Fus., 55, 053011 (2015). [Preview Abstract] |
Monday, October 21, 2019 2:12PM - 2:24PM |
CO7.00002: Resolving a convergence issue with local gyrokinetic simulations Justin Ball, Stephan Brunner, Ajay C. J. Flux-tube gyrokinetic simulations are often used to model turbulence in toroidal plasmas. Their simulation domain follows a narrow bundle of field lines and typically extends one poloidal turn in length. However, this work and others show that such simulations may not be fully converged. In many parameter regimes, turbulent self-interaction can occur through the parallel boundary condition. This is not physical unless the flux-tube corresponds to a full flux surface, which is often computationally prohibitive. Symptoms of such self-interaction include flow shear layers at mode rational surfaces and staircase structures in the plasma profiles. We have observed such features with both kinetic and adiabatic electrons, ion and electron-scale turbulence, and toroidal and slab geometry. Ultimately, this self-interaction is observed to reduce the heat flux and can even cause non-convergence with respect to the size of the domain perpendicular to the magnetic field. To resolve these issues, we turn to a recommendation from the original paper on flux-tube boundary conditions [Beer et al. (1995)] - by increasing the parallel length of the domain to two or three poloidal turns, the self-interaction can be eliminated. This enables a small flux-tube to accurately model turbulence in tokamaks. [Preview Abstract] |
Monday, October 21, 2019 2:24PM - 2:36PM |
CO7.00003: Gyrokinetic Simulation of Turbulent Transport for I-mode Edge Plasmas Hongwei Yang, Tianchun Zhou, Yong Xiao, Zhihong Lin I-mode is an attractive candidate for tokamak operation with good energy confinement similar to H-mode but poor particle confinement similar to L-mode [1-2]. Additional benefits about I-mode include no impurity accumulation and free of ELMs. Many tokamaks, such as Alcator C-Mod, AUG, DIII-D and EAST, have achieved I-mode operation recently. However, theory and simulation remains insufficient to explain the I-mode formation and its transport behavior. In this work, an electrostatic gyrokinetic simulation using GTC code is carried out for the I-mode physics. Linear simulations find that two comparable instabilities co-exist for both short wavelength and long wavelength modes. The passing electron response cannot be treated adiabatically for those mode numbers close to that of weekly-coherent-mode (WCM), which is shown to account for the I-mode formation and transport behavior. Nonlinear simulations give a turbulent heat transport level consistent with the experimental value. But the particle transport from the simulation remains elusive from the experiment, which requires further investigation. References: [1] D. G. Whyte et al. 2010 Nuclear Fusion, 50(10), 105005. [2] A. E. Hubbard et al. 2011 Physics of Plasmas, 18(5), 056115. [Preview Abstract] |
Monday, October 21, 2019 2:36PM - 2:48PM |
CO7.00004: Formation of a High Pressure Staircase Pedestal with Suppressed Edge-Localized-Modes in the DIII-D Tokamak Arash Ashourvan, Raffi Nazikian, W Guttenfelder, SR Haskey, BA Grierson, J Candy, D Eldon, CC Petty, E Belli, GR McKee, C Lasnier We observe the formation of a high-pressure two-step staircase pedestal ($\approx $ 16-20 kPa) in the DIII-D tokamak when large amplitude Edge-Localized-Modes are suppressed using resonant magnetic perturbations. The pedestal oscillates between the staircase and a single step structure every 40-60 ms, correlated with oscillations in the heat and particle flux to the divertor. Gyrokinetic analysis using the CGYRO code shows that when the heat and particle flux to the divertor decreases, the pedestal broadens and the E\texttimes B shear at the mid-pedestal decreases, triggering a transport bifurcation from Kinetic-Ballooning-Mode (KBM) to Trapped-Electron-Mode (TEM) limited transport that flattens the density and temperature profiles at mid-pedestal and results in the formation of the staircase pedestal. The reverse transition from staircase to one-step pedestal takes place as the heat and particle fluxes to the divertor increase. Our results suggest that in the reactor-scale tokamaks for which the efficacy of ExB shear is reduced (e.g. ITER), enhanced ion-scale transport can be locally contained with the formation of staircase pedestal, leading to the increase in pedestal pressure and improved confinement. [Preview Abstract] |
Monday, October 21, 2019 2:48PM - 3:00PM |
CO7.00005: A Non-Perturbative Eigenvalue Code for Gyrokinetic Simulation of Alfven Instabilities in Tokamaks Yue-Yan Li, Shuang-Hui Hu, Yong Xiao Drift Alfven Energetic Particle Stability (DAEPS), a non-perturbative eigenvalue code under intense development, is a comprehensive linear instability FEM code to investigate the physics of various unstable Alfven modes observed in toroidal fusion plasmas., which can be excited by either energetic particle or thermal particles. The energetic particle and thermal particles are treated on an equal footing in DAEPS. DAEPS can calculate accurately and efficiently eigen frequency and growth rate, as well as the asymptotic behavior of drift Alfven wave instability based on an eigenvalue approach for different boundary conditions. The model equations of DAEPS include essential physics ingredients such as plasma non-uniformity, pressure anisotropy, field line curvature, finite Larmor radius, wave-particle interaction, and etc. We here show the current development status of the DAEPS code and its capabilities on a number of prominent physics scenarios, e.g., BAE, BAAE and KBM excited by thermal particle, TAE excited by passing energetic particle, and $\alpha $TAE excited by trapped energetic particle, which are carefully benchmarked with other codes and theories within satisfactory accuracy. [Preview Abstract] |
Monday, October 21, 2019 3:00PM - 3:12PM |
CO7.00006: Verification and UQ activities of the Particle-In-Cell code hPIC for Near Surface Plasma Conditions Mohammad Mustafa, Davide Curreli, Pablo Seleson, Cory Hauck, David Bernholdt A crucial step of the verification of Particle-in-Cell codes is systematic estimation of error produced by simulation models. Here we report a series of verification tests of the Particle-in-Cell code hPIC for plasma sheath and plasma boundary problems under various conditions. Furthermore a preliminary Uncertainty Quantification has been performed on the code. The dependency of physical parameters such as the IEAD (Ion Energy-Angle Distribution) of the ions crossing a plasma sheath have been investigated. However, due to the limited number of the empirical or semi-empirical formulas available in the literature that can describe the floating wall potential at high values of magnetic field or IEAD, the method of manufactured solution has been preliminarily investigated. The approach of bootstrap sampling has been used in uncertainty quantification to ensure covering a wide range of numerical and physical variable and keeping the computational intensity as low as possible. Both random number and pseudo-random number generators have been used to study the effect of particle noise of propagated uncertainties. Overall, a robust convergence has been found, and the numerical bound has been determined. [Preview Abstract] |
Monday, October 21, 2019 3:12PM - 3:24PM |
CO7.00007: Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals Jason F. Parisi, Felix I. Parra, Colin M. Roach, Carine Giroud, Nobuyuki Aibi, Michael Barnes, Plaven G. Ivanov Electron temperature gradient (ETG) physics is shown to dominate the linear gyrokinetic spectrum in JET-ILW pedestals where the ion temperature is measured. Local linear gyrokinetic simulations of JET pedestal shots 82550, 92167, and 92174, demonstrate that with the exception of kinetic ballooning modes (KBMs), microinstabilities are driven mainly by the electron temperature gradient --- many of these instabilities exist at transport-relevant scales. The ion temperature gradient (ITG) instability is subdominant or absent, and both KBMs and ITG are shown to be suppressed by $\mathbf{ E} \times \mathbf{ B}$ shear. Electron temperature gradients are particularly large, causing ETG instabilities to be driven at perpendicular scales larger than in the core by a factor of $R_0/L_{Te} > \sqrt{m_i T_{0i} / m_e T_{0e}}$. Results show ETG instability dominating at $k_y \rho_i > 0.5$, which is mainly toroidal at low $k_y$ and slab at larger $k_y$. The toroidal ETG mode has a sufficiently large radial wavenumber that electron finite Larmor radius (FLR) effects become important; that is, $k_y \rho_i \sim 1$, but $ K_x \rho_e \sim 1$, where $K_x$ is the effective radial wavenumber. We also calculate the ETG stability boundary for general perpendicular and parallel wavenumbers. [Preview Abstract] |
Monday, October 21, 2019 3:24PM - 3:36PM |
CO7.00008: Gyrokinetic investigation of the ASDEX Upgrade I-mode pedestal Karl Stimmel, Alejandro Bañón Navarro, Tim Happel, Daniel Told, Tobias Görler, Elisabeth Wolfrum, James Martin Collar, Rainer Fischer, Philip Schneider, Frank Jenko Characterizing pedestal turbulence in the tokamak I-mode is a crucial step in understanding how particle and heat transport decouple during I-mode operation. This work models an ASDEX Upgrade I-mode discharge for the first time via linear and nonlinear gyrokinetic simulations with the GENE code. Experimental measurements from ASDEX-Upgrade discharge $\#30865$ are used as simulation inputs for four scenarios at two pedestal locations and two time phases at 3.11 s and 3.80 s which correspond to L-mode and I-mode regimes. A microtearing mode which is absent in linear L-mode simulations is found in I-mode simulations at two radial positions, and ion-scale instabilities are characterized for all four scenarios linearly. Computed nonlinear heat flux values approach experimental measurements in three of the four cases, and heat transport is found to be dominated by ion-scale electrostatic turbulence. Electrostatic potential oscillation frequencies, as well as potential-temperature and potential-density crossphases are compared linearly and nonlinearly, and agreement is found at wavenumber ranges corresponding with peaks in the simulated heat flux spectra at one radial position for L-mode and I-mode. [Preview Abstract] |
(Author Not Attending)
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CO7.00009: The Effect of External Driven Sources on the Edge Radial Electric Field Guoliang XIAO Intensive work has been conducted to investigate the role of E$\mathrm{×}$B flow shearing for L-H transition. It has been shown that this flow shear is responsible for the turbulence suppression. An effective method to achieve the external active control of E$\mathrm{×}$B velocity shear is required to accelerate the transition process. Recently, a body of work has pointed out that the turbulence regulation by shear flow also play an important role during ELM mitigation. At present, active control techniques for the radial electric profile is the electrode immersed in the device or the outer magnetic perturbation coils. In the HL-2A H mode plasma, different methods have been found to modify the radial electric profile, namely, E$\mathrm{×}$B velocity shear, such as SMBI, LBO impurity seeding and LHCD. This paper reports the obvious modification and different effects of these external source input on the E$\mathrm{×}$B velocity shear. And the discrepancy is due to the external source input modified the different term of the E$\mathrm{×}$B velocity shear. LHCD modified the ion diamagnetic term ${\mathrm{\nabla }E}_{r}^{\nabla P_{i}}$ of the $\gamma_{E\mathrm{×}B}$ due to the non-resonant collisional absorption of the high N$_{\mathrm{//\thinspace }}$components of lower hybrid wave at the plasma edge. On the other hand, different from the experiment with LHCD, it has been observed that change of $\gamma_{E\mathrm{×}B}$ with LBO impurity seeding and SMBI is attributed to the poloidal velocity component $\mathrm{\nabla }E_{r}^{V_{\theta }}$ and the toroidal velocity component ${\mathrm{\nabla E}}_{\mathrm{r}}^{\mathrm{V}_{\mathrm{\varphi }}}\mathrm{\nabla }\mathrm{E}_{\mathrm{r}}^{\mathrm{V}_{\mathrm{\theta }}}$ due to the localized cooling effect. These results indicate that the edge flow shear can be regulated to achieve L-H transition and ELM mitigation by alternating proper external source input. [Preview Abstract] |
Monday, October 21, 2019 3:48PM - 4:00PM |
CO7.00010: Modeling of Edge Harmonic Oscillation in DIII-D QH-mode Discharges A.Y. Pankin, Xi Chen, J.R. King, K.H. Burrell, A.M. Garofalo, R.J. Groebner, S.E. Kruger, G.R. McKee, T. Rafiq, Z. Yan In this research, the extended MHD NIMROD code is used to simulate the dynamics of EHOs during the early stage of the QH-discharge 163518. Edge Harmonic Oscillations (EHOs) observed in the DIII-D experiment are successfully reproduced in these nonlinear NIMROD simulations. Similar to the previous NIMROD results of broadband QH regime in another DIII-D discharge [J.R. King \textit{et al}. Phys. of Plasmas (2017) 055902], we show that the rotation is essential for the formation of the saturated EHO states. For these long simulations that are comparable to the transport times, a correct balance between particle and energy sources and sinks becomes important. The simulation with a more advanced transport model that has been implemented in NIMROD yields a more realistic H-mode pedestal transport. The model includes a description of sources and radial profiles for particle and energy transport coefficients. It is shown that the relaxation of plasma profiles depends on the initial profiles and instability saturation levels. In order to facilitate the model validation, the synthetic BES diagnostic has been implemented in the NIMROD framework. A comparison between the density perturbations obtained in the simulations with the experimental BES measurements show qualitatively similar behavior. [Preview Abstract] |
Monday, October 21, 2019 4:00PM - 4:12PM |
CO7.00011: Machine Learning and Data Analytics for Reduction, Extraction, Enhancement, and Analysis in Plasma and Fusion Systems Craig Michoski, David Hatch, Todd Oliver, Milos Milosavljevic, Salomon Janhunen, Gabriele MErlo, Mike Kotschenreuther Advancements in machine learning (ML), artificial intelligence (AI), advanced data analytics (ADA), and uncertainty quantification (UQ) provide tools to evaluate classical plasma and fusion systems in new, and often conceptually revolutionary ways. As examples, uncertainty quantification (UQ) and Gaussian process regression are used to examine the advanced \emph{analytics} of plasma profiles, revealing unexpected features (e.g. signed gradients) in diagnostic data. Next, multifidelity frameworks can be used to develop unbiased low variance \emph{reduced} order estimators of expensive models (e.g. EDIPIC, GENE, XGC), that are propagated forward to minimize uncertainties in measurable quantities. In addition, deep learning networks are used to \emph{extract} predictive models (e.g. parameterized equations representing derived plasma subsystems, such as, for example, the Dimits shift, or experimental diagnostics) from data, where new, hidden structures within the data can be extracted to develop more accurate, cheaper, and more stable modeling frameworks. Finally, theoretical (i.e. physical) models can be \emph{enhanced} with data -- e.g. experimental data used to extend MHD type models) ; or vice versa, enhancing experimental data acquisition with theory and models. [Preview Abstract] |
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