Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session BI02: Invited: Magnetic Fusion: TurbulenceLive
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Chair: Jeffrey Parker, University of Wisconsin |
Monday, November 9, 2020 9:30AM - 10:00AM Live |
BI02.00001: Continuum Gyrokinetic Simulations of Edge Plasmas in Single-Null Geometries Invited Speaker: Mikhail Dorf The first continuum gyrokinetic calculations of edge plasmas in a single-null geometry are presented for the cases of 4D axisymmetric transport and 5D electrostatic ion scale turbulence. The simulations are performed with the finite-volume code COGENT, which was designed to handle the geometrical complexity of the tokamak edge. In particular, in order to facilitate simulations of highly-anisotropic microturbulence in the presence of strong magnetic shear and a magnetic X-point, a numerical algorithm utilizing a locally field-aligned multiblock coordinate system has been developed. In this approach, the toroidal direction is divided into blocks, such that within each block the cells are field-aligned and a non-matching (non-conformal) grid interface is allowed at block boundaries. The toroidal angle corresponds to the ``coarse'' field-aligned coordinate, whereas the poloidal cross-section, comprised of the radial and poloidal directions, is finely gridded to resolve short-scale perpendicular turbulence structures and to support accurate re-mapping (interpolation) at block boundaries. The simulation model solves the long-wavelength limit of the full-F gyrokinetic equation for ion species, coupled to the quasi-neutrality equation for electrostatic potential variations, where a fluid model is used for the electron response. The 4D transport calculations, including the effects of fully-nonlinear Fokker-Planck collisions and ad-hoc anomalous radial transport, demonstrate values of radial electric field and toroidal rotation comparable to those observed on the DIII-D facility. The 5D simulations explore cross-separatrix ion temperature gradient (ITG) turbulence in the presence of a self-consistent radial electric field and elucidate the effects of magnetic-shear stabilization in the X-point region. [Preview Abstract] |
Monday, November 9, 2020 10:00AM - 10:30AM Live |
BI02.00002: Investigating magnetic fluctuations in tokamak SOL turbulence using gyrokinetic simulations Invited Speaker: Noah Mandell Understanding turbulent transport physics in the tokamak edge and scrape-off layer (SOL) is critical to developing a successful fusion reactor. The dynamics in these regions plays a key role in determining the L-H transition, the pedestal height and the heat load to the vessel walls. Large-amplitude fluctuations, magnetic X-point geometry, and plasma interactions with material walls make modeling turbulence in the edge/SOL more challenging than in the core region, requiring specialized gyrokinetic codes. Electromagnetic effects can also be important in the edge/SOL region due to steep pressure gradients and non-adiabatic electron dynamics, which can result in line bending due to coupling of perpendicular dynamics with kinetic shear Alfven waves. However, all gyrokinetic results in the SOL to date have assumed electrostatic dynamics, due in part to numerical challenges like the Ampere cancellation problem. We present the first nonlinear electromagnetic gyrokinetic results of turbulence on open field lines in the tokamak SOL, obtained using the Gkeyll full-$f$ continuum gyrokinetic code. The results, which use a model helical SOL geometry and NSTX-like parameters, show that even strong magnetic turbulence with fluctuations up to $\delta B_\perp/B \sim 1\%$ can be handled robustly. Line-tracing visualizations show that field lines are pushed and bent by radially-propagating blobs. Preliminary comparisons to electrostatic simulations show that including electromagnetic effects can produce larger, more intermittent relative density fluctuations, but somewhat surprisingly, less radial transport in some cases. We also examine how the magnetic geometry influences the importance of electromagnetic effects via the connection length and magnetic shear. [Preview Abstract] |
Monday, November 9, 2020 10:30AM - 11:00AM Live |
BI02.00003: Understanding pedestal transport through gyrokinetic and edge modeling Invited Speaker: David Hatch We report on a broad study combining the capabilities of gyrokinetic codes (GENE and CGYRO) and edge fluid codes (SOLPS and UEDGE) to identify the transport mechanisms active in pedestals spanning multiple devices (DIII-D, JET, C-Mod), modes of operation (H-mode, I-mode, QH-mode), fueling / heating levels, and wall materials. The gyrokinetic codes can analyze the instabilities and transport that arise in the pedestal while the edge codes provide the best possible estimate of particle sources. This study was carried out from perspective of the so-called transport `fingerprint' conceptual framework, which compares basic physical signatures of prospective pedestal instabilities with the breadth of available experimental data, including frequency spectra, fluctuation scales and amplitudes, transport ratios, and inter-ELM profile evolution. Edge modeling determined that edge transport barriers typically lie in a regime in which heat diffusivity far exceeds particle diffusivity: De/$\chi $e $\ll $ 1, which has major implications for the role of various pedestal instabilities. In conventional ELMy H-modes, microtearing modes and ETG turbulence dominate the electron heat transport; neoclassical dominates ion heat and impurity transport; and several candidates, including KBM, remain to explain the (small) particle transport. The presence of significant ion-scale electrostatic turbulence generally results in interesting variations on the standard H-mode pedestal theme. Detailed comparisons were carried out with experimental fluctuation data. Notably, simulations of microtearing modes quantitatively match distinctive frequency bands for multiple discharges on both JET and DIII-D. This study expands our understanding the transport mechanisms that determine many important properties of edge transport barriers and lays a foundation for predicting their behavior in future devices. [Preview Abstract] |
Monday, November 9, 2020 11:00AM - 11:30AM Live |
BI02.00004: Strong Reversal of Simple Isotope Scaling Laws in Tokamak Edge Turbulence Invited Speaker: Emily Belli The role of the nonadiabatic electron drive in regulating the hydrogenic isotope mass scaling of gyrokinetic turbulence in tokamaks is assessed in the transition from ion-dominated core transport regimes to electron-dominated edge transport regimes. Experiments often show confinement improving with increasing ion mass. However, simple gyroBohm-scaling theoretical arguments (that ignore electron dynamics) predict that the turbulent ion energy flux scales with the square root of the ion mass, implying that the global confinement degrades with increasing ion mass. Using nonlinear gyrokinetic simulations of DIII-D, we illustrate a remarkable transition in the turbulent isotope scaling towards the plasma L-mode edge. The transition is controlled by finite electron-to-ion mass-ratio dependence of the nonadiabatic electron response, dominantly generated by the parallel motion, which represents a correction to bounce-averaging of the electrons. The nonadiabatic electron drive strongly regulates the turbulence levels and plays a key role in favorably altering -- and in the case of the DIII-D edge, reversing -- the simple gyroBohm scaling. A novel isotope mass scaling law is proposed which describes the electron-to-ion mass ratio dependence of the turbulent energy flux and reversal from naive gyroBohm scaling in the edge. The finite electron-mass correction is larger for light ions and higher safety factor so that, while it is weak in the core, it dominates the mass scaling in the edge. These results may have favorable implications for global energy confinement and for the power threshold for the L to H mode transition in a reactor like ITER from hydrogen to deuterium to deuterium-tritium, consistent with recent experimental observations. [Preview Abstract] |
Monday, November 9, 2020 11:30AM - 12:00PM Live |
BI02.00005: Destabilization of High-Field-Side Micro-Instabilities by Large Shafranov Shift in Present and Future Devices Invited Speaker: Xiang Jian A new gyrokinetic study of internal transport barrier (ITB) stability shows that large Shafranov shift can destabilize high-field-side (HFS) instabilities in addition to stabilizing conventional drift-ballooning modes. Recent analysis [1] of a typical DIII-D high $\beta $p discharge shows that while the high local Shafronov shift (as quantified by $\alpha $ \textasciitilde -q2Rd$\beta $/dr) in the ITB region is able to suppress all the conventional drift-ballooning instabilities, it also destabilizes micro-tearing modes (MTM), which become the unique instability limiting the ITB kinetic gradient. Interestingly, the destabilized MTM is found to be a slab-like mode whose eigenfunction peaks in the high field side (HFS), with a mode structure requiring extremely high grid resolution to accurately capture. These results provide the first direct validation of MTM transport levels as predicted by nonlinear gyrokinetics for measured core ITB parameters. Moreover, this finding demonstrates that there are potential limits to confinement improvements that can be achieved through $\alpha $ stabilization, independent of global stability considerations. Extrapolation to future tokamak regimes suggests that while the HFS MTM mode is likely to be stabilized due to reduced collisionality, other electrostatic slab-like HFS modes will be destabilized by large values of $\alpha $ and act as dominant instabilities. A detailed gyrokinetic analysis shows how the effective ``squeezing'' of the bad curvature region by large $\alpha $ (the mechanism which leads to stabilization of conventional drift-ballooning modes) also opens a window to destabilization of HFS modes. This work supported by US DOE under DE-SC0018287. [1] X. Jian et al., Phys. Rev. Lett. 123, 225002 (2019) [Preview Abstract] |
Monday, November 9, 2020 12:00PM - 12:30PM Live |
BI02.00006: Progress in theoretical understanding of the Dimits shift and the tertiary instability of drift waves Invited Speaker: Hongxuan Zhu A natural way to control turbulence in magnetic fusion devices is to take advantage of zonal flows, which form spontaneously and can reduce the turbulence level. Zonal flows can even suppress turbulence completely in a certain parameter range where drift-wave instabilities would otherwise develop. But exploiting this effect, which is known as the Dimits shift, requires understanding of its basic physics, which has been unclear. In our work [1, 2], a generic understanding of the Dimits shift in electrostatic drift-wave turbulence is obtained by studying the tertiary instability of a zonal flow within reduced turbulence models. We show that tertiary modes are localized near extrema of the zonal-flow velocity $U(x)$ with respect to the radial coordinate $x$. These modes can be described as quantum harmonic oscillators with complex frequencies, so their spectrum can be readily calculated. The corresponding growth rate $\gamma_{\rm TI}$ is derived within the modified Hasegawa--Wakatani model. We show that $\gamma_{\rm TI}$ equals the primary-instability growth rate plus a term that depends on the local flow ``curvature'' $U''(x)$; hence, the instability threshold is shifted compared to that in homogeneous turbulence. This shift is the Dimits shift, which we find explicitly in the Terry--Horton limit [3], and our analytic predictions agree well with results of numerical simulations. Our theory of the tertiary instability also extends to other turbulence models. For example, the key features of the tertiary instability of ion-temperature-gradient mode [4] are reproduced by our theory and verified by gyrokinetic simulations.\\ $[1]$ H. Zhu, Y. Zhou, and I. Y. Dodin, Phys. Rev. Lett. 124, 055002 (2020).\\ $[2]$ H. Zhu, Y. Zhou, and I. Y. Dodin, arXiv:2004.03739.\\ $[3]$ D. A. St-Onge, J. Plasma Phys. 83, 905830504 (2017).\\ $[4]$ B. N. Rogers, W. Dorland, and M. Kotschenreuther, Phys. Rev. Lett. 85, 5336 (2000). [Preview Abstract] |
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