Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session DI02: Magnetic Confinement Fusion IILive Streamed
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Chair: Donald Spong, Oak Ridge National Lab Room: Ballroom 100 B |
Monday, October 17, 2022 3:00PM - 3:30PM |
DI02.00001: Validation of gyrokinetic edge and scrape-off layer turbulence simulations vs TCV experiments Invited Speaker: Philipp Ulbl Turbulence in the edge and scrape-off layer (SOL) of magnetic confinement fusion devices is of upmost importance for the feasibility of fusion energy. Two major aspects of a future fusion reactor, confinement and heat exhaust, are predominantly affected by turbulence, requiring the development of predictive edge turbulence codes. |
Monday, October 17, 2022 3:30PM - 4:00PM |
DI02.00002: A Gyrokinetic moment-based method to model the plasma boundary of fusion devices at arbitrary collisionality Invited Speaker: Baptiste J Frei We present a new first-principles model that allows for the proper simulation of the tokamak plasma boundary [1,2]. Developed onto a set of fluid-like equations that retain the gyrokinetic Coulomb collision operator, the gyro-moment (GM) model offers a modeling framework able to describe the fluctuations occurring on a large range of spatial scales present in the plasma boundary where the plasma collisionality can vary by orders of magnitude. The GM model contains the core gyrokinetic model and the fluid and gyrofluid models used for scrape-off layer simulations as particular limits. We illustrate the analytical and numerical capabilities of the GM model. We demonstrate that the GM approach can correctly retrieve the properties of microinstabilities that develop at low plasma collisionality, strongly sensitive to kinetic features, in perfect agreement with the GK continuum GENE code. At the same time, we show that the GM model correctly retrieves the fluid limit at high collisionality by using advanced GK collision operators, such as the GK Coulomb collision operator. Furthermore, we prove that the GM approach is numerically efficient for the simulations in plasma regimes that range from the low-collisionality banana regime in the H-mode pedestal to the high-collisionality regime of the scrape-off layer in L-mode discharges. Our first nonlinear simulations show that, due to deviations in the linear growth rates and zonal flow damping [3,4,5], turbulent transport levels in the boundary can be largely underestimated if commonly used collision operators are used. |
Monday, October 17, 2022 4:00PM - 4:30PM |
DI02.00003: A probabilistic approach for the computation of local and nonlocal transport Invited Speaker: Diego Del-Castillo-Negrete Understanding transport is one of the main challenges of the controlled nuclear fusion program. Although significant progress has been made, the complexity of the problem calls for novel methods to overcome limitations of current approaches. The models of interest in this presentation are Fokker-Planck equations in which local (e.g., diffusive) transport is represented by differential operators and nonlocal transport by integro-differential operators (e.g., Landau-fluid and non-diffusive turbulent transport closures). Computational approaches for these problems can be classified as deterministic (e.g., finite-difference) and stochastic (e.g., Monte-Carlo). Although extensively used, continuum deterministic methods face stability and scalability challenges especially in the case of nonlocal operators that result in non-sparse matrices. On the other hand, particle-based stochastic methods face poor convergence due to statistical sampling. Here we present an alternative novel approach based on the Feynman-Kac theory that establishes a link between the Fokker-Planck equation and the stochastic differential equation of the underlying stochastic process. Intuitively, the method is a hybrid approach based on the stochastic representation of the local, or nonlocal, transport process, but with the actual computation reduced to the deterministic evaluation of mathematical expectations (i.e., integrals) bypassing the need of sampling individual stochastic orbits. The resulting algorithm is unconditionally stable, and parallelizable [Yang, et al., J. of Comput. Phys. 444, 110564 (2021) and https://arxiv.org/pdf/2205.00516.pdf]. We present applications to the initial value and the exit time problems for Fokker-Planck equations. Physics problems of interest include local and nonlocal anisotropic transport in toroidal geometry, confinement, and production rate of runaway electrons. |
Monday, October 17, 2022 4:30PM - 5:00PM |
DI02.00004: Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient Invited Speaker: Toby Adkins Electromagnetic (EM) instabilities and turbulence driven by the electron-temperature gradient are considered in a local slab model of a tokamak-like plasma, with constant equilibrium gradients (including magnetic drifts, but no magnetic shear). Derived in a low-beta asymptotic limit of gyrokinetics, the model describes perturbations at scales both larger and smaller than the electron inertial length de, but below the ion Larmor scale ρi, capturing both electrostatic and EM regimes of turbulence. The well-known electrostatic instabilities --- slab and curvature-mediated ETG --- are recovered, and a new instability is found in the EM regime, called the Thermo-Alfvenic instability (TAI). It exists in both a slab version (destabilising kinetic Alfven waves) and a curvature-mediated version, which is a cousin of the (electron-scale) kinetic ballooning mode (KBM). The curvature-mediated TAI is shown to be dominant at the largest scales covered by the model (greater than de but smaller than ρi), its physical mechanism hinging on the fast equalisation of the total temperature along perturbed magnetic field lines (in contrast to KBM, which is pressure balanced). A priori critical-balance estimates suggest that the TAI-driven heat-flux scales more steeply with the temperature gradient than that due to electrostatic ETG turbulence, giving rise to stiffer transport. Numerical results on the saturation of the resultant turbulence and the role of zonal flows and zonal fields are presented. |
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