Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session TI02: MFE IV: Edge and Scrape-Off Layer PlasmasInvited Session Live
|
Hide Abstracts |
Chair: Mikhail Dorf, Lawrence Livermore National Laboratory Room: Ballroom C |
Thursday, November 11, 2021 9:30AM - 10:00AM |
TI02.00001: Collisionless transport mechanisms for thermal quench in stochastic magnetic fields open at the wall boundary Invited Speaker: Min-Gu Yoo The break of closed magnetic surfaces due to locked modes can result in a very broad stochastic layer and lead to a rapid loss of energy confinement in tokamaks [1]. Here we report on a first-principles-based simulation study of plasma transport in stochastic magnetic fields based on a global gyrokinetic model to address some specific challenges of thermal quench in tokamak disruption [2]. Even though magnetic field lines become stochastic and open at the wall, the majority of electrons were found to be confined in the system due to trapping by the magnetic mirror force and positive electrostatic potential developed in the stochastic layer. In this study, we present a comprehensive understanding of the dynamics of passing and trapped electrons and the 3-D topology of the stochastic layer, taking into account the consistent coupling of electron and ion dynamics through the ambipolar electric field. The 3-D ambipolar potential builds up in the stochastic layer to maintain quasi-neutrality in the plasma during the thermal quench. The associated ExB vortices mix particles across the stochastic open field lines, providing a collisionless detrapping of electrons that plays a major role in the loss of high-energy electrons along favorable open field lines. In addition, the 3-D electric field also drives a significant perpendicular transport, directly contributing to the thermal quench. As a result, the electron temperature decreases steadily in the typical thermal quench time scale of milliseconds. |
Thursday, November 11, 2021 10:00AM - 10:30AM |
TI02.00002: Experimental Characterization of the Electric Sheath at Divertor Surfaces Using Micro-Engineered Targets on DiMES at DIII-D Invited Speaker: Shota Abe We report an experimental determination of the characteristic magnetic pre-sheath (MPS) width, LMPS, in the divertor of the DIII-D tokamak by performing detailed measurements of the azimuthal and polar ion impact angles using micro-engineered targets. Analytical and numerical studies reported the wide MPS formation for several ion gyro radii, which dominates the sheath potential structure rather than the classical Debye sheath, when the grazing magnetic field (< 5º) enters into the divertor surface under conventional Tokamak and ITER-like plasma parameters [1]. A validation of the theoretical sheath models supports its applicability to ITER and pilot plant divertors. The divertor sheath width is defined by an approximated MPS potential Φ ∝ exp(-2z/LMPS), where z is the distance from the surface. The sheath potential critically controls the ion trajectory of low-Z species (D, T, He, and C), as well as the prompt re-deposition of high-Z species. Hence, LMPS is a critical parameter to successfully predict plasma-materials interactions. We exposed specially engineered micro-trenches (30×30×2-4 µm) to L-mode D plasmas via DiMES. Deposition patterns of C impurities resulting from the D ion shadowing effect on the trench floors were measured by energy-dispersive X-ray spectroscopy and compared with a gross erosion calculated by the Monte Carlo micro-patterning and roughness code. The C deposition profiles showed that the erosion was maximized for the azimuthal direction of φ = -40° (referenced to BT) and polar angle of θ = 80°. The erosion simulation using input of D ion angle distributions calculated for the assumption k = 3 reproduced the experimental C deposition profiles. This result verified a kinetic modeling result, k ~ 3 [1]. [1] D. Coulette, G. Manfredi, Plasma Phys. Control. Fusion 58 (2016) 025008 |
Thursday, November 11, 2021 10:30AM - 11:00AM |
TI02.00003: Particle pinch in the tokamak edge Invited Speaker: Ben Zhu Particle pinch, a process in which electrons/ions are somehow transported up (rather than down) the plasma pressure gradient, is a fundamental and yet not fully understood process in plasma physics, partially in the tokamak edge. An understanding of this process is vital not only because it would enable the capability to predict density profiles from first-principle theory, but also it is a key step to untangle some of the longstanding problems in tokamak physics, e.g., pedestal formation in the LH transition, density limit, absence of particle transport barrier in I-mode, and so on. Recently, a significant progress on the particle pinch research in the tokamak edge has been made by using GDB model – a drift-reduced Braginskii based 3D electromagnetic turbulence model which can self-consistently evolve both plasma and flow quantities in full annulus (including both closed flux region and the scrape-off-layer). A robust particle pinch has been observed in flux driven 3D global edge simulations when a narrow particle source is located near the last closed flux surface. The inward particle flux, accompanied by outward heat flux, is carried by fluctuating ExB drift in these fully nonlinear simulations. With a simplified 1D linear model, toroidal ITG mode is identified as the possible dominate pinch driver, even in the negative density gradient (hence, negative ηe and ηi) range. Moreover, the neoclassical effect, namely Pfirsch-Schluter ion heat flux is found to have a significant impact on the final steady-state density profile. |
Thursday, November 11, 2021 11:00AM - 11:30AM |
TI02.00004: Effects of neutral transport on plasma scrape-off layer turbulence in gyrokinetic simulations Invited Speaker: Tess Bernard Interactions between the plasma and neutral particles play a crucial role in determining the exhaust characteristics of tokamak plasmas. We present the first coupling of a continuum full-f gyrokinetic turbulence model with a 6D continuum model for kinetic neutrals, carried out using the Gkeyll code. Our objective is to improve the first-principles understanding of the role of neutrals in plasma fueling, detachment, and their interaction with blobby transport and edge plasma profiles. Previous gyrokinetic simulations with kinetic neutrals included only adiabatic electrons. We include both gyrokinetic ions and electrons in our simulations, allowing us to explore the effect of neutrals on density profiles. Our model incorporates electron-impact ionization, charge exchange, and wall recycling boundary conditions. Ionization and charge exchange interactions are coupled as sources and sinks to the gyrokinetic species, which avoids the statistical noise convergence properties associated with hybrid continuum-Monte Carlo codes. The model has been successfully verified with analytical predictions and benchmarked with the DEGAS2 Monte Carlo neutral code. We have carried out simulations for a scrape-off layer (SOL) in simplified geometry with NSTX parameters. Compared to simulations without neutrals, these tests exhibit a reduction in density gradients and fluctuation levels, increased skewness and kurtosis of the PDFs, increased autocorrelation times, and a decrease in the heat flux to the divertor. These results point towards a complex interaction between neutral effects and plasma turbulence that cannot be accurately captured by transport codes used to model exhaust characteristics in future devices and highlight the importance of coupling first-principles turbulence models with neutrals in predictive SOL modeling. |
Thursday, November 11, 2021 11:30AM - 12:00PM |
TI02.00005: Full-f electromagnetic gyrokinetic turbulence simulations of the edge and scrape-off layer of ASDEX Upgrade with GENE-X Invited Speaker: Dominik Michels Understanding and predicting the effects of turbulence in the edge and scrape-off layer (SOL) of magnetic fusion devices is known to be a critical step on the path towards a truly predictive capability. While there has been progress along these lines in recent years, especially with the help of Braginskii-type fluid codes, the development of full-f electromagnetic gyrokinetic codes for the edge and SOL in general diverted geometries remains the ultimate goal. |
Thursday, November 11, 2021 12:00PM - 12:30PM |
TI02.00006: Experimental study of the role of the edge radial electric field on the access to H-mode at ASDEX Upgrade Invited Speaker: Ulrike Plank The underlying mechanism for the transition from L- to H-mode (LH transition) has been of interest for experimental and theoretical studies since the discovery of the H-mode. The gradients of the radial electric field, Er , at the plasma edge are often considered to be responsible for the edge transport barrier and the access to H-mode. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2025 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700