Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Plasma Physics
Monday–Friday, October 30–November 3 2023; Denver, Colorado
Session YI01: MFE: Stellarators and 3D PhysicsInvited Session
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Chair: Elizabeth Paul, Columbia University Room: Plaza F |
Friday, November 3, 2023 9:30AM - 10:00AM |
YI01.00001: Core density profile control by energetic ion anisotropy in LHD Invited Speaker: Masaki Nishiura Experimental and theoretical studies on thermal and particle transport in fusion plasmas have been intensively carried out because of their impact on the performance of fusion reactors [1]. Controls of density profiles in the LHD were investigated by using tangential and perpendicular neutral beam injectors. The LHD plasmas have not yet achieved a high particle confinement state of particles except for a temporal state of density peaking created by a pellet injection. However, we found that reduced anisotropy of stored energy for energetic ion En_perp/En_para enhances the inward transport, resulting in an electron density peaking successfully in the core plasma of the LHD for the first time. The low En_perp/En_para (< 0.4) regime, where the reference [2] examined the particle transport, results in a flattening profile (dn/dr~0 [/m^4]) for electron and a hollow profile (dn/dr~0 or slightly positive) for impurity carbon, while the new En_perp/En_para regime of 0.47 or higher can access to peaked density profiles (dn/dr~-6 [/m^4]). At the same time, density peaking profiles rise in neutron flux and stored energy. On the other hand, the rise in the anisotropy flattens the density profile, which leads to an increased turbulence amplitude in the spectral range below 500 kHz in the core plasma. |
Friday, November 3, 2023 10:00AM - 10:30AM |
YI01.00002: Reduction (or enhancement) of stellarator turbulence by impurities Invited Speaker: José M García-Regaña The deliberate injection of impurities into the plasma core has been proven to increase the energy confinement time, and to reduce the turbulent fluctuations and the turbulent ion heat diffusivity in the Large Helical Device (LHD) [1] and in Wendelstein 7-X (W7-X) [2]. However, in general, impurities are neglected in the context of gyrokinetic modeling of bulk plasma turbulence in stellarators. In this talk, we present a systematic study of the impact of impurities on stellarator bulk turbulence, and in particular on the turbulent heat fluxes in W7-X. By means of nonlinear multispecies gyrokinetic simulations with the code stella [3], we demonstrate that if the impurity density gradient and the bulk ion density gradient are parallel (resp. antiparallel), impurities can significantly reduce (resp. enhance) turbulent heat losses. We will explain that this is caused by the intrinsic capacity of impurities to modify bulk plasma turbulence, and not by trivial effects such as dilution or changes in the bulk plasma profiles. For the relevant situation where the impurity and bulk ion density gradients are parallel, the heat flux exhibits a minimum at a certain impurity concentration. These results show the potential of impurities for enabling access to enhanced confinement regimes in stellarators. |
Friday, November 3, 2023 10:30AM - 11:00AM |
YI01.00003: Neoclassical transport due to resonant magnetic perturbations in DIII-D and NSTX Invited Speaker: Priyanjana Sinha Resonant magnetic perturbations (RMPs) are applied to mitigate or suppress the instabilities present in the plasma called edge localized modes (ELMs) which arise because of the steep pressure gradient at the edge in H-mode plasmas. The RMPs often result in a decrease in the plasma density, also referred to as density pump-out, which adversely affects the fusion performance. In this study, the role of neoclassical transport in density pump-out and heat flux in the presence of RMPs is investigated in DIII-D and NSTX plasmas. The drift kinetic code NEO with the enhanced capability to handle non-axisymmetric magnetic geometry is used here to evaluate the neoclassical transport properties where RMPs are applied. The magnetic field provided as an input to NEO is calculated using extended magnetohydrodynamic code M3D-C1 and includes the nonlinear resistive plasma response in realistic geometry and with realistic values of resistivity. The study performed here indicates a dramatic increase of the neoclassical particle and energy fluxes for main ions in the presence of the RMPs and is on the same order as experimentally inferred fluxes, suggesting that neoclassical transport plays an important role in edge transport in such cases. The calculated neoclassical fluxes in DIII-D plasmas are found to be closely correlated with the observations of density pump-out over a range of RMP spectra. These calculations show that nonlinear MHD simulations are essential at high RMPs to satisfactorily model the perturbed magnetic geometry in the pedestal region. The study of transport dynamics of impurities is important as they can lead to fuel dilution, radiative cooling which deteriorates the plasma performance, and has also been analyzed using this framework. Our study indicates that the atomic number (Z), collisionality of the impurity and perturbation field strength determines the effect of RMPs on neoclassical impurity transport. We extrapolate our results to further predict the impurity transport and its control in ITER. |
Friday, November 3, 2023 11:00AM - 11:30AM |
YI01.00004: Experimental Evidence for Equilibrium Pressure Surfaces supported by ExB Drifts in the Island Divertor of Wendelstein 7-X Invited Speaker: Erik R Flom For the first time, direct experimental evidence is presented for equilibrium pressure surfaces formed on the magnetic flux surfaces of the island divertor at Wendelstein 7-X (W7-X). These isobars are observed up to moderate densities (e> = 3 x 1019 m-3) and are supported by strong ExB drift transport in the island. Observed hollow temperature profiles, mapped to the target sheath, induce an electric field pointing towards the center of the island of several kV/m. These measurements were obtained with the thermal helium beam diagnostic in the magnetic islands of the “standard” ι = 2π (5/5) configuration. At low density, the presence of isobars conforming to the magnetic flux surfaces in the island is confirmed. In a parallel-dominated, sheath-limited regime, such isobars would be expected. However, these isobars are experimentally observed at higher densities than predicted by EMC3-EIRENE, sustained by poloidal ExB fluid drifts not included in the code. We present evidence of a significant transition in transport regimes beyond the critical density mentioned above, consistent with a reduction in poloidal drifts at increasing density. These poloidal ExB drifts also transport particles into the magnetic shadow of the divertor, shortcutting transport through the island. Evidence for these drift effects in the form of transport asymmetries corresponding to the expected directions of poloidal ExB drifts between lower and upper divertor modules are shown, as well as estimations of drift velocities. Such investigations into the fundamental local plasma properties in the island divertor and associated particle and energy transport are required to understand the functionality of the island divertor as an exhaust concept. Despite not reflecting the drift physics, comparisons to EMC3-EIRENE modeling are presented to show that profile differences at different positions around the island can be attributed to differences not just in the magnetic structure, but the competing parallel/radial transport channels as well. These results contribute to extrapolation from W7-X to future divertor scenarios in a fusion reactor. |
Friday, November 3, 2023 11:30AM - 12:00PM |
YI01.00005: Turbulence optimization of the HSX stellarator via small coil-current modifications Invited Speaker: Michael J Gerard Turbulent transport commonly limits confinement in fusion devices. In stellarators, the 3D structure of the confining magnetic field provides the potential for reducing transport by optimizing the field geometry. To date, turbulence-optimized configurations have been obtained primarily for idealized or synthetic geometries, and no such configurations have been validated against experiments. In order to make stellarators competitive, turbulence optimization needs to be demonstrated in existing devices. To that end, it is shown numerically (M. J. Gerard et al. 2023 Nucl. Fusion 63 056004) that small changes in the magnetic field of a stellarator equilibrium can result in a significant reduction in trapped-electron-mode (TEM) activity in the Helically Symmetric eXperiment (HSX). This is accomplished by numerically generating > 106 stellarator equilibria by modifying individual coil currents. A hierarchy of models is used to reduce this set of configurations to ~ 102 promising experimental-candidate configurations. In so doing, it is found that TEM growth rates can be regulated by modifying trapped-electron resonances via their precessional drifts across helically-linked magnetic trapping wells. Moreover, it is found that by elongating the plasma cross-section, TEM growth rates can be reduced without sacrificing helical symmetry. Considering configurations of varying elongation, nonlinear simulations reveal that the reduction in growth rates is correlated with an increase in the density gradient at which a sharp increase in heat flux is observed. Three configurations are identified in which the highest heat flux is a factor of two larger than the lowest heat flux, with a 30 % reduction from the standard configuration achieved by a moderate increase in elongation. These results are compared against quasilinear models to determine what impact the linear stabilization has on transport. This provides a set of candidate configurations that will be explored in future experiments on HSX. |
Friday, November 3, 2023 12:00PM - 12:30PM |
YI01.00006: Streamlined Stellarator Design: Single-Stage Optimization with Fixed Boundary Equilibria Invited Speaker: Rogerio Jorge The quest for practical and economically viable fusion devices demands innovative approaches to overcome design challenges. With their complex coil systems and steady-state operation capabilities, stellarators have emerged as promising magnetic confinement devices. However, the traditional two-stage design process optimizes magnetic fields and coils separately, which usually leads to a time-consuming design process and can result in suboptimal coil configurations. In this work [1], we devised a novel single-stage optimization method using fixed boundary equilibria, allowing for simultaneous optimization of physics goals and engineering constraints. This approach creates a streamlined combined plasma-coil optimization process in stellarator design by incorporating both the plasma boundary and coil shapes as degrees of freedom in the optimization. Our innovative method is adaptable to various vacuum and finite plasma pressure stellarator equilibria, offering improved efficiency and reduced computational time compared to conventional techniques. We will explore the methodology, applications, and implications for the future of fusion devices in plasma physics, showing applications of this method to quasi-symmetric, quasi-isodynamic, and particular designs using a minimal set of coils. Such findings usher in a new era of streamlined stellarator design, paving the way for more efficient and effective magnetic confinement fusion devices. |
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