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 GI02: MFE I: Plasma-Material Interactions and StelleratorsInvited Live
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Chair: Aaron Bader, University of Wisconsin - Madison Room: Ballroom C |
Tuesday, November 9, 2021 9:30AM - 10:00AM |
GI02.00001: Mitigation of plasma-material interactions with low-Z powders in DIII-D H-mode discharges Invited Speaker: Florian Effenberg First experiments with multiple low-Z powder injections in DIII-D H-mode discharges demonstrated enhanced divertor dissipation with a rapid reduction of divertor electron temperature from 30 eV to below 10 eV, increasing divertor neutral pressure from 0.9 to 9 mTorr, and transition into stable detachment during the injection. In some cases, the plasma was observed to detach during powder injection, and in all cases, good energy confinement (Wmhd=0.7 MJ, τE=0.11 s) was maintained. |
Tuesday, November 9, 2021 10:00AM - 10:30AM |
GI02.00002: Plasma Facing Components with Capillary Porous System and Liquid Metal Coolant Flow Invited Speaker: Andrei Khodak Liquid metal (LM) can create renewable protective surfaces on plasma facing components, with an additional advantage of deuterium pumping and tritium extraction if liquid lithium (LL) is used. LM can also be utilised as an efficient coolant, driven by the Lorentz force created with the help of the magnetic field in fusion devices. Capillary porous systems can serve as a conduit of LM and simultaneously provide stabilisation of the LM flow, protecting against spills into the plasma. Recently within the framework of the U.S. DoE domestic LM divertor PFC design project, we proposed a combination of a fast-flowing LM cooling system with a porous plasma facing wall (CPSF) [1]. The system takes an advantage of a magnetohydrodynamics (MHD) velocity profile, as well as attractive LM properties to promote efficient heat transfer from plasma to LL at low pumping energy cost, relative to the incident heat flux on the PFC. In case of disruption leading to excessive heat flux from plasma, LL evaporation can stabilize the temperature due to high evaporation heat and apparent vapor shielding [2]. |
Tuesday, November 9, 2021 10:30AM - 11:00AM |
GI02.00003: Access to an improved confinement regime with reduced turbulence by boron powder injection in the Large Helical Device Invited Speaker: Federico Nespoli The Impurity Powder Dropper is used in the Large Helical Device to inject controlled amounts of sub-millimetric boron powder into the plasma, performing a real time boronization of the plasma facing components. During boron powder injection experiments, wall recycling and impurity content are observed to decrease both on a shot-to-shot basis, and in real time. Furthermore, a novel improved confinement regime, characterized by reduced turbulence fluctuations, is observed to be triggered by the powder injection, at constant line-averaged electron density and input power. The plasma stored energy, electron and ion temperature are increased in the order of 20%, and up to 50%. Simultaneously, the amplitude of the turbulent density fluctuations is reduced to half its value before powder injection, across most of the plasma cross section. While lower frequency ion temperature gradient-like (ITG) fluctuations are substantially damped, higher frequency modes appear in the range 100 < f [kHz] < 200. Dynamic transport analysis shows a reduction, by up to 50%, of both ion and electron thermal conductivity in the plasma edge ρ ≥ 0.5, and the energy confinement time is also observed to increase. |
Tuesday, November 9, 2021 11:00AM - 11:30AM |
GI02.00004: Stellarator Simplification using Permanent Magnets Invited Speaker: Caoxiang Zhu We report the status of the permanent magnet stellarator project. Non-planar coils are the most complicated and expensive part of a stellarator. Permanent magnets provide a novel method to produce optimized stellarator configurations using very simple coils. The new concept for generating 3D fields using permanent magnets has led to the world's first project examining the use of permanent magnets for stellarators, which has been funded by ARPA-E and FES and will be located at Princeton Plasma Physics Laboratory. The project will design and construct a half-period of the magnet structure for a possible stellarator concept that would use components from NCSX, including the toroidal field coils and vacuum vessel, together with an array of neodymium magnets. Two new codes, MAGPIE and FAMUS, have been developed to design the magnets. MAGPIE provides the geometry information and FAMUS optimizes the magnet arrangements. By using the two codes, the target quasi-axisymmetric equilibrium with improved energetic particle confinement can be realized by uniform cuboidal magnets in a limited number of discrete polarizations together with planar toroidal field coils. A post-office-box structure will be used to mount the magnets. An automated system has been developed within the Virtual Engineering framework of the ANSYS suite of codes to integrate the MAGPIE-FAMUS code set with engineering analysis codes. The magnet positions will be adjusted iteratively and an array of correction magnets will be installed to minimize error fields with tolerance. The methods used and the results from the design effort will be described in detail and the status of the construction activity will be summarized. A table-top PM stellarator project (MUSE) has also been designed for basic experiments, and construction is underway. |
Tuesday, November 9, 2021 11:30AM - 12:00PM |
GI02.00005: New approaches to stellarator optimization using expansion in aspect ratio Invited Speaker: Matt Landreman To achieve good orbit confinement, recent stellarators such as HSX and W7-X have been designed using optimization, with numerical calculation of a 3D MHD equilibrium at each objective function evaluation. Here we present new design methods which reduce the computational cost of each objective evaluation by orders of magnitude, while also providing new insights into the space of solutions [1-3]. These benefits are made possible by an expansion in large local aspect ratio [4]. The expansion enables a direct construction of stellarator shapes with quasisymmetry, a symmetry hidden in the field strength that provides good orbit confinement. This construction makes it possible to achieve quasisymmetry to much higher accuracy than reported before. The aspect ratio expansion also permits a precise understanding of how many unique quasisymmetric configurations are possible (close to the magnetic axis). Due to the reduction of the MHD equilibrium equations by the expansion, stellarator configurations can be evaluated far faster than in traditional optimization, enabling wide surveys over parameter space. Many figures of merit can be calculated directly from the near-axis solutions, including Mercier stability [5] and all the geometric quantities in the gyrokinetic equation [6-7]. This new approach to design can be used in concert with traditional stellarator optimization, the former providing initial conditions for the latter. The near-axis approach has also enabled the first simultaneous plasma-and-coil optimizations of quasisymmetric stellarators using analytic derivatives [8-9]. |
Tuesday, November 9, 2021 12:00PM - 12:30PM |
GI02.00006: Combined plasma-coil optimization approaches Invited Speaker: Sophia A Henneberg Approaches for combined plasma–coil optimization for designing stellarators are investigated and a new method for calculating free-boundary equilibria for multiregion relaxed magnetohydrodynmics is proposed (Henneberg, et al., (2021). JPP, 87(2), 905870226). Stellarators exploit three-dimensional magnetic fields to generate external rotational transform -- rotational transform solely generated by the coils’ magnetic field. This reduces or even eliminates the need for generating toroidal plasma currents, which can lead to instabilities such as disruptions. However, the three-dimensionality can involve some drawbacks, e.g., more complicated coils are typically needed compared to the axisymmetric case. Achieving favorable plasma and coil properties simultaneously is a major challenge in stellarator design. Nonetheless, with careful exploitation of the large design space, the apparent disadvantages can be diminished. With this work we investigate the mathematical structure of the various inter-related calculations that underpin the integrated stellarator optimization problem to better understand how the equilibrium calculation, the coil calculation, and the optimization calculation communicate with each other. The proposed free-boundary equilibrium calculation differs from existing methods in how the boundary-value problem is posed and will reduce the free-boundary equilibrium calculation to something comparable to a fixed-boundary calculation. |
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