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Allen H. Boozer - One of the best experts on this subject based on the ideXlab platform.
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curl free magnetic fields for Stellarator optimization
Physics of Plasmas, 2019Co-Authors: Allen H. BoozerAbstract:This paper describes a new and efficient method of defining an annular region of a curl-free magnetic field with specific physics and coil properties that can be used in Stellarator design. Three statements define the importance: (1) Codes can follow an optimized curl-free initial state to a final full-pressure equilibrium. The large size of the optimization space of Stellarators, approximately 50 externally produced distributions of magnetic field, makes success in finding a global optimum largely determined by the starting point. (2) The design of a Stellarator is actually improved when the central region of the plasma has rapid transport with the confinement provided by a surrounding annulus of magnetic surfaces with low transport. (3) The Stellarator is unique among all fusion concepts, inertial as well as magnetic, in not using the plasma itself to provide an essential part of its confinement concept. This permits reliable computational design, which opens a path to faster, cheaper, and more certain achievement of fusion energy.This paper describes a new and efficient method of defining an annular region of a curl-free magnetic field with specific physics and coil properties that can be used in Stellarator design. Three statements define the importance: (1) Codes can follow an optimized curl-free initial state to a final full-pressure equilibrium. The large size of the optimization space of Stellarators, approximately 50 externally produced distributions of magnetic field, makes success in finding a global optimum largely determined by the starting point. (2) The design of a Stellarator is actually improved when the central region of the plasma has rapid transport with the confinement provided by a surrounding annulus of magnetic surfaces with low transport. (3) The Stellarator is unique among all fusion concepts, inertial as well as magnetic, in not using the plasma itself to provide an essential part of its confinement concept. This permits reliable computational design, which opens a path to faster, cheaper, and more certain ...
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curl free magnetic fields for Stellarator optimization
arXiv: Plasma Physics, 2019Co-Authors: Allen H. BoozerAbstract:This paper describes a new and efficient method of defining an annular region of a curl-free magnetic field with specific physics and coil properties that can be used in Stellarator design. Three statements define the importance: (1) Codes can follow an optimized curl-free initial state to a final full-pressure equilibrium. The large size of the optimization space of Stellarators, approximately fifty externally-produced distributions of magnetic field, makes success in finding a global optimum largely determined by the starting point. (2) The design of a Stellarator is actually improved when the central region of the plasma has rapid transport with the confinement provided by a surrounding annulus of magnetic surfaces with low transport. (3) The Stellarator is unique among all fusion concepts, inertial as well as magnetic, in not using the plasma itself to provide an essential part of its confinement concept. This permits reliable computational design, which opens a path to faster, cheaper, and more certain achievement of fusion energy.
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Stellarator Coil Design and Plasma Sensitivity
Physics of Plasmas, 2010Co-Authors: Allen H. BoozerAbstract:The rich information contained in the plasma response to external magnetic perturbations can be used to help design Stellarator coils more effectively. We demonstrate the feasibility by first developing a simple direct method to study perturbations in Stellarators that do not break Stellarator symmetry and periodicity. The method applies a small perturbation to the plasma boundary and evaluates the resulting perturbed free-boundary equilibrium to build up a sensitivity matrix for the important physics attributes of the underlying configuration. Using this sensitivity information, design methods for better Stellarator coils are then developed. The procedure and a proof-of-principle application are given that (1) determine the spatial distributions of external normal magnetic field at the location of the unperturbed plasma boundary to which the plasma properties are most sensitive, (2) determine the distributions of external normal magnetic field that can be produced most efficiently by distant coils, and (3) choose the ratios of the magnitudes of the efficiently produced magnetic distributions so the sensitive plasma properties can be controlled. Using these methods, sets of modular coils are found for the National Compact Stellarator Experiment (NCSX) which are either smoother or can be located much farther from the plasma boundary than those of the present design.
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What is a Stellarator
Physics of Plasmas, 1998Co-Authors: Allen H. BoozerAbstract:A Stellarator is a toroidal plasma confinement concept that uses effects that arise in the absence of toroidal symmetry to maintain the magnetic configuration without the need for current drive. The largest magnetic fusion machines under construction are Stellarators, and the plasma parameters achieved in Stellarators are second only to those in tokamaks. Stellarators are poised for rapid progress toward showing the feasibility of fusion power. The physics and mathematical concepts that are required to understand Stellarators are reviewed.
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Quasi-helical symmetry in Stellarators
Plasma Physics and Controlled Fusion, 1995Co-Authors: Allen H. BoozerAbstract:The quasi-helical Stellarator, which was invented by Nuhrenberg and Zille in 1988, is a toroidal fusion concept that has the excellent alpha confinement of the axisymmetric tokamak without the disadvantages of major disruptions and current drive. Quasi-helical Stellarators demonstrate the flexibility that exists in the tokamak and related toroidal configurations to circumvent critical problems in obtaining fusion power.
J.f. Lyon - One of the best experts on this subject based on the ideXlab platform.
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Development of a Robust Class of Quasi-Poloidal Compact Stellarator
2015Co-Authors: D. J. Strickler, J.f. Lyon, S. P. Hirshman, D. A. Spong, L. A. Berry, M. J. Cole, B. E. Nelson, A. S. Ware, D. E. WilliamsonAbstract:The Stellarator optimization code STELLOPT [1] has been used in low aspect ratio Quasi-Poloidal Stellarator (QPS) [2] design to determine the shape of the outer magnetic flux surface, together with internal plasma pressure and current profiles, that produce desirable physics properties such as confined particle drift trajectories and plasma MHD stability at <> 2%. The integration of the COILOPT [3] model, based on explicit representations for modular coils and coil geometry constraints, into the Stellarator optimization package STELLOPT, provides a unique and important computational tool [4] for the design of compact Stellarators. This self-consistent analysis ensures that physics and engineering criteria are simultaneously targeted in the full-pressure, full-current plasma/coil configuration. In this paper we describe a method that is based on including, in addition to the usual plasma confinement and stability properties at the full value of beta, a vacuum condition that drives the combined optimization of the plasma and coil configuration into a region of parameter space with improved flexibility and robustness. In the STELLOPT code, the optimization is formulated as a least-squares minimization of a target 2 = i(x)2, where the individual components, i, are nonlinear functions of th
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Compact Stellarator Path to DEMO
Bulletin of the American Physical Society, 2007Co-Authors: J.f. LyonAbstract:The US fusion program is considering what new facilities are needed in addition to ITER and existing facilities to prepare the way to a DEMO reactor. The broad issues are familiar: sustainment of an ignited/high-Q plasma in steady state, avoidance of disruptions and large variations in power flux to the wall, adequate confinement of thermal plasma and alpha-particles, control of a burning plasma, particle and power handling (divertors), etc. Stellarators have key reactor advantages: steady-state high-plasma-density operation without external current drive or disruptions, stability without a close conducting wall or active feedback systems, and low recirculating power. While Stellarators are at present less developed than tokamaks because of the higher parameters obtained in large tokamaks, D-T operation in TFTR and JET, and the decision to proceed with ITER, Stellarators do present an attractive alternate path to a DEMO reactor. Compact Stellarators, with moderate plasma aspect ratios, good confinement, and high-beta potential lead to the smaller-size reactors that are favored by the US utility industry. The ARIES group, which had previously analyzed different tokamaks, an ST, an RFP and a modular Stellarator (SPPS) as reactors, has recently examined compact Stellarators. The ARIES-CS study established that compact Stellarators can be economically competitive with tokamaks as reactors, but face serious issues with divertors, as do other toroidal concepts.
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Bootstrap current in quasi-symmetric Stellarators
Fusion Science and Technology, 2006Co-Authors: Andrew Ware, Donald A. Spong, Lee A. Berry, S. Hirshman, J.f. LyonAbstract:This work examines bootstrap current in quasi-symmetric Stellarators with a focus on the impact of bootstrap current on the equilibrium properties of Stellarator configurations. In the design of the Quasi-Poloidal Stellarator (QPS), a code was used to predict the bootstrap current based on a calculation in an asymptotically collisionless limit. This calculation is believed to be a good approximation of the bootstrap current for low-collisionality plasmas but is expected to be higher than the actual bootstrap current for more collisional plasmas. A fluid moments approach has been developed to self-consistently calculate viscosities and neoclassical transport coefficients. The viscosities and transport coefficients can be used to calculate the bootstrap current for arbitrary collisionality and magnetic geometry. The bootstrap current calculations from the two codes were done for low-density, electron cyclotron-heated (ECH) plasmas and high-density, ion cyclotron-heated (ICH) plasmas for a range of configurations, and provide a benchmark for the moments code and a test of the range of validity of the collisionless code. In the configurations examined here, namely, QPS, the National Compact Stellarator Experiment, the Helically Symmetric Experiment, the Large Helical Device, and the Wendelstein-7X Stellarator, the bootstrap currents predicted from the two codes agree qualitatively for both ICH and ECH profiles.
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ORNL Stellarator divertor studies
1995Co-Authors: James A. Rome, D. K. Lee, A.c. England, J.f. LyonAbstract:Oak Ridge National Laboratory (ORNL) is studying various aspects of divertors in different Stellarators. We are looking at a local island divertor (LID) on the CHS helical system, and potential designs of divertors that use islands for modular Stellarators such as W7-AS and the modular helias-like heliac MHH chosen for the US Stellarator Power Plant Study. Although the helical CHS configuration is quite different from the modular W 7-AS configuration both rely on the island structure outside the last closed flux surface (LCFS) to aid in funneling escaping plasma flux and to protect the leading edges of the divertor plates. However, the way that the islands are used is quite different.
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Assessment of Stellarators as reactors
1993Co-Authors: J.f. LyonAbstract:Stellarators have significant operational advantages over tokamaks as ignited steady-state reactors: no dangerous disruptions, no need for continuous current drive and power recirculated to the plasma, less severe constraints on the plasma parameters and profiles, and access from the inboard side for easier maintenance. The US is starting a multi-year multi-institutional Stellarator reactor study whose purpose is to ``identify and assess the feasibility of critical issues and their consequences for development of the Stellarator concept as a steady-state fusion reactor.`` The activities during the first year are focusing on physics optimization and selection of one or more Stellarator coil configurations for more detailed engineering design evaluation. The physics team is focusing on torsatron modularization, modular Stellarators with lower aspect ratio, the divertor geometry, development of transport models, and overall system studies. The engineering team is studying design issues relating to minimizing the inboard thickness of the blanket and shields, the feasibility of the superconducting magnets, and maintenance schemes.
P. Xanthopoulos - One of the best experts on this subject based on the ideXlab platform.
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the parallel boundary condition for turbulence simulations in low magnetic shear devices
Plasma Physics and Controlled Fusion, 2018Co-Authors: Mike F Martin, P. Xanthopoulos, Matt Landreman, Noah Mandell, W DorlandAbstract:Flux tube simulations of plasma turbulence in Stellarators and tokamaks typically employ coordinates which are aligned with the magnetic field lines. Anisotropic turbulent fluctuations can be represented in such field-aligned coordinates very efficiently, but the resulting non-trivial boundary conditions involve all three spatial directions, and must be handled with care. The standard 'twist-and-shift' formulation of the boundary conditions (Beer et al 1995 Phys. Plasmas 2 2687) was derived assuming axisymmetry and is widely used because it is efficient, as long as the global magnetic shear is not too small. A generalization of this formulation is presented, appropriate for studies of non-axisymmetric, Stellarator symmetric configurations, as well as for axisymmetric configurations with small global shear. The key idea is to replace the 'twist' of the standard approach (which accounts only for global shear) with the integrated local shear. This generalization allows one significantly more freedom when choosing the extent of the simulation domain in each direction, without losing the natural efficiency of field line-following coordinates. It also corrects some errors associated with naive application of axisymmetric boundary conditions to non-axisymmetric configurations. Simulations of Stellarator turbulence that employ the generalized boundary conditions generally require much less resolution than simulations that use the conventional, axisymmetric boundary conditions. We also demonstrate the surprising result that (at least in some cases) an easily implemented but manifestly incorrect formulation of the boundary conditions does not change important predicted quantities, such as the turbulent heat flux.
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TEM turbulence optimisation in Stellarators
Plasma Physics and Controlled Fusion, 2015Co-Authors: J. H. E. Proll, H. E. Mynick, P. Xanthopoulos, Samuel Lazerson, B J FaberAbstract:With the advent of neoclassically optimised Stellarators, optimising Stellarators for turbulent transport is an important next step. The reduction of ion-temperature-gradient-driven turbulence has been achieved via shaping of the magnetic field, and the reduction of trapped-electron mode (TEM) turbulence is addressed in the present paper. Recent analytical and numerical findings suggest TEMs are stabilised when a large fraction of trapped particles experiences favourable bounce-averaged curvature. This is the case for example in Wendelstein 7-X (Beidler et al 1990 Fusion Technol. 17 148) and other Helias-type Stellarators. Using this knowledge, a proxy function was designed to estimate the TEM dynamics, allowing optimal configurations for TEM stability to be determined with the STELLOPT (Spong et al 2001 Nucl. Fusion 41 711) code without extensive turbulence simulations. A first proof-of-principle optimised equilibrium stemming from the TEM-dominated Stellarator experiment HSX (Anderson et al 1995 Fusion Technol. 27 273) is presented for which a reduction of the linear growth rates is achieved over a broad range of the operational parameter space. As an important consequence of this property, the turbulent heat flux levels are reduced compared with the initial configuration.
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Controlling Turbulence in Present and Future Stellarators
Physical review letters, 2014Co-Authors: P. Xanthopoulos, H. E. Mynick, Per Helander, Y Turkin, G. G. Plunk, Frank Jenko, Tobias Görler, Daniel Told, T. Bird, J. H. E. ProllAbstract:Turbulence is widely expected to limit the confinement and, thus, the overall performance of modern neoclassically optimized Stellarators. We employ novel petaflop-scale gyrokinetic simulations to predict the distribution of turbulence fluctuations and the related transport scaling on entire Stellarator magnetic surfaces and reveal striking differences to tokamaks. Using a stochastic global-search optimization method, we derive the first turbulence-optimized Stellarator configuration stemming from an existing quasiomnigenous design.
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Optimizing Stellarators for Turbulent Transport
Physical review letters, 2010Co-Authors: H. E. Mynick, N. Pomphrey, P. XanthopoulosAbstract:Up to now, the term "transport-optimized" Stellarators has meant optimized to minimize neoclassical transport, while the task of also mitigating turbulent transport, usually the dominant transport channel in such designs, has not been addressed, due to the complexity of plasma turbulence in Stellarators. Here, we demonstrate that Stellarators can also be designed to mitigate their turbulent transport, by making use of two powerful numerical tools not available until recently, namely, gyrokinetic codes valid for 3D nonlinear simulations and Stellarator optimization codes. Two initial proof-of-principle configurations are obtained, reducing the level of ion temperature gradient turbulent transport from the National Compact Stellarator Experiment baseline design by a factor of 2-2.5.
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nonlinear gyrokinetic simulations of ion temperature gradient turbulence for the optimized wendelstein 7 x Stellarator
Physical Review Letters, 2007Co-Authors: P. Xanthopoulos, F Merz, T Gorler, F JenkoAbstract:Ion-temperature-gradient turbulence constitutes a possibly dominant transport mechanism for optimized Stellarators, in view of the effective suppression of neoclassical losses characterizing these devices. Nonlinear gyrokinetic simulation results for the Wendelstein 7-X Stellarator [G. Grieger et al., in Proceedings of the IAEA Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1990 (IAEA, Vienna, 1991) Vol. 3, p. 525]---assuming an adiabatic electron response---are presented. Several fundamental features are discussed, including the role of zonal flows for turbulence saturation, the resulting flux-gradient relationship, and the coexistence of ion-temperature-gradient modes with trapped ion modes in the saturated state.
C S Chang - One of the best experts on this subject based on the ideXlab platform.
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verification of the global gyrokinetic Stellarator code xgc s for linear ion temperature gradient driven modes
Physics of Plasmas, 2019Co-Authors: M D J Cole, R Hager, Toseo Moritaka, Julien Dominski, R Kleiber, S Ku, S Lazerson, J Riemann, C S ChangAbstract:XGC (X-point Gyrokinetic Code) is a whole-volume, total-f gyrokinetic particle-in-cell code developed for modeling tokamaks. In recent work, XGC has been extended to model more general 3D toroidal magnetic configurations, such as Stellarators. These improvements have resulted in the XGC-S version. In this paper, XGC-S is benchmarked in the reduced delta-f limit for linear electrostatic ion temperature gradient-driven microinstabilities, which can underlie turbulent transport in Stellarators. An initial benchmark of XGC-S in tokamak geometry shows good agreement with the XGC1, ORB5, and global GENE codes. A benchmark between XGC-S and the EUTERPE global gyrokinetic code for Stellarators has also been performed, this time in the geometry of the optimized Stellarator Wendelstein 7-X. Good agreement has been found for the mode number spectrum, mode structure, and growth rate.
R C Wolf - One of the best experts on this subject based on the ideXlab platform.
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magnetic configuration effects on the wendelstein 7 x Stellarator
Nature Physics, 2018Co-Authors: A Dinklage, C D Beidler, P Helander, G Fuchert, H Maasberg, K Rahbarnia, Sunn T Pedersen, Y Turkin, R C WolfAbstract:The two leading concepts for confining high-temperature fusion plasmas are the tokamak and the Stellarator. Tokamaks are rotationally symmetric and use a large plasma current to achieve confinement, whereas Stellarators are non-axisymmetric and employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic field of a Stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport leads to the appearance of a net toroidal plasma current, the so-called bootstrap current. Here, we analyse results from the first experimental campaign of the Wendelstein 7-X Stellarator, showing that its magnetic-field design allows good control of bootstrap currents and collisional transport. The energy confinement time is among the best ever achieved in Stellarators, both in absolute figures (τE > 100 ms) and relative to the Stellarator confinement scaling. The bootstrap current responds as predicted to changes in the magnetic mirror ratio. These initial experiments confirm several theoretically predicted properties of Wendelstein 7-X plasmas, and already indicate consistency with optimization measures. Results from the first experimental campaign of the Wendelstein 7-X Stellarator demonstrate that its magnetic-field design grants good control of parasitic plasma currents, leading to long energy confinement times.
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magnetic configuration effects on the wendelstein 7 x Stellarator
Nature Physics, 2018Co-Authors: A Dinklage, C D Beidler, P Helander, G Fuchert, H Maasberg, K Rahbarnia, Sunn T Pedersen, Y Turkin, R C WolfAbstract:The two leading concepts for confining high-temperature fusion plasmas are the tokamak and the Stellarator. Tokamaks are rotationally symmetric and use a large plasma current to achieve confinement, whereas Stellarators are non-axisymmetric and employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic field of a Stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport leads to the appearance of a net toroidal plasma current, the so-called bootstrap current. Here, we analyse results from the first experimental campaign of the Wendelstein 7-X Stellarator, showing that its magnetic-field design allows good control of bootstrap currents and collisional transport. The energy confinement time is among the best ever achieved in Stellarators, both in absolute figures (τE > 100 ms) and relative to the Stellarator confinement scaling. The bootstrap current responds as predicted to changes in the magnetic mirror ratio. These initial experiments confirm several theoretically predicted properties of Wendelstein 7-X plasmas, and already indicate consistency with optimization measures. Results from the first experimental campaign of the Wendelstein 7-X Stellarator demonstrate that its magnetic-field design grants good control of parasitic plasma currents, leading to long energy confinement times.
