Compact optimized stellarators offer novel solutions for confining high-β plasmas and developing magnetic confinement fusion. The three-dimensional plasma shape can be designed to enhance the magnetohydrodynamic (MHD) stability without feedback or nearby conducting structures and provide driftorbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the low aspect ratio, high β limit, and good confinement of advanced tokamaks. Quasiaxisymmetric equilibria have been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4-4.4 and average elongation ∼1.8. Even with bootstrap-current consistent profiles, they are passively stable to the ballooning, kink, vertical, Mercier, and neoclassicaltearing modes for β > 4%, without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at β = 4% (the rest is from the coils); thus the equilibrium is much less non-linear and is more controllable than similar advanced tokamaks. The enhanced stability is a result of 'reversed' global shear, the spatial distribution of local shear, and the large fraction of externally generated transform. Transport simulations show adequate fast-ion confinement and thermal neoclassical transport similar to equivalent tokamaks. Modular coils have been designed which reproduce the physics properties, provide good flux surfaces, and allow flexible variation of the plasma shape to control the predicted MHD stability and transport properties.
Magnetohydrodynamic (MHD) equilibrium states with imposed axisymmetric boundary are computed in which a spontaneous bifurcation develops to produce an internal three-dimensional (3D) configuration with a helical structure in addition to the standard axisymmetric system. Equilibrium states with similar MHD energy levels are shown to develop very different geometric structures. The helical equilibrium states resemble saturated internal kink mode structures. The essential confinement of particles and energy in magnetically enclosed plasmas is described by magnetohydrodynamics (MHD). The tokamak is the leading fusion energy research concept in which the plasma is expected to be contained in an essentially axisymmetric configuration with relatively small ripple effects from the finite toroidal coils. Symmetry of the equilibrium state along the toroidal coordinate grants the tokamak many appealing properties such as toroidal mass flow, which is known to reduce MHD instability and reduce non-MHD transport of particles and energy, as well as important conservation properties of single particles, thus enhancing confinement on various scales.It is shown in this Letter that despite imposing axisymmetry (toroidal symmetry) at the edge of the plasma, enforced by the geometry of the coil system, the preferred lowest energy state of MHD equilibrium can be nonaxisymmetric in the plasma center. The computation of threedimensional (3D) helical cores constitutes a paradigm shift for the description of tokamak equilibria that opens the way for the application of theoretical and simulation tools developed for stellarator analysis of MHD stability, guiding center particle orbits, kinetic stability, wave propagation or heating, neoclassical transport, gyrokinetics, etc., to determine the impact of these novel 3D states on a large range of magnetic confinement physics phenomena.The ignorable toroidal angle coordinate in axisymmetric magnetic confinement systems allows a simplified description of the MHD equilibrium state through the GradShafranov equation [1,2]. A more sophisticated approach invokes the minimization of the MHD energy to achieve an equilibrium state which can be naturally extended to model 3D systems with the imposition of nested magnetic flux surfaces and a single magnetic axis [3][4][5][6][7][8][9]. Tokamak devices, though nominally axisymmetric, display internal plasma reorganization phenomena that can break the symmetry of the system. In the tokamak à configuration variable (TCV) [10], a transition is observed where core sawteeth relaxations are replaced by global oscillations with low poloidal and toroidal mode numbers [11,12]. In these discharges, the inverse rotational transform q profiles are nearly flat or slightly reversed. One possible explanation for this transition is that q min , the minimum value of safety factor q within the plasma, becomes greater than unity. Equilibria with q min near unity are of interest in the present contribution. Similarly saturated ideal modes in the MAST device have also been re...
In the Large Helical Device (LHD), the highest operational averaged beta value has been expanded from 3.2% to 4% in the last 2 years by increasing the heating capability and exploring a new magnetic configuration with a high aspect ratio. Although the magneto-hydrodynamic (MHD) stability properties are considered to be unfavourable in the new high aspect configuration, the heating efficiency due to neutral beams and the transport properties are expected to be favourable in a high-beta range. In order to clarify the effect of the global ideal MHD unstable mode on the operational regimes in helical systems, especially the beta gradients in the peripheral region and the beta value, the MHD analysis and the transport analysis are performed in a high-beta range of up to 4% in LHD. In a high-beta range of more than 3%, the maxima of the observed thermal pressure gradients at a low order rational magnetic surface in the peripheral region are marginally unstable to the low-mode-number ideal MHD instability. Though a gradual degradation of the local transport in the region has been observed as beta increases, a disruptive degradation of the local transport does not appear in the beta range up to 4%.
A quasi-isodynamic stellarator with poloidally closed contours of the magnetic field strength B (Mikhailov 2002 Nucl. Fusion 42 L23) has been obtained by an integrated physics optimization comprising MHD and neoclassical theory. For a configuration with six periods and aspect ratio approximately 12, a main result is the attainability of an essentially MHD-stable high-β (β ≈ 0.085) plasma with low neoclassical transport, approximately vanishing bootstrap current in the long-mean-free-path regime and excellent α-particle confinement.
The GENE/GIST code package is developed for the investigation of plasma microturbulence, suitable for both stellarator and tokamak configurations. The geometry module is able to process typical equilibrium files and create the interface for the gyrokinetic solver. The analytical description of the method for constructing the geometric elements is documented, together with several numerical evaluation tests. As a concrete application of this product, a cross-machine comparison of the anomalous ion heat diffusivity is presented.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.