Island divertor configuration design for a quasi-axisymmetric stellarator CFQS

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The equilibrium of the Chinese first quasi-axisymmetric stellarator (CFQS) has been investigated by the HINT code. It is found that the stochastization of magnetic field lines expands with the increase in the volume-averaged beta value 〈β〉 in the plasma boundary. In the high-β regime, the generation of large magnetic islands at rational surfaces not only leads to an effective shrinkage of the plasma confinement region but also increases the outward shift of the magnetic axis. With bootstrap current effects, the low-order islands spread over the whole plasma area, leading to a stochastization of magnetic field lines due to island chain overlapping. However, for a flat pressure profile, the magnetic islands are significantly suppressed so that the magnetic surfaces can be fairly maintained. Moreover, the magnetohydrodynamic (MHD) instabilities in the optimized CFQS configurations have been simulated by the MEGA code. The linear properties of unstable resistive MHD modes are studied. The results show that in the CFQS the main MHD behaviour is dominated by the resistive ballooning modes with strong mode coupling. The mode structure and resistivity scaling are consistent with related theories.

Section: Calculations Of the Cfqs Equilibriummentioning

confidence: 99%

The three-dimensional equilibrium with magnetic islands and MHD instabilities in the CFQS quasi-axisymmetric stellarator

Wang

Shimizu

et al. 2021

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“…10. This configuration has been designed by S. Okamura, et.al [51]. The confinement region is bounded by an island bundle with ι=2/5, which determines the last closed flux surface (LCFS).…”

Section: (D)-(f)mentioning

confidence: 99%

“…In addition to the standard quasi-axisymmetric configuration shown in figure 1, the island bundle divertor (IBD) configuration can be also achieved without breaking the quasiaxisymmetry in the core confinement region by adding a weak toroidal field to the quasi-axisymmetry, displayed in figure 10. This configuration has been designed by S. Okamura, et al [49]. The confinement region is bounded by an island bundle with ι = 2/5, which determines the last closed flux surface (LCFS).…”

Section: Configuration Preparties In Fixed and Free-boundary Equilibriamentioning

confidence: 99%

Configuration characteristics of the Chinese First Quasi-axisymmetric Stellarator

Liu

Shimizu

et al. 2020

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The Chinese First Quasi-axisymmetric Stellarator (CFQS) will be the first operational quasi-axially symmetric stellarator in the world. The physical and engineering complexities led to the cancellation of two famous quasi-axisymmetric stellarators, CHS-qa and NCSX. Therefore, the major mission of the CFQS is to experimentally achieve the canonical quasi-axisymmetric configuration. The CFQS has been designed to possess a number of advanced features in fixed and free-boundary equilibria. It is a compact stellarator with an aspect ratio R/a ∼4.0. The neoclassical diffusion coefficient is similar to that of tokamaks in the collisionless regime. The MHD equilibrium of the CFQS configuration is stable up to volume-averaged normalized pressure β ∼1.1%. A region of the second ballooning stability exists in this facility with a large region of plasma, becoming second stable for β ∼2.7% in free-boundary equilibria. The gap between the first and second stability boundaries is very narrow, which is greatly beneficial for the CFQS operation in the second stable regime with high β plasma. A modular coil system with 16 coils is designed which robustly reproduces the standard quasi-axisymmetric magnetic field.

“…An equilibrium with 2/4 magnetic islands was found in the simulation results with the HINT code [10]. 12 planar TFC have been designed to adjust the rotational transform profile [54], healing magnetic islands. Moreover, active control of islands is also available by electron cyclotron heating and tailoring the pressure profile [55,56].…”

Section: Effects Of Bootstrap Current On Neoclassical Transport and Jmentioning

confidence: 99%

Effects of bootstrap current on magnetic configuration in Chinese first quasi-axisymmetric stellarator

Liu

zhang

et al. 2023

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Neoclassical properties in quasi-axisymmetric (QA) stellarators are analogous to these in tokamaks. Consequently, a substantial bootstrap current could significantly modify the MHD equilibrium properties of a QA stellarator, which is an important characteristic in this type of stellarator. This paper is dedicated to systemically investigate the effects of bootstrap current on the magnetic configuration in Chinese First Quasi-axisymmetric Stellarator (CFQS). For the first time, self-consistent bootstrap currents in free-boundary equilibria are calculated with an accurate Fokker–Planck neoclassical numerical mode in CFQS. Several important results are achieved: 1) As the bootstrap current grows with increasing volume-averaged normalized pressure β, the magnetic shear develops in the bulk plasma and meanwhile, a deep magnetic well is robustly sustained, which leads to improved stabilization of interchange modes up to β~2.0%. 2) In the analysis of global ideal MHD instability, as the bootstrap current rises to 39kA (β~1.3%), external kink modes become destabilized and the unstable mode with m/n= 2/1 is dominant. 3) From β=0 to 1.5%, the bootstrap current hardly changes the quasi-axisymmetric property and a low neoclassical transport is maintained. However, as β is enhanced beyond 2.0%, the substantial bootstrap current gives rise to an increase of non-quasi-axisymmetric magnetic field components, which weakens the neoclassical transport properties. 4) An increase of the negative magnetic shear at the core region by the bootstrap currents has a favorable effect on the properties of J (second adiabatic invariant). The maximum-J region can be extended by raising bootstrap currents.