Shoichi Okamura

A key feature of the stellarator is its inherent three-dimensional magnetic configuration, which leads to numerous differences in plasma physics between stellarators and tokamaks. The influence of this three-dimensional geometry on ion temperature gradient (ITG) modes in the Chinese First Quasi-axisymmetric Stellarator is investigated through linear gyrokinetic simulations using the gyrokinetic Vlasov code. Across different radial and toroidal positions, the ITG mode exhibits a strong toroidal dependence and a weak radial dependence. The eigenmode localization is primarily determined by the perpendicular wavenumber k ⊥ 2 and the normal curvature κ y : the ITG mode peaks where κ y is negative and k ⊥ 2 is minimized. For ITG modes with k x = 0 , the growth rate mainly exhibits a toroidal dependence. As the toroidal angle increases, the peak growth rate decreases and shifts to the low k y ρ i region. For modes with k x ≠ 0 , the most unstable modes acquire a finite k x at all three radial positions for ϕ = 90 ∘ , and at ρ = 0.8 for ϕ = 45 ∘ . This finite k x alters the value of k ⊥ 2 and modifies its distribution along the field line, thereby changing the eigenmode structure. By combining the eigenmode structure with the overall distributions of magnetic geometry parameters, the variation of the growth rate across different spatial positions can be qualitatively estimated.

Using the nonlinear 3D equilibrium code HINT, we investigate the suppression of magnetic field stochasticity and stochasticity-induced radial electron heat transport by an externally driven current in the Chinese First Quasi-axisymmetric Stellarator configuration. For β0=3%, a Gaussian-profile driven current with I0 = −12 kA effectively suppresses stochasticity within the confinement region, increasing the normalized volume-averaged β while maintaining a magnetic well depth of 7%. The suppression mechanism is attributed to the current reducing the rotational transform inside the n/m = 2/4 rational surface, which weakens or eliminates coupling among low-order satellite island chains. Radial heat transport is evaluated via Rechester–Rosenbluth theory. The radial thermal conductivity χr in the stochastic region decreases with increasing magnitude of I0. In the low-density case, the χr is reduced from 498 m2/s to below 0.1 m2/s; in the high-density case, it decreases from 44 m2/s to below 0.01 m2/s. These results confirm that externally driven current effectively suppresses stochasticity-driven radial heat transport in quasi-axisymmetric configurations.

The Chinese First Quasi-axisymmetric Stellarator (CFQS) is being constructed as an international joint project between Southwest Jiaotong University (China) and the National Institute for Fusion Science (Japan), aiming to prove the inherent advantages of the quasi-axisymmetric (QA) magnetic configuration in confining plasmas. The CFQS project is divided into two stages. The first one focuses on testing the feasibility and accuracy of modular coils for realization of the QA topology, which is conducted on the CFQS-Test (CFQS-T) device and operated at low magnetic field strength of 0.1 T. The second stage is scheduled to delve into plasma properties of high parameters at relatively high magnetic field (1 T) in a QA stellarator. The latter stage will involve installation of dedicated support structures, such as coil cases, central poles and pillars, to withstand the strong electromagnetic force of 1 T operation. By the end of July 2024, the assembly of CFQS-T was completed with a maximum deviation of ∼2.96 mm, meeting the design requirement. Auxiliary systems, including power supply, vacuum pumping, magnetron, central control, water cooling and diagnostic systems, have also been installed and commissioned accordingly. In August 2024, the first QA magnetic configuration was successfully achieved in CFQS-T. This paper outlines the construction progress of CFQS and the preliminary experimental results attained with the CFQS-T device.

The interaction between energetic particles (EPs) and resistive ballooning modes (RBMs) in quasi-axisymmetric stellarator (QAS) plasmas is investigated via MHD hybrid simulations using the MEGA code for the Chinese First QAS configuration. Simulations show RBMs ( m = 9 / n = − 4 ) are stabilized by EPs at low beta values ( β h < 0.8 % ), which is consistent with theoretical predictions. However, as β h exceeds 0.8%, the RBM is suppressed and a new EP-driven reversed shear Alfvén eigenmode (RSAE)-like instability ( m = 4 / n = − 2 ) becomes dominant, with significantly increased frequency and growth rate. A resistivity ( η ) scan at high β h (1.5%) reveals a transition: a mode exhibits RSAE characteristics at low η , while transitioning to a resistivity-driven ballooning mode at high η . This suggests the EP-driven instability observed in initial high-resistivity simulations originates from the RSAE branch but its structure is strongly modified, displaying pronounced ballooning features. This work highlights the dual role of EPs and the crucial influence of resistivity on instabilities in QAS, providing insights for high-beta operation.

In this study, the impact of a non-inductive current drive, such as electron cyclotron current drive, on three-dimensional (3D) magnetic islands in the high-β equilibrium of the Chinese First Quasi-axisymmetric Stellarator (CFQS) was investigated using the HINT code. In the case of a high-β equilibrium (volume-averaged plasma beta <β> ∼ 0.74% and bootstrap current I _bs∼ 24.5 kA), two m/n = 4/2 rational surfaces with large magnetic islands develop (Wang et al 2021 Nucl. Fusion 61 036021). The islands can be effectively controlled using a constant or a Gaussian current density profile, depending on the direction and amplitude of the current. With a constant current density amounting to a total current of −6 kA, the rotational transform profile can be modified such that the m/n = 4/2 rational surface is eliminated and the island is suppressed. For the Gaussian current density profile, the magnetic island can also be suppressed using a smaller total current of ∼−2 kA to adjust the iota profile. These results suggest that in the CFQS stellarator, the external current drive might be an efficient approach for controlling 3D magnetic islands and consequently improving plasma confinement.

In a quasi-axisymmetric stellarator, a significant bootstrap current will result in the generation of low-order rational surfaces and three-dimensional (3D) magnetic islands. In this paper, the influence of plasma density profiles on the equilibrium magnetic islands for the Chinese first quasi-axisymmetric stellarator (CFQS) is investigated using the HINT code. It is found that the flattening of the core plasma density profile leads to a significant suppression of magnetic islands. When the peaking factor of plasma density is 1.19, complete suppression of magnetic islands occurs while maintaining excellent integrity of the magnetic surface even with the volume-averaged plasma beta <β> increase up to 2%. On the other hand, during the transition of a plasma density profile from flat to hollow, there is a reversal in the core bootstrap current, resulting in reduction of rotational transform values to pass through the rational surface. Hence, formation of magnetic islands in the core region. Therefore, effective inhibition of CFQS’s magnetic islands can be achieved by appropriately controlling density profiles through methods like gas injection.

The Chinese first quasi-axisymmetric stellarator (CFQS), which will be the first quasi-axisymmetric (QA) stellarator in the world, is now under construction. The primary task of the CFQS project is to realize a QA configuration and to examine its physical properties. Based on this task, two important issues were investigated in this work in order to estimate the robustness of the CFQS design from a physical perspective. One was the toroidal field (TF) ripple due to the discreteness of modular coils (MCs) which could potentially degrade the charged particle confinement in the CFQS configuration. The other was a possible MC misalignment in the assembly that would affect the magnetic field and charged particle confinement in the CFQS. Moreover, since the stellarator symmetry might be broken by the MC misalignment, such a case was also investigated in this work. By performing a magnetic field line tracing and charged particle orbit tracing calculation, it was found that the TF ripple does not affect the confinement property significantly and the magnetohydrodynamics equilibrium was robust against possible MC misalignments. These results are helpful in defining the reasonable tolerance of assembly accuracy.