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Orbit-following simulations of fast-ion transport and losses due to the Alfvén eigenmode burst in the Large Helical Device
Full text linkORCID 0000-0002-5364-805XOrbit-following simulations of fast-ion transport and losses with time-dependent electromagnetic perturbations are performed to clarify the roles of Alfvén eigenmodes (AEs) and the low-frequency magnetohydrodynamic (MHD) mode observed in the kinetic-MHD hybrid simulation of AE bursts in the Large Helical Device. Fast-ion pressure profile flattening in the kinetic-MHD hybrid simulation can be reproduced by an orbit-following simulation with only the primary single AE of the time-dependent amplitude following the kinetic-MHD hybrid simulation result, while orbit-following simulations with constant AE amplitude of average level during AE burst cannot reproduce the fast-ion pressure profile flattening observed. The effects of other modes are negligible on the fast-ion pressure profile flattening. The fast-ion losses in kinetic-MHD hybrid simulation can be reproduced by an orbit-following simulation with time-dependent amplitude when the low-frequency MHD mode is considered in addition to multiple AEs. This indicates the synergetic effect of multiple AEs and the low-frequency MHD mode on fast-ion losses.journal articl
Controlling three-dimensional magnetic island appearance with external current drive in the Chinese first quasi-axisymmetric stellarator
Full text linkIn 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 ∼ 0.74% and bootstrap current Ibs∼ 24.5 kA), two m/n = 4/2 rational surfaces with large magnetic islands develop (Wang et al 2021 Nucl. Fusion61 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.journal articl
Turbulence Transition in Magnetically Confined Hydrogen and Deuterium Plasmas
Full text linkORCID 0000-0003-3930-4434In this study, we discovered a turbulence transition in a large helical device. The turbulence level and turbulence-driven energy transport decrease to a specific transition density and increase above it. The ruling turbulences below and above the transition density were ion-temperature gradient (ITG) and resistive-interchange (RI) turbulences, consistent with the predictions of gyrokinetic theory and two-fluid MHD model, respectively. Isotope experiments on hydrogen (H) and deuterium (D) clarified the role of transitions. In the ITG regime, turbulence levels and energy transport were comparable in the H and D plasmas. In contrast, in the RI regime, they were clearly suppressed in the D plasma. The results provide crucial knowledge for understanding isotope effects and future optimization of stellarator and heliotron devices.journal articl
Electron Heating in High Mach Number Collisionless Shocks
Full text linkORCID 0000-0002-3643-9205The energy partition in high Mach number collisionless shock waves is central to a wide range of high-energy astrophysical environments. We present a new theoretical model for electron heating that accounts for the energy exchange between electrons and ions at the shock. The fundamental mechanism relies on the difference in inertia between electrons and ions, resulting in differential scattering of the particles off a decelerating magnetically dominated microturbulence across the shock transition. We show that the self-consistent interplay between the resulting ambipolar-type electric field and diffusive transport of electrons leads to efficient heating in the magnetic field produced by the Weibel instability in the high Mach number regime and is consistent with fully kinetic simulations.journal articl
Origin of Line Broadening in Fading Granules: Influence of Small-scale Turbulence
Full text linkORCID 0000-0002-4669-5376In the quiet region of the solar photosphere, turbulent convective motions of the granular flows naturally drive the subgranular-scale flows. However, evaluating such small-scale velocities is challenging because of the limited instrumental resolution. Our previous study, R. T. Ishikawa et al., found line-broadening events during the fading process of granules; however, their physical mechanism has remained unclear. In the present study, we observed the fading granules with the Hinode-SOT/SP and performed spectral line inversions. Moreover, we investigated the broadening events of synthesized spectra in fading granules reproduced by the MURaM simulation. Our results demonstrated that the small-scale turbulent motions are excited in the fading process, and such turbulent flows contribute to line broadening. The spectral line widths can be potential tracers of the photospheric turbulent flows.journal articl
Kinetic Theory with Casimir Invariants—Toward Understanding of Self-Organization by Topological Constraints
Full text linkORCID 0000-0002-9165-6616A topological constraint, characterized by the Casimir invariant, imparts non-trivial structures in a complex system. We construct a kinetic theory in a constrained phase space (infinite-dimensional function space of macroscopic fields), and characterize a self-organized structure as a thermal equilibrium on a leaf of foliated phase space. By introducing a model of a grand canonical ensemble, the Casimir invariant is interpreted as the number of topological particles.journal articl
Suppression of equilibrium magnetic islands by density profile effect in quasi-axisymmetric stellarator plasmas
Full text linkORCID 0009-0008-3053-8713In 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 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.journal articl
Performance of ITER pressure gauges during deuterium operation in the large helical device
Full text linkDuring deuterium campaigns on the heliotron large helical device (LHD), ITER pressure gauges with different cathode materials were used to measure the neutral pressure in the sub-divertor region. Throughout these campaigns, it was observed that the performance of a LaB6 cathode was unsatisfactory during deuterium operation. Conversely, measurements taken with pressure gauges with a ZrC cathode performed well throughout the deuterium pulses. The ITER pressure gauge with the ZrC cathode could be operated with a very high electron current of 800 µA, thus improving the lower detection limit of the neutral pressure in LHD. With this design it was also possible to avoid jumps in the ion current within strong magnetic fields, improving the accuracy of the measurement from 15% uncertainty to 5%. These features allowed very precise neutral pressure measurements to be made in a fusion device with magnetic confinement. The problems with the lifetime of the ZrC cathode reported in Mackel et al (2023 Fusion Eng. Des. 189 113439) were not relevant here, as the total runtime in the magnetic field was about 60 h, less than the expected lifetime of the cathode of 350 h.journal articl
Simulation study of energetic-ion mass dependence on nonlinear development of lower hybrid wave instabilities
Full text linkORCID 0000-0003-2691-5068In both space and fusion plasmas, energetic ions with a ring-like velocity distribution are present in various ion species with different masses. We study how the nonlinear development of lower hybrid wave (LHW) instabilities, driven by the ring-like energetic ions, depends on the mass of the energetic ions, by using the electromagnetic particle-in-cell simulation with a model where the energetic ions are continuously injected into a plasma. As expected from linear theory, the initial growth of the LHWs slows down as the energetic-ion mass (M) increases. However, long-time simulations have revealed that as M increases, the LHWs eventually grow to larger amplitudes. This is because the velocity distribution of the energetic ions, influenced by spreading due to wave development and reformation due to injection, can maintain a steep gradient when M is large. As a result, in a large M plasma, a greater part of the injected energy is converted to the energies of the electric field and background ions. In addition, it has been found that a larger value of M leads to an increase in the amplitudes of the second and third harmonics of the LHWs because of enhanced nonlinear wave-wave coupling.journal articl