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Gyrokinetic PIC Study on RMP Affected Neoclassical Transport in Toroidal Plasmas
Full text linkIn magnetically confined fusion plasmas, the breaking of ‘magnetic flux-surfaces’ due to resonant magnetic perturbations (RMPs) can generate magnetic islands and alter field topology to significantly impact plasma confinement and transport. This work investigates the effect of magnetic islands on neoclassical radial energy transport within the core plasma of an analytic circular tokamak using the XGC-S global gyrokinetic particle-in-cell code. Findings from our simulations revealed substantial enhancements in electron neoclassical radial energy diffusivity in and around the islands, in addition to a newly observed two-peak structure at the O/X-points and outer island boundary in the electron diffusivity profile.journal articl
Steady compressible 3D Euler flows in toroidal volumes without continuous Euclidean isometries
Full text linkWe demonstrate the existence of smooth three-dimensional vector fields where the cross product between the vector field and its curl is balanced by the gradient of a smooth function, with toroidal level sets that are not invariant under continuous Euclidean isometries. This finding indicates the existence of steady compressible Euler flows, either influenced by an external potential energy or maintained by a density source in the continuity equation, that are foliated by asymmetric nested toroidal surfaces. Our analysis suggests that the primary obstacle in resolving Grad’s conjecture regarding the existence of nontrivial magnetohydrodynamic equilibria arises from the incompressibility constraint imposed on the magnetic field.journal articl
Quantum-inspired information entropy in multifield turbulence
Full text linkAn information entropy for turbulence systems with multiple field quantities is formulated, as a new paradigm to explore the nonlinear dynamics and pattern formations. Combining quantum state descriptions in quantum mechanics into the turbulence field analysis, the von Neumann entropy (vNE) and the entanglement entropy (EE) are derived from a density matrix for the turbulence state in terms of the multifield singular value decomposition (MFSVD). Applying the information-theoretic entropy analyses to spatio-temporal dynamics in turbulent plasmas with phase-transition–like behavior, we discover a new nontrivial transition threshold regarding the vNE, which significantly deviates from the transition threshold of the field energy considered in the conventional approaches. These findings provide us with physically more diverse classifications of the turbulence state from the new perspective of “information”, in addition to the energetics of turbulent vortices. It is also revealed that the EE for nonlinear interactions in turbulence extracts the information regarding the strength of nonlinear mode couplings and the direction of net energy transfer. A plausible application of the EE to the turbulence measurements is demonstrated, as well as the associated reconstruction technique for fluctuation fields.journal articl
Evaluation on the operational state of turboexpanders in a helium refrigerator for nuclear fusion experimental devices using principal component analysis
Full text linkORCID 0000-0002-1989-867XThe operational state of helium turboexpanders has been evaluated using principal component analysis (PCA). For the evaluation, two models which were single and multiple ones were utilized. The single and the multiple models assess a single turboexpander and seven turboexpanders, respectively. In the model development, we used measurement data of helium turboexpanders for a cryogenic system in the Large Helical Device. Using the two models, the changes in the turboexpanders' condition before and after the occurrence of the trouble were clarified. Consequently, the PCA is very useful to monitor the turboexpanders. Comparing the two models, the single is more practical than the multiple one.journal articl
Quantitative Analysis of Doppler-Free Spectra via the Collisional-Radiative Model
No full textWe have constructed a simulation model of the Doppler free spectra for the hydrogen Balmer α line. We are introducing the laser excitation process into the collisional-radiative model of hydrogen atoms to see how much saturation can be achieved under realistic plasma conditions and laser power density. Results show that the simulated spectra were able to successfully model Lamb dips and peaks utilizing this method, with the simulated plasma and laser parameters showing good agreement to the ones used in the experiment. This model has additionally helped illustrate further insight into how plasma parameters can affect the spectral characteristics of Lamb dips and peaks.journal articl
Direct observation of coexisting local and nonlocal turbulence in a magnetically confined plasma
No full textORCID 0000-0003-1088-8237Observing mediating structures in nonlocal transport is a cross-disciplinary challenge that spans disordered materials and turbulent fluids. The difficulty stems from the transient nature of mediators and nonlocal responses to probes, which renders indirect diagnostic and numerical simulations the most commonly used probing strategies. This study introduces a direct method using high-resolution spatiotemporal diagnostics in magnetically confined plasma to isolate two turbulence regimes: high-frequency local turbulence, which scales with temperature gradients and carries heat flux, and low-frequency nonlocal turbulence, which acts as a mediator and rapidly links distant regions. We observe temporal ordering as follows: immediately after heating, nonlocal turbulence appears, followed by local turbulence carrying the heat pulse. Shorter heating shifts the balance toward the nonlocal component. The impact of the mediator is quantified by the scaling v ∝ s−1.06 between the heat-pulse speed v and duration s, demonstrating that stronger departures from steady state enhance nonlocal transport. Correspondingly, the intensity ratio of the nonlocal-to-local turbulence increases as s shortens. This direct observation of nonlocal transport offers insights into mediator-enabled pathways and advances the understanding of nonlocal dynamics.journal articl
Measurements of Optical Emission from Singly Ionized Er Ions at LHD for Laboratory Assessment of Atomic Data Relevant to Opacity of Kilonovae
No full textThe atomic data of heavy elements, especially rare-earth metals, plays a crucial role in enhancing our understanding and interpreting kilonova spectra and underlying astrophysical processes. Among these elements, Erbium (Er) is particularly intriguing because it is important for opacities of the kilonova observed in 2017 (GW170817). In order to assess the atomic data, optical spectra of Er ions were precisely measured in 385 − 400nm at Large Helical Device (LHD). In the present experiment, Er was injected into the core plasma of LHD through carbon pellets containing Er powders. The electron density and temperature of the Er-contained C pellet ablation cloud were obtained to be 1.6 × 1022 m−3 and 1.4 eV using the Stark broadening of a C II line and the Boltzmann plot of Er II lines, respectively. Transition probabilities of observed Er II lines were assessed using the Boltzmann plot analysis. Recent measurements with laser-induced breakdown spectroscopy (LIBS) of an Er II line at 393.86 nm were confirmed by the present work.journal articl
Optical vortex-based spectroscopy for particle diagnostics
No full textORCID 0000-0002-0602-0665This study presents an approach for plasma flow diagnostics employing optical vortex beams with helical wavefronts. Although flow velocities perpendicular to the optical axis cannot be measured using conventional laser spectroscopy, this limitation is overcome herein by applying the azimuthal Doppler effect inherent in optical vortices. Specifically, we demonstrate quantitative velocity measurements of argon metastable atoms in a radio-frequency plasma using optical vortex laser absorption spectroscopy. Moreover, by introducing asymmetry into the beam intensity distribution, the asymmetric optical vortex laser-induced fluorescence method enables the determination of three-dimensional flow velocity vectors from a single laser path. Overall, these results indicate the promise of this diagnostic approach for complex plasma flows with limited optical access.journal articl
Suppression of resistive ballooning mode by energetic ions in quasi-axisymmetric stellarator plasmas
No full textThe 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 (βh < 0.8%), which is consistent with theoretical predictions. However, as βh 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 βh (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.journal articl
Energetic particle driven Alfvén eigenmodes and associated energetic particle redistribution in a tokamak burning plasma
No full textORCID 0000-0001-9323-8285Energetic-particle driven Alfvén eigenmodes (AEs) and associated redistribution of energetic particles in phase space are investigated using kinetic-magnetohydrodynamic (MHD) hybrid simulations for a tokamak burning plasma with weakly reversed magnetic shear. The initial MHD equilibrium and plasma profile are those of ITER, which was studied in Todo and Bierwage (2014 Plasma Fusion Res. 9 3403068). Two types of kinetic-MHD hybrid simulations are performed and the results are compared. In the first type of simulation (KEP-MHD simulation), energetic particles are simulated using the gyrokinetic particle method, while in the second type (KEPTI-MHD simulation), both energetic particles and bulk ions are simulated using the gyrokinetic particle method. In the KEP-MHD simulation result, it is observed that multiple toroidal AEs (TAEs) with toroidal mode number n ∼ 15 are the most unstable in the linear growth phase, while reversed shear AEs (RSAEs) with low-n dominate in the nonlinear phase. In the KEPTI-MHD simulation result, it is observed that beta-induced AEs with n ∼ 15 have the largest linear growth rate, while TAEs with n ∼ 10 dominate in the nonlinear phase. The redistribution of energetic particles is substantial and comparable between the two simulations.
Redistribution of energetic alpha particles and beam deuterons in phase space is analyzed for the KEP-MHD simulation. In the early nonlinear phase, local flattening regions are formed around the resonances with the AEs. Staircase-like structures are formed along the E ′ = const. lines for the dominant AEs with amplitudes of radial MHD velocity normalized by the Alfvén velocity vr/vA ∼ 4×10−4, where E′ is the conserved quantity of the wave-particle interaction. As the amplitudes of the AEs increase to vr/vA ∼ 10−3, the resonance regions broaden and overlap each other leading to the formation of a single flattened region. In the KEP-MHD simulation, it is observed that the steep gradient regions in phase space formed by the multiple TAEs excite the low-n RSAE that transports energetic alpha particles near the plasma center.journal articl