Nonlinear excitation of energetic particle driven geodesic acoustic mode by resonance overlap with Alfvén instability in ASDEX Upgrade

The Alfvén instability nonlinearly excited the energetic-particle-driven geodesic acoustic mode on the ASDEX-Upgrade tokamak, as demonstrated experimentally. The mechanism of the energetic-particle-driven geodesic acoustic mode excitation and the mode nonlinear evolution is not yet fully understood. In the present work, a first-principles simulation using the MEGA code investigated the mode properties in both the linear growth and nonlinear saturated phases. Here we show that the simulation successfully reproduced the excitation and coexistence of these two modes, and agreed with the experimental results well. Conclusive evidence showed that the resonance overlap is the excitation mechanism of the energetic-particle-driven geodesic acoustic mode. In the linear growth phase, energetic particles that satisfied different resonance conditions excited the Alfvén instability, which then caused energetic particle redistribution in phase space. These redistributed energetic particles caused resonance overlap, exciting the energetic-particle-driven geodesic acoustic mode in the nonlinear phase.journal articl

Kinetic effects of thermal ions on internal kink modes in tokamak plasmas

Linear growth of internal kink mode is investigated using a kinetic-MHD hybrid simulation model under realistic tokamak conditions. By comparing purely fluid (single-fluid MHD) simulations with kinetic thermal ion simulations using various coupling schemes, it is demonstrated that thermal-ion effects—including finite orbit width and ion pressure anisotropy—can significantly stabilize the internal kink mode. The maximum perturbation of distribution function aligns with resonance regions and near the passing-trapped boundary, indicating outward transport and redistribution of thermal ions. The net positive energy transfer from the mode to thermal ions leads to a reduction in growth rate. These results underscore the importance of incorporating thermal ion kinetics when modeling internal kink instabilities in fusion plasmas.journal articl

Development of Data Assimilation System for Temperature and Density Control in Helical Plasmas

We develop a real-time adaptive predictive control system based on data assimilation (DA) for the temperature and density of helical fusion plasmas. The DA-based control approach enables the harmonious integration of measurement, heating, fueling, and simulation and can provide a flexible platform for adaptive model predictive control. The core part of the control system, ASTI, is built upon the integrated simulation code TASK3D and a data assimilation framework DACS. DACS integrates adaptation of the predictive model (digital twin) to the actual system using real-time measurements and control estimation that is robust against model and observation uncertainties. We perform numerical experiments using ASTI to control the electron temperature profile and density of a virtual plasma generated by TASK3D. The results demonstrate that ASTI can effectively drive the virtual plasma state toward the target state while bridging the gap between the digital twin and the virtual plasma. Furthermore, the numerical experiments clarify the effects of hyperparameters in the DA-based control approach on control performance.journal articl

Machine learning-based hydrogen recycling model for predicting rovibrational distributions of released molecular hydrogen on tungsten materials via molecular dynamics simulations

Understanding the hydrogen recycling process is crucial for comprehending the behavior of detached plasma in nuclear fusion devices. To achieve this, a molecular dynamics (MD) model is being developed to predict the distribution of translational energies and rovibrational states of hydrogen atoms and molecules released from the plasma-facing materials. Neutral transport simulations, utilizing distributions obtained from the MD model as boundary conditions, are also a powerful tool for analyzing the impact of recycled hydrogens on edge plasma. However, the MD model requires significant computational resources to obtain distributions under varying material and irradiation conditions such as material temperature and incident energy. Therefore, developing effective models that seamlessly integrate neutral transport simulation with hydrogen recycling models is crucial. Machine learning techniques are employed to develop predictive models capable of forecasting distributions of energies and rovibrational states of released hydrogen atoms and molecules. Furthermore, a model considering the incident energy distribution (shifted-Maxwellian) is developed by integrating the monochromatic distribution with the shifted-Maxwellian distribution.journal articl

Nonlinear Gaussian process tomography with imposed non-negativity constraints on physical quantities for plasma diagnostics

ORCID　0000-0002-6479-049XWe propose a novel tomographic method, nonlinear Gaussian process tomography (nonlinear GPT), that uses the Laplace approximation to impose constraints on non-negative physical quantities, such as the emissivity in plasma optical diagnostics. While positive-valued posteriors have previously been introduced through sampling-based approaches in the original GPT method, our alternative approach implements a logarithmic Gaussian process (log-GP) for faster computation and more natural enforcement of non-negativity. The effectiveness of the proposed log-GP tomography is demonstrated through a case study using the Ring Trap 1 device, where log-GPT outperforms existing methods, standard GPT, and the minimum Fisher information methods in terms of reconstruction accuracy. The results highlight the effectiveness of nonlinear GPT for imposing physical constraints in applications to an inverse problem.journal articl

Helium plasma-induced surface modifications of unirradiated and pre-ion irradiated tungsten at LHD divertor leg position

ORCID　0000-0001-5089-3642Understanding helium (He) plasma-induced tungsten (W) surface modifications and the effect of irradiation defects on He plasma-induced W surface modifications under a real divertor environment are important for the operation of fusion reactors. In this study, two iron (Fe) ions pre-irradiated in W samples with dislocation loops and voids, and two unirradiated W samples were exposed to He plasma at the divertor leg position of the Large Helical Device. The gross erosion rate is 1.0 × 1020 atoms m−2 s−1, 1.0 × 1020 atoms m−2 s−1, 9.3 × 1019 atoms m−2 s−1 and 7.4 × 1019 atoms m−2 s−1 for W9, W10, W11 and W12, respectively. The surface of each sample after the exposure was different at the strike point and the two regions on either side of the strike point, the scrape-off layer (SOL) region and the private region. The typical He plasma-induced structures in the SOL region are stripe structure, sawtooth structure and non-undulating structure, which are collectively called He-structures in the present study. At the strike point, the typical He plasma-induced structures are dense W protrusions. In the private flux region, the typical He plasma-induced structures are semi-formed He-structures. The formation of these structures is dependent on the grain orientation. Pinholes were observed in these structures. No significant difference in He plasma-induced structures is found between the pre-irradiated W and the unirradiated W. The formation of non-undulating structure and stripe structure is discussed based on the observed semi-formed He-structures.journal articl

液体窒素中における水平面上の遷移沸騰熱伝達 ―遷移沸騰域における定常的な沸騰曲線とその曲線に現れるヒステリシスの観測―

ORCID　0000-0002-5682-3343An open question has been existing whether hysteresis appears in transition boiling heat transfer. Herein, steady-state experiments were conducted using liquid nitrogen as the test fluid to obtain boiling curves for film and transition boiling regimes. In the experiments, steady-state boiling curves were obtained not only in the film boiling regime but also in the transition boiling regime, and hysteresis in the transition boiling regime was clearly observed. Steady transition boiling was observed when the superheat gradually decreased from the film boiling regime, whereas a higher heat flux was observed when the superheat increased. This hysteresis continued until the minimum heat flux temperature.journal articl

Dynamic response of atomic processes in detached helium plasma induced by high-density transient pulse in Magnum-PSI

Pulsed plasma experiments simulating the transient divertor heat loads caused by edge-localized modes (ELMs) were conducted in Magnum-PSI, a linear plasma device capable of generating ITER-relevant conditions. The dynamic responses of detached recombining helium (He) plasmas to the pulsed plasma were observed through time-resolved laser Thomson scattering measurements and optical emission spectroscopy. Experimental results indicate that He atom depletion occurs in high-density pulsed plasmas due to a reduced mean-free-path and increased plasma pressure. This limits plasma-neutral interactions, suppressing energy dissipation via direct excitation and electron–ion recombination processes. The He II and He I emissions were found to peak sequentially, suggesting that doubly-ionized He ions recombine first, followed by the recombination of singly-ionized He ions. These findings suggest that the neutral penetration and recombination efficiency are significantly reduced in pulsed plasmas, challenging heat flux mitigation in fusion divertors. The results provide critical insights into the behavior of He ash transported by ELMs and plasma detachment under high-density pulsed conditions, highlighting the need for further studies on neutral fueling strategies in ITER-relevant environments.journal articl

Thermal Runaway of a REBCO Coil Co-Wound With a Copper Tape Immersed in Liquid Nitrogen/Hydrogen

ORCID　0000-0003-1454-8117Aiming at the establishment of design criteria for cryostable HTS magnets, we have investigated the behavior of thermal runaway of REBCO three-turn coils immersed in liquid nitrogen and liquid hydrogen. Sample 1 consisted of a REBCO tape with a copper layer 0.1 mm thick and a co-wound copper tape 0.2 mm thick, and Sample 2 consisted of the REBCO tape only. Each conductor was wound in a groove of a G10 plate, and only an upper side of the coil was cooled with cryogen. In order to simulate local degradation, the testing part, 0.1 m long, was damaged by bending, and critical currents at 77 K were reduced to less than 1/10 of the original value of 120 A. In liquid nitrogen, thermal runaway of Sample 1 occurred at 107 A after wide propagation of a normal zone, while that of Sample 2 occurred at 57 A before propagation of a normal zone. In liquid hydrogen, thermal runaway of both samples occurred before propagation of a normal zone, and the amount of heat generation of Sample 1 when starting thermal runaway was 1.5–1.8 times as high as that of Sample 2, which was lower than the ratio of the wetted areas. The co-wound copper tape was less effective for the short normal zone, and it worked effectively as a bypass-current path after wide propagation of a normal zone.journal articl

Benchmark tests of the efficient electrostatic particle-in-cell simulation code on various high-performance computing systems

ORCID　0000-0001-7641-6908The “up3bd” code, which is an efficient three-dimensional (3D) electrostatic particle-in-cell (PIC) simulation code for study of transport dynamics in fusion boundary layer plasmas or other plasmas in nature, has been tested on various high-performance computing (HPC) systems which consist of different types of processor. The results of benchmark tests indicate that the up3bd code works faster on processors in which cache memory is larger or memory bandwidth is broader. Also, the types of computations that each architecture excels at and struggles with has been revealed.journal articl