Showing 1–37 of 37 results for author: Nakamura, H

The performance of the TA$\times$4 surface detector array: 4.3 years of the first-half expansion

Authors: Telescope Array Collaboration, R. U. Abbasi, T. Abu-Zayyad, M. Allen, J. W. Belz, D. R. Bergman, F. Bradfield, I. Buckland, W. Campbell, B. G. Cheon, K. Endo, A. Fedynitch, T. Fujii, K. Fujisue, K. Fujita, M. Fukushima, G. Furlich, A. Gálvez Ureña, Z. Gerber, N. Globus, T. Hanaoka, W. Hanlon, N. Hayashida, H. He, K. Hibino , et al. (105 additional authors not shown)

Abstract: The Telescope Array (TA) experiment aims to reveal the origin of ultra-high-energy cosmic rays (UHECRs) by observing air showers using surface detectors (SDs), which spread over an area of approximately 700 km$^2$, and fluorescence detectors (FDs) viewing the skies above the SD array. The TA experiment has been observing UHECRs since 2008, and has reported an indication of clustering in the arriva… ▽ More The Telescope Array (TA) experiment aims to reveal the origin of ultra-high-energy cosmic rays (UHECRs) by observing air showers using surface detectors (SDs), which spread over an area of approximately 700 km$^2$, and fluorescence detectors (FDs) viewing the skies above the SD array. The TA experiment has been observing UHECRs since 2008, and has reported an indication of clustering in the arrival directions of cosmic-ray events with energy greater than 57 EeV. To improve the exposure for anisotropy studies of UHECRs, the TA$\times$4 upgrade was designed to expand the observational area by approximately 2,000 km$^2$ with 500 additional SDs. Half of the planned upgrade, consisting of 257 SDs, was completed, and the newly installed array began operation in 2019. In addition to the expanded SD array, two FD stations were constructed for the TA$\times$4 experiment. In this paper, we present a study of the performance of the expanded SD array, including the energy resolution, angular resolution, and effective aperture, over the first 4.3 years of data acquisition. While the effective aperture varied initially due to changing detector states, it has stabilized since June 2023 with more than 90\% operational SDs. Furthermore, a new inter-tower trigger system was implemented to connect six new communication towers to form two geographically separated arrays, increasing the effective aperture. The time variation of this effective aperture, the resulting total exposure of approximately 3,500 km$^2$~sr~yr, and a comparison with the original TA SD array are presented to demonstrate the performance of the expanded array. △ Less

Submitted 26 June, 2026; originally announced June 2026.

Comments: 24 pages, 9 figures, submitted to Nuclear Inst. and Methods in Physics Research, A

Deep Learning-Accelerated Dynamic Kinetic Monte Carlo Simulation for Hydrogen Transport in Tungsten

Authors: Seiki Saito, Keisuke Takeuchi, Hiroaki Nakamura, Yasuhiro Oda, Kazuo Hoshino, Yuki Homma, Shohei Yamoto, Yuki Uchida

Abstract: In magnetic confinement fusion reactors, hydrogen plasma irradiation causes material saturation and recycling, where hydrogen released from the tungsten wall significantly impacts the peripheral plasma. Kinetic Monte Carlo (kMC) simulations are essential for investigating the dynamic balance between incident and emitted fluxes at the atomic scale. However, standard kMC frameworks are inadequate fo… ▽ More In magnetic confinement fusion reactors, hydrogen plasma irradiation causes material saturation and recycling, where hydrogen released from the tungsten wall significantly impacts the peripheral plasma. Kinetic Monte Carlo (kMC) simulations are essential for investigating the dynamic balance between incident and emitted fluxes at the atomic scale. However, standard kMC frameworks are inadequate for handling realistic material complexities, such as polycrystalline structures and dynamic evolution under irradiation, being computationally bottlenecked by continuous transition parameter updates. Conventionally, evaluating migration barriers in disordered systems (e.g., grain boundaries) relies on computationally prohibitive on-the-fly atomistic calculations like the Nudged Elastic Band (NEB) method. Here, we present a deep learning-accelerated Dynamic kMC framework that eliminates this reliance. Our approach integrates a three-stage deep learning pipeline: a pix2pix model for predicting local 3D potential energy distributions, a U-Net for extracting hydrogen trapping sites, and a 3D-CNN for directly evaluating migration barriers. To achieve macroscopic timescales, we implemented a hierarchical spatial index combined with a differential local-update algorithm operating in O(1) complexity. This architecture restricts recalculations to the immediate vicinity of moving atoms, accelerating updates. Demonstrated on a large-scale realistic polycrystalline tungsten model, the framework successfully reproduces preferential hydrogen trapping along grain boundaries, bridging the gap between atomic-scale accuracy and macroscopic timescales for full-scale plasma-wall interaction simulations. △ Less

Submitted 4 June, 2026; v1 submitted 1 June, 2026; originally announced June 2026.

Pulse magnet of 10 T for power laser experiments with x-ray free-electron laser diagnostics

Authors: Akihiko Ikeda, Kosuke Noda, Yutaro Yamanaka, Yuma Urabe, Keiichiro Kawai, Yasuhiro H. Matsuda, Hirotaka Nakamura, Ryusuke Yamamoto, Yoshiki Naito, Yasuhiro Kuramitsu, Kai Taketoshi, Naoki Yamagata, Norimasa Ozaki, Tatiana Pikuz, Yoichi Sakawa, Takayoshi Sano, Ryosuke Kodama, Taichi Morita, Tomoya Ogawa, Kohei Miyanishi, Toshinori Yabuuchi, Rigon Gabriel, Bakandreas Stavros, Koenig Michel, Bruno Albertazzi

Abstract: The importance of investigating magnetized plasmas/solids in extreme conditions has grown over the last decades, particularly in the field of high energy density physics (HEDP), such as laboratory astrophysics and inertial confinement fusion. However, up to now, the unique capabilities of an X-ray free-electron laser (XFEL), such as high brilliance and low divergence have never been exploited for… ▽ More The importance of investigating magnetized plasmas/solids in extreme conditions has grown over the last decades, particularly in the field of high energy density physics (HEDP), such as laboratory astrophysics and inertial confinement fusion. However, up to now, the unique capabilities of an X-ray free-electron laser (XFEL), such as high brilliance and low divergence have never been exploited for this type of research. In this paper, we present the first platform developed at SACLA, Japan, that combines a high-power optical laser for generating matter under extreme conditions of pressure and temperature, an XFEL probe, and an external magnetic field. The high current is produced using a 2 kV, 4.8 kJ pulsed power system giving a maximum current of 10 kA which is synchronized with the optical laser and XFEL in a vacuum environment. It flows through a split-pair coil to generate a high magnetic field (10 T at 6 kA) which has 1 cm access every 45$^{\circ}$ in the equatorial plane and 90$^{\circ}$ in the poloidal one. This platform offers new opportunities to study high-energy-density matter in strong magnetic fields, including shock propagation, instability growth, and turbulent plasma dynamics. △ Less

Submitted 25 May, 2026; originally announced May 2026.

Comments: 6 pages, 7 figures

Development of a 3D-CNN-based Prediction Model for Migration Barriers in Plasma-Wall Interactions

Authors: Seiki Saito, Keisuke Takeuchi, Hiroaki Nakamura, Yasuhiro Oda, Kazuo Hoshino, Yuki Homma, Shohei Yamoto, Yuki Uchida

Abstract: Understanding the long-term transport of hydrogen isotopes in plasma-facing materials, such as tungsten, is critical for the steady-state operation of magnetic confinement fusion reactors. However, dynamically updating the transition parameters for kinetic Monte Carlo (kMC) simulations as the atomic structure evolves under continuous plasma irradiation remains a severe computational bottleneck. Co… ▽ More Understanding the long-term transport of hydrogen isotopes in plasma-facing materials, such as tungsten, is critical for the steady-state operation of magnetic confinement fusion reactors. However, dynamically updating the transition parameters for kinetic Monte Carlo (kMC) simulations as the atomic structure evolves under continuous plasma irradiation remains a severe computational bottleneck. Conventionally, calculating these migration barriers requires the iterative and computationally expensive Nudged Elastic Band (NEB) method. To overcome this limitation, this article presents a highly efficient surrogate model for predicting migration barriers using a three-dimensional Convolutional Neural Network (3D-CNN), establishing the final component necessary to realize on-the-fly molecular dynamics (MD) and kMC hybrid simulations. The proposed deep learning model takes a two-channel volumetric input, the local three-dimensional potential energy distribution and the voxelized spatial coordinates of the initial and final trapping sites, to directly output the migration barrier as a scalar value. Trained on a comprehensive dataset of tungsten-hydrogen configurations evaluated using the Embedded Atom Method (EAM) potential, the model demonstrated robust predictive accuracy, achieving a Mean Absolute Error (MAE) of 0.124 eV and a high coefficient of determination of 0.890. Furthermore, utilizing GPU acceleration, the inference time is reduced to approximately 2.7 milliseconds per barrier, achieving a speed-up ratio of over 23,000 compared to conventional NEB calculations. This extraordinary acceleration effectively resolves the computational barrier of transition rate evaluations, paving the way for large-scale, dynamic modeling of plasma-wall interactions. △ Less

Submitted 4 June, 2026; v1 submitted 7 April, 2026; originally announced April 2026.

FDTD Simulation of O-X Mode Conversion Process in Non-uniform Magnetized Plasma

Authors: Chenxu Wang, Ryota Usui, Hiroaki Nakamura, Hideki Kawaguchi, Kubo Shin

Abstract: Electron Bernstein Waves (EBWs) are electrostatic waves than can propagate in overdense plasmas without density cutoff, making them suitable for high density plasma heating. Since EBWs cannot be directly launched from vacuum, mode conversion processes such as O-X-B conversion are required. In this study, the O-X mode conversion process is investigated using the finite difference time-domain (FDTD)… ▽ More Electron Bernstein Waves (EBWs) are electrostatic waves than can propagate in overdense plasmas without density cutoff, making them suitable for high density plasma heating. Since EBWs cannot be directly launched from vacuum, mode conversion processes such as O-X-B conversion are required. In this study, the O-X mode conversion process is investigated using the finite difference time-domain (FDTD) method in a magnetized plasma with non-uniform density. In particular, the dependence of mode conversion characteristics on the incident angle of the injected wave is studied. The results show that an optimal incident angle exists at which the wave propagates without significant attenuation and strong electric field enhancement is observed near the upper hybrid resonance (UHR) layer. When the incident angle deviates from this optimal angle, an evanescent region appears, resulting in attenuation of the wave. These results demonstrate that angular optimization is essential for efficient wave propagation toward the UHR region and EBW excitation. △ Less

Submitted 31 March, 2026; originally announced March 2026.

Comments: 6 pages, 6 figures

Correction scheme for molecular total energies from quantum phase estimation under limited qubit resources

Authors: Nobuki Inoue, Hisao Nakamura

Abstract: We propose a practical method for accurately evaluating molecular total energies using a hybrid approach that integrates fault-tolerant quantum computers with classical computing. Our scheme consists of two complementary components: quantum dominant orbital selection (QDOS) and subspace dynamical correlation (SDC). QDOS extracts only the essential active orbitals from the complete active space (CA… ▽ More We propose a practical method for accurately evaluating molecular total energies using a hybrid approach that integrates fault-tolerant quantum computers with classical computing. Our scheme consists of two complementary components: quantum dominant orbital selection (QDOS) and subspace dynamical correlation (SDC). QDOS extracts only the essential active orbitals from the complete active space (CAS) configuration interaction (CI) state on a quantum computer, yielding a compact active space suitable for classical CASCI calculations. SDC then evaluates dynamical-correlation corrections for the CASCI energy using this compact state, which remains tractable on classical machines. To demonstrate that the CAS energy obtained on a quantum computer can be post-corrected by SDC, we examine two frameworks: multireference perturbation theory and tailored coupled-cluster theory. Our scheme enables effective treatment of relatively large molecular systems by combining limited quantum and classical resources. △ Less

Submitted 11 June, 2026; v1 submitted 3 March, 2026; originally announced March 2026.

Journal ref: J. Comp. Chem., 47, 14 (2026)

Nonlinear Anisotropy in Phase-Tuned Wide-Gap Halides

Authors: L. Landivar Scott, L. M. Vogl, C. Klenke, S. Puri, H. Nakamura

Abstract: Silver iodide (AgI) thin films offer a compelling platform for studying nonlinear optical phenomena due to their intrinsic noncentrosymmetric lattice and direct band gap. Here, we investigate the nonlinear optical properties of AgI thin films grown by physical vapor deposition that selectively produce zincblende (\zbAgI) and wurtzite (\wzAgI) phases. Using a combination of polarization-resolved se… ▽ More Silver iodide (AgI) thin films offer a compelling platform for studying nonlinear optical phenomena due to their intrinsic noncentrosymmetric lattice and direct band gap. Here, we investigate the nonlinear optical properties of AgI thin films grown by physical vapor deposition that selectively produce zincblende (\zbAgI) and wurtzite (\wzAgI) phases. Using a combination of polarization-resolved second harmonic generation (SHG) and two-photon photoluminescence (2PPL) spectroscopy, we identify clear phase- and morphology-dependent anisotropic nonlinear responses. Triangular \zbAgI $(111)$ flakes exhibit sixfold SHG symmetry and isotropic 2PPL emission, while rod-shaped \wzAgI $(101)$ samples display twofold-symmetric patterns in both SHG and 2PPL, which are explained by theories that integrate explicit polarization analysis using second- and third-order nonlinear susceptibilities. We estimate $χ^{(2)}_\mathrm{eff}$ of 0.45 pm/V (\zbAgI) and 0.16 pm/V (\wzAgI), respectively, after correcting for multiple internal reflections and generation of SHG within the AgI film and optical interference effect in the dielectric layers. These results identify AgI as a useful single-composition halide platform for studying phase-dependent nonlinearity and establish a realistic methodology for evaluating nonlinear susceptibilities in layered materials or thin films supported by a substrate. △ Less

Submitted 9 July, 2026; v1 submitted 26 December, 2025; originally announced December 2025.

Journal ref: Phys. Rev. B 114, 045408 (2026)

Consideration on relation between penetrated power and topological charge of millimeter-wave vortex in magnetized plasma

Authors: Chenxu Wang, Hideki Kawaguchi, Hiroaki Nakamura, Shin Kubo

Abstract: It was demonstrated that the vortex field of a hybrid mode can propagate in the mag-netized plasma region where plane waves are unable to propagate due to the cut-off condition. In this study, the dependence of the penetrated power of injected as millime-ter-wave vortices of the hybrid mode in magnetized plasma is analyzed using the Fi-nite-Difference Time-Domain (FDTD) method. The effect of the r… ▽ More It was demonstrated that the vortex field of a hybrid mode can propagate in the mag-netized plasma region where plane waves are unable to propagate due to the cut-off condition. In this study, the dependence of the penetrated power of injected as millime-ter-wave vortices of the hybrid mode in magnetized plasma is analyzed using the Fi-nite-Difference Time-Domain (FDTD) method. The effect of the radius size of the cor-rugated waveguide on the penetrated power is described, revealing its significant con-tribution to reducing the deviation of topological charge in the hybrid mode. Further-more, it was found that the penetrated power of the vortex field in magnetized plasma strongly depends on the topological charge l and the deviation of topological charge △ Less

Submitted 24 January, 2025; originally announced January 2025.

Comments: 7 pages, 3 figures

Specific Heat Anomalies and Local Symmetry Breaking in (Anti-)Fluorite Materials: A Machine Learning Molecular Dynamics Study

Authors: Keita Kobayashi, Hiroki Nakamura, Masahiko Okumura, Mitsuhiro Itakura, Masahiko Machida

Abstract: Understanding the high-temperature properties of materials with (anti-)fluorite structures is crucial for their application in nuclear reactors. In this study, we employ machine learning molecular dynamics (MLMD) simulations to investigate the high-temperature thermal properties of thorium dioxide, which has a fluorite structure, and lithium oxide, which has an anti-fluorite structure. Our results… ▽ More Understanding the high-temperature properties of materials with (anti-)fluorite structures is crucial for their application in nuclear reactors. In this study, we employ machine learning molecular dynamics (MLMD) simulations to investigate the high-temperature thermal properties of thorium dioxide, which has a fluorite structure, and lithium oxide, which has an anti-fluorite structure. Our results show that MLMD simulations effectively reproduce the reported thermal properties of these materials. A central focus of this work is the analysis of specific heat anomalies in these materials at high temperatures, commonly referred to as Bredig, pre-melting, or $λ$-transitions. We demonstrate that a local order parameter, analogous to those used to describe liquid-liquid transitions in supercooled water and liquid silica, can effectively characterize these specific heat anomalies. The local order parameter identifies two distinct types of defective structures: lattice defect-like and liquid-like local structures. Above the transition temperature, liquid-like local structures predominate, and the sub-lattice character of mobile atoms disappears. △ Less

Submitted 16 December, 2024; originally announced December 2024.

Mechanisms for a Spring Peak in East Asian Cyclone Activity

Authors: Satoru Okajima, Hisashi Nakamura, Akira Kuwano-Yoshida, Rhys Parfitt

Abstract: The frequency of extratropical cyclones in East Asia, including those traveling along the Kuroshio off the south coast of Japan, maximizes climatologically in spring in harmony with local enhancement of precipitation. The springtime cyclone activity is of great socioeconomic importance for East Asian countries. However, mechanisms for the spring peak in the East Asian cyclone activity have been po… ▽ More The frequency of extratropical cyclones in East Asia, including those traveling along the Kuroshio off the south coast of Japan, maximizes climatologically in spring in harmony with local enhancement of precipitation. The springtime cyclone activity is of great socioeconomic importance for East Asian countries. However, mechanisms for the spring peak in the East Asian cyclone activity have been poorly understood. This study aims to unravel the mechanisms, focusing particularly on favorable conditions for relevant cyclogenesis. Through a composite analysis based on atmospheric reanalysis data, we show that cyclogenesis enhanced around the East China Sea under anomalously strengthened cyclonic wind shear and temperature gradient, in addition to enhanced moisture flux from the south, is important for the spring peak in the cyclone activity in East Asia. In spring, climatologically strengthened cyclonic shear north of the low-level jet axis and associated frequent atmospheric frontogenesis in southern China and the East China Sea serve as favorable background conditions for low-level cyclogenesis. We also demonstrate that climatologically enhanced diabatic heating around East Asia is pivotal in strengthening of the low-level jet through a set of linear baroclinic model experiments. Our findings suggest the importance of the seasonal evolution of diabatic heating in East Asia for that of the climate system around East Asia from winter to spring, encompassing the spring peak in the cyclone activity and climatological precipitation. △ Less

Submitted 30 July, 2024; originally announced July 2024.

Comments: 30 pages, 12 figures

Reactive Molecular Dynamics Simulation on DNA Double Strand Breaks Induced by Hydrogen Elimination

Authors: Hiroaki Nakamura, Kento Ishiguro, Ayako Nakata, Shunsuke Usami, Seiki Saito, Susumu Fujiwara

Abstract: We propose a scar model to reproduce how double-strand breaks occur in the telomeric DNA damaged by the effect of the $β$-decay of tritium to helium. In this scar model, the two hydrogens bonded to the 5$^\prime$ carbon connecting the pent saccharides and phosphate are removed. Molecular dynamics simulations using the reactive force field are carried out for 10 cases for the telomeric DNA consisti… ▽ More We propose a scar model to reproduce how double-strand breaks occur in the telomeric DNA damaged by the effect of the $β$-decay of tritium to helium. In this scar model, the two hydrogens bonded to the 5$^\prime$ carbon connecting the pent saccharides and phosphate are removed. Molecular dynamics simulations using the reactive force field are carried out for 10 cases for the telomeric DNA consisting of 16 base pairs (32 nucleotides). It results in double-strand breaks (DSBs) being observed for structures with more than 24 scars. For 16 scar cases, only single-strand breaks (SSB) are observed. Moreover, in the case of $\left\{16, 0 \right\}$ and $\left\{ 0, 16 \right\},$ where only one of the strands had scars, SSB occurs only in the scarred strand. Secondly, in the $\left\{16, 8 \right\}$ and $\left\{ 8, 16 \right\}$ cases, DSBs occurs. Therefore, we conclude that the following conditions are necessary for DSBs:(i) Scars must occur on both the L and R strands. (ii) A large number of scars (24 or more) must occur in close proximity to each other. △ Less

Submitted 8 July, 2024; originally announced July 2024.

Journal ref: Jpn. J. Appl. Phys. 63 12SP09 (2024)

The study of propagation characteristics of millimeter-wave vortex in magnetized plasma by using FDTD Method

Authors: Chenxu Wang, Hideki Kawaguchi, Hiroaki Nakamura, Shin Kubo

Abstract: It is pointed out that millimeter-wave vortex may contribute an efficient plasma heating since it was found that the millimeter-wave vortex can propagate in magnetized plasma even in which the normal plane wave is in cut-off condition. Then, it was assumed that the vortex field was the Laguerre-Gaussian (L-G) mode which is free-space solution, but the generation and stable propagation of the L-G m… ▽ More It is pointed out that millimeter-wave vortex may contribute an efficient plasma heating since it was found that the millimeter-wave vortex can propagate in magnetized plasma even in which the normal plane wave is in cut-off condition. Then, it was assumed that the vortex field was the Laguerre-Gaussian (L-G) mode which is free-space solution, but the generation and stable propagation of the L-G mode vortex are not easy in the millimeter frequency range. On the other hand, it is known that millimeter-wave hybrid mode of cylindrical corrugated waveguide has also vortex property. In this paper, we investigate propagation characteristics of millimeter-wave vortex of a hybrid mode of cylindrical corrugated waveguide in the magnetized plasma by using three dimensional numerical simulations with finite-difference time-domain (FDTD) method. △ Less

Submitted 5 July, 2024; originally announced July 2024.

Comments: 9 pages, 5 figures

Molecular dynamics simulation for coalescence of vacancies in tungsten crystal

Authors: Sotaro Tsuru, Hiroaki Nakamura, Yuki Goto, Miyuki Yajima, Seiki Saito, Shunsuke Usami

Abstract: We performed molecular dynamics simulations of coalescence of two vacancies in a tungsten (W) crystal to elucidate the effect of temperature and hydrogen atoms. Simulations were performed for two types of vacancy structures, $\mathrm{V}_9 + \mathrm{W}_1 + \mathrm{V}_9$ and $\mathrm{V}_{10} + \mathrm{W}_4 + \mathrm{V}_{10}$ ($\mathrm{V}_{n}$ means that a vacancy corresponds to the absence of $n$ W… ▽ More We performed molecular dynamics simulations of coalescence of two vacancies in a tungsten (W) crystal to elucidate the effect of temperature and hydrogen atoms. Simulations were performed for two types of vacancy structures, $\mathrm{V}_9 + \mathrm{W}_1 + \mathrm{V}_9$ and $\mathrm{V}_{10} + \mathrm{W}_4 + \mathrm{V}_{10}$ ($\mathrm{V}_{n}$ means that a vacancy corresponds to the absence of $n$ W atoms, and $\mathrm{W}_{m}$ indicates that there are $m$ W atoms between two vacancies) in various cases of temperature and hydrogen atom concentration. Under the vacancy structure $\mathrm{V}_9 + \mathrm{W}_1 + \mathrm{V}_9$, we observed vacancy coalescence for all the cases of the temperature and the number of hydrogen atoms. Evaluating the potential energy required for removing one of the W atoms between two vacancies, we found that high temperature and existing hydrogen atoms in the vacancies facilitate vacancy coalescence, and that under the structure $\mathrm{V}_{10} + \mathrm{W}_4 + \mathrm{V}_{10}$, hydrogen atoms facilitate vacancy coalescence most strongly when the number is around 45 to 54 in each vacancy. △ Less

Submitted 2 July, 2024; originally announced July 2024.

Comments: 12 pages, 5 figures

Performance evaluation of electron multiplier tubes as a high-intensity muon beam monitor of accelerator neutrino experiments

Authors: Takashi Honjo, Yosuke Ashida, Oderich F. Auersperg-Castell, Megan Friend, Ian Heitkamp, Atsuko K. Ichikawa, Masaki Ishitsuka, Nao Izumi, Sohei Kasama, Shigeru Kashiwagi, Yuma Kawamura, Tatsuya Kikawa, Takuya Kobata, Tsunayuki Matsubara, Manabu Miyabe, Kiseki D. Nakamura, Hina Nakamura, Yukine Sato, Ken Sakashita, Yoshihiro Seiya, Kouchi Takifuji, Atsushi Tokiyasu, Tatsuya Yamamoto, Kazuhiro Yamamoto, Kenji Yasutome

Abstract: Upgrade work towards increasing the beam intensity of the neutrino beamline at J- PARC is underway. Monitoring tertiary muon beams is essential for stable operation of the beamline. Accordingly, we plan to replace the present muon monitor sensors with electron multiplier tubes (EMTs). We investigated the radiation tolerance and linearity response of EMTs using a 90 MeV electron beam. An EMTs was i… ▽ More Upgrade work towards increasing the beam intensity of the neutrino beamline at J- PARC is underway. Monitoring tertiary muon beams is essential for stable operation of the beamline. Accordingly, we plan to replace the present muon monitor sensors with electron multiplier tubes (EMTs). We investigated the radiation tolerance and linearity response of EMTs using a 90 MeV electron beam. An EMTs was irradiated with electrons up to 470 nC. EMTs show higher radiation tolerance than the Si sensors which are presently used as one of the muon monitor detectors for the T2K long-baseline neutrino experiment at J-PARC. The integrated charge yield decrease is found to be less than 8% after a beam irradiation equivalent to 132 days of operation at the future J-PARC beam power of 1.3 MW. The EMTs show linearity better than $\pm$5% up to the future beam intensity. The observed yield decrease is likely due to dynode deterioration based on the detailed investigation. The studies described here confirm that EMTs can be used as a high-intensity muon beam monitor. From the reported results, we are proceeding with the installation in the J-PARC neutrino beamline. △ Less

Submitted 16 January, 2025; v1 submitted 9 May, 2024; originally announced May 2024.

Comments: 20 pages,19 figures

Journal ref: Prog. Theor. Exp. Phys., 2024(12), 123H01 (2024)

Scintillator ageing of the T2K near detectors from 2010 to 2021

Authors: The T2K Collaboration, K. Abe, N. Akhlaq, R. Akutsu, A. Ali, C. Alt, C. Andreopoulos, M. Antonova, S. Aoki, T. Arihara, Y. Asada, Y. Ashida, E. T. Atkin, S. Ban, M. Barbi, G. J. Barker, G. Barr, D. Barrow, M. Batkiewicz-Kwasniak, F. Bench, V. Berardi, L. Berns, S. Bhadra, A. Blanchet, A. Blondel , et al. (333 additional authors not shown)

Abstract: The T2K experiment widely uses plastic scintillator as a target for neutrino interactions and an active medium for the measurement of charged particles produced in neutrino interactions at its near detector complex. Over 10 years of operation the measured light yield recorded by the scintillator based subsystems has been observed to degrade by 0.9--2.2\% per year. Extrapolation of the degradation… ▽ More The T2K experiment widely uses plastic scintillator as a target for neutrino interactions and an active medium for the measurement of charged particles produced in neutrino interactions at its near detector complex. Over 10 years of operation the measured light yield recorded by the scintillator based subsystems has been observed to degrade by 0.9--2.2\% per year. Extrapolation of the degradation rate through to 2040 indicates the recorded light yield should remain above the lower threshold used by the current reconstruction algorithms for all subsystems. This will allow the near detectors to continue contributing to important physics measurements during the T2K-II and Hyper-Kamiokande eras. Additionally, work to disentangle the degradation of the plastic scintillator and wavelength shifting fibres shows that the reduction in light yield can be attributed to the ageing of the plastic scintillator. △ Less

Submitted 26 July, 2022; originally announced July 2022.

Comments: 29 pages, 18 figures. Prepared for submission to JINST

Code O-SUKI-N 3D: Upgraded Direct-Drive Fuel Target 3D Implosion Code in Heavy Ion Inertial Fusion

Authors: H. Nakamura, K. Uchibori, S. Kawata, T. Karino, R. Sato, A. I. Ogoyski

Abstract: The Code O-SUKI-N 3D is an upgraded version of the 2D Code O-SUKI (Comput. Phys. Commun. 240, 83 (2019)). Code O-SUKI-N 3D is an integrated 3-dimensional (3D) simulation program system for fuel implosion, ignition and burning of a direct-drive nuclear-fusion pellet in heavy ion beam (HIB) inertial confinement fusion (HIF).The Code O-SUKI-N 3D consists of the three programs of Lagrangian fluid impl… ▽ More The Code O-SUKI-N 3D is an upgraded version of the 2D Code O-SUKI (Comput. Phys. Commun. 240, 83 (2019)). Code O-SUKI-N 3D is an integrated 3-dimensional (3D) simulation program system for fuel implosion, ignition and burning of a direct-drive nuclear-fusion pellet in heavy ion beam (HIB) inertial confinement fusion (HIF).The Code O-SUKI-N 3D consists of the three programs of Lagrangian fluid implosion program, data conversion program, and Euler fluid implosion, ignition and burning program. The Code O-SUKI-N 3D can also couple with the HIB illumination and energy deposition program of OK3 (Comput. Phys. Commun. 181, 1332 (2010)). The spherical target implosion 3D behavior is computed by the 3D Lagrangian fluid code until the time just before the void closure of the fuel implosion. After that, all the data by the Lagrangian implosion code are converted to the data for the 3D Eulerian code. In the 3D Euler code, the DT fuel compression at the stagnation, ignition and burning are computed. The Code O-SUKI-N 3D simulation system provides a capability to compute and to study the HIF target implosion dynamics. △ Less

Submitted 30 April, 2021; originally announced May 2021.

Comments: 51 pages, 21 figures. Computer code is also available with an example output directory structure. arXiv admin note: substantial text overlap with arXiv:1812.07128

Supernova Model Discrimination with Hyper-Kamiokande

Authors: Hyper-Kamiokande Collaboration, :, K. Abe, P. Adrich, H. Aihara, R. Akutsu, I. Alekseev, A. Ali, F. Ameli, I. Anghel, L. H. V. Anthony, M. Antonova, A. Araya, Y. Asaoka, Y. Ashida, V. Aushev, F. Ballester, I. Bandac, M. Barbi, G. J. Barker, G. Barr, M. Batkiewicz-Kwasniak, M. Bellato, V. Berardi, M. Bergevin , et al. (478 additional authors not shown)

Abstract: Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants -- neutron stars and black holes -- are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-colla… ▽ More Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants -- neutron stars and black holes -- are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-collapse supernovae is not yet well understood. Hyper-Kamiokande is a next-generation neutrino detector that will be able to observe the neutrino flux from the next galactic core-collapse supernova in unprecedented detail. We focus on the first 500 ms of the neutrino burst, corresponding to the accretion phase, and use a newly-developed, high-precision supernova event generator to simulate Hyper-Kamiokande's response to five different supernova models. We show that Hyper-Kamiokande will be able to distinguish between these models with high accuracy for a supernova at a distance of up to 100 kpc. Once the next galactic supernova happens, this ability will be a powerful tool for guiding simulations towards a precise reproduction of the explosion mechanism observed in nature. △ Less

Submitted 20 July, 2021; v1 submitted 13 January, 2021; originally announced January 2021.

Comments: 21 pages, 7 figures. Article based on thesis published as arXiv:2002.01649. v2: added references and some explanations in response to reviewer comments

Journal ref: Astrophys.J. 916 (2021) 15

The Hyper-Kamiokande Experiment -- Snowmass LOI

Authors: Hyper-Kamiokande Collaboration, :, K. Abe, P. Adrich, H. Aihara, R. Akutsu, I. Alekseev, A. Ali, F. Ameli, L. H. V. Anthony, A. Araya, Y. Asaoka, V. Aushev, I. Bandac, M. Barbi, G. Barr, M. Batkiewicz-Kwasniak, M. Bellato, V. Berardi, L. Bernard, E. Bernardini, L. Berns, S. Bhadra, J. Bian, A. Blanchet , et al. (366 additional authors not shown)

Abstract: Hyper-Kamiokande is the next generation underground water Cherenkov detector that builds on the highly successful Super-Kamiokande experiment. The detector which has an 8.4~times larger effective volume than its predecessor will be located along the T2K neutrino beamline and utilize an upgraded J-PARC beam with 2.6~times beam power. Hyper-K's low energy threshold combined with the very large fiduc… ▽ More Hyper-Kamiokande is the next generation underground water Cherenkov detector that builds on the highly successful Super-Kamiokande experiment. The detector which has an 8.4~times larger effective volume than its predecessor will be located along the T2K neutrino beamline and utilize an upgraded J-PARC beam with 2.6~times beam power. Hyper-K's low energy threshold combined with the very large fiducial volume make the detector unique, that is expected to acquire an unprecedented exposure of 3.8~Mton$\cdot$year over a period of 20~years of operation. Hyper-Kamiokande combines an extremely diverse science program including nucleon decays, long-baseline neutrino oscillations, atmospheric neutrinos, and neutrinos from astrophysical origins. The scientific scope of this program is highly complementary to liquid-argon detectors for example in sensitivity to nucleon decay channels or supernova detection modes. Hyper-Kamiokande construction has started in early 2020 and the experiment is expected to start operations in 2027. The Hyper-Kamiokande collaboration is presently being formed amongst groups from 19 countries including the United States, whose community has a long history of making significant contributions to the neutrino physics program in Japan. US physicists have played leading roles in the Kamiokande, Super-Kamiokande, EGADS, K2K, and T2K programs. △ Less

Submitted 1 September, 2020; originally announced September 2020.

Comments: 6 pages, prepared as Snowmass2021 LOI

Applications of Quantum Computing for Investigations of Electronic Transitions in Phenylsulfonyl-carbazole TADF Emitters

Authors: Qi Gao, Gavin O. Jones, Mario Motta, Michihiko Sugawara, Hiroshi C. Watanabe, Takao Kobayashi, Eriko Watanabe, Yu-ya Ohnishi, Hajime Nakamura, Naoki Yamamoto

Abstract: A quantum chemistry study of the first singlet (S1) and triplet (T1) excited states of phenylsulfonyl-carbazole compounds, proposed as useful thermally activated delayed fluorescence (TADF) emitters for organic light emitting diode (OLED) applications, was performed with the quantum Equation-Of-Motion Variational Quantum Eigensolver (qEOM-VQE) and Variational Quantum Deflation (VQD) algorithms on… ▽ More A quantum chemistry study of the first singlet (S1) and triplet (T1) excited states of phenylsulfonyl-carbazole compounds, proposed as useful thermally activated delayed fluorescence (TADF) emitters for organic light emitting diode (OLED) applications, was performed with the quantum Equation-Of-Motion Variational Quantum Eigensolver (qEOM-VQE) and Variational Quantum Deflation (VQD) algorithms on quantum simulators and devices. These quantum simulations were performed with double zeta quality basis sets on an active space comprising the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the TADF molecules. The differences in energy separations between S1 and T1 ($ΔE_{st}$) predicted by calculations on quantum simulators were found to be in excellent agreement with experimental data. Differences of 16 and 88 mHa with respect to exact energies were found for excited states by using the qEOM-VQE and VQD algorithms, respectively, to perform simulations on quantum devices without error mitigation. By utilizing error mitigation by state tomography to purify the quantum states and correct energy values, the large errors found for unmitigated results could be improved to differences of, at most, 3 mHa with respect to exact values. Consequently, excellent agreement could be found between values of $ΔE_{st}$ predicted by quantum simulations and those found in experiments. △ Less

Submitted 30 July, 2020; originally announced July 2020.

Comments: 18 pages, 14 figures

Journal ref: npj Computational Materials 7, 70 (2021)

Three-Dimensional Euler Fluid Code for Fusion Fuel Ignition and Burning

Authors: Hiroki Nakamura, Shigeo Kawata, Ken Uchibori, Takahiro Karino

Abstract: The document describes a numerical algorithm to simulate plasmas and fluids in the 3 dimensional space by the Euler method, in which the spatial meshes are fixed to the space. The plasmas and fluids move through the spacial Euler mesh boundary. The Euler method can represent a large deformation of the plasmas and fluids. On the other hand, when the plasmas or fluids are compressed to a high densit… ▽ More The document describes a numerical algorithm to simulate plasmas and fluids in the 3 dimensional space by the Euler method, in which the spatial meshes are fixed to the space. The plasmas and fluids move through the spacial Euler mesh boundary. The Euler method can represent a large deformation of the plasmas and fluids. On the other hand, when the plasmas or fluids are compressed to a high density, the spatial resolution should be ensured to describe the density change precisely. The present 3D Euler code is developed to simulate a nuclear fusion fuel ignition and burning. Therefore, the 3D Euler code includes the DT fuel reactions, the alpha particle diffusion, the alpha particle deposition to heat the DT fuel and the DT fuel depletion by the DT reactions, as well as the thermal energy diffusion based on the three-temperature compressible fluid model. △ Less

Submitted 8 April, 2020; originally announced April 2020.

Comments: 20 pages, 2 figures

Coupling Effects in Multi-Stage Laser Wake-field Acceleration of Electrons

Authors: Zhan Jin, Hirotaka Nakamura, Naveen Pathak, Yasuo Sakai, Alexei Zhidkov, Keiichi Sueda, Ryosuke Kodama, Tomonao Hosokai

Abstract: Staging laser wake-field acceleration is considered as a necessary technique for developing full-optical jitter-free electron accelerators. Splitting of the acceleration length into several technical parts with their lengths smaller than the dephasing length and with independent laser drivers allows generation of stable, reproducible acceleration fields. Temporal and spatial coupling of pre-accele… ▽ More Staging laser wake-field acceleration is considered as a necessary technique for developing full-optical jitter-free electron accelerators. Splitting of the acceleration length into several technical parts with their lengths smaller than the dephasing length and with independent laser drivers allows generation of stable, reproducible acceleration fields. Temporal and spatial coupling of pre-accelerated electron bunches for their injection in the acceleration phase of a successive laser pulse wake field is the key part of the staging laser-driven acceleration. Here, characterization of the coupling is performed with dense, stable, a narrow energy band <3% and energy selectable electron beams with charges ~1.6 pC and energy ~10 MeV generated from a laser plasma cathode. Cumulative focusing of electron bunches in a low density pre-plasma, exhibiting the Budker- Bennett effect, is shown to result in the efficient injection of electrons even with a long distance between the injector and the booster in the laser pulse wake. Measured characteristics of electron beams modified by the booster wake field agree well with those obtained by multidimensional particle-in-cell simulations. △ Less

Submitted 4 August, 2019; originally announced August 2019.

Computational Investigations of the Lithium Superoxide Dimer Rearrangement on Noisy Quantum Devices

Authors: Qi Gao, Hajime Nakamura, Tanvi P. Gujarati, Gavin O. Jones, Julia E. Rice, Stephen P. Wood, Marco Pistoia, Jeannette M. Garcia, Naoki Yamamoto

Abstract: Currently available noisy intermediate-scale quantum (NISQ) devices are limited by the number of qubits that can be used for quantum chemistry calculations on molecules. We show herein that the number of qubits required for simulations on a quantum computer can be reduced by limiting the number of orbitals in the active space. Thus, we have utilized ansätze that approximate exact classical matrix… ▽ More Currently available noisy intermediate-scale quantum (NISQ) devices are limited by the number of qubits that can be used for quantum chemistry calculations on molecules. We show herein that the number of qubits required for simulations on a quantum computer can be reduced by limiting the number of orbitals in the active space. Thus, we have utilized ansätze that approximate exact classical matrix eigenvalue decomposition methods (Full Configuration Interaction). Such methods are appropriate for computations with the Variational Quantum Eigensolver algorithm to perform computational investigations on the rearrangement of the lithium superoxide dimer with both quantum simulators and quantum devices. These results demonstrate that, even with a limited orbital active space, quantum simulators are capable of obtaining energy values that are similar to the exact ones. However, calculations on quantum hardware underestimate energies even after the application of readout error mitigation. △ Less

Submitted 23 August, 2019; v1 submitted 25 June, 2019; originally announced June 2019.

Bright X-ray radiation from plasma bubbles in an evolving laser wakefield accelerator

Authors: M. S. Bloom, M. J. V. Streeter, S. Kneip, R. A. Bendoyro, O. Cheklov, J. M. Cole, A. Doepp, C. J. Hooker, J. Holloway, J. Jiang, N. C. Lopes, H. Nakamura, P. A. Norreys, P. P. Rajeev, D. R. Symes, J. Schreiber, J. C. Wood, M. Wing, Z. Najmudin, S. P. D. Mangles

Abstract: We show that the properties of the electron beam and bright x-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion leng… ▽ More We show that the properties of the electron beam and bright x-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion length. With a 200~TW laser pulse this results in an x-ray beam with median photon energy of \unit[20]{keV}, $> 6\times 10^{8}$ photons above \unit[1]{keV} per shot and a peak brightness of $\unit[3 \times 10^{22}]{photons~s^{-1}mrad^{-2}mm^{-2} (0.1\% BW)^{-1}}$. △ Less

Submitted 6 May, 2020; v1 submitted 16 October, 2017; originally announced October 2017.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Accel. Beams 23, 061301 (2020)

Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

Authors: M. J. V. Streeter, S. Kneip, M. S. Bloom, R. A. Bendoyro, O. Chekhlov, A. E. Dangor, A. Döpp, C. J. Hooker, J. Holloway, J. Jiang, N. C. Lopes, H. Nakamura, P. A. Norreys, C. A. J. Palmer, P. P. Rajeev, J. Schreiber, D. R. Symes, M. Wing, S. P. D. Mangles, Z. Najmudin

Abstract: We report on the depletion and power amplification of the driving laser pulse in a strongly-driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power amplification is c… ▽ More We report on the depletion and power amplification of the driving laser pulse in a strongly-driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including localized redshift and subsequent group delay dispersion at the laser pulse front. △ Less

Submitted 21 June, 2018; v1 submitted 15 October, 2017; originally announced October 2017.

Comments: 6 pages, 4 figures. Published in PRL

Journal ref: Phys. Rev. Lett. 120, 254801 (2018)

Insight of breaking of powerful axisymmetrically-polarized laser pulses in under-dense plasma

Authors: Nobuhiko Nakanii, Tomonao Hosokai, Naveen C. Pathak, Shinichi Masuda, Alexei G. Zhidkov, Hiroki Nakahara, Kenta Iwasa, Yoshio Mizuta, Naoki Takeguchi, Takamitsu P. Otsuka, Keiichi Sueda, Hirotaka Nakamura, Michiaki Mori, Masaki Kando, Ryosuke Kodama

Abstract: Interaction of axisymmetrically-polarized (radially or azimuthally-polarized), relativistically intense laser pulses (ALP) with under-dense plasma is shown experimentally to be different from the interaction of conventional Gaussian pulses. The difference is clearly observed in distinct spectra of scattered laser light as well as in appearance of a strong side emission of second harmonic in the vi… ▽ More Interaction of axisymmetrically-polarized (radially or azimuthally-polarized), relativistically intense laser pulses (ALP) with under-dense plasma is shown experimentally to be different from the interaction of conventional Gaussian pulses. The difference is clearly observed in distinct spectra of scattered laser light as well as in appearance of a strong side emission of second harmonic in the vicinity of focus spot. According 3D particle-in-cell simulations, this is a result of instability in the propagation of ALP in under-dense plasma. Laser wakefield acceleration of electrons by ALP, therefore, is less efficient than that by Gaussian laser pulses but ALP may be interesting for efficient electron self-injection. △ Less

Submitted 2 November, 2015; originally announced November 2015.

Comments: 10 pages, 5 figures

Journal ref: Phys. Rev. E 94, 063205 (2016)

Calibration of Time Of Flight Detectors Using Laser-driven Neutron Source

Authors: S. R. Mirfayzi, S. Kar, H. Ahmed, A. G. Krygier, A. Green, A. Alejo, R. Clarke, R. R. Freeman, J. Fuchs, D. Jung, A. Kleinschmidt, J. T. Morrison, Z. Najmudin, H. Nakamura, P. Norreys, M. Oliver, M. Roth, L. Vassura, M. Zepf, M. Borghesi

Abstract: Calibration of three scintillators (EJ232Q, BC422Q and EJ410) in a time-of-flight (TOF) arrangement using a laser drive-neutron source is presented. The three plastic scintillator detectors were calibrated with gamma insensitive bubble detector spectrometers, which were absolutely calibrated over a wide range of neutron energies ranging from sub MeV to 20 MeV. A typical set of data obtained simult… ▽ More Calibration of three scintillators (EJ232Q, BC422Q and EJ410) in a time-of-flight (TOF) arrangement using a laser drive-neutron source is presented. The three plastic scintillator detectors were calibrated with gamma insensitive bubble detector spectrometers, which were absolutely calibrated over a wide range of neutron energies ranging from sub MeV to 20 MeV. A typical set of data obtained simultaneously by the detectors are shown, measuring the neutron spectrum emitted from a petawatt laser irradiated thin foil. △ Less

Submitted 15 June, 2015; originally announced June 2015.

Journal ref: Rev. Sci. Instrum. 86, 073308 (2015)

Selective Deuterium Ion Acceleration Using the Vulcan PW Laser

Authors: AG Krygier, JT Morrison, S Kar, H Ahmed, A Alejo, R Clarke, J Fuchs, A Green, D Jung, A Kleinschmidt, Z Najmudin, H Nakamura, P Norreys, M Notley, M Oliver, M Roth, L Vassura, M Zepf, M Borghesi, RR Freeman

Abstract: We report on the successful demonstration of selective acceleration of deuterium ions by target-normal sheath acceleration (TNSA) with a high-energy petawatt laser. TNSA typically produces a multi-species ion beam that originates from the intrinsic hydrocarbon and water vapor contaminants on the target surface. Using the method first developed by Morrison, et al.,$^{1}$ an ion beam with $>$99$\%$… ▽ More We report on the successful demonstration of selective acceleration of deuterium ions by target-normal sheath acceleration (TNSA) with a high-energy petawatt laser. TNSA typically produces a multi-species ion beam that originates from the intrinsic hydrocarbon and water vapor contaminants on the target surface. Using the method first developed by Morrison, et al.,$^{1}$ an ion beam with $>$99$\%$ deuterium ions and peak energy 14 MeV/nucleon is produced with a 200 J, 700 fs, $>10^{20} W/cm^{2}$ laser pulse by cryogenically freezing heavy water (D$_{2}$O) vapor onto the rear surface of the target prior to the shot. Within the range of our detectors (0-8.5$^{\circ}$), we find laser-to-deuterium-ion energy conversion efficiency of 4.3$\%$ above 0.7 MeV/nucleon while a conservative estimate of the total beam gives a conversion efficiency of 9.4$\%$. △ Less

Submitted 10 April, 2015; v1 submitted 26 January, 2015; originally announced January 2015.

Comments: 5 pages, 5 figures

Characterisation of deuterium spectra from laser driven multi-species sources by employing differentially filtered image plate detectors in Thomson spectrometers

Authors: A. Alejo, S. Kar, H. Ahmed, A. G. Krygier, D. Doria, R. Clarke, J. Fernandez, R. R. Freeman, J. Fuchs, A. Green, J. S. Green, D. Jung, A. Kleinschmidt, C. L. S. Lewis, J. T. Morrison, Z. Najmudin, H. Nakamura, G. Nersisyan, P. Norreys, M. Notley, M. Oliver, M. Roth, J. A. Ruiz, L. Vassura, M. Zepf , et al. (1 additional authors not shown)

Abstract: A novel method for characterising the full spectrum of deuteron ions emitted by laser driven multi-species ion sources is discussed. The procedure is based on using differential filtering over the detector of a Thompson parabola ion spectrometer, which enables discrimination of deuterium ions from heavier ion species with the same charge-to-mass ratio (such as C6+, O8+, etc.). Commonly used Fuji I… ▽ More A novel method for characterising the full spectrum of deuteron ions emitted by laser driven multi-species ion sources is discussed. The procedure is based on using differential filtering over the detector of a Thompson parabola ion spectrometer, which enables discrimination of deuterium ions from heavier ion species with the same charge-to-mass ratio (such as C6+, O8+, etc.). Commonly used Fuji Image plates were used as detectors in the spectrometer, whose absolute response to deuterium ions over a wide range of energies was calibrated by using slotted CR-39 nuclear track detectors. A typical deuterium ion spectrum diagnosed in a recent experimental campaign is presented. △ Less

Submitted 14 September, 2014; v1 submitted 13 August, 2014; originally announced August 2014.

Comments: 7 pages, 7 figures

Journal ref: Rev. Sci. Instrum. 85, 093303 (2014)

Local representation of N-body Coulomb energy with path integrals

Authors: Takanori Sugihara, Junichi Higo, Haruki Nakamura

Abstract: We represent N-body Coulomb energy in a localized form to achieve massive parallelism. It is a well-known fact that Green's functions can be written as path integrals of field theory. Since two-body Coulomb potential is a Green's function of Poisson equations, it reduces to a path integral of free scalar field theory with three spatial dimensions. This means that N-body one also reduces to a path… ▽ More We represent N-body Coulomb energy in a localized form to achieve massive parallelism. It is a well-known fact that Green's functions can be written as path integrals of field theory. Since two-body Coulomb potential is a Green's function of Poisson equations, it reduces to a path integral of free scalar field theory with three spatial dimensions. This means that N-body one also reduces to a path integral. We discretize real space with a cubic lattice and evaluate the obtained multiple integrals approximately with the Markov-chain Monte Carlo method. △ Less

Submitted 18 April, 2014; v1 submitted 4 March, 2014; originally announced March 2014.

Comments: 4 pages, 3 figures. Accepted for JPSJ Letters

Journal ref: J. Phys. Soc. Jpn., Vol.83, No.6, 063001 (2014)

Formation and Classification of Amorphous Carbon by Molecular Dynamics Simulation

Authors: Atsushi M. Ito, Arimichi Takayama, Seiki Saito, Hiroaki Nakamura

Abstract: By using molecular dynamics simulation, formation mechanisms of amorphous carbon in particular sp${}^3$ rich structure was researched. The problem that reactive empirical bond order potential cannot represent amorphous carbon properly was cleared in the transition process from graphite to diamond by high pressure and the deposition process of amorphous carbon thin films. Moreover, the new potentia… ▽ More By using molecular dynamics simulation, formation mechanisms of amorphous carbon in particular sp${}^3$ rich structure was researched. The problem that reactive empirical bond order potential cannot represent amorphous carbon properly was cleared in the transition process from graphite to diamond by high pressure and the deposition process of amorphous carbon thin films. Moreover, the new potential model which is based on electron distribution simplified as a point charge was developed by using downfolding method. As a result, the molecular dynamics simulation with the new potential could demonstrate the transition from graphite to diamond at the pressure of 15 GPa corresponding to experiment and the deposition of sp${}^3$ rich amorphous carbon. △ Less

Submitted 4 July, 2012; originally announced July 2012.

Comments: International Symposium ISPlasma 2012, submitted to Jpn. J. Appl. Phys

Hybrid Simulation between Molecular Dynamics and Binary Collision Approximation Codes for Hydrogen injection onto Carbon Materials

Authors: Seiki Saito, Atsushi M. Ito, Arimichi Takayama, Takahiro Kenmotsu, Hiroaki Nakamura

Abstract: Molecular dynamics (MD) simulation with modified Brenner's reactive empirical bond order (REBO) potential is a powerful tool to investigate plasma wall interaction on divertor plates in a nuclear fusion device. However, MD simulation box's size is less than several nm for the performance of a computer. To extend the size of the MD simulation, we develop a hybrid simulation code between MD code usi… ▽ More Molecular dynamics (MD) simulation with modified Brenner's reactive empirical bond order (REBO) potential is a powerful tool to investigate plasma wall interaction on divertor plates in a nuclear fusion device. However, MD simulation box's size is less than several nm for the performance of a computer. To extend the size of the MD simulation, we develop a hybrid simulation code between MD code using REBO potential and binary collision approximation (BCA) code. Using the BCA code instead of computing all particles with a high kinetic energy for every step in the MD simulation, considerable computation time is saved. By demonstrating a hydrogen atom injection on a graphite by the hybrid simulation code, it is found that the hybrid simulation code works efficiently in a large simulation box. △ Less

Submitted 25 June, 2010; v1 submitted 23 June, 2010; originally announced June 2010.

Comments: 5 pages, 5 figures

Extension of the simulation code ACAT to treat real atomic positions

Authors: Arimichi Takayama, Seiki Saito, Atsushi M. Ito, Takahiro Kenmotsu, Hiroaki Nakamura

Abstract: We have investigated plasma-surface interactions with molecular dynamics (MD) simulations. It, however, is high cost computation and is limited to simulations for materials of nanometer order. In order to overcome the limitation, a complementary model based on binary collision approximation (BCA) can be established. We employed a BCA-based simulation code ACAT and extended to handle any structure… ▽ More We have investigated plasma-surface interactions with molecular dynamics (MD) simulations. It, however, is high cost computation and is limited to simulations for materials of nanometer order. In order to overcome the limitation, a complementary model based on binary collision approximation (BCA) can be established. We employed a BCA-based simulation code ACAT and extended to handle any structure involving crystalline and amorphous. The extended code, named "ACaT", stores all positions of projectile and target atoms and velocities of recoil atoms, so it can be combined with the MD code. It also holds the potential to reproduce channeling phenomena. Thus it is expected to be useful for evaluation of channeling effects. △ Less

Submitted 18 May, 2010; originally announced May 2010.

Incident angle dependence of reactions between graphene and hydrogen atom by molecular dynamics simulation

Authors: Seiki Saito, Atsushi Ito, Hiroaki Nakamura

Abstract: Incident angle dependence of reactions between graphene and hydrogen atoms are obtained qualitatively by classical molecular dynamics simulation under the NVE condition with modified Brenner reactive empirical bond order (REBO) potential. Chemical reaction depends on two parameters, i.e., polar angle $θ$ and azimuthal angle $φ$ of the incident hydrogen. From the simulation results, it is found t… ▽ More Incident angle dependence of reactions between graphene and hydrogen atoms are obtained qualitatively by classical molecular dynamics simulation under the NVE condition with modified Brenner reactive empirical bond order (REBO) potential. Chemical reaction depends on two parameters, i.e., polar angle $θ$ and azimuthal angle $φ$ of the incident hydrogen. From the simulation results, it is found that the reaction rates strongly depend on polar angle $θ$. Reflection rate becomes larger with increasing $θ$, and the $θ$ dependence of adsorption rate is also found. The $θ$ dependence is caused by three dimensional structure of the small potential barrier which covers adsorption sites. $φ$ dependence of penetration rate is also found for large $θ$. △ Less

Submitted 14 December, 2009; originally announced December 2009.

Comments: 4 pages, 7 figures

Parallelization of Markov chain generation and its application to the multicanonical method

Authors: Takanori Sugihara, Junichi Higo, Haruki Nakamura

Abstract: We develop a simple algorithm to parallelize generation processes of Markov chains. In this algorithm, multiple Markov chains are generated in parallel and jointed together to make a longer Markov chain. The joints between the constituent Markov chains are processed using the detailed balance. We apply the parallelization algorithm to multicanonical calculations of the two-dimensional Ising mode… ▽ More We develop a simple algorithm to parallelize generation processes of Markov chains. In this algorithm, multiple Markov chains are generated in parallel and jointed together to make a longer Markov chain. The joints between the constituent Markov chains are processed using the detailed balance. We apply the parallelization algorithm to multicanonical calculations of the two-dimensional Ising model and demonstrate accurate estimation of multicanonical weights. △ Less

Submitted 1 July, 2009; v1 submitted 15 December, 2008; originally announced December 2008.

Comments: 15 pages, 5 figures, uses elsart.cls

Journal ref: J. Phys. Soc. Jpn. 78 (2009) 074003

Some properties of the resonant state in quantum mechanics and its computation

Authors: Naomichi Hatano, Keita Sasada, Hiroaki Nakamura, Tomio Petrosky

Abstract: The resonant state of the open quantum system is studied from the viewpoint of the outgoing momentum flux. We show that the number of particles is conserved for a resonant state, if we use an expanding volume of integration in order to take account of the outgoing momentum flux; the number of particles would decay exponentially in a fixed volume of integration. Moreover, we introduce new numeric… ▽ More The resonant state of the open quantum system is studied from the viewpoint of the outgoing momentum flux. We show that the number of particles is conserved for a resonant state, if we use an expanding volume of integration in order to take account of the outgoing momentum flux; the number of particles would decay exponentially in a fixed volume of integration. Moreover, we introduce new numerical methods of treating the resonant state with the use of the effective potential. We first give a numerical method of finding a resonance pole in the complex energy plane. The method seeks an energy eigenvalue iteratively. We found that our method leads to a super-convergence, the convergence exponential with respect to the iteration step. The present method is completely independent of commonly used complex scaling. We also give a numerical trick for computing the time evolution of the resonant state in a limited spatial area. Since the wave function of the resonant state is diverging away from the scattering potential, it has been previously difficult to follow its time evolution numerically in a finite area. △ Less

Submitted 6 August, 2007; v1 submitted 10 May, 2007; originally announced May 2007.

Comments: 20 pages, 12 figures embedded

Journal ref: Prog.Theor.Phys.119:187-222,2008

Design and optimization of tapered structure of near-field fiber probe based on FDTD simulation

Authors: Hiroaki Nakamura, Tetsuya Sato, Hirotomo Kambe, Keiji Sawada, Toshiharu Saiki

Abstract: The finite-difference time-domain method was employed to simulate light propagation in tapered near-field fiber probes with small metal aperture. By conducting large-volume simulations, including tapered metal-cladding waveguide and connected optical fiber waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture… ▽ More The finite-difference time-domain method was employed to simulate light propagation in tapered near-field fiber probes with small metal aperture. By conducting large-volume simulations, including tapered metal-cladding waveguide and connected optical fiber waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The high collection efficiency of a double-tapered probe was reproduced and was ascribed to the shortening of the cutoff region and the efficient coupling to the guiding mode of the optical fiber. The dependence of the efficiency on the tapered structure parameters was also examined. △ Less

Submitted 4 October, 2000; v1 submitted 3 October, 2000; originally announced October 2000.

Comments: 4 Pages including 3 Figures

Spatial Resolution of Near-Field Scanning Optical Microscopy with Sub-wavelength Aperture

Authors: H. Nakamura, K. Sawada, H. Kambe, T . Saiki, T. Sato

Abstract: The finite-difference time-domain (FDTD) method is employed to solve the three dimensional Maxwell equation for the situation of near-field microscopy using a sub-wavelength aperture. Experimental result on unexpected high spatial resolution is reproduced by our computer simulation. The finite-difference time-domain (FDTD) method is employed to solve the three dimensional Maxwell equation for the situation of near-field microscopy using a sub-wavelength aperture. Experimental result on unexpected high spatial resolution is reproduced by our computer simulation. △ Less

Submitted 1 November, 1999; originally announced November 1999.

Comments: 2 pages, 3 figures

Report number: ICCP5, P2-26