arXiv subjects

Masaki Nishiura

Publications and source records attributed to Masaki Nishiura.

6 recordsLinked to original sources

arXiv2026

Review of LHD-ECRH activities and beyond

The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) played a key role in the progress of helical fusion research from the first plasma in 1998 to the final discharge on December 25, 2025. This paper presents a historical review of ECRH activities on LHD, highlighting the stepwise advances in hardware, including gyrotrons, transmission lines, and launcher systems, together with the exploration of advanced plasma regimes such as overdense heating and long-pulse steady-state operation. Beyond plasma heating, the ECRH system was also used as a high-power probe for advanced diagnostics, including collective Thomson scattering (CTS) as well as correlation electron cyclotron emission (CECE), and as a flexible actuator for transport studies and real-time control experiments. Finally, we discuss how the technological assets and operational expertise accumulated over the nearly three decades of the LHD program are being further developed and applied to next-generation helical devices, particularly CHD and CHD-U, as well as to broader gyrotron and millimeter-wave applications.

physics.plasm-ph↗

arXiv2026

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

We 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 (RT-1) device, where log-GPT outperforms existing methods, standard GPT, and the Minimum Fisher Information (MFI) methods in terms of reconstruction accuracy. The results highlight the effectiveness of nonlinear GPT for imposing physical constraints in applications to an inverse problem.

physics.plasm-ph↗

arXiv2025

High performance beam transport with multi-stage acceleration system and its application to plasma potential measurement in fusion plasmas

In accelerators, ion beams are often accelerated using electrostatic accelerating tubes. This paper reports on a method to improve the beam transport efficiency without adding new components to the beam transport system. High beam currents often suffer from a beam loss in a transport line. When Au negative ion beams are injected into a tandem accelerator, numerical simulations of low-energy ion beam transport have found that the beam loss increases significantly when the Au or Cu negative ion beam current exceeds 100 $μ$A due to space-charge effects. We found that the transport efficiency is significantly improved by remaining constant beam energy accelerated at the multi-stage accelerator tube and by providing an electrostatic lens effect. In the heavy ion beam probe system (HIBP) system of the Large Helical Device (LHD) for plasma potential measurement, the negative ion beam current injected into the tandem accelerator could be increased by a factor of 3.6. As a result, the output of Au$^+$ beam current at the tandem accelerator used to measure the LHD plasma potential was increased from about 3 $μ$A to 12$ μ$A and it was demonstrated that the average electron density in the plasma could be measured up to $1.75\times10^{19}m^{-3}$. This method is effective and widely applicable to improve the performance of low-energy heavy-ion beam transport systems output from the first stage of tandem accelerators and ion sources by adding a lens effect to the multi-stage accelerator tubes.

physics.acc-ph↗

arXiv2023

Valley filters using graphene blister defects from first principles

Valleytronics, which makes use of the two valleys in graphenes, attracts considerable attention and a valley filter is expected to be the central component in valleytronics. We propose the application of the graphene valley filter using blister defects to the investigation of the valley-dependent transport properties of the Stone--Wales and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition from the $\mathbf{K}$ valley to the $\mathbf{K}^\prime$ valleys is completely suppressed in some defects. Using a large bipartite honeycomb cell including several carbon atoms in a cell and replacing atomic orbitals with molecular orbitals in the tight-binding model, we demonstrate analytically and numerically that the symmetry between the A and B sites of the bipartite honeycomb cell contributes to the suppression of the intervalley transition. In addition, the universal rule for the atomic structures of the blisters suppressing the intervalley transition is derived. Furthermore, by introducing additional carbon atoms to graphenes to form blister defects, we can split the energies of the states at which resonant scattering occurs on the $\mathrm{K}$ and $\mathrm{K}^\prime$ channel electrons. Because of this split, the fully valley-polarized current will be achieved by the local application of a gate voltage.

cond-mat.mes-hall↗

arXiv2016

Anisotropy in broad component of H$α$ line in the magnetospheric device RT-1

Temperature anisotropy in broad component of H$α$ line was found in the ring trap 1 (RT-1) device by Doppler spectroscopy. Since hot hydrogen neutrals emitting a broad component are mainly produced by charge exchange between neutrals and protons, the anisotropy in the broad component is the evidence of proton temperature anisotropy generated by betatron acceleration.

physics.plasm-ph↗

arXiv2015

Observation of particle acceleration in laboratory magnetosphere

The self-organization of magnetospheric plasma is brought about by inward diffusion of magnetized particles. Not only creating a density gradient toward the center of a dipole magnetic field, the inward diffusion also accelerates particles and provides a planetary radiation belt with high energy particles. Here, we report the first experimental observation of a 'laboratory radiation belt' created in the Ring Trap 1 (RT-1) device. By spectroscopic measurement, we found an appreciable anisotropy in the ion temperature, proving the betatron acceleration mechanism which heats particles in the perpendicular direction with respect to the magnetic field when particles move inward. The energy balance model including the heating mechanism explains the observed ion temperature profile.

physics.plasm-ph↗