To properly study plasma fluctuations and to develop new plasma processing technologies, precise calibration of probes is required. We exploit plasma fluctuations properties to develop a novel simple method for calibrating the radial position of a probe in a linear magnetized plasma device. We determine the probe radial position with respect to the plasma column axis by using a previously calibrated reference probe. This study provides the experimental evidence that plasma fluctuations can be used not only to investigate plasma physics but also to determine the radial position of probes.

Spatial profile of oxygen negative ions produced during the afterglow phase of an inductively coupled radiofrequency (RF) plasma is experimentally investigated, motivated by the development of a gridded ion thruster incorporating both positive and negative ion sources. It is found that the negative ion density exhibits a peak approximately 40–100 mm downstream of the inductive plasma source, depending on the working gas pressure. It is presented that the axial profile of the negative ions in the afterglow mimics the plasma density profile observed during the RF-ON phase, in which the peak of the positive ion density is formed downstream of the source exit as a result of charge exchange collisions of spontaneously accelerated positive ions.

A tomography system in the PANTA device found fluctuation pattern distortion during a cycle of the solitary wave oscillations, and global moment vector analysis quantified the degree of the distortion successfully. The method can be refined by help of Fourier-Rectangular Function Expansion to analyze local property of the distortion. This article present the method, called the local moment vector analysis, which can evaluate the local distortion of fluctuation patterns, with the results obtained by applying the local method to the solitary wave oscillations.

This article presents the first experimental results from a novel configuration in Heliotron J, which features a shallow magnetic well in the core of the plasma. A broadband coherent fluctuation of approximately 10 kHz is observed in a low-β electron cyclotron heated (ECH) plasma at rotational transform ι/2π = 0.55 in this configuration, while no mode appears in a deep magnetic well configuration. An electron cyclotron emission measurement shows the mode is excited in the core region where the magnetic well is shallow. This indicates Heliotron J is able to sustain viable plasmas with an edge-hill, although these can be destabilised even at low density and pressure.

Runaway electron (RE) driven kinetic instabilities have been observed in the QUEST spherical tokamak. A strong correlation between decay of the plasma current and activity of kinetic instabilities was first found with quasi-periodic behaviour during current quench. An efficient mitigation of RE by interaction with kinetic instabilities was experimentally demonstrated.

A non-evaporable getter (NEG) pump was experimentally installed in the QUEST spherical tokamak to enhance particle exhaust capability during long-pulse, high wall temperature operation (473 K). Activation and pumping performance tests were conducted using hydrogen gas injection and quadrupole mass spectrometry. During activation at 773 K, hydrogen desorption was clearly observed, whereas impurity species (mass number 18, 28, 32, 44) desorption remained strongly suppressed. Pumping tests at 473 K demonstrated hydrogen-selective exhaust with an effective pumping speed of approximately 150 L/s. These results demonstrate that NEG pumps can provide a controllable and impurity-compatible particle exhaust method in fusion-relevant environments. In particular, the suppression of impurity release during activation suggests the possibility of in situ regeneration without significant degradation of plasma conditions due to impurity contamination, enabling continuous operation without isolation. This operational flexibility highlights the potential of NEG pumps as assist tools for particle control in steady-state plasma operation.

Plasma experiments on electron cyclotron resonance heating (ECRH) using a high-power optical vortex beam (OVB) have been demonstrated for the first time in the Heliotron J stellarator/heliotron device. The OVB is generated by a spiral phase plate in a 70 GHz ECRH system. Plasmas are successfully sustained up to the second harmonic extraordinary (X) mode cut-off density. Virtually identical performance in terms of stored energy and density limits confirms the injection of the OVB as a heating tool comparable to Gaussian beams. These first results showed that the OVB is a possible method for the research on wave and plasma physics in magnetically confined fusion systems.

A high-speed floating potential measurement system has been developed for investigating turbulence at frequencies above the ion cyclotron frequency in the PANTA linear plasma device. The system is designed to have a phase-flat response up to 1 MHz, enabling accurate measurement of the phase relationship between floating potential and density fluctuations extending into the high-frequency regime. Initial results show that the attenuation of gain in the high-frequency range is significantly reduced compared to conventional measurements. A cross-phase analysis between the estimated electric field and density fluctuations shows a clear difference.

A new torus device named SOLEIL, equipped with a copper internal coil (IC), has been constructed and plasma experiments have been initiated. A unique feature of SOLEIL is that the magnetic field structure of the divertor tokamak plasma, with either a single null point or double null points, can be generated without plasma current. Various shapes of the last-closed flux surface (LCFS) can be formed including both positive and negative triangularity. The primary objective of the device is to study the dependence of the scrape-off layer (SOL) flow and impurity transport in the SOL and divertor regions on the LCFS shape. In the design of the device, it was intended to make the IC as compact as possible, to minimize its impact on the plasma confinement. Dismountable toroidal field coils were also designed to be compact taking into account space constraints. The main parameters are as follows: the major radius of the IC is 0.18 m, the minor radius of the IC cover is 0.031 m, the nominal IC current is 8 kA⋅turns, and the nominal toroidal field is 0.0875 T at R = 0.18 m. The assembly was completed in January 2025 and the first plasma was generated in February 2025.

Microscale turbulence drives not only particle and heat transport but also energy exchange between different particle species. Previous local gyrokinetic studies have shown that turbulent energy exchange can exceed collisional exchange in weakly collisional plasmas, and that ion temperature gradient (ITG) turbulence may hinder ion heating by alpha-heated electrons. In addition, it has been clarified that trapped electron mode (TEM) turbulence transfers energy from electrons to ions, thereby enhancing ion heating. In this work, we extend these studies by examining the impact of turbulent energy exchange on the global temperature profiles at a steady state using the one-dimensional transport solver GOTRESS. For the case of DIII-D discharge #128913, turbulent energy exchange has minimal influence on temperature profiles. However, in the case of enhanced electron heating in a DIII-D-like tokamak plasma, energy transfer from hot electrons to cold ions driven by TEM turbulence becomes comparable to, or even exceeds, the collisional contribution, leading to a significant increase in the ion temperature profile. For ITER Baseline and SPARC standard H-mode scenarios, the turbulent energy exchange is largely compensated by the collisional one, producing only small effects. These results indicate that the impact of turbulent energy exchange on the global temperature profiles in steady‐state conditions of future fusion reactor scenarios is expected to be negligibly small, although it can become significant in situations such as plasma start-up phases, where the heating power is strongly unbalanced between electrons and ions.

Ionization waves exhibiting chaotic oscillations were periodized by modulating the discharge voltage. The oscillations were made periodic by applying modulation to the discharge voltage using a square wave. Furthermore, the dynamic behavior when the duty cycle of the square wave was varied was investigated. The behavior of the transition threshold separating the chaotic and periodic states was investigated. A difference was confirmed between the modulation value causing the transition from the chaotic state to the periodic state when the amplitude of the modulation voltage (square wave) was increased, and the modulation value causing the transition from the periodic to the chaotic state when the amplitude was decreased. The degree of periodization of the orbit was quantitatively evaluated using the largest Lyapunov exponent and the CH diagram, which confirmed the transition from the chaotic to the periodic state. Furthermore, chaotic periodization was possible with a duty cycle close to 50%.

The so-called “ion-ion” plasma, which consists primarily of negative and positive ions, has been observed near the extraction aperture in negative hydrogen ion (H⁻) sources with a substantial amount of surface-produced negative ions, according to several experiments. In this study, to elucidate the characteristics of ion-ion plasmas with substantial surface H⁻ production, the relationship between surface-produced negative ion density and electric potential is investigated using three-dimensional Particle-In-Cell simulations with the KEIO-BFX code. The results reveal that the plasma in a negative ion source can be classified into four distinct regions based on the relationship between H⁻ density and electric potential: Region (I), where H⁻ follows the Boltzmann relation; Region (II), where H⁺ follows the Boltzmann relation; Region (III), a transition region between the plasma region and the beam acceleration region; and Region (IV), the beam acceleration region. In negative ion sources with surface-produced H⁻ ions, plotting the H⁻ density against the electric potential offers a useful method for identifying the plasma meniscus, which is crucial for optimizing extracted beam quality and ensuring proper operation of beam extraction systems.

In a two-ion-species plasma, a magnetosonic wave splits into two distinct modes: a low-frequency mode and a high-frequency mode. Nonlinear solitary waves corresponding to these modes can accelerate heavy ions in a plasma composed of hydrogen (H) and a heavier ion species (denoted by b). In this study, we theoretically examine the dependence of the heavy-ion acceleration on the heavy-ion mass (m_b) and the difference between cyclotron frequencies Ω_H and Ω_b. When the pulse amplitude is fixed, the heavy-ion acceleration weakens as m_b increases for both the low- and high-frequency-mode pulses. However, for the low-frequency-mode pulse, a maximum attainable amplitude exists, and this amplitude increases as the ratio Ω_H/Ω_b becomes larger. Consequently, in a plasma containing heavy ions with larger m_b, the low-frequency-mode pulse can have a larger amplitude, leading to the enhancement of the heavy-ion acceleration. These results indicate that the low-frequency-mode pulse can accelerate, for instance, oxygen (O) ions in an H-O plasma more effectively than helium (He) ions in an H-He plasma.

Understanding the interaction between plasma turbulence and zonal flow is essential for structure formation and transport regulation in magnetically confined plasmas. Using experimental data from the PANTA linear plasma device, we reduce the high-dimensional dynamics of numerous modes into two variables: turbulence and zonal. We then identify prominent intermittent bursts localized in the zonal-flow component—a phenomenon that, to the best of our knowledge, has not been reported previously. These bursts appear as impulsive, positive-going, low-frequency excursions superimposed on rapid background fluctuations. To address this non-stationarity, we investigate two approaches: a model-driven approach and a data-driven approach. We use a generalized Predator-Prey (P-P) model identified via an adaptive extended Kalman filter to track the evolution of time-varying physical parameters, and we also apply a Time-Varying Coefficient Vector Autoregressive model (TV-VAR) to achieve high-fidelity tracking of the sharp, transient burst dynamics. A distinct reorganization of the directed coupling structure is observed following burst events, including a transition to a regime in which zonal-flow regulation of turbulence becomes transiently dominant. These results indicate that time-varying interaction measures may provide informative features for burst forecasting and offer a basis for understanding regime transitions in transient plasma dynamics.

The electron cyclotron emission (ECE) diagnostics is planned to be installed in the CHD stellarator/heliotrons for electron temperature measurement. The radiation profile of ECE is simulated by raytracing code, TRAVIS. Due to the insufficient optical thickness, the radiation temperature can be deviate from the electron temperature. To recon­struct the electron temperature profile, we have developed the analysis method based on the Bayesian framework, ODAT-SE. The method examined for several scenarios, including the wall reflections and energetic electrons. We have accurately reconstructed the electron temperature profile in all scenarios if the electron density profile is priorly given.

The linear plasma device TPD-II was reactivated and upgraded to enable pulsed plasma and liquid-metal flow experiments. As an initial study, we performed pulsed plasma experiments using a capacitor bank system, focusing on the influence of transient recycling particles. Time-resolved measurements showed that the ion current at the target responded on a time scale longer than that of the pulse duration, attributed to slower transport of ions compared to that of electrons. In contrast, floating potentials responded on the same time scale as the pulse, reflecting the behavior of fast electrons. At higher pulse power, a transient drop in ion current and an increase in floating potential were observed after the input of pulse to the target, suggesting a rapid decrease in electron temperature and the onset of electron-ion recombination. Filtered high-speed imaging revealed enhanced He I emissions due to the recombination processes at the period. These results demonstrate the utility of the upgraded TPD-II in exploring transient plasma-neutral interactions relevant to divertor physics in future fusion reactors.

In this paper, an experimental consistency check for a theory describing the interplay between micro-scale turbulence and meso- and macro-scale flows is presented. The experimental data obtained in the JFT-2M tokamak are used for the consistency check. After L-H transition, a solitary radial electric field well, driving an E × B flow structure in the electron diamagnetic direction, forms. Turbulence is regulated in the flow shear regions, while it nearly remains essentially unchanged at the peak of the flow structure where the shear is zero. In a theory, this type of turbulence redistribution, called hill-trapping, is predicted to occur when the adiabatic parameter α is larger than unity. In the experiment, α ∼ 40 ≫ 1 is given, supporting the theory. An eigenmode model analysis for the turbulence system is performed for the experimental parameters, and the occurrence of the hill-trapping is found.

Understanding the interactions of multi-ion species is essential for analyzing particle transport in divertor plasmas. This study focuses on the development of a Coulomb collision operator utilizing the Nanbu collision algorithm for modeling Coulomb interactions among multi-ion plasmas composed of hydrogen isotope ions and electrons. Simulations initialized with a Maxwellian velocity distribution examine the energy relaxations and momentum exchanges among particle species. The simulation results confirm energy conservation and reveal energy relaxation patterns, with equilibrium timescales influenced by interspecies mass differences. Incorporating particles with unequal statistical weights improves computational efficiency, reducing simulation time while maintaining the accuracy of these relaxation dynamics. This approach accelerates the Coulomb interactions in multi-ion species plasmas, making it an effective tool in fusion plasma research.

This study investigates the influence of finite ion temperature on the linear growth rate of the parallel velocity gradient (PVG)-driven instability. Parallel (toroidal) flows are prevalent in magnetically confined fusion plasmas, where external momentum sources such as neutral beam injection (NBI) can act as a potential driver for PVG instabilities in both conventional and spherical tokamaks. Accurate transport modeling using quasi-linear models such as TGLF or QuaLiKiz requires a fundamental understanding of the characteristics of PVG modes in warm plasmas. The linear stability analysis is conducted to include the finite ion temperature effects. Depending on the radial profiles of ion temperature and parallel velocity, the dominant unstable modes can be categorized as either ion temperature gradient (ITG) or PVG-driven modes. These two instabilities are found to be mutually exclusive in their parameter spaces. Both modes become excited when the compression becomes negative. The ITG and PVG modes are each strengthened by the gradient that drives the other, up to the point where the driving gradient exceeds the instability threshold of the respective mode. In the PVG-dominant regime, increasing the temperature ratio enhances the compression and thereby exerts the stabilizing effects, whereas the temperature gradient contributes to PVG growth until the ITG thresholds is reached.

In this study, an isentropic scaling law is developed for direct-drive implosions of solid spherical targets—a setup frequently employed in Fast Ignition Realization Experiments—in order to estimate densities at maximum compression. Adiabatic theories usually cannot explain shock-driven compression due to entropy production; however, differences among similar implosion scenarios can be approximated by isentropic laws. Numerous one-dimensional implosion simulations support for the proposed isentropic scaling law, especially regarding the relationship between confinement time and density at maximum compression.

This study investigated the effects of heavy-ion beam (HIB) pulse waveform and irradiation time on the implosion process of a multilayer fuel target designed for heavy-ion inertial fusion (HIF). Achieving high beam intensity, beam quality, and precise timing synchronization is challenging owing to the space charge effect. Multilayer fuel targets with low-density foam layers may alleviate these beam condition requirements and help achieve robust, uniform implosion. Using a 1-D spherical coordinate hydrodynamic model, this study evaluated implosion dynamics and variation using the coefficient of variation. Results showed that multilayer targets offer enhanced robustness against beam irradiation time variations compared to conventional designs. The Pb pusher configuration consistently demonstrated superior stability, attributed to the high density and atomic number of Lead that influences transport and hydrodynamics. The foam layer also plays a crucial role in mitigating the adverse effects of timing fluctuations. These findings suggest that multilayer fuel targets, particularly with a Pb pusher, provide a compelling advantage for practical applications by offering improved robustness and simplifying parameter optimization procedures compared to conventional HIF targets.

Numerical simulations were conducted for a pyroelectric fusion device with three electrode geometries to investigate proton generation, acceleration, and impact with a solid ¹¹B target. Evaluation of the electric field distributions showed that the maximum potential scales with the crystal temperature difference as ϕ_max ≃ 238 ΔT [kV]. Analysis of electron-impact ionization clarified the spatial regions where protons are generated and begin to accelerate, revealing that a considerable fraction of protons is laterally deflected by the electric field structure and fail to reach the target. The proton impact rate was systematically examined by varying the electrode geometry and heating conditions. The results indicate that the impact rate is maximized at moderate temperature differences, whereas excessively strong electric fields shift the ionization region outward and reduce proton delivery to the target. Further analysis suggests that rapid electron acceleration near the electrode shortens the residence time within the energy range favorable for ionization, thereby suppressing proton generation under high heating conditions. These results demonstrate that proton impact efficiency in pyroelectric fusion devices is strongly governed by the interplay between electrode geometry, ionization location, and operating temperature. Appropriate control of these parameters is therefore essential for improving target-directed proton irradiation.

Hydrogenated amorphous carbon (a-C:H) films were deposited on silicon wafers using a pseudo-spark discharge (PSD) method. A mixture of argon and acetylene (C₂H₂) was employed as the process gas. PSD generates high-density plasma and forms a diffuse discharge due to the hollow cathode effect. The film structure was analyzed using Raman spectroscopy, while film hardness was measured using nanoindentation. Surface hybridization states and contamination levels were examined by X-ray photoelectron spectroscopy. This study investigates the optimal distance between the anode and the deposition substrate, as well as the optimal flow rate ratio using acetylene diluted with argon. The results indicated that the optimal substrate distance was 40 mm and the ideal flow rate ratio (Ar/(Ar + C₂H₂)) was 1%. Under these conditions, the a-C:H films exhibited a G-band position of 1,571 cm⁻¹, a full width at half maximum of 114 cm⁻¹ for the G peak, a deposition rate of 155 nm/h, and a hardness of 5.3 GPa.

In this study, we present a comprehensive description of a single crystal diamond-based detector (SCDD) system equipped with a ⁶Li thermal neutron converter and its radiation detection processes. Based on these processes, several pulse shape discrimination (PSD) methods applicable to the performance evaluation of breeding blankets are proposed. A basic PSD method, using either pulse width or charge integral alone, enables discrimination between gamma-rays and energetic ions produced by the ⁶Li(n,α)³H reaction. This method is therefore suitable for measuring the tritium production rate in a thermal neutron irradiation field. An advanced PSD method, utilizing both pulse width and a specific pulse shape index, effectively rejects gamma-ray signals and extracts pulses originating from fast neutrons and energetic ions produced by the ⁶Li(n,α)³H reaction. Post-analysis of the extracted signals demonstrates that both the tritium production rate and the fast neutron flux can be evaluated simultaneously in a mixed radiation field using a single SCDD. In addition to the intrinsic advantages of SCDD, such as compact size, excellent radiation hardness, and high temperature operability, the PSD methods developed in this study enable broader applications not only in breeding blanket performance evaluation but also across a wide range of scientific research fields.