To diagnose energetic and bulk ions in fusion plasmas, we have developed a millimeter-wave collective Thomson scattering (CTS) diagnostic system utilizing a megawatt-class gyrotron, originally designed for electron cyclotron heating (ECH), in the Large Helical Device (LHD). This paper presents CTS measurements of energetic and anisotropic ions produced by neutral beam injection (NBI), along with a method for analyzing CTS spectra. In addition, we report on the development of a 303 GHz gyrotron intended for CTS diagnostics, which enables lower background radiation levels and improved signal-to-noise ratio (SNR) in the detection of scattered signals.which is expected to reduce background radiation and improve the signal-to-noise ratio in CTS measurements. Furthermore, a high-performance notch filter has been developed to block stray radiation from the gyrotron beam within the vacuum vessel, preventing it from entering the receiver as noise. This type of notch filter is employed not only in CTS measurements but also in electron cyclotron emission (ECE) and correlation ECE diagnostics. This paper outlines these developments and their application to energetic ion diagnostics in LHD plasmas.
This article reviews the technological details of a 2-m Schwob-Fraenkel soft X-ray multichannel spectrometer (SOXMOS) which has been installed in the Large Helical Device (LHD) to study impurity behaviors. Experimental setup of the SOXMOS operated in the duo-multichannel detector mode is described together with the peripheral equipment and remote control systems customized for the LHD. The absolute wavelength is carefully calibrated using the positions of reference lines based on the two different fitting methods. The data acquisition and registration are completely automated, and the secondary physical data resulting from the wavelength calibration and the peak detection are also made available for all the LHD users. The SOXMOS data have been widely used for various research topics relevant to impurity transport as well as atomic physics.
This study employs an integrated analytical hierarchy process (AHP) and SWOT analysis to systematically evaluate strategic pathways for Chinese traditional oil and gas companies that are transitioning to the fusion energy sector. By constructing a hybrid AHP-SWOT model, the relative importance of internal strengths and weaknesses along with external opportunities and threats is quantitatively assessed. The weights of each factor are determined and the priority of strategic decisions is derived accordingly. The results indicate that traditional oil and gas companies in China have significant advantages in engineering capabilities, financial strength, and completeness of the industrial chain, while policy support and demand for energy transition provide crucial external opportunities. Based on these findings, differentiated strategic recommendations are proposed to offer actionable insights for corporate decision-makers and policymakers in the context of the China energy transition.
The LHD experiment started in 1998 at the National Institute for Fusion Science and has continued for nearly 30 years. The experimental data obtained in these experiments are stored as physical data in the Kaiseki Data Server and made available to the public using an open data server. Through the experience of developing and operating this server, this paper discusses the problems that can occur in data management systems in long-term projects such as the LHD experiment and how they can be improved.
The STEP Programme began in 2019 with the mission to deliver a UK prototype fusion energy plant, targeting 2040, and a path to commercial viability of fusion. The STEP Prototype Plant (SPP) aims to demonstrate power plant characteristics by producing at least 100 MW_e net electrical output, being fuel self-sufficient, demonstrating high grade heat production, and a route to commercial availability. In this paper we give an overview of the concept design stage and discuss lessons learnt that are applicable to wider fusion power plant design. Concepts were initially generated using a combination of systems and equilibrium codes, that gave staring points for more detailed study and evaluation. From 69 different concepts explored, a down-selection was made to the SPP-1 baseline design. Further development identified challenges around the lifetime of the centre column magnet and the realisation of the blanket. This led to a pivot to the new SPP-2 design, increasing the major radius from 3.6 m to 4.275 m to increase the centre column lifetime from 0.22 FPY (Full Power Years) to 0.6 FPY , and changing from liquid lithium to a solid lithium oxide breeder blanket. The STEP programme will now look to increase the design definition and confidence as it moves to preliminary design.
A comprehensive numerical investigation was performed to optimize neon (Ne) soft X-ray emission (Ysxr) from a spherical plasma focus (SPF) device using the Lee code. As a prerequisite for predictive modeling, the code was benchmarked against the experimentally measured discharge current waveform of the 135 kJ SPF (N. V. Zavyalov et al., 2013). In this numerical study using ‘equivalent straight length’ technique referred to ‘shortcut’ simply fits a straight length equivalent to the curved run-down length of this SPF with the Lee code (for cylindrical electrodes), the computed current trace is first fitted to the measured one at 14.3 Torr deuterium-tritium (D-T) gas and the obtained best-fitted values of the model parameters are found as f_m=0.022, f_c=0.55, f_mr=0.14, f_cr= 0.72 . These values were then adopted in a systematic optimisation study of the pinch-plasma conditions. The simulations covered Ne fill pressures from 0.5 to 7 Torr, anode and cathode radii at fixed ratios of b/a=1.8125 and b/a=1.41 , and charging voltages from 15 to 30 kV. In all the cases, the plasma pinch temperature was constrained to the characteristic Ne emission window of (2.3–5) × 106 K. For each electrode geometry the code identified a unique combination of fill pressure and charging voltage that maximized Ysxr. At moderate electrode sizes and bank voltages of 25 kV the SPF is predicted to deliver peak Ne soft X-ray (SXR) yields exceeding 5 kJ per shot with efficiencies greater than 4
The electron cyclotron heating (ECH) system has played a key role in plasma heating and control since the earliest stages of the Large Helical System (LHD) project. In this paper, we will show the history of the strategic installation of gyrotrons, which are the core components of the ECH system, into the LHD and the progress of increasing ECH power. In the LHD, total injection power has been increased by repeatedly installing and replacing gyrotrons, and in particular, 1 MW-class gyrotrons, which began operation in 2007 and have led to a significant increase in the ECH power of the LHD. For these gyrotrons, the frequencies of 77–154 GHz were selected to initiate and heat the plasma under a magnetic field strength of 2.75 T for the standard magnetic configuration in the LHD. With the installation of the gyrotrons and a significant increase in millimeter wave power per tube, due to the stepped anode voltage rising, simultaneous ECH injection power into the LHD reached 5.6 MW.
The kinematic viscosity of reversed-field pinch fusion plasmas plays an important role in the physical understanding of helical self-organized states. Unlike previous studies that relied on correlation-based methods and simulations, this work employs Bayesian model comparison to objectively rank candidate viscosity models, accounting for model complexity and goodness-of-fit using data from the RFX-mod experiment. The Bayesian approach, exhibiting several advantages over conventional model comparison tools, reveals a clear preference for the perpendicular viscosity model.
Fabrication of multilayered plasma facing components (PFC) in fusion devices is associated with the production of joints between dissimilar materials. This overview discusses existing techniques for joining beryllium to steels, common fabrication issues and possible solutions. The following two primary joining techniques are discussed in detail: hot isostatic pressing (HIP) and brazing. An evaluation of the mechanical properties of the joints is provided, with limited comparison where relevant.
Experiments in the HIDRA stellarator show that the inferred lithium net-loss suppression factor Reff, which absorbs effects from redeposition, recycling, and slow evaporation, remains between 0.91 and unity across hydrogen and helium plasmas. This implies a lithium lifetime far longer than would be expected from the evaporation rate alone. Lithium surface temperature locking appears only intermittently, and high-temperature operation consistently drives lithium release that overwhelms the available fueling. As a result, the plasma transitions quickly to a lithium-dominated regime, preventing sustained operation in vapor-shielding-relevant conditions. Despite HIDRA’s relatively low densities and particle fluxes, these findings highlight that unmitigated lithium vapor readily contaminates the confined plasma, underscoring the need for active capture or control to achieve stable lithium-based plasma–material interaction regimes.
Direct measurements of nuclear reaction cross sections are not feasible for all target nucleus mass regions and incident particle energies due to experimental difficulties, such as stable target availability, energy resolution, and reaction threshold constraints. In such kind of situation, nuclear cross sections must be evaluated using empirical systematics or theoretical model calculations. In this work, a new empirical formula is presented for the calculation of (n,3n) reaction cross sections induced by 14–15 MeV neutrons, developed within the framework of the statistical model with inclusion of the reaction Q_n -value. It is obtained through a least-squares fit to experimental data for even–even heavy nuclei in the mass range 146≤ A≤ 238 . The results of the present empirical formula are compared with both experimental data and TALYS 1.95 code calculations. The systematic analysis presented in this paper provides a reference for the (n,3n) reaction channel, particularly in view of the lack of literature data at neutron energies of 14–15 MeV.
Research on the simulation of energetic particle driven instabilities in the Large Helical Device (LHD) has been actively conducted in recent decades. The progress achieved has been substantial since the last comprehensive review by Todo et al. [29]. Since then, many new simulations have been conducted to investigate the energetic particle driven instabilities in LHD. The simulation studies are mainly focused on the energetic particle driven geodesic acoustic modes (EGAMs), covering both linear properties and nonlinear frequency chirping, as well as the energy channel and anomalous bulk ion heating. In addition, there are also notable studies on the Alfvén eigenmodes (AEs). The energetic particle transport caused by AEs is investigated in different ways. The interchange modes are also simulated under different conditions. In addition to the published works mentioned above, there are also new findings. Both the low-frequency and high-frequency branches of the EGAM are investigated, and various types of energetic particle velocity distribution functions are modeled with different charge-exchange loss rates. The transition between the low-frequency and high-frequency branches of EGAM is determined by the slope of the energetic particle velocity distribution. The low-frequency branch is excited under the condition of a slowing-down distribution, while the high-frequency branch is excited with a bump-on-tail distribution. Furthermore, the bulk pressure perturbation and the energetic particle pressure perturbation are in anti-phase for the low-frequency branch, while they are in-phase for the high-frequency branch.
Understanding turbulence behavior in high-temperature plasmas is crucial for elucidating the mechanisms behind anomalous transport phenomena. The correlation electron cyclotron emission (cECE) technique is widely used to measure electron temperature fluctuations associated with turbulence in high-temperature plasmas. A new radial cECE system has been installed in the Large Helical Device (LHD), incorporating tunable bandpass filters (BPFs). Effective spectral decorrelation was obtained by increasing the center frequency separation between two BPFs. As an initial result using the cECE system, relatively low-coherence electron temperature fluctuations were observed in the LHD, with an estimated fluctuation level below 1
Suppressing the magnetohydrodynamic (MHD) effect in the liquid tritium breeder is key to ensuring the safe and stable operation of the tritium breeding blanket in a fusion reactor.To address this issue, this study proposes a novel strategy for regulating the MHD effect by doping with solid insulating particles.Based on the finite element method, a three-dimensional rectangular duct MHD model is constructed, in which solid insulating Al₂O₃ particles are assumed to be uniformly distributed in the liquid lithium-lead alloy and interact with the velocity field and the electromagnetic field.By varying the particle quantity and the applied magnetic field strength, the effects of particles on the velocity field, electric potential distribution, and MHD pressure drop are simulated and analyzed.The results show that the introduction of solid insulating particles significantly optimizes the flow characteristics.Under a magnetic field of 2 T, the velocity reduction after adding 128 alumina particles with a diameter of 5 mm is 24.7
This study evaluates the mechanical safety of the armour and helium inlet in the CFETR Central Solenoid Model Coil (CSMC). The analysis considers multi-physics coupling fields during steady-state operation. We adopted an electromagnetic-structural indirect coupling method consistent with ASME analytical design criteria and ITER magnet design specifications. A simplified electromagnetic model was first established in ANSYS Maxwell. We then used a current filament mapping approach to transfer the electromagnetic forces to the structural model. Static structural analysis was performed to obtain the stress distribution under multi-field coupling. The results, evaluated through stress linearization, show that the Nb3Sn armour and helium inlet meet the safety margins required by ASME and ITER standards. The NbTi armour also satisfies the design requirements, provided its yield strength is increased through cold work hardening. The simulation results indicate that the current CSMC design is adequate under the condition of multi-field coupling at steady operation state. These findings provide a technical reference for the engineering of the CFETR CSMC and the design of cooling interfaces for Cable-in-Conduit Conductors (CICC) in large-scale superconducting magnets.
We have constructed collisional-radiative (CR) models for tungsten ions, Wq+, with q = 18–25, including ionizing and recombining processes as well as collisional excitation/deexcitation and radiative decay, and applied the models to analyze extreme ultraviolet (EUV) spectra measured in magnetically confined fusion plasmas with a tungsten pellet injection into the Large Helical Device (LHD). The CR models are constructed as hybrids with fine-structure levels for lower states and relativistic configuration-averaged levels for higher states to include autoionizing states. The dielectronic recombination process is treated as dielectronic capture to the autoionizing states and radiative decay from the autoionizing states in the CR model. We calculated EUV spectra of the tungsten ions for ionizing and recombining plasmas. Measured spectra at the 2–4 nm region were used to estimate the charge state distribution of tungsten ions in LHD plasmas by comparing calculated emission peaks of n = 4–5 transitions of Wq+ with q = 22–25. Using the estimated ion distribution, the synthesized spectral profile at 4.5–7 nm shows characteristic profiles similar to the measured so-called unresolved transition array (UTA) due to n = 4–4 transitions. We also synthesized the spectra at 10–35 nm, where the UTA due to n = 5–5 transitions appeared, and we partly reproduced the UTA with synthesized spectra. The CR models for the tungsten ions were validated with the measured spectra of LHD plasmas.
This study reports the development of CO_2 laser interferometers for electron density measurements on the large helical device (LHD). Two types of interferometers using approximately 10 μ m are developed. One is an imaging two-color laser interferometer (I-TCI) for measuring density profiles and macroscopic fluctuations of MHD instabilities, and the other is a single channel phase-modulated dispersion interferometer (PMDI) designed for reliable density monitoring. The diagnostic principles, systems, and analysis techniques are outlined, and representative measurement results are presented. The I-TCI system was also designed to operate in the deuterium experiments performed on LHD from 2017 until 2022. Shielding against neutron and γ -ray irradiation was necessary to prevent damage to the I-TCI detection system. Design strategies for neutron and γ -ray shielding are presented in the appendices.
We derive an empirical scaling law for the external RMP (Resonant Magnetic Perturbation) amplitude required to completely suppress the resistive interchange MHD instabilities of m/n = 1/1 (m and n are the poloidal and toroidal mode numbers, respectively), which are typically observed in the Large Helical Device (LHD). We also derive the scaling law for the penetration threshold of the external RMP. Both scaling laws are found to have a strong dependence on the beta value and the normalized Larmor radius at the resonant rational surface. On the dependence on the collision parameters, the scaling laws for the external RMP amplitude to suppress the instability and the RMP penetration threshold show an inverse dependence. The above scaling laws suggest that it is easier to completely suppress the resistive interchange instability of the m/n = 1/1 mode in the LHD without degrading the plasma confinement performance due to the RMP penetration with a lower collision frequency, even with the same beta discharges.
Experiments on physics issues in burning plasma were conducted in the deuterium campaign of Large Helical Devices (LHD). One is an isotope mixing experiment, and the other is the collisionless energy transfer from energetic particles to bulk ions. The important finding for ion mixing in the LHD experiment is that the ITG turbulence (but not TEM turbulence) contributes to the isotope and ion mixing, which is beneficial for controlling the isotope ratio (deuterium and tritium) and helium ash exhaust. The experimental identification of energy transfer from energetic particles to bulk ions through Landau damping and transit-time damping suggests a possible ion-heating process for the ion-ITB plasma with ( T_i/T_e> 1 ) through collisionless energy transfer from alpha particles to bulk ions through energetic particle-driven instability, so-called alpha channeling.
This paper revisits and extends previous simulation studies of pressure-driven magnetohydrodynamic (MHD) stability in high-beta plasmas in the Large Helical Device (LHD). In the conventional MHD model, resistive ballooning modes become unstable at low magnetic Reynolds numbers, while ideal interchange modes become unstable at high magnetic Reynolds numbers. Although these instabilities are initially destabilized in the peripheral region, their nonlinear development causes their impact to extend into the core region, ultimately resulting in core collapse. In contrast, kinetic-MHD hybrid simulations that include kinetic thermal and energetic ions demonstrate that the instabilities remain confined to the peripheral region, allowing high-beta plasmas to be sustained in agreement with experimental observations. These findings highlight the critical role of kinetic ions in accurately modeling the stability and confinement of high-beta plasmas in three-dimensional magnetic configurations.
