In magnetically confined fusion plasmas, the breaking of ‘magnetic flux-surfaces’ due to resonant magnetic perturbations (RMPs) can generate magnetic islands and alter field topology to significantly impact plasma confinement and transport. This work investigates the effect of magnetic islands on neoclassical radial energy transport within the core plasma of an analytic circular tokamak using the XGC-S global gyrokinetic particle-in-cell code. Findings from our simulations revealed substantial enhancements in electron neoclassical radial energy diffusivity in and around the islands, in addition to a newly observed two-peak structure at the O/X-points and outer island boundary in the electron diffusivity profile.

The ion temperature in an ECR plasma is determined from the ratio of wavelength-integrated spectra obtained using a spectrometer with sufficient time resolution. The ion temperature during the intermittent negative spikes in floating potential is extracted using the conditional averaging method. For the first time, we have successfully observed a decrease in ion temperature during intermittent events.

We evaluate the performance of Gaussian Process Regression (GPR) in estimating the current-voltage characteristics of a Langmuir probe, as well as its first and second derivatives. The results show good agreement between the estimated and measured data. When comparing GPR with the conventional Savitzky-Golay filter, we find that GPR is comparable to the Savitzky-Golay filter in terms of the accuracy of the estimated data. The uncertainty of the estimated data is also evaluated, and the results indicate that GPR underestimates the uncertainty of the electron current. This is likely due to the assumption of a homoscedastic noise model in the standard GPR.

In the linear plasma experimental device NUMBER, the densities of ions and atoms in helium plasma were estimated using passive spectroscopic measurements. The collisional-radiative models were employed to derive the densities of atoms and ions from the measured line intensities of each species. The estimated densities of helium species showed that (1) the atom density is one order of magnitude smaller than that estimated from gas pressure before discharge, (2) the density of singly charged ions is one order of magnitude smaller than that of atoms, (3) the doubly charged ions have a density 4 orders of magnitude smaller than the singly charged ions.

This study presents the first detailed investigation of confinement times in non-adiabatic traps during the axial injection of thermal plasma from the mirror edge, using particle trajectory calculations. Plasma is supplied from a coaxially positioned plasma source through an orifice with a ring-shaped aperture. The results of the analysis show that the longest confinement time occurs when the ring radius of the aperture is approximately equal to the ion Larmor radius at the mirror region. Under these conditions, the confinement time is found to be approximately four times the time it takes for particles to traverse the device length at thermal velocity.

We demonstrate the estimation of electrostatic potential fluctuations in dynamically varying Kelvin-Helmholtz turbulence using multi-scale convolutional neural network. The turbulence field is obtained from simulations based on a reduced fluid model in cylindrical magnetized plasmas. The target turbulence shows limit-cycle oscillations, and coherent and spiral structures are generated and annihilated repeatedly. High accuracy of the prediction is realized for the electrostatic potential field, and the estimation of the particle flux calculated from the predicted potential agrees with the answer with 98.4% accuracy. Behavior of the prediction accuracy is also discussed by changing the hyper parameters, such as the number of filters and the size of the training data.

The core-degradation effect due to tungsten (W) - a proposed plasma-facing material for the operation of future nuclear fusion reactors such as ITER and DEMO - calls for the study of sputtered W. In this study, sputtered W from a point source in the linear divertor plasma simulator, NAGDIS-II, was investigated. A hyperspectral imaging (HSI) camera was used to image the spatial profile of the sputtered W in helium and argon mixture plasma with different incident ion energies. By applying the Abel transform and fittings with several functions and comparing with the theoretical value, it is found that the ionization effect is not obvious but the geometrical spreading effect is dominant for the axial decay of the local W emission profile.

This Letter presents a Lagrangian formulation of reduced magnetohydrodynamics (RMHD) for Alfvénic fluctuations in a uniform background magnetic field. The RMHD equations are derived in Lagrangian coordinates through the least action principle. We also demonstrate that cross helicity conservation is naturally tied to fluid element relabeling symmetry.

Broadening of hydrogenic spectral lines is an important tool in spectroscopic diagnostics of various laboratory and astrophysical plasmas. We review recent analytical advances in five areas. First, we review a new method for spectroscopic diagnostics of tokamak edge plasmas based on a peculiar Stark broadening of hydrogen or deuterium spectral lines emitted by the injected neutral beam. Second, we review the analytical solution for the magnetic-field-caused narrowing of hydrogenic spectral lines under a circularly polarized electromagnetic wave. Third, we review analytical results concerning the Stark-Zeeman broadening of the Lyman-alpha line in plasmas. Forth, we review the effect of helical trajectories of electrons in strongly magnetized plasmas on the width of hydrogen/deuterium spectral lines. Fifth, we review recent analytical advances in the area of the intra-Stark spectroscopy: three different new methods, based on the emergent phenomenon of the Langmuir-wave-caused structures (“L-dips”) in the line profiles, for measuring super-strong magnetic fields of the GigaGauss range developing during relativistic laser-plasma interactions. We also review the rich physics behind the L-dips phenomenon – because there was a confusion in the literature in this regard.

A hydrogen secondary gas feeding experiment was conducted with hydrogen plasma. A rollover of the electron density and a monotonic decrease in the electron temperature were observed as the amount of the secondary gas increased. The vibrational distribution and temperature of ground electronic hydrogen molecules were evaluated based on the Fulcher-α band spectroscopy. To analyze the contribution of molecular activated recombination (MAR) to plasma particle loss, the reaction rates of the dissociative attachment (DA) and ion conversion (IC) of vibrationally excited hydrogen molecules were calculated. The reaction rate of IC was approximately two orders of magnitude greater than that of DA and significantly increased with the onset of the density rollover. The IC reaction rate remained high even as the electron density decreased. This analysis is limited to the first reactions of MAR; however, the significance of IC-MAR is strongly indicated.

The accuracy of magnetic field analysis including eddy current is important in MHD equilibrium reconstruction of tokamak plasmas. In a small toroidal plasma experimental device TOKASTAR-2, the eddy current calculations were done with an axisymmetric model of the vacuum vessel though its vacuum vessel has periodic three-dimensionality every 90 degrees of toroidal angle due to large horizontal ports. The three-dimensional (3D) eddy current magnetic field is evaluated by 3D magnetic field calculations using ANSYS for the first time in TOKASTAR-2. The results are compared with the conventional axisymmetric magnetic field calculation and measurements using magnetic probes located inside and outside of the vacuum vessel. The resistivity of the vacuum vessel model in ANSYS is modified to reduce the error from the experimental values. Using the developed model, the effect of the presence of the port on the eddy current magnetic field is evaluated. The results show that the toroidal-average of the eddy current magnetic field becomes smaller by presence of the ports but the non-uniformity in the toroidal direction is relatively small. This implies that the effects of the port would be introduced in an axisymmetric model by using poloidally nonuniform resistivity to suppress the eddy current on the midplane.

Predicting and preventing abrupt plasma termination incidents pose considerable challenges in nuclear fusion research. In the Large Helical Device (LHD), this occurrence is referred to as radiative collapse. During radiative collapse, impurity particles induce energy dissipation via radiation, hindering the maintenance of plasma discharges. Our approach aims to predict radiative collapse by analyzing the visible light emitted during such events. LHD uses approximately ten cameras to continuously observe plasma discharges, resulting in the accumulation of substantial video data from previous experiments. Using these images, convolutional neural network (CNN) models were trained to identify discharge states and subsequently applied to plasma discharge videos of the plasma discharges as a predictor. As a result, a determination model was developed, capable of discerning between stable and collapsed plasma discharge states with an accuracy of 91.5% ± 4% using plasma discharge images. Notably, this model demonstrated the potential to predict radiative collapse approximately three frames (66–132 ms) in advance. An examination of the model’s focal points revealed consistency with findings from prior research.

Based on the successful results of three 77 and two 154 GHz gyrotrons development and their contributions to large helical device (LHD) plasma experiments, a new 154/116 GHz dual-frequency gyrotron was developed. The optimal combination of cavity oscillation modes for dual-frequency oscillations at 154 and 116 GHz and optimal designs for the electron gun, cavity, mode converter, RF transmission mirrors, output window, and collector were determined. In an experimental test of the 154/116 GHz dual-frequency gyrotron, maximum powers of 1.66 and 1.34 MW were achieved at 154.05 and 116.15 GHz with pulse widths of 2.5 ms, respectively.

The transition condition from an attached state to a detached state of magnetic confinement plasmas has been investigated by a data-drive approach in LHD. This transition is defined as a binary classification problem of two states, and Support Vector Machine together with Exhaust Search has been applied. The boundary between detachment and attachment in the physical parameter space has been identified as a decision function comprising radiation and heating power, magnetic field and the resonant magnetic flux. While resonant magnetic perturbation (RMP) secures stable detached plasmas, it has been found that the featured parameter is not externally applied RMP itself but the plasma response to RMP. The present approach gives a robust separation boundary even for the extended operation with radiation enhancement by neon gas puff. Anomaly detection with a singular value decomposition has been also applied to the temporal behavior and identified pre- and post-relationships of each physical parameter in time. Emissions from carbon impurities with low ionization potential start to change prior to the RMP penetration and then the drop of ion saturation current, that is the transition to detachment, happens. These temporal sequences do not necessarily mean causality but are helpful for approach to physical inference.

We developed a bounce-time (BT)-based orbit-following Monte-Carlo code as an extension of the OFMC code in QST. In the BT-based method, we take a bounce time as a time step of the orbit following. The time step is ~ 100 times longer than the gyro period which is a typical time step for the conventional guiding-center (GC) method. In the BT-based method, an accurate and simple estimation of a poloidal projection of the bounce orbit and a staying time are essential. An expression for the orbit gives us an orbit shape by a small calculation with the difference of less than 1% of the minor radius, compared with the GC method with the same fast ion parameters. And an approximate expression for the staying time also gives us the staying time with a good accuracy for our purpose. We can see a good agreement between calculation results for the BT-based method and those for the GC method in an axisymmetric condition. The BT-based method is 70–140 times faster than the GC method, depending on the slowing-down time.

To optimize the design of a helical fusion reactor by varying the shape of the magnetic coils, several requirements related to the performance of the reactor should be satisfied under various constraints. To address this multi-objective optimization problem, we utilized Gaussian process regression (GPR) for machine learning to develop a surrogate model capable of predicting the dependence of the objective functions on the parameters representing the coil shape. This study demonstrates that the dependence of objective functions, such as plasma volume and the Mercier criterion, on the shape of helical coil windings can be predicted by GPR.

The Bohm criterion for two-ion-species plasmas in a scrape-off layer–divertor system is investigated using the anisotropic-ion-pressure plasma fluid scheme and the virtual divertor model, which does not require the boundary condition of the incident flow speed at a divertor target. The flow velocities of the two ion species at the target obtained from our fluid simulations agree well with those obtained from earlier particle-in-cell simulations for both collisionless and collisional cases. The so-called Bohm criterion can be self-consistently established without direct treatment of the sheath region. Examining the time evolution of the flow velocity at the target, we infer that the Bohm criterion is not related to sheath potential formation but is the stability condition for an equilibrium solution of the quasi-neutral plasma region.

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

To evaluate the delamination mechanism of the joint interfaces of a plasma-facing component, a new approach using the finite element analysis (FEA) applying the cohesive zone model (CZM) is proposed. The parametric study on the maximum traction τ_max, which is one of the principal CZM parameters, was conducted for compensating the lack of material data. Monotonic heat loading was applied to the surface up to 20 MW/m² in 1 second. The traction-separation law was assumed to be bilinear, which represents the relation between the representative crack stress and its opening displacement used for CZM. In the parametric study, three assumptions of τ_max were defined, (1) equal to the weaker bulk strength (Copper), (2) considering temperature dependency, and the average value of the strength ratio of the interface to bulk copper, and (3) considering as well as (2) but the lowest value of the ratio. Results of the parametric study suggest shear stress-governed (mode Ⅱ) delamination without vertical crack propagation in tungsten monoblock. Meanwhile, the joint interface shows compression, which means the interface remains in contact. Therefore, it is suggested that the degradation of cooling capability does not happen during the heating process unless vertical cracks in tungsten do not propagate into the interface.

A pulsed power discharge experiment was conducted to investigate the compositional effect on plasma parameters of a mixed gas plasma of argon (Ar) and helium (He) gases as the heavy and light species, respectively. As the plasma parameters, electron temperature, drift velocity, and ion density were estimated for different compositions of Ar and He. Electron temperature and drift velocity were estimated by line-pair and time-of-flight methods, respectively. Ion density was estimated by Faraday cup method. Line-pair method results obtained by Ar lines and He lines at each composition show that Ar and He are in different partial local thermal equilibrium (PLTE) states in the mixed gas. Different relaxation times between different atomic species confirm the deviation of LTE. Similar drift velocity estimated by Ar and He lines separately at each composition shows that the plasma is a homogenous mixture. Drift velocity decreases as the increment of the Ar percentage in the mixture since the average mass of the mixture increases.

Different forms of the Stark effect can affect the emission of line shapes in a plasma. Alongside the fluctuating microfield created by plasma ions and electrons, one often observes the fingerprints of oscillating electric fields. All these dynamic Stark effects result from random and collective motion of the particles but also from oscillating fields applied from outside the plasma by radiofrequency or laser sources. We here use a computer simulation to accurately capture the complex dynamics affecting the emission of a hydrogen atom in a laboratory or fusion plasma.

We recently reported the quantity values of line intensity for H₂O near 1.393 µm in a manner traceable to the International System of Units (SI). This paper briefly describes how to determine the reliable values of the line intensity, mainly focusing on the SI-traceability.

The temperature dependent shape of quasi-continuum unresolved transition array (UTA) spectra from highly charged heavy ions has been examined based on the experimental spectra recorded in the Large Helical Device plasmas. The observed spectral shape of the n=4–4 UTA emission strongly depends on the electron temperature especially for the lanthanide elements with the atomic numbers of 63 - 66. As the temperature decreases, the UTA position moves to shorter wavelength and the UTA bandwidth becomes narrower. Eventually, characteristic narrowed spectra with the lines of palladium-like and silver-like ions are observed at the lowest peak temperature of a few hundred eV. The temperature dependence of the UTA shape can be explained by the change in ion abundance and the wavelength distributions of the weighted transition probabilities calculated with the Flexible Atomic Code (FAC). A collisional-radiative modeling of the narrowed spectrum for terbium ions is tried based on the FLYCHK code and the FAC. As a result of slight intentional shifts of the calculated line positions, the measured spectrum matches qualitatively with the simulation for the electron temperature of 230 eV.

We investigate extreme-ultra-violet (EUV) emission from laser-produced tin plasmas for its application to microlithography. Strong emission occurs through 4d-4f and 4p-4d transitions, which appear as an unresolved transition array (UTA) due to the effect of configuration interaction (CI). Emissions from 8 to 13 times ionized tin overlap in the same λ=13.5 nm band, and both singly and multiply excited states of each ion contribute to the emission. We develop the collisional radiative model of tin ions, taking into account an appropriate set of atomic states that have a large population to contribute to the emission. The model is being validated through comparisons between the calculated and observed spectrums.

The Stark broadening of a Be II line (1s²3d ²D– 1s²4f ²F, 467.339 nm) under a magnetic field is evaluated with the divertor plasma of ITER in mind. The electron and ion perturbers are treated in the impact and static approximations, respectively. The perturbation term due to the magnetic field is included in the static approximation. The results show that the Stark broadening comes to be significantly large when the density is higher than 10²¹ m^－3, and the ion temperature would be overestimated if the Stark broadening is not taken into account.

We observed the EUV emission spectra from laser-produced tungsten plasmas. The spectral structures were compared by optical thickness due to the critical density difference of 1 × 10²¹ and 4 × 10²¹ cm^－3 with laser wavelengths of 1064 nm and 532 nm, respectively. We found some spectral structure changes, an increase of emission in 1 - 3 nm region under an optically thick condition while a decrease of a peak near 5 nm for unresolved transition array of 4d - 4f transitions. Some 1.3 - 2.5 nm peaks were attributed to charge states higher than W³⁰⁺. We showed some dependence of the spectral behavior for the EUV emission.

Tungsten spectroscopic studies have attracted much attention, because tungsten will be used as a plasma-facing component in ITER and future DEMO reactors. However, spectral line data of tungsten ions in low to intermediate charge states, such as W⁸⁺-W²⁶⁺, is still lacking. To accumulate spectral line data of W⁸⁺-W²⁶⁺, it is very important to identify the charge state and transition of Unresolved Transition Array (UTA) spectrum, as well as discrete line spectrum. In this study, we investigated electron temperature dependence of a UTA spectrum around 300Å for an advanced understanding of spectral line shape. As a result, the UTA spectrum contains W¹⁷⁺- W²⁷⁺ and emission line from 5s-5p transition and its satellite line from 5s²-5s5p transition are strongly emitted. It was suggested that the UTA spectrum around 300Å will be useful for diagnostics of ITER edge plasma.

Spectroscopic studies of emissions released from tungsten ions combined with a pellet injection technique have been conducted in the Large Helical Device for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. Emission lines were explored for the observation of low to intermediate charge states in the range of W¹⁰⁺ to W²⁰⁺, and the line spectra of W¹³⁺ were observed for the first time in fusion plasma experiments. The wavelengths of the observed W¹³⁺ lines were 243.1 Å, 247.6 Å, 248.3 Å, and 249.1Å in the extreme ultraviolet wavelength range, and all of them were emission from the 4 f ¹³5s² - 4 f ¹³5s5p transitions.

To optimize the plasma performance of fusion devices using tungsten plasma-facing components (PFCs), it is crucial to study the transport of tungsten impurities. This work focuses on the observation of line emissions from various charge states of tungsten ions in high-temperature plasmas utilizing Extreme Ultraviolet (EUV) spectrometers. Line emissions from W²⁶⁺-W³²⁺ ions are observed in the 45-55Å band at electron temperatures below 2.0 keV, while line emissions from W³⁴⁺-W⁴⁵⁺ ions appear in the 55-70Å band at electron temperatures above 3.0 keV. Additionally, radial profiles of W-UTA spectra in plasma with different electron temperature show that as the electron temperature decreases from 5.0 keV to 3.0 keV, the peak position of W²⁶⁺-W³²⁺ions move inward from ρ~ 0.5 to ρ~0.2. The line intensity profiles of W⁴²⁺-W⁴⁵⁺ ions accumulate within a narrow region of plasma core, specifically at ρ < 0.4. This study provides essential experimental data to support further research on tungsten impurity transport, control of tungsten content, and the enhancement of plasma performance.

Complex spectra of tungsten (W) and molybdenum (Mo) unresolved transition arrays (W-UTA and Mo-UTA) are frequently observed with isolated W, Mo and copper (Cu) lines at 20-150Å wavelength range in EAST plasmas with limiter configuration. The high-Z impurity ions of W, Mo and Cu enter the plasma through an unavoidable plasma-wall interaction at inboard Mo first wall and outboard W/Cu limiter. The spectra were observed with a fast-time-response extreme ultraviolet (EUV) spectrometer and line identification was carefully performed referring our previous works. As a result, W-UTA at 46-64Å and Mo-UTA at 68-96Å are found to be composed of W²⁷⁺-W⁴⁵⁺ and Mo¹⁶⁺-Mo³⁰⁺ ions, respectively. The Cu spectra from Cu¹⁰⁺-Cu¹³⁺ ions are also found in wavelength range of 135-150 Å. In the analysis radial profiles of the impurity spectra are taken into account in determining the charge state in addition to the time behavior.

We described the spectral behavior of the water-window soft x-ray emission in detail by dual laser pulse irradiation. We also observed the spatial separation of the soft x-ray emission and fast ions produced using dual pulse irradiation. Soft x-ray emission distribution was almost isotropic, and the fast ions were emitted to the target normal along the pre-pulse laser axis. The soft x-ray emission was maximized at a delay time of 50 ns between the pre-pulse, and the main laser heating pulse when the pre-plasma was irradiated at a distance of 50 µm above the target.

In double pulse cross-laser irradiation configuration, we measured the extreme ultraviolet (EUV) spectral purity and emission energy around 6.x nm wavelength in laser produced Gd plasmas. It was found that the spectral purity of 4.1% within a 0.6% bandwidth (Δλ = 6.74 - 6.78 nm) at 6.76 nm to the total emission between wavelengths of 5 - 9 nm was improved by producing a low-density Gd plasma target, compared to the spectral purity of 1.6% for solid-Gd target plasma. A low-density plasma must be produced before the main heating laser pulse to enhance the spectral purity. We also reported the delay time dependence of the spectral purity irradiated by the 6-ns, 1-µm main laser pulse irradiation under the cross-laser-injection scheme.

We demonstrated the efficient water-window extreme ultraviolet (EUV) source using a regenerative liquid metal target irradiated by a 10-Hz, 150-ps, solid-state laser at a wavelength of 1.064 µm. A regenerative liquid bismuth (Bi) target with a diameter of 30 µm was continuously injected into a vacuum. The number of photons was about 1 × 10¹³ photons/(sr · shot) in the spectral region from 2.3 to 4.4 nm, which was attributed to be n = 4 – n = 4 (Δn = 0) and n = 4 – n = 5 (Δn = 1) transitions. The fast ion was also observed with a maximum energy of 140 keV from a hot, dense Bi plasma.