Keisuke Fujii

Disentangling the complex interplay of neutral fueling and transport considerations in the edge plasma pedestal region can be informed by direct measurements of the energy-resolved local neutral densities from passive Dα spectroscopy. In this work, we employed the MAST upgrade charge exchange spectroscopy system passive background viewing fan to assess the feasibility of resolving both the higher energy thermal neutral component due to charge exchange reactions with thermal ions and the cold component due to edge neutral recycling. The spectral coverage, resolution, and dynamic range of the CCD adequately capture the entire Dα line profile. The outer midplane tangential viewing fan of 19 lines of sight provides good spatial resolution of the edge plasma region. To infer the 1D radial profiles of the thermal and cold neutral densities, we used a modified version of the FIDASIM Monte Carlo neutral modeling package to develop a forward model of the line integrated spectroscopic signals. In combination with a numerical optimization scheme, the model enables a direct comparison of the spectrally resolved Dα profiles to the experiment data for a given range of assumptions on the cold neutral densities in the scrape-off layer. Quantitatively good agreement is obtained in recovering the 1D cold and thermal density profiles in the edge pedestal region as well as the spectral Dα line shape details, with typically observed cold and thermal neutral densities at the outer midplane separatrix of ∼3–5 × 1015 and 5–7 × 1014 m−3, respectively, for high-field side fueled plasmas. Inboard of the pedestal region the model consistently underestimates the measured cold component line-integrated intensity, suggesting that further characterization of the viewing geometry, reflections impact and/or FIDASIM model assumptions, is needed.

Oak Ridge National Laboratory (ORNL) has been developing RF-driven, linear plasma devices for plasma–material interaction (PMI) studies. The physics basis for the Material Plasma Exposure eXperiment (MPEX), under construction at ORNL, was established through experimental measurements on the Proto-MPEX device. The Proto-MPEX device has recently been reconfigured as the “Proto-Lite” platform, with the removal of auxiliary electron and ion heating capabilities and the reduction of the number of magnet coils from 13 to 8. Proto-Lite is a risk-reduction platform for MPEX, where helicon wave RF-technologies can be tested, plasma scenarios developed, new workforce members trained, diagnostic systems explored, and control architectures vetted, in addition to short-pulse PMI target exposures. The diagnostic suite for Proto-Lite largely consists of systems that were ported from Proto-MPEX or that are being readied for deployment on MPEX. The current Proto-Lite diagnostic set includes (typical achieved range; data rate): double Langmuir probes (Te ∼ 1–20 eV, ne ∼ 1018–1020 m−3; 1 kHz), infrared camera imaging (T ∼ 20–150 °C; 50 Hz full frame), filtered visible camera imaging (1–18 kfps), neutral pressure gauges (P ∼ 10−4–10−2 Torr; 100 Hz), residual gas mass spectroscopy (1–100 amu), survey (400–900 nm; 1 Hz), and high-resolution optical emission spectroscopy (200 Hz). Plasma exposed samples are removed via a vacuum load-lock for ex situ analysis. This paper will give an overview of experimental measurements from Proto-Lite diagnostic systems and describe how those systems are contributing to risk-reduction research for MPEX.

We present a conceptual design for High-Dynamic-Range Spectroscopy (HDRS) to measure the penetration of neutrals into the confined plasma of the SPARC tokamak. HDRS detects these penetrating neutrals—hereafter core neutrals—in the far wings of the Balmer-series spectrum, rather than relying on spatial inversion, which is strongly affected by scrape-off layer (SOL) emission. These neutrals undergo charge exchange with high-temperature confined ions, producing significantly greater Doppler broadening than the SOL neutral emission. While this diagnostic concept has been applied to multiple devices, applying it to SPARC will pose unique challenges due to the device’s high density and strong magnetic field. Stark broadening at divertor densities exceeding 1021 m−3 and Zeeman splitting at the ∼12 T magnetic field both broaden the edge emission toward the wavelengths where the core-neutral emission appears, and the metallic first wall can reflect bright divertor light into core-viewing sightlines. To quantify these effects, we developed a framework that couples plasma backgrounds from the scrape-off layer plasma simulation-ITER code, OpenADAS atomic data, and Cherab/Raysect ray tracing and used it to produce line-integrated spectra for illustrative chords and derive preliminary spectrometer requirements. The synthetic spectra identify a Dα core-neutral shoulder visible around 1–3 nm away from the line center, embedded in the Stark tail of the cold edge emission; Dγ, by contrast, remains Stark-dominated and provides an independent constraint on the edge electron density. The resulting spectrometer concept targets a resolving power of 20 000–40 000, a 7.5–15 nm bandwidth, and a 105–106:1 dynamic range.

Ion cyclotron range of frequencies (ICRF) heating is essential for creating plasma in next-generation fusion devices. ICRF antennas often produce localized wall erosion, reducing reliability, material survivability, and overall plasma performance. We show that the wave physics' gyrotropy is among the causes of toroidal and poloidal asymmetries in these erosion spots, and that this source of asymmetry can be compensated for either by controlling the poloidal phasing or by modifying the limiter shapes.

The purpose of the 5th International Atomic Energy Agency technical meeting on fusion data processing, validation and analysis (FDPVA) (Ghent University, Ghent, Belgium, 12–15 June 2023) was to provide a platform during which a set of topics relevant to FDPVA were discussed with the view of meeting the needs of next step fusion devices such as ITER. The validation and analysis of experimental data obtained from diagnostics used to characterize fusion plasmas are crucial for a knowledge-based understanding of the physical processes governing the dynamics of these plasmas. This paper presents the recent progress and achievements in the domain of plasma diagnostics data analysis and synthetic diagnostics reported at the meeting, including concept description of new devices; fusion databases; integrated data analysis; inverse problems; uncertainty propagation, verification and validation; probabilistic methods and machine learning. The relevant results underline trends observed in the current major fusion confinement devices.

We present an application of the Portable Diagnostic Package (PDP) on the Wisconsin High-temperature-superconducting Axisymmetric Mirror (WHAM), which integrates an optical emission spectroscopy (OES) system and an active Thomson scattering (TS) system. The OES system facilitates a comprehensive impurity line survey and enables flow measurements through the Doppler effect observed on impurity lines. Plasma rotation profiles were successfully derived from doubly charged carbon lines. The TS system enabled the first measurements of the electron temperature in commissioning plasmas on WHAM. Notably, the PDP was installed, commissioned, and used to obtain OES and TS data within ∼6 months from project start, enabled by its designed portability and standardized interfaces. These results demonstrate the PDP’s potential to accelerate diagnostic readiness and advance experimental plasma studies.

Oak Ridge National Laboratory (ORNL) and Realta Fusion successfully executed a one-year collaborative effort to deploy the Portable Diagnostic Package (PDP) on the Wisconsin HTS Axisymmetric Mirror (WHAM) and to assess future diagnostic needs. The PDP consists of an optical emission spectroscopy (OES) subsystem and a Thomson scattering (TS) subsystem. Within the first two months, the PDP OES subsystem was installed, integrated with WHAM timing, and handed over to trained Realta personnel. It immediately delivered multi-channel impurity surveys and the first spatially resolved plasma-rotation profiles inferred from C III Doppler shifts. The TS subsystem was commissioned within the first six months; transportation of the class IV laser system, approval of the safety plan, and the integration with WHAM timing with 100 ns resolution. The TS subsystem provided the first localized electron temperature measurements in WHAM commissioning shots. Leveraging these results, the team has discussed a diagnostic concept map for Realta’s next-step facility, Anvil, identifying line-of-sight constraints, required temporal resolution, and hardware upgrades.

Hydrogen atoms penetrating deep inside high-temperature magnetically confined plasmas by repetitive charge-exchange collisions result in a particle source of the plasma, which affects the plasma transport significantly. In this paper, we present an approximate solution of the fluid equations for neutral transport and an analytical representation of the neutral penetration length, in a simplified plasma geometry. This analysis predicts a power-law decay in the Balmer-α line wings which reflects the velocity distribution of the neutral atoms, with the power-law exponent analytically represented as well. These scaling laws are compared with a simple Monte–Carlo simulation and spectroscopic observations of Large Helical Device plasmas. Since the Balmer-α line wings are experimentally accessible, our formulation opens the possibility to quickly estimate the neutral penetration length from spectroscopic observations.

Understanding the structures of the unresolved transition array (UTA) observed in extreme ultraviolet (EUV) spectra from many-electron atoms is crucial for various applications, including fusion science and nanolithography. To measure the fine structure of the UTA from tungsten and tin at around 5 and 13.5 nm at the Tokyo electron beam ion trap, we developed a high-resolution EUV spectrometer. The designed spectrometer achieves a resolving power of λ/dλ > 5000 at 5 and 13.5 nm. The fabricated large-area grating was experimentally examined at beamline BL5B of the UVSOR synchrotron facility to evaluate the diffraction efficiencies and their variation across the ruled area. The measured diffraction efficiencies are 0.65% ± 0.07% at 5 nm (second order) and 7.9% ± 0.2% at 13.5 nm (first order). The variation in the diffraction efficiency across the ruled area is 2.2%, 13.6%, and 10.0% in zeroth, first, and second order diffractions, respectively. The discrepancies in diffraction efficiencies between the experiments and the calculations were 5.2%, 29%, and 35% for the zeroth, first, and second diffraction orders, respectively.

Optimal elemental configuration search in crystal is a crucial task to discovering industrially important materials such as lithium-ion battery cathodes. In this paper we present application of quantum approximate optimization algorithm, the representative near-term quantum algorithm for combinatorial optimization, to finding the most stable elemental configuration in a crystal, using Au-Cu alloys as an example. After expressing the energy of the crystal in the form of the Ising model through the cluster expansion method combined with first-principles calculations, we numerically perform QAOA with three types of parameter optimization. As a result, we have demonstrated that the optimal solution can be sampled with a high probability for crystals containing up to 32 atoms. Our results could pave the way for optimal elemental configuration search in a crystal using a near-term quantum computer.