Tohru Mizuuchi

The impact of helical magnetic ripples on the transport of plasma and carbon impurities in the scrape-off layer (SOL) of a Heliotron J device was investigated using the EMC3-EIRENE code. Comparisons with a simplified one-dimensional fluid model revealed that the transport behavior varied with the Knudsen number ( K ), defined as the ratio of the ion–ion collisional mean-free path to the ripple scale length. In the weakly collisional regime ( K ≫ 1 ), the qualitative trends suggested by the fluid model indicate that the magnetic mirror effects become dominant, the conductive heat flux governs the heat transport, and the impurity density follows a Boltzmann distribution. Conversely, in the collisional regime ( K ≪ 1 ), the transport exhibits nozzle-like fluid behavior, convective heat flux governs heat transport, and impurities couple to the bulk plasma. These characteristics underscore the importance of charged-particle transport along magnetic field lines in the SOL of fusion devices, where magnetic ripples fluctuate significantly with changes in the magnetic field configuration.

The divertor structures of Heliotron J were investigated by analyzing the three-dimensional magnetic field and plasma transport simulations. The magnetic field of Heliotron J typically exhibits a characteristic resonant structure, where the stable and unstable manifolds emanating from specific X -points determine the divertor legs and the boundary of a confined core region. Regions zoned according to the divertor helical period provide information about the plasma; the boundary of the shadowed region determines the plasma-occupied volume, and the first multi-fold layer determines the fluid acceleration in an island-like configuration. The divertor legs guide the transport of charged particles outside the core. Density accumulates, and the fluid velocity decreases at the intersection points of the legs. The particle and heat depositions on the target plate can be inferred from the lobe structures and their relative toroidal locations on the divertor legs. We found that a chaotic divertor structure can potentially reduce the peak particle and flux densities by creating multiple strike lines, which is preferred for the design of future devices.

We develop a systematic configuration-search methodology for the Heliotron J helical-axis heliotron, combining a Bayesian exploration approach with field-line tracing and evaluation of magnetic field structural parameters. The methodology enables efficient exploration of an enormous coil current parameter space ( ∼ 10 11 possible combinations) and is consistent to VMEC equilibrium calculations. As a demonstration, the methodology is applied to search three magnetic configurations: σ = + 1 (uniform ripple bottom), σ = − 1 (uniform ripple top), and quasi-omnigenous configurations. The σ = + 1 exploration successfully identifies configurations with a factor-of-two reduction in ε eff 3 / 2 , associated with an increase in the bumpiness (toroidal mirror) component. The σ = − 1 exploration yields configurations with worse ε eff 3 / 2 ,as expected, providing as a contrasting reference to the σ = + 1 case. A practical omnigenous configuration was not found, suggesting that the inherent toroidicity cannot be overcome through coil current optimization alone. The developed framework provides a practical methodology for systematic magnetic configuration search within a single device, providing the way for element-wise experimental studies of how individual magnetic field components affect confinement in stellarator/heliotron devices.

Magnetic islands are formed at the plasma edge in Heliotron J, a stellarator/heliotron device, by controlling the rotational transform, and the effect of the magnetic islands on the poloidal flow profile was investigated using a Q-band Doppler reflectometer. Comparative measurements were performed for the magnetic configurations with magnetic islands at the plasma edge, referred to as ‘island configuration,’ and those without the magnetic islands, referred to as ‘non-island configuration.’ In all configurations, poloidal flow in the electron diamagnetic direction was observed at the edge in electron cyclotron heated plasmas. In the non-island configuration, the sheared flow maximum was inside the last closed flux surface (LCFS), whereas in the island configuration, the sheared flow was formed around the magnetic island independent of the LCFS position, and increases in electron temperature, T e , and density, n e , were observed at the edge. The fine control of the magnetic island position around the LCFS shows that the poloidal flow structure, particularly the flow shear position, correlates with the position of magnetic islands. An increase in T e and n e is observed at the edge only when the magnetic islands are located outside the LCFS. These results indicate that the edge magnetic islands are a possible means to control the poloidal flow shear position and the edge plasma parameters.

In this study, we observed a distinct low-frequency (LF) fluctuation in the n/m = 4/9 configuration under electron cyclotron heating (ECH) and neutral beam injection heating—conditions favorable for the L–H transition in Heliotron J [Sano et al., Nucl. Fusion 45, 1557 (2005)]. This LF mode, with a frequency of approximately 14 kHz, was detected during the L–H transition using Doppler reflectometry (DR), electron cyclotron emission (ECE), magnetic probes (MP), Dα emission, and edge divertor probes (DP). It modulates edge turbulence amplitude and extends radially, with toroidal and poloidal mode numbers of n/m = 1/3. It is found that the LF mode detected by ECE is correlated with the perturbation of the Dα emission, suggesting the mode contributes to the cross-field edge transport. At the L–H transition, while the LF mode persists, modulation of edge turbulence is suppressed, as evidenced by the reduced amplitudes in Dα emission and DP signals. These findings reinforce the importance of turbulence in the edge transport modulation induced by the LF mode.

In magnetically confined fusion reactors, the Zeeman effect can sometimes be utilized in spatially resolved spectroscopy to localize the observed line emission. To apply this method to divertor plasma spectroscopy, the relative magnitude of the Zeeman splitting to linewidth for hydrogen atomic emission lines in the visible to the near-infrared range was evaluated. It was found that the magnitude becomes the largest for the Paschen-α (Paα) emission line (1875 nm, 3–4). The observability of the Zeeman splitting on the Paα emission line spectrum was confirmed by an experiment using a hydrogen pellet ablation plasma in Heliotron J, which is relevant to the divertor plasma in terms of magnetic field strength (1.2 T), electron temperature (∼1 eV), and electron density (3.6 × 1020 m−3).

A synthetic diagnostic of x-ray bremsstrahlung radiation and a Monte Carlo radiation transport simulation were carried out to obtain the electron energy distribution in stochastic acceleration experiments in Heliotron J, a mid-sized heliotron-type magnetic confinement device. Three sets of LaBr3(Ce) scintillator and photomultiplier tubes were installed in Heliotron J in three directions relative to the magnetic field line, co, counter, and perpendicular, to determine the velocity distribution of the high-energy electrons. These systems are positioned about 5 m away from the vacuum chamber and shielded by lead blocks and magnetic shields to reduce the influence of stray radiation and magnetic fields. The vacuum chamber of Heliotron J is made of stainless steel with a 3D helical shape. Since its x-ray shielding effect is not negligible when obtaining the x-ray energy distribution in the vacuum chamber, the Monte Carlo radiation transport code particle and heavy ion transport code system was applied to Heliotron J to clarify the shielding effect. Given the vacuum chamber and coil shape information and x-ray energy distribution expected in the vacuum chamber, this code can calculate x-ray energy distribution considering the shielding effect of Heliotron J. This simulation model was based on computer aided design data of Heliotron J devices. The x-ray energy distribution in the vacuum vessel was adapted until the simulated and measured x-ray spectra outside the vacuum vessel match with each other. The shapes of resultant distribution have two types of shape: power-law distribution at more than 450 keV and Maxwell distribution at less than 450 keV. At a higher energy range, x-ray bremsstrahlung energy distribution is consistent with the characteristic of stochastic acceleration.

In this study, we observe avalanche-like electron thermal transport in Heliotron J under plasma conditions that exhibit power degradation to the central electron cyclotron heating (ECH). The newly installed GHz sampling electron cyclotron emission diagnostic allows for the observation of the long-distance radial propagation of electron temperature fluctuations. We find that the electron temperature fluctuations are associated with avalanches, because the Te fluctuations (i) propagate from the core to the edge with a speed comparable to the diamagnetic drift velocity, (ii) exhibit a 1/f power-law scaling in the frequency spectrum and a Hurst exponent close to 1, and (iii) dominate in the ECH deposition location and spread to the edge as the heating power increases. Furthermore, the electron heat avalanches can spread to the scrape off layer region when they are enhanced, i.e., the Te fluctuations correlate with the Dα emission, which also has a frequency spectrum that scales with a 1/f power law.

A signal separation system is constructed on the multi-pass Thomson scattering system of Heliotron J to solve the problem of overlapping scattered light signals for the electron temperature anisotropy measurement. The phenomenon of overlapping scattered light signals is relieved by operating the signal separation system. A Raman scattering experiment is undertaken to verify the separation effect of the signal separation system. Two scattered light signals corresponding to two adjacent incidences of one laser shot were extended to 104 ns. Moreover, we applied the multi-pass Thomson scattering system with signal separation system to the electron temperature anisotropy measurement. No anisotropy was observed within the error bars in the initial experiment.

We report the development of a new interferometer with two stable, high-power, 320 GHz solid-state sources in Heliotron J. A heterodyne Michelson interferometer optical scheme is employed. Two solid-state oscillators are utilized as sources with a fixed frequency at 320 GHz and frequency tunable of 312–324 GHz. Quasi-optical techniques are used for beam transmission. The beam is elongated in the vertical direction with two off-axis parabolic mirrors and injected into the plasma as a sheet beam for the multi-channel measurement (>5 ch.). Passing through the plasma, the beam is reflected at a retroreflector-array installed at the vacuum chamber wall. The retroreflector-array is a bunch of retroreflector structures, which can suppress the beam refraction caused by plasma without much space inside a vacuum chamber unlike a single retroreflector and can facilitate the system design. The source, detectors, and the retroreflector-array are tested to evaluate their basic performance on a tabletop experiment.