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Three-dimensionally localization of radiative cooling due to nitrogen (N2) seeding for divertor detachment was detected experimentally. Since the localization along some magnetic field lines induces toroidal asymmetry of heat load reduction on divertor plates, it should be avoided for fusion reactors. The three-dimensionally localized structure was extracted using Principal Component Analysis (PCA) from two-dimensional radiation images measured with an InfraRed imaging Video Bolometer (IRVB). By applying PCA to 34 images each in N2 seeded plasmas with toroidally-asymmetric heat load reduction and in neon (Ne) seeded plasmas with toroidally-symmetric heat load reduction, a radiation feature in N2 seeded plasmas was found as one of the principal components (PC). The three-dimensional transport code EMC3-EIRENE indicated that the ionization in one of the divertor legs is enhanced in nitrogen seeding compared with Ne seeding due to the difference in the first ionization energy. The magnetic field lines from the divertor leg were along the extracted radiation structure and were terminated by the divertor where the heat load decreased due to the N2 seeding. These results indicate that three-dimensionally localized structure of radiative cooling was detected experimentally., source:https://doi.org/10.1016/j.nme.2022.101294, identifier:0000-0003-1586-1084

We report the observation of arcing damage on the diagnostic shutter during the glow discharge wall conditioning in LHD. The diagnostic system has no experience of plasma discharge produced by electron or ion cyclotron resonance heating or neutral beam injection. The arc trails were observed on the aluminum surface but not on the stainless steel although both materials were exposed to the glow discharge with the same duration. The difference in work functions between two materials may be a cause to divide the conditions of arcing ignition., source:https://doi.org/10.1585/pfr.16.1202061, identifier:0000-0001-6090-5010

Extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) wavelength spectra including line emissions released from neon (Ne) ions ranging from low to high charge states observed simultaneously in a single discharge are summarized for contribution to compile a fundamental spectral dataset for the Ne-seeded divertor heat load reduction experiments in Large Helical Device (LHD). NeIX and NeX lines were observed in the EUV wavelength range of 10∼50 Å and NeIII-NeVIII lines were observed in the VUV wavelength range of 400∼1000 Å. The temporal evolutions of the line intensities exhibited different behaviors between the edge emissions of NeIII-NeVIII with the ionization potential, Ei, of 63∼239 eV and the core emission of NeX with Ei of 1362 eV. NeIX with Ei of 1196 eV exhibited a marginal behavior between the edge emission and the core emission., source:https://doi.org/10.1585/pfr.16.2402006, identifier:0000-0002-1171-8603

From the viewpoint of the development of molecular response systems, stimulus-induced switching of multi-conformational/multi-configurational overcrowded ethylenes are interesting, whose properties could be manipulated by understanding the detailed isomerization paths. Anthraquinodi-methane (AQD) ring-flip is usually a very fast process, and thus less studied experimentally. Herein, we studied AQDs with dibenzo- and tribenzocycloheptatrienylidene units, which have large enough steric hindrance to retard the AQD ring-flip to allow determination of the Delta G(not equal) value experimentally. Their thermal isomerization was also scrutinized using the artificial force induced reaction method to elucidate the intermediates. Based on the structural unsymmetry in a newly prepared AQD, one of the isomers that undergoes a reversible conformational change via AQD ring-flip was isolated and analyzed by X-ray for the first time.

Turbulence spreading into the edge stochastic magnetic layer induced by magnetic fluctuation is observed at the sharp boundary region in the large helical device. The density fluctuation excited at the sharp boundary region with a large pressure gradient does not propagate into the boundary region due to the blocking of turbulence spreading by the large second derivative of the pressure gradient. Once the magnetic fluctuation appears at the boundary, the density fluctuation begins to penetrate the edge stochastic layer and the second derivative of the pressure gradient also decreases. The increase of density fluctuation in this layer results in the broadening and reduction of the peak divertor heat load. It is demonstrated that magnetic fluctuation plays a key role in controlling the turbulence spreading at the boundary of plasma which contributes to the reduction of divertor heat load., source:M. Kobayashi, K. Tanaka, K. Ida, Y. Hayashi, Y. Takemura, and T. Kinoshita Phys. Rev. Lett. 128, 125001 – Published 23 March 2022, source:https://link.aps.org/doi/10.1103/PhysRevLett.128.125001, identifier:0000-0002-0990-7093

In order to explore the compatibility of good core plasma performance with divertor heat load mitigation, the interaction between cold edge plasma and core plasma transport, including the edge transport barrier (ETB), has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with resonant magnetic perturbation (RMP) field application. The RMP application introduces a widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens, the resistive pressure gradient-driven MHD mode is excited, which degrades the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (Wp) unchanged, showing clear core-edge coupling. After a gradual increase of density fluctuation during the MHD activity, a spontaneous increase of Wp and the recovery of ETB are observed while the detachment is maintained. Then, the coherent MHD mode ceases and ELM-like bursts appear. In the improved mode, impurity decontamination occurs, and the divertor heat load increases slightly. Key controlling physical processes in the interplay between core and cold edge plasma are discussed. A comparison between deuterium and hydrogen plasmas shows that hydrogen plasmas exhibit similar features to the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination towards higher confinement, etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at a higher radiation level., source:Citation M. Kobayashi et al 2022 Nucl. Fusion 62 056006, source:https://doi.org/10.1088/1741-4326/ac42f3, identifier:0000-0002-0990-7093

In the Large Helical Device, the divertor detachment has been attempted by application of resonant magnetic perturbation (RMP) field and Ne gas puffing in electron cyclotron resonance- heated discharges for compatibility of high central electron temperature and low divertor heat load. Two kinds of divertor detachment phases were observed. The first one appeared transiently just after the Ne gas puffing (1st detachment), and the second one appeared steadily in the latter half of the discharge (2nd detachment). Space-resolved extreme ultraviolet spectroscopy revealed that NeVI-NeVIII emissions increased slightly outside the last closed flux surface (LCFS), while NeIX and NeX emissions increased inside the LCFS in the 1st detachment phase. Although in the 1st detachment the divertor heat load was significantly reduced, the central electron temperature also decreased because the Ne ions were penetrated inside the LCFS as a radiation source. In the 2nd detachment phase, NeVI-NeVIII emissions increased outside the LCFS while NeIX and NeX emissions kept low intensity inside the LCFS. In this phase, low divertor heat load and high central electron temperature were obtained simultaneously because the Ne ions were localized outside the LCFS as a radiation source. The profile measurements of Ne emission show that the edge island structure created by the RMP application impacts on the impurity emission distribution, where the peak of the emission shifts radially stepwise as the detachment proceeds., source:https://doi.org/10.1585/pfr.17.2402022, identifier:0000-0002-1171-8603

Divertor detachment using higher-Z (Kr) and/or lower-Z (Ne) is investigated. Using Kr+Ne superimposed seeding, plasma radiation could be enhanced at the upstream region in the edge plasma with suppression of impurity accumulation toward the core plasma. Here, the pre-seeded Kr emission was drastically enhanced after the subsequent Ne seeding. Moreover, the detachment using Kr+Ne seeding could be stably sustained while the detachment using single species seeding is short-lived. Reduction of edge electron temperature due to Ne seeding can promote the Kr emission at the upstream region. It indicates the availability of multi-species impurity seeding with different cooling rate., source:https://doi.org/10.1585/pfr.15.1402051, identifier:0000-0003-1586-1084

Feasibility of an ion sensitive probe was evaluated in high electron density (> 5 × 1019 m−3) helium plasmas produced in the Magnum-PSI device. The ion sensitive probe showed that the ion temperature was ∼ 1 eV and almost equal to the electron temperature. Increasing the neutral pressure to efficiently lead to the electron-ion recombination processes, the electron currents were collected at the ion collector. A secondary electron emission in the guard electrode might have an effect on the ion current., source:https://doi.org/10.1585/pfr.14.1202135, identifier:0000-0001-6090-5010

A new type of self-sustained divertor oscillation is discovered in the Large Helical Device stellarator, where the peripheral plasma is detached from material diverters by means of externally applied perturbation fields. The divertor oscillation is found to be a self-regulation of an isolated magnetic field structure (the magnetic island) width induced by a drastic change in a poloidal inhomogeneity of the plasma radiation across the detachment-attachment transitions. A predator-prey model between the magnetic island width and a self-generated local plasma current (the bootstrap current) is introduced to describe the divertor oscillation, which successfully reproduces the experimental observations, source:T. Kobayashi, M. Kobayashi, Y. Narushima, Y. Suzuki, K. Y. Watanabe, K. Mukai, and Y. Hayashi Phys. Rev. Lett. 128, 085001, source:https://doi.org/10.1103/PhysRevLett.128.085001, identifier:0000-0001-5669-1937