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Development of Two-Color Laser Imaging Interferometer Using CO2 Laser and Quantum Cascade Laser in the Large Helical Device
Get PDF0000-0003-3930-4434CO2 laser interferometers are a promising option for high-density plasma measurements. However, in low- and middle-density measurements, noise due to mechanical vibrations is a serious problem. To remove this noise, we developed a two-color laser imaging interferometer using a CO2 laser and quantum cascade (QC) laser, called the CO2/QC laser imaging interferometer, through benchtop experiments and installed it in the Large Helical Device (LHD). Benchtop experiments provided optical design guidelines for the CO2/QC laser imaging interferometer to minimize the influence of the unstable output wavelength of the QC laser. The optical system in LHD was designed according to this guideline, and the vibration noise was successfully reduced to 2.80 × 1018 m-3. We also demonstrate measurement examples of hollowed and peaked electron density profiles evaluated using Abel inversion and macro-scale instability. This is the first study to present the measurement results of high-temperature plasma using a CO2/QC two-color laser interferometer. The study outcomes provide important insights for the development of two-color laser interferometers in future fusion devices.journal articl
Recent Progress on Identifications of Spectral Lines from Tungsten Ions in Low and High Ionization Stages Using Laboratory Plasmas for Fusion Research and Its Application to Plasma Diagnostics
No full text0000-0003-4139-7721Tungsten spectra in visible, vacuum ultra-violet (VUV) and extreme ultra-violet (EUV) wavelength ranges have been studied in LHD, EAST and HL-2A toroidal devices for magnetic-confinement fusion research. Tungsten spectra from low-ionized tungsten ions are observed in VUV and EUV ranges, e.g. W VI at 605.926 Å and W VII at 216.219 Å and 261.387 Å, and Doppler temperature of the W5+ ion and edge tungsten influx rate of the W6+ ion are analyzed. Ionization stages of unresolved transition array mainly consisting of W24+-W33+ ions at 15–70 Å are determined at each small wavelength interval of 0.1 Å based on radial profile measurement. The density profile is analyzed for W24+ and W27+ ions in addition to highly ionized ions, e.g. W44+ and W45+. Several magnetic dipole forbidden lines are observed, e.g. W XXVII and W XXVIII, and the density profile is also analyzed for W27+ ions. Contributions are made on tungsten diagnostics for fusion research through the present studies.journal articl
Hydrogen isotope effect on self-organized electron internal transport barrier criticality and role of radial electric field in toroidal plasmas
Get PDF0000-0001-5669-1937Self-organized structure formation in magnetically confined plasmas is one of the most attractive subjects in modern experimental physics. Nonequilibrium media are known to often exhibit phenomena that cannot be predicted by superposition of linear theories. One representative example of such phenomena is the hydrogen isotope effect in fusion plasmas, where the larger the mass of the hydrogen isotope fuel is the better the plasma confinement becomes, contrary to what simple scaling models anticipate. In this article, threshold condition of a plasma structure formation is shown to have a strong hydrogen isotope effect. To investigate the underlying mechanism of this isotope effect, the electrostatic potential is directly measured by a heavy ion beam probe. It is elucidated that the core electrostatic potential transition occurs with less input power normalized by plasma density in plasmas with larger isotope mass across the structure formation. This observation is suggestive of the isotope effect in the radial electric field structure formation.journal articl
Direct observation of the non-locality of non-diffusive counter-gradient electron thermal transport during the formation of hollow electron-temperature profiles in the Large Helical Device
Get PDF0000-0002-2983-5920A heating source with off-axis electron cyclotron heating (ECH) alone produced a plasma with a quasi-steady-state hollow electron-temperature profile in the Large Helical Device. The clear formation of this quasi-steady-state hollow electron-temperature profile can be explained by adding the outward heat convection term to the diffusion term, as a simple model to describe the electron heat flux, using the energy conservation equation. In addition, we directly observed the non-locality of the non-diffusive (convective) contribution in transient electron thermal transport in the condition that power-modulated on-axis ECH was applied to the plasma sustained by off-axis ECH. The experimentally evaluated flux-gradient relation shows two different positive values of the electron heat flux at zero temperature gradient by going back and forth between positive and negative temperature gradient regions in the transport hysteresis phenomenon.journal articl
Difference of co-extracted electron current and beam acceleration in a negative ion source with hydrogen-isotope ions
Get PDF0000-0001-9781-231XImprovement of the performance on a hydrogen/deuterium negative ion source for a nuclear fusion device is reported. In particular, the suppression of the co-extracted electron current, Ie, is an important issue to ensure the stable beam acceleration. Improvement of the Ie has been confirmed by optimizing the magnetic field of the electron deflection magnet in the extraction grid. Two other new methods for reduction of the Ie were validated. The first was an electron fence whose rods were set between the rows of apertures on a plasma grid. The electron and negative ion current ratio, approximately Ie/Iacc, was greatly improved from 0.7 to 0.25 in deuterium. The second was an outer iron yoke which enhanced the magnetic flux density 19% inside the arc discharge chamber. The Ie/Iacc using the outer yoke decreased by 0.1 compared with using a normal magnetic filter in a deuterium operation. These attempts have improved the total deuterium injection beam power of 8.4 MW by three negative ion based NBIs.journal articl
トリチウムによるDNA損傷のメカニズム――二本鎖切断の蛍光顕微鏡観察およびシミュレーション
No full textトリチウム（三重水素，3HあるいはTと表記）は，極めて低いエネルギーのβ線と反ニュートリノを放出する放射性の水素同位体である．自然界では地球に降り注ぐ宇宙線と大気との核反応により生成される．また原子炉でも生成される．生体試験用のトレーサーや蛍光物質を用いたライトなどにも利用されており，高純度のトリチウムは，核融合反応の燃料にもなる．福島第一原子力発電所の処理水中にも存在しており，社会的関心を集めている．
トリチウム由来のβ線の飛程は水中や細胞中で数ミクロン程度と短い．そのため，外部被ばくが問題となることはなく，内部被ばくに対する防護が重要となる．我々は，トリチウムが生体分子へ与える影響を計算機シミュレーションで解き明かすことにより，生体分子の損傷機構を明らかにすることを目指している．そこで，計算手法およびシミュレーション精度の確認のため，単純な系で生体分子の損傷速度を定量的に評価する実験技術の開発を進めている．
実験では，蛍光顕微鏡を用いたDNA一分子観察法により，トリチウム水中に浮遊するDNAの二本鎖切断メカニズムを定量的に明らかにしつつある．具体的には，滅菌環境下でトリチウム水およびトリチウムを含まない注射用水中におけるDNAの平均長さの経時変化を，蛍光顕微鏡で観察した．その結果，注射用水と比べて高濃度トリチウム水中では，DNA二本鎖切断が速やかに起こることがわかった．一方で，1 kBq/cm3程度のトリチウム濃度では有意な照射効果が見られないことを確認した．
トリチウムを含む化合物が生体内に取り込まれると，化合物中のトリチウムがDNA分子中の軽水素と置き換わることがある．このことは，メダカや大腸菌を使った実験で確かめられている．トリチウムに特有の壊変効果として，DNA分子中の軽水素に置換したトリチウムが3Heにβ壊変することによる化学結合の切断が挙げられる．法令による排水中の濃度限度（60 Bq/cm3）におけるトリチウムと軽水素の比はT/H=5×10－13と極めて小さく，置換トリチウムの影響が現れるとは考えにくい．一方で，「どの程度の濃度以上であれば置換トリチウムの影響が顕著になるのか？」という問いに対して，現時点では必ずしも明確な答えはない．そこで我々はトリチウムの壊変効果に着目し，DNAから置換トリチウムが除去されることに伴うDNA部分構造の変化を，分子動力学シミュレーションにより明らかにする．
我々の戦略として，まずDNAよりも分子構造の単純な高分子の計算から始め，続いてDNAの計算を行った．高分子の分子動力学シミュレーションの結果，除去される水素の割合が大きいほど，高分子の熱安定性と構造安定性が低下することがわかった．また，二重結合や共役結合の生成など，化学結合の変化を確認することもできた．さらに，テロメア二重らせんDNAの分子動力学シミュレーションの結果，グアニンのアミノ基中の水素が除去されることにより，水素結合が消失し二重らせん構造が崩れる様子を明らかにすることができた．
今後は，反応力場を用いた分子動力学シミュレーションにより，β壊変によるDNA二本鎖切断のメカニズムの解明といった展開が期待される．
本記事の長さは通常の「最近の研究から」欄記事の規定を超過しておりますが，編集委員会の判断によりこのまま掲載しています．journal articl
Self-Sustained Divertor Oscillation Driven by Magnetic Island Dynamics in Torus Plasma
Get PDF0000-0001-5669-1937A 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 observationsjournal articl
A Novel Approach for Data Analysis Based on Visualization of Phase Space Distribution Function in Plasma Turbulence Simulations
Get PDF0000-0002-1953-530XGyrokinetic simulations are important for analyzing magnetically confined plasmas. However, the data obtained from the gyrokinetic simulations are time-series of a five-dimensional phase space distribution function, making analyzing the transport phenomena extremely difficult because of its high dimensionality and large data size. We propose a novel method for analyzing such phase space distribution functions. First, the two-dimensional velocity space distribution function is mapped into the wavenumber space and visualized as an image. This enables us to easily capture the global features and the features of the individual velocity space distribution functions. Second, we apply similarity analysis based on the local features of images and cluster analysis based on distances between images and the velocity space distribution function. The proposed method enables us to automatically extract similar structures in the velocity space distribution function and quantify the duration of these structures.journal articl
Theoretical calculation of cesium deposition and co-deposition with electronegative elements on the plasma grid in negative ion sources
Get PDFWe studied the work function of cesium deposition and co-deposition with the electronegative element on the plasma grid (PG) using the first-principles calculations. The impurity particles may exist in the background plasma and vacuum chamber wall, and the work function of the PG will be affected. The results indicate that the minimum work functions of pure cesium deposition on Mo (110), W (110), and Mo (112) are reached at a partial monolayer. They are 1.66 eV (σ = 0.56 θ), 1.69 eV (σ = 0.75 θ), and 1.75 eV (σ = 0.88 θ), respectively. An appropriate co-deposition model consisting of cesium with electronegative elements can further decrease the work function. The coverage of cesium and electronegative elements are both 0.34 θ in all the co-deposition models. The F-Cs co-deposition model where the Cs atom and F atom are aligned along the surface normal obtains the lowest work function. They are 1.31 eV for F-Cs on Mo (110), and 1.23 eV for F-Cs on W (110), respectively. The change in work function is linearly related to the change in dipole moment density with a slope of −167.03 VÅ. For pure cesium deposition, two factors control the change in dipole-moment density, one is the electron transfer between adsorbates and the substrate, and another one is the restructuring of surface atoms. There are two additional factors for the co-deposition model. One is the intrinsic dipole moment of the double layer, the other is the angle between the intrinsic dipole moment and the surface. The latter two factors play important roles in increasing the total dipole moment.journal articl