[en] A spectroscopic measurement is performed for a plasma in a rotating radiation belt, which accompanies the complete divertor detachment recently realized in the Large Helical Device (LHD). The population distribution over the exited levels is experimentally determined from the Balmer series line intensities and is compared with the result of the collisional-radiative model calculation to determine the electron temperature T_e and density n_e. No reasonable pair of T_e and n_e is found when either the ionizing plasma or the recombining plasma is assumed. A good fitting is obtained under an assumption of the ionization balance plasma with T_e=1.8 eV and n_e=2 x 10²⁰ m^-3. These parameters are confirmed through analysis of a high-wavelength-resolution measurement for the Balmer series lines which clearly exhibit the Stark broadening effect. (author)

[en] Back-illuminated CCD detectors have been used for visible, VUV and x-ray spectroscopy. The results are compared with those generated by conventional detection systems such as the photodiode array with image intensifier or micro-channel plate. The use of the back-illuminated CCD demonstrated excellent qualities including low noise, a high level of spectral resolution and a greatly extended output dynamic range. Detailed results of the comparison are also discussed. (author)

[en] We have revised the collisional-radiative (CR) model code of neutral helium (T. Fujimoto, JQSRT 21, 1979). The spin-orbit interaction gives rise to mixing of the wavefunctions of the singlet and triplet states. The degree of the mixing depends on the magnetic field, and at the field strength of the level-anticrossings complete mixing, or complete breakdown of the L-S coupling scheme, occurs. We have approximately incorporated this effect into the code. We have reviewed the excitation cross section data for electron impacts. For transitions starting from the ground state, the recent assessment by the group led by Dr. de Heer is judged satisfactory. For transitions from the metastable levels the assessment by the same group appears rather conservative; there remains a question about the cross section values near the threshold. For transitions between different-l levels within the same multiplicity and same n, a semi-empirical formula based on the Born cross section gives a good agreement with experiment. Proton impacts are also considered for these transitions. We compare the new cross sections with those used in the original version. These cross sections for transitions starting from the metastable levels are fitted by analytical formulas and the parameter values are given. We also give parameter values for the excitation rate coefficient for these transitions as well as for transitions starting from the ground state. With all the above revisions incorporated into the CR model code, we have calculated the energy loss rates and the line intensity ratios for the purpose of plasma diagnostics, where the effect of a magnetic field is noted. The calculated population distribution over excited levels are compared with experiment, and a tentative conclusion is drawn concerning the excitation cross section from the metastable level. (author)

[en] Emission lines in the visible/UV wavelength ranges are observed with 80 lines of sight which cover an entire poloidal cross section of the plasma in the Large Helical Device. The emitted light is received with optical fibers having 100 μm diameter and is guided into a 1.33 m Czerny-Turner-type spectrometer based on spherical mirrors for collimating and focusing. A charge-coupled device having 13.3x13.3 mm² area size is used as the detector and the spectra from all the lines of sight are recorded perpendicularly to the wavelength dispersion. The spectrometer is equipped with optics located in front of the entrance slit to correct the difference between the meridional and sagittal focal points, and thus the astigmatism, which otherwise causes severe cross talk between adjacent optical fiber images on the detector, is corrected. Consequently, simultaneous spectral measurement with 80 lines of sight is realized. The Zeeman splitting of a neutral helium line, λ667.8 nm (2 ¹P-3 ¹D), which is caused by the magnetic field for plasma confinement, is measured with the spectrometer. Though the obtained line profile is in general a superposition of several components on the same line of sight, they can be separated according to their different splitting widths. The two-dimensional poloidal distribution of the helium line intensity is obtained with the help of a tomographic technique

[en] A spectroscopic measurement is made for the plasma in the rotating radiation belt accompanying the complete divertor detachment which is recently realized in the Large Helical Device (LHD). The Balmer series lines of neutral hydrogen are clearly observed. The population distribution over the exited levels is determined from the Balmer series line intensities and is compared with the result of the collisional-radiative model calculation to determine the electron temperature T_e and density n_e. No reasonable pair of T_e and n_e is found when either the ionizing plasma or the recombining plasma is assumed. A good fitting is obtained under an assumption of the ionization balance plasma with T_e=1.8 eV and n_e=2x10²⁰ m^-3. The mechanism to sustain such a low temperature plasma inside the high temperature confinement region is unclear. (author)

[en] Fully three-dimensional simulation of neutral particle transport is applied to analyses of the density profiles of neutral hydrogen in the plasma periphery and in the divertor region on the large helical device (LHD). For considering the three-dimensionally complicated geometry of the LHD plasma and the vacuum vessel, a detailed three-dimensional grid model is formed. In order to simulate the effect of the full toroidal geometry of the plasma, a new wall 'warp wall' is introduced. The calculations of three-dimensional profile of neutral densities in the grid model prove the availability of the warp walls for detailed analyses of neutral transport. The simulation strongly suggests that baffle plates installed in the inner region located between two helical coils in the vacuum vessel are cost-effective in achieving divertor detachment and efficient particle removal and control in the plasma periphery. (author)

[en] Results are described on NBI and ECH plasma response after hydrocarbon pellet injection in CHS. The hydrocarbon pellet injection indicated that the stored energy of the NBI discharge successfully increased for the pellet diameter of smaller than 0.32 mm, although other discharges with larger diameter of the pellet collapsed with a rapid density rise and a large increment of radiation loss. After the pellet injection the density profiles largely changed from flat and hollow profiles to peaked profiles for both cases of the NBI and ECH plasmas. These peaked profiles were maintained until the end of the heating pulse, whereas the edge density decreases. A radial position where the pellet penetrated and a fueling efficiency are also obtained with the pellet size and the target density. (author)

[en] Wall conditioning is essential for controlling hydrogen recycling and the amount of intrinsic impurities to achieve high-performance plasmas in magnetically confined fusion devices. A technique for wall conditioning, boronization, is coating the chamber walls with boron. To examine the effectiveness of boronization, it is essential to investigate where and how much boron is deposited onto the plasma-facing materials (PFM). BH spectral emission is a good diagnostic of boron deposition onto PFM. BH (or boron deuteride, BD) molecular bands have been confirmed in discharges after fresh boronization. Kawate et al. (2022 NucFu) performed impurity powder-dropping experiments (Nagy et al. 2018 RScI) with boron powders in the Large Helical Device toward real-time wall conditioning. Our spatially-resolved spectroscopic measurements of BH molecular bands suggest deposition and desorption of boron on the divertor plates.

[en] The hydrogen pellet injection has been expected as a fuel feeding method for thermonuclear fusion reactors. In the present paper, two elementary processes that govern particle feeding characteristics, ablation of solid hydrogen and homogenization of ablation plasma, are discussed. A solid hydrogen pellet is vaporized by heat flux from impinging ions and electrons in a hot plasma. A high-density plasmoid created from the solid hydrogen is strongly influenced by the confining magnetic field and is merged to background plasma. The production of ablation plasma from a hydrogen pellet differs so much from the plasma production by laser ablation. I addition to the physics of particles feeding by hydrogen pellet injection, diagnostic methods of the pellet ablation process and hydrogen pellet injection technologies are introduced. (J.P.N.)

[en] A detailed spectral profile of the Lyman-α line of neutral hydrogen, i.e., the transition from n = 2 to the ground state, where n is the principal quantum number, was measured with a vacuum ultraviolet spectrometer for the plasma in the Large Helical Device. Self-reversal was observed in the spectral profile when the plasma density was increased with repetitive injection of hydrogen pellets. A one-dimensional radiation transport model was used for creating the Lyman-α spectral profile, for which an emission and absorption medium with a slab geometry and constant plasma parameters were assumed. The population density of the n = 2 level generally has a peaked spatial profile even with constant ground state density because of the reabsorption effect which is essential for the emergence of self-reversal in the spectral profile. We used the n = 2 level population distribution in the medium by Molisch et al. [Radiation Trapping in Atomic Vapours, Oxford University, Oxford, 1998] and evaluated the Lyman-α spectral profile as a function of the optical thickness. The observed line profile was found to be well fitted, for example, with a ground state density of 5.2 × 10¹⁸ m^-3 and a medium thickness of 10 cm when a Lorentzian profile having a full width at half maximum of 0.0018 nm is adopted for the emission and absorption coefficients. (author)