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Experimental study on boron distribution and transport at plasma-facing components during impurity powder dropping in the Large Helical Device
Get PDF0000-0002-1021-0322Toward real-time wall conditioning, impurity powder dropping experiments with boron powder were performed in the 22nd experimental campaign of the Large Helical Device. To examine the deposition and desorption process of boron, we focus on boron hydride (BH) molecules which presumably populate near plasma-facing components. We performed spatially-resolved spectroscopic measurements of emission by boron ions and BH molecules. From the measurement, we found that BH and B+ were concentrated on the divertor viewing chord, which suggest boron deposition in the divertor region. By comparing Hγ emissions with and without boron injection, neutral hydrogen shows uniform reduction in the SOL region, whereas less reduction of neutral hydrogen is confirmed in the divertor region. Although emissions from BH and B+ increased linearly, emissions by B0 and B4+ became constant after the middle of the discharge. Continuous reduction of carbon density in the core plasma was confirmed even after B0 and B4+ became constant. The results may show reduction of hydrogen recycling and facilitation of impurity gettering by boron in the divertor region and thus effective real-time wall conditioning.journal articl
A comprehensive evaluation of the thermal neutron detection efficiency by a single crystal CVD diamond detector with a LiF thermal neutron converter
Get PDFORCID 0000-0003-0920-2154Thermal neutron detection efficiency of the single crystal CVD diamond detector (SDD) equipping 1.9 μm-thick 6LiF thermal neutron converter, which induces the energetic ions through the 6Li(n,α)3H reaction, was evaluated comprehensively using a charge-integral based pulse shape discrimination (PSD) method and radiation transport calculation by PHITS. The PSD method applied in this work successfully separated the pulses induced by energetic tritons, alpha particles and other types of radiation. The detection efficiency of tritons was consistent with the radiation transport calculation, although that of alpha particles was lower than the estimation. The overall thermal neutron detection efficiency was obtained by summing detection efficiencies of tritons and alpha particles. Approximately 29% of 6Li(n,α)3H reaction occurred in an effective 1.9 μm-thick 6LiF thermal neutron converter could be detectable as the pulses of tritons (∼ 73%) and alpha particles (23%).journal articl
Reliable long-term operation of superconducting bus lines for the LHD
Get PDFThe Large Helical Device (LHD) is an experimental device for helical type fusion plasma in National Institute for Fusion Science and plasma experiments over 150,000 shots have been successfully conducted during twenty long-term plasma experimental campaigns. The LHD has two kinds of superconducting magnets and nine flexible superconducting bus lines with an average length of 55 m, which are utilized as a part of the current feeder system between the coils and the power sources. The superconducting bus lines consist of a pair of aluminum stabilized NbTi/Cu compacted stranded cable insulated electrically and coaxial five corrugated stainless steel tubes with two layers of vacuum insulations. The nominal current is 32 kA and the withstand voltage is 5 kV in 77 K gas helium. From the first experimental campaign, the superconducting bus lines have been stably operated at steady state by using automatic control. It is also confirmed that the status of the superconducting bus lines are kept good thanks to appropriate maintenances. As the results, the reliable operation of the superconducting bus lines has been achieved during the plasma experimental campaigns without any serious failure and the total operational time of the steady state cooling is approximately 58,000 hours.journal articl
Kinetic construction of the high-beta anisotropic-pressure equilibrium in the magnetosphere
Get PDF0000-0002-0783-0838A theoretical model of the high-beta equilibrium of magnetospheric plasmas was constructed by consistently connecting the (anisotropic pressure) Grad–Shafranov equation and the Vlasov equation. The Grad–Shafranov equation was used to determine the axisymmetric magnetic field for a given magnetization current corresponding to a pressure tensor. Given a magnetic field, we determine the distribution function as a specific equilibrium solution of the Vlasov equation, using which we obtain the pressure tensor. We need to find an appropriate class of the distribution function for these two equations to be satisfied simultaneously. Here, we consider the distribution function that maximizes the entropy on the submanifold specified by the magnetic moment. This is equivalent to the reduction of the canonical Poisson bracket to the noncanonical one having the Casimir corresponding to the magnetic moment. The pressure tensor then becomes a function of the magnetic field (through the cyclotron frequency) and flux function, satisfying the requirement of the Grad–Shafranov equation.journal articl
Improving Gyrokinetic Field Solvers toward Whole-Volume Modeling of Stellarators
Get PDF0000-0003-2513-0453We develop novel numerical schemes for electrostatic field solvers toward the whole-volume gyrokinetic simulation of stellarators. The gyrokinetic Poisson equation should be solved for complicated magnetic fields in the stellarator without assuming nested flux surfaces and toroidal symmetry. The developed schemes enable us to generate suitable unstructured meshes and obtain the solutions within a limited numerical cost for general magnetic field structures. These schemes will be integrated and utilized in X-point Gyrokinetic Code - Stellarator (XGC-S).journal articl
Qualitative Improvement of Zeff Diagnostic Based on Visible Bremsstrahlung Profile Measurement by Changing Observation Cross Section in LHD
Get PDF0000-0002-6337-4080Full vertical profiles of visible bremsstrahlung continuum have been measured for diagnostics of effective ion charge, Zeff , with parallel optical fiber arrays at horizontally elongated plasma cross section in the Large Helical Device (LHD). Measured bremsstrahlung profiles were asymmetric due to non-uniform bremsstrahlung emissions located in the stochastic magnetic field layer. Then, the Zeff profile analysis has been done only for a magnetic configuration with relatively thin stochastic magnetic field layer, i.e. for plasmas at magnetic axis position of Rax = 3.60 m. The effect of the non-uniform bremsstrahlung emission disappears in the lower half profile at Rax = 3.60 m, while the Zeff profile analysis is extremely difficult in other magnetic configurations due to the presence of the non-uniform bremsstrahlung emission over the entire vertical profile from top to bottom. To improve the difficult situation in the Zeff diagnostic the fan array optical fiber system was newly installed at vertically elongated plasma cross section for full horizontal profile measurement. It is found that the non-uniform bremsstrahlung emissions disappear from the signal and the observed bremsstrahlung profile is almost symmetric between inboard and outboard radial profiles. The analysis on the Zeff profile has been carried out based on the plasma equilibrium database in LHD, TSMAP. A preliminary result is obtained in high-density discharges, which shows a flat Zeff profile and values of Zeff ∼ 1.journal articl
Initial Results of Hydrogen and Deuterium Beam Ion Simultaneous Transport due to Toroidal Alfvén Eigenmode in the Large Helical Device
Get PDF0000-0003-0755-2433The behavior of energetic particles (EPs) associated with toroidal Alfvén eigenmode (TAE) activities during the combined injection of hydrogen and deuterium beams was investigated in the Large Helical Device (LHD). The enhanced transports of both proton and deuteron with TAE activities were simultaneously observed by a tangentially viewing and mass and energy resolved neutral particle analyzer (E||B-NPA). At the timing of the TAE bursts with the mode number n = 1, both proton and deuteron were transported to the outboard and observed with the similar energies of 137 - 138 keV. At the peak amplitude of the magnetic fluctuations measured by the Mirnov coils, the mixed frequencies of 64 kHz and 29 kHz were identified, and the observed frequencies did not chirp down. The observed timings of the transported hydrogen and deuterium were just after the magnetic fluctuations of 64 kHz and 29 kHz, respectively. By adapting the cross-correlation analysis, the delay times from the magnetic fluctuation to the detection of EPs by E||B-NPA are estimated to 95 μs and 145 μs for hydrogen and deuterium, respectively. These delays are considered to be the time of the radial transport, and the time delays depended on the velocities of the transported EPs.journal articl
Development of a Wide Dynamic Range Neutron Flux Measurement Instrument Having Fast Time Response for Fusion Experiments
Get PDF0000-0002-7959-6155A wide-range neutron flux measurement instrument is developed herein for monitoring the total neutron emission rate and yield of the Large Helical Device (LHD) during deuterium experiments implemented from March 2017 in the National Institute for Fusion Science (NIFS), Japan. The instrument is designed for and installed on the Neutron Flux Monitoring (NFM) system, which measures the counting rate using a 235U Fission Chamber. By combining the pulse counting and Campbell methods, the Digital Signal Processing Unit (DSPU) realized a wide dynamic range of over six orders of magnitude from 1 × 103 counts/s (cps) to 5 × 109 cps. This study explains and discusses how the instrument is developed, including topics from the predevelopment activities to the verification test at the Kyoto University Critical Assembly (KUCA). Experimental results in the LHD using the finished products suggest that the NFM system works well during deuterium experiments.journal articl
The three-dimensional equilibrium with magnetic islands and MHD instabilities in the CFQS quasi-axisymmetric stellarator
Get PDF0000-0001-8485-895XThe equilibrium of the Chinese first quasi-axisymmetric stellarator (CFQS) has been investigated by the HINT code. It is found that the stochastization of magnetic field lines expands with the increase in the volume-averaged beta value 〈β〉 in the plasma boundary. In the high-β regime, the generation of large magnetic islands at rational surfaces not only leads to an effective shrinkage of the plasma confinement region but also increases the outward shift of the magnetic axis. With bootstrap current effects, the low-order islands spread over the whole plasma area, leading to a stochastization of magnetic field lines due to island chain overlapping. However, for a flat pressure profile, the magnetic islands are significantly suppressed so that the magnetic surfaces can be fairly maintained. Moreover, the magnetohydrodynamic (MHD) instabilities in the optimized CFQS configurations have been simulated by the MEGA code. The linear properties of unstable resistive MHD modes are studied. The results show that in the CFQS the main MHD behaviour is dominated by the resistive ballooning modes with strong mode coupling. The mode structure and resistivity scaling are consistent with related theories.journal articl
To chieve both economic activity and suppression of COVID-19--Based on the analogy between the SIR model and controlled fusion--
Get PDF 感染症学でよく用いられているSusceptible-Infectious-Recovered (SIR)モデルの感染者の式がD-T核融合反応のHe灰の式に類似していることに着目して、この感染症を制圧する方法を提案した[1][2]。即ち、SIRモデルの隔離項の回復時間を隔離時間と解釈して、感染性を有してから隔離するまでの時間を２〜３日とすれば、外出制限なしで原理的には可能であることを意味する。例として東京の第３波に隔離制御を適用した場合や、次の第４波の解析も行いワクチン接種との比較見当も行う。また、COVID-19を制圧するのに必要な隔離に要する時間から検査の頻度を求めることができることも示す。これを実現するためには幅広い検査と検査頻度が重要で、それらを実現するための色々な方法の考察も行う。departmental bulletin pape