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Demonstration of aneutronic p-11B reaction in a magnetic confinement device
Get PDFORCID 0000-0003-4555-1837Aneutronic fusion using commonly available fuel such as hydrogen and boron 11 (11B) is one of the most attractive potential energy sources. On the other hand, it requires 30 times higher temperature than deuterium–tritium fusion in a thermonuclear fusion reactor condition. Development of techniques to realize its potential for the experimental capability to produce proton-boron 11 (p-11B) fusion in the magnetically confined fusion device using neutral beam injection is desired. Here we report clear experimental exploration and measurements of p-11B fusion reactions supported by intense hydrogen beams and impurity powder dropper installed in the magnetic confinement plasma Large Helical Device. We measured a significant amount of fusion alpha particle emission using a custom designed alpha particle detector based on a passivated implanted planar silicon detector. Intense negative-ion-based hydrogen beam injectors created a large population of up to 160 keV energetic protons to react with the boron-injected plasma. The p-11B alpha particles having MeV energy were measured with the alpha particle detector which gave a fusion rate in a good agreement with the global p-11B alpha emission rate calculated based on classical confinement of energetic proton, using experimentally obtained plasma parameters.journal articl
Full-torus impurity transport simulation in boron powder injection experiments in the Large Helical Device
Get PDFORCID 0000-0003-0655-7347The toroidal distribution of boron deposition on plasma-facing components (PFCs) in boron powder injection using an impurity power dropper (IPD) was investigated by full-torus simulation and observations in a systematic plasma density-scan experiment. The images of the ablation of dropped boron powders observed with a visible CCD camera were consistently explained by the simulations of the ablation positions of the boron powders considering the size distribution. Simulations assuming full-torus boron deposition on the PFCs did not reproduce the observed intensity profile of boron emission lines for higher plasma densities. It indicated that the density of boron deposited on PFCs installed toroidally far from the IPD was low for higher plasma densities due to the change in the ablation positions of the boron powders toward the outboard side. The experimental results verified the previous full-torus simulation of the toroidal distribution of the boron deposition in both lower and higher plasma densities.journal articl
Emergence of Optical Activity and Surface Morphology Changes in Racemic Amino Acid Films Under Circularly Polarized Lyman-α Light Irradiation
Get PDFORCID 0000-0002-0990-7093The homochirality of life remains one of the most enigmatic issues in the study of the origin of life. A proposed mechanism for symmetry breaking involves irradiation by circularly polarized light (CPL). To investigate the photoreaction of amino acids under CPL irradiation, a vacuum ultraviolet (VUV) CPL irradiation system was developed at the synchrotron light source UVSOR-III. Hydrogen Lyman-α CPL (121.6 nm) is considered a potential asymmetric source in space. Therefore, racemic alanine film samples were irradiated with Lyman-α CPL to explore the photoreaction of biomolecules. Circular dichroism (CD) spectra measurements revealed that irradiation with right- (left-) handed CPL induced a positive (negative) anisotropy factor g in the wavelength range of 180–240 nm. However, the spectra differed from those of enantiopure alanine, exhibiting broad wavelength ranges and no sign change. Liquid chromatography-mass spectrometry (LC–MS) measurements indicate formation of larger molecules, such as oligomeric alanine adducts or modified oligomers after the Lyman-α CPL irradiation. Additionally, CPL irradiation considerably changes the microstructure of the alanine film surface, leading to the formation of circular network aggregates on the scale of 100 nm. The morphology changes in the alanine film and/or the formation of the larger molecules could be possible causes of the modified anisotropy factor spectra compared to those of enantiopure alanine. These findings highlight the need for further research on the photoreaction of biomolecules in solid states under VUV CPL irradiation, particularly in the photoionization energy range, to validate the cosmic scenario of homochirality.journal articl
Impact of helium and hydrogen plasma exposure on surface damage and erosion of tungsten
Get PDFORCID 0000-0002-4453-953XThe impact of helium plasma exposure on the tungsten surface damage structure development and erosion has been investigated by comparing the impact of hydrogen plasma exposure. Crystal orientation dependence of the undulating surface structure formation and erosion rate is observed on the plasma-exposed tungsten surface independently from the plasma species. The top surface of the plasma exposed tungsten has a tendency to {100} plane independently from the initial surface orientation. Although hydrogen and/or helium cause no erosion in tungsten under incident ion energy exposure conditions below the sputtering threshold, inevitable minute impurities, like oxygen, play an essential role in erosion, and significant erosion can be observed even at 30 eV.journal articl
Self-consistent transport simulation of boron dust particle injection in the peripheral plasma in Large Helical Device
Get PDFORCID 0000-0003-0655-7347The trajectories and the ablation positions of boron dust particles dropped from an impurity powder dropper in the peripheral plasma in the Large Helical Device (LHD) were calculated using a three-dimensional edge plasma simulation code (EMC3-EIRENE) and a dust transport simulation code (DUSTT). The simulation shows that the trajectory of the boron dust particles is deflected at the upper divertor leg due to the effect of the hydrogen plasma flow, and the ablation positions of the dust particles in an ergodic layer change toward the outboard side of the torus for higher plasma densities. The effect of the boron ion flow in the divertor leg on the deflection is investigated by coupling the two codes self-consistently. The simulation predicts that the boron ions in the divertor leg, which are produced by sputtering on the divertor plates, which do not affect the change in the ablation positions. It also shows that the ablation positions move toward the inboard side and approach the Last Closed Flux Surface (LCFS) in case of increased boron dust drop rates, which is caused by the lowered plasma flow in the upper divertor leg due to the lowered electron temperature by radiation cooling by the dropped dust particles.journal articl
Effect of Magnetic Field Distribution on Recovery Currents of NbTi Superconducting Conductors
Get PDFORCID 0000-0003-1454-8117NbTi conductors for helical coils of the Large Helical Device were developed to satisfy cold-end stability. Their recovery currents were measured with a conductor test facility with 9 T split coils at the National Institute for Fusion Science. The measured recovery currents were higher by 15 to 20 % than that calculated from Maddockʼs equal area theorem with the measured conductor resistance and heat transfer. We have proposed an analytical method to estimate the recovery current in a finite magnetic field using the temperature distribution that is calculated with representative thermal conductivity, resistivity, and heat transfer. In order to check the validity of this method, we carried out simulation with a finite-difference method. The results revealed that the proposed analytical method is applicable with slight underestimation as long as the resistivity can be fitted by a function of temperature only. In addition, the necessary length of the magnetic field higher than 95 % is around three times of the temperature characteristic length of the conductor for measurement of recovery currents with an overestimation of less than 5 %.journal articl
Potential impact of surface microstructure change on reduction of emission current in tungsten filament
Get PDFORCID 0000-0002-4453-953XWhen a helium beam is extracted, a significant decrease in arc current is observed in the filament arc ion source of the Neutral Beam Injector. Microstructural changes in the tungsten filament surface of the ion source are observed by electron microscopy to investigate the cause of the reduction of arc current. Helium bubbles with 100–800 nm diameters appear immediately beneath the surface within 1 μm, and the bubbles develop into surface hole structures with surface erosion by sputtering. The decrease in arc current with helium beam extraction can be explained by a reduced effective surface area, due to the increase in hole porosity. It has been shown that the reduction of the arc current can be recovered by removing the helium-irradiation-affected layer by sputtering with argon discharge.journal articl
Simulation Analysis of Transport of Boron Dust Particles Injected by Impurity Powder Dropper in the Large Helical Device
Get PDFORCID 0000-0003-0655-7347Boron dust particles were injected by an impurity powder dropper to improve plasma confinement and perform wall conditioning in the Large Helical Device. A fast-framing camera for monitoring dust particle trajectories in the peripheral plasma detected a change in the ablation positions of the dust particles depending on the plasma density and heating power. An analysis using a three-dimensional edge plasma simulation code (EMC3-EIRENE) and a dust transport simulation code (DUSTT) was applied to understand these observations. The simulations proved that the dust particle trajectories are more deflected toward the outboard side of the torus by the effect of the plasma flow in an upper divertor leg for higher plasma densities and higher plasma heating powers. The simulation successfully reproduced observations of the change in the ablation positions in the peripheral plasma.journal articl
Enhancement of ECCD by the current condensation effect for stabilizing large magnetic islands caused by neoclassical tearing modes in tokamak plasmas
Get PDFORCID 0000-0001-8221-9804The radio frequency current condensation effect reported in Reiman and Fisch (2018 Phys. Rev. Lett.121 225001) is modeled in the nonlinear resistive magnetohydrodynamic code. A series of numerical investigations have been performed to investigate the enhancement of electron cyclotron current drive (ECCD) by the current condensation effect during the control of neoclassical tearing mode (NTM) in tokamak plasmas. In the numerical model, both the parallel transport and the perpendicular transport of electron temperature are considered. The EC driven current and driven perturbed electron temperature can nonlinearly evolve within the given magnetic configuration and eventually reach saturation states. The input power threshold of ECCD and the fold bifurcation phenomenon are numerically verified via nonlinear simulations. The numerical results show good agreements with the analytical results. Moreover, spatial distributions of EC current for the two solutions at different condensed level are displayed. The control effectiveness of ECCD for large NTM islands has been evaluated while considering the current condensation effect. While taking into account current condensation effect, for a sufficiently large input power, a larger island can be more effectively stabilized than a smaller one, which suggests a reassessment of the previous idea that the ECCD should always be turned on as early as possible. The potential physics mechanism behind the ECCD control have all been discussed in detail. Furthermore, the condensation effect is found to have favorable effects on the radial misalignment of ECCD. In the consideration of the situation for extremely localized control needs, a highly peaked heating profile is adopted to verify that the fold bifurcation phenomenon still exists and the current condensation effect can still take effect in this extreme condition.journal articl
Effect of metal impurities on the adsorption energy of cesium and work function of the cesiated Mo (001) surface
Get PDFBased on the DFT method, the effects of copper and tungsten impurities present in the negative ion source of neutral beams on the cesiated surface were studied, including their effects on the adsorption energy of cesium on the surface and the surface work function. The results indicate that copper impurities significantly increase the average adsorption energy of cesium, whereas tungsten has limited enhancement on the average adsorption energy of cesium and may even reduce it. The work function calculations show that, at cesium coverages below 4/16 θ, copper impurities cause a significant increase in the work function. However, at cesium coverages above 6/16 θ, high-coverage copper impurities lead to a further decrease in the work function, causing the cesium coverage corresponding to the lowest work function to shift towards higher cesium coverage. Under any tungsten impurity and cesium coverage, tungsten impurities can significantly increase the surface work function, with the maximum increase reaching 0.50 eV. The dipole moment density analysis shows that in most cases, impurities significantly reduce the dipole moment density of the cesiated surface, except when the coverage of cesium and copper impurities is above 6/16 θ. The charge transfer results show that the copper impurity layer has more positive charge compared to the tungsten impurity layer, which significantly affects the dipole moment density of the surface system. In addition, adsorbed atoms cause electrons in the molybdenum atomic layer to migrate to the surface, resulting in the molybdenum substrate having a pronounced negative dipole moment.journal articl