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Propagation properties of electron cyclotron waves with helical wavefronts in magnetized plasma
Get PDF0000-0002-2983-5920Propagation properties of an optical vortex with a helical wavefront in cold uniform magnetized plasma are theoretically investigated in an electron cyclotron range of frequencies. The effects of the helical wavefront of the optical vortex on the wave fields in magnetized plasma are described. These effects become significant as the topological charge of the optical vortex increases or the distance from the phase singularity point becomes small. The different properties of propagation are also confirmed in propagation of Laguerre–Gaussian beams by three-dimensional simulations with the finite element method.journal articl
Measurements of radial profile of isotope density ratio using bulk charge exchange spectroscopy
Get PDF0000-0002-5113-9710A bulk charge exchange spectroscopy (BCXS) system using a grism (grating prism) spectrometer has been applied to measure the profile of the deuterium (D) fraction in deuterium and hydrogen (H) mixture plasma in the Large Helical Device. The observed spectrum can be fitted with four Gaussian functions successfully by reduction of free parameters for the least-squares fit. The plasma flow velocity and ion temperature profile measured by charge exchange spectroscopy using carbon impurity are used for estimation of the wavelength shift of hot components to reduce the free parameter. The ion temperature is used to estimate the apparent wavelength shift due to the energy dependent emission cross section only and is not used to set the Doppler width for H and D in the fitting. The sensitivity of the evaluated D fraction on the velocity is increased for a higher D fraction. The error of the D fraction is calculated from the error in the fitted parameter and sensitivity on the velocity of the hot component. The difference in the profile and time trace of the D fraction with D pellet and H pellet injection was observed clearly by BCXS using a grism spectrometer.journal articl
Sensitivity Improvement of Infrared Imaging Video Bolometer for Divertor Plasma Measurement
Get PDF0000-0003-1586-1084The sensitivity of an infrared imaging video bolometer (IRVB) was improved for the measurement of relatively low energy plasma radiation from the viewpoint of the metal foil absorber material. The photon energy of the radiation was considered up to 1 keV for the divertor plasma measurement. The thickness of the foil absorber was evaluated not only for conventional heavy elements, e.g., platinum, but also for light elements by the relation between the photon energy and attenuation length and by mechanical strength. A heat-transfer calculation using ANSYS suggested that light elements with practical foil thickness provide a higher temperature rise of the foil absorber compared with heavier elements with practical foil thickness. The maximum of the temperature rise was evaluated using He–Ne laser irradiation onto absorber samples. The material dependence of the temperature rise has a similar tendency between calculations and experiments. Experimentally, the sensitivity of the IRVB improved from 280 to 110 µW/cm2 using titanium with 1 µm thickness compared with conventional platinum with 2.5 µm thickness. Consequently, the signal-to-noise ratio of the IRVB could be improved from 2.8 to 9.1.journal articl
Feasibility study of tokamak, helical and laser reactors as affordable fusion volumetric neutron sources
Get PDF0000-0001-6028-8980The applicability of tokamak, helical and laser fusion reactors as a volumetric fusion neutron source has been examined using the systems codes that have been utilised for the conceptual design of DEMO and commercial reactors in Japan. This study has clarified the characteristics of reactor-based volumetric neutron sources that can be designed based on the current physics and engineering basis with a reasonable running cost (∼5B Yen/year). Although the achievable neutron flux is 2–3 orders lower than that of accelerator driven neutron sources, tokamak and helical neutron sources can provide a much larger irradiation area for the test of large components. Laser neutron sources have both high operability and tritium breeding capacity. These reactor-based neutron sources also serve as an integrated test bed of the entire reactor system.journal articl
Assessment of W density in LHD core plasmas using visible forbidden lines of highly charged W ions
Get PDF0000-0002-5302-073XVisible magnetic-dipole (M1) lines can serve as a novel diagnostic mean of tungsten in ITER and future DEMO reactors. Here a local tungsten density in core plasmas of the large helical device (LHD) is successfully assessed with the measurement of visible M1 lines emitted by W26+ and W27+. By such a measurement, (i) the radial profile of total tungsten density in the LHD core plasmas with a line-averaged electron density of ∼4 × 1019 m−3 and central electron temperature of ∼1 keV is found to be a hollow, and (ii) the total tungsten density is found to be decreased in the whole region with an increase in the electron temperature due to the onset of reheat mode. The scheme with visible M1 lines used in the present work can be also applied for the tungsten density measurement at the ITER edge plasma.journal articl
Demonstration of reduced neoclassical energy transport in Wendelstein 7-X
Get PDF0000-0002-4395-239XResearch on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.journal articl
Predictive analysis for triton burnup ratio in HL-2A and HL-2M plasmas
Get PDF0000-0003-4555-1837The expected triton burnup ratio was analyzed based on numerical simulation to study the feasibility of demonstrating energetic particle confinement through 1 MeV triton burnup experiments in HL-2A and HL-2M. Calculations were conducted using LORBIT, a collisionless Lorentz orbit code, and FBURN, a neutron emission calculation code based on the classical confinement of energetic particles. First, the orbit loss and radial distribution of the tritons were evaluated using the LORBIT code. The LORBIT code revealed that all tritons were lost within ∼10−6 s in HL-2A, whereas in HL-2M, most of the tritons were still confined at 10−3 s. The FBURN code calculated the deuterium–tritium neutron emission rate using the radial distribution of 1 MeV tritons. The predictive analysis found that nearly no deuterium–tritium neutrons remained in HL-2A at a plasma current of 160 kA. Also, in HL-2M, a significant triton burnup ratio could be obtained at the relatively high plasma currents of 1MA, 2MA, and 3MA. This analysis predicts that the triton burnup ratio exceeds 1% under relatively high plasma current conditions.journal articl
Development of a 56 GHz ECH system for deuterium plasma experiments of a low magnetic field in LHD
Get PDFWe have completed establishing a 56 GHz electron cyclotron heating (ECH) system with a new gyrotron to realize high β plasma experiments with pure deuterium gas in the Large Helical Device (LHD). This new ECH system made it possible to conduct the relatively low magnetic field experiment around 1 T with pure deuterium gas because tangential neutral beam injection (NBI) systems of deuterium beams did not have enough power to initiate plasma in the LHD. We succeeded in initiating and sustaining pure deuterium plasma in the magnetic field of 1 T by using the 56 GHz ECH system and the deuterium NBI (D-NBI). This new ECH system contributed to expanding the new experimental regime in the LHD.journal articl
Modeling of Stacked Single-Layer Coils Wound with Bi2223 Tapes Using a Lumped Constant Circuit
Get PDF0000-0002-1989-867XStacked single-layer (SSL) coils wound with Bi2223 HTS tapes were modeled with a lumped circuit. Using the model, we investigated the effect of the joint resistance at coil terminals on the generated voltage for the SSL coils. The investigation shows that the joint resistance must be reduced as much as possible to restrain the voltage occurred in the SSL coils and each joint resistance should be uniform to make the current distribution uniform in the SSL coils. Additionally, we investigated the effect of inductance on the current distribution in the SSL coils and the effect of the tape's degradation on the coil performance. As a result, the current distribution of the SSL coils can be uniform due to the joint resistance even though the SSL coils have different inductances in each coil winding. Further, the current distribution and the voltage of the SSL coils are influenced by the tape's degradation.journal articl
Numerical Simulation Study of the Magnetic Flux Tube Expansion on the Divertor Plasma Parameters by the LINDA Code
Get PDF0000-0003-2019-4001In this research, we investigate the effect of magnetic flux tube expansion on the divertor plasma parameters by using the fluid code “LINDA”. A comparison between the cylindrical flux tube (without the magnetic flux expansion) and the expansion magnetic flux tube has been undertaken. The aim of the study is to understand the impact of magnetic field expansion on the divertor physics by using the LINDA fluid code. The plasma density (ni) is decreased and parallel velocity is increased (ui||) toward the target plate with the expansion of magnetic field lines near the target plate. The heat and particle fluxes are reduced significantly on the target plate in the case of the expansion mesh configuration. For the case of cylindrical mesh, advection becomes stronger with the decreasing distance from the target plate. In the case of expansion mesh, diffusion is stronger with the decreasing distance from the target plate. These outcomes clearly indicate the effect of the magnetic field structure on the divertor plasma parameters.journal articl