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Beam modulation and bump-on-tail effects on Alfvén eigenmode stability in DIII-D
Get PDF0000-0002-7911-2739Beam modulation effects on Alfvén eigenmode stability have been investigated in a recent DIII-D experiment and show that variations in neutral beam modulation period can have an impact on the beam driven Alfvén eigenmode spectrum and resultant fast ion transport despite similar time-averaged input power. The experiment was carried out during the current ramp phase of L-mode discharges heated with sub-Alfvénic 50–80 kV deuterium neutral beams that drive a variety of Alfvén eigenmodes unstable. The modulation period of two interleaved beams with different tangency radii was varied from shot to shot in order to modify the relative time dependent mix of the beam pitch angle distribution as well as the persistence of a bump-on-tail feature near the injection energy (a feature confirmed by imaging neutral particle analyzer measurements). As the beam modulation period is varied from 7 ms to 30 ms on/off (typical full energy slowing down time of τslow ≈ 50 ms at mid-radius), toroidicity-induced Alfvén eigenmodes (TAEs) located in the outer periphery of the plasma become intermittent and coincident with the more tangential beam. Core mode activity changes from reversed shear Alfvén eigenmodes (RSAEs) to a mix of RSAE and beta-induced Alfvén eigenmodes. Discharges with 30 ms on/off period do not have a persistent bump-on-tail feature, have the lowest average mode amplitude and least fast ion transport. Detailed analysis of an individual TAE using TRANSP kick modeling (Monte Carlo evolution of the distribution function with probabilistic 'kicks' by the AEs) and the resistive MHD code with kinetic fast ions, MEGA, find no strong role of energy gradient drive due to bump-on-tail features. Instead, the observed TAE modulation with interleaved beams is likely a pitch angle dependent result combined with slowing down of the tangential beam between pulses. For the conditions investigated, bump-on-tail contributions to TAE drive were found to be 5% or less of the total drive at any given time.journal articl
W-band millimeter-wave back-scattering system for high wave number turbulence measurements in LHD
Get PDF0000-0001-5473-2109A 90 GHz W-band millimeter-wave back-scattering system is designed and installed for measuring electron scale turbulence (kρs ∼ 40). A
metal lens relay antenna is used for in-vessel beam focusing, and a beam diameter of less than 40mm is achieved in the plasma core region.
This antenna can be steered at an angle of 159○ ± 6○, which almost covers the plasma radius. The estimated size of the scattering volume is ∼105mm at the edge and 135mm at the core, respectively. A 60m corrugated waveguide is used to achieve a low transmission loss of ∼8 dB. A heterodyne detection system for millimeter-wave circuits with probing power modulation can distinguish the scattered signal from
background noise.journal articl
Line identification of boron and nitrogen emissions in extreme- and vacuum-ultraviolet wavelength ranges in the impurity powder dropping experiments of the Large Helical Device and its application to spectroscopic diagnostics
Get PDFAn impurity powder dropper was installed in the 21st campaign of the Large Helical Device experiment (Oct. 2019–Feb. 2020) under a collaboration between the National Institute for Fusion Science and the Princeton Plasma Physics Laboratory for the purposes of real-time wall conditioning and edge plasma control. In order to assess the effective injection of the impurity powders, spectroscopic diagnostics were applied to observe line emission from the injected impurity. Thus, extreme-ultraviolet (EUV) and vacuum-ultraviolet (VUV) emission spectra were analyzed to summarize observable impurity lines with B and BN powder injection. Emission lines released from B and N ions were identified in the EUV wavelength range of 5–300 Å measured using two grazing incidence flat-field EUV spectrometers and in the VUV wavelength range of 300–2400 Å measured using three normal incidence 20 cm VUV spectrometers. BI–BV and NIII–NVII emission lines were identified in the discharges with the B and BN powder injection, respectively. Useful B and N emission lines which have large intensities and are isolated from other lines were successfully identified as follows: BI (1825.89, 1826.40) Å (blended), BII 1362.46 Å, BIII (677.00, 677.14, 677.16) Å (blended), BIV 60.31 Å, BV 48.59 Å, NIII (989.79, 991.51, 991.58) Å (blended), NIV 765.15 Å, NV (209.27, 209.31) Å (blended), NVI 1896.80 Å, and NVII 24.78 Å. Applications of the line identifications to the advanced spectroscopic diagnostics were demonstrated, such as the vertical profile measurements for the BV and NVII lines using a space-resolved EUV spectrometer and the ion temperature measurement for the BII line using a normal incidence 3 m VUV spectrometer.journal articl
Density dependence of ion cyclotron emission from deuterium plasmas in the large helical device
Get PDFIon cyclotron emission (ICE) driven by perpendicular neutral beam-injected (NBI) deuterons, together with the distinctive ICE driven by tangential NBI, have been observed from heliotron–stellarator plasmas in the large helical device (LHD). Radio frequency radiation in the lower hybrid range has also been observed Saito K. et al (2018 Plasma Fusion Res. 13 3402043), with frequency dependent on plasma density. Here we focus on recent measurements of ICE from deuterium plasmas in LHD, which show substantial variation in spectral character, between otherwise similar plasmas that have different local density in the emitting region. We analyse this variation by means of first principles simulations, carried out using a particle-in-cell (PIC) kinetic approach. We show, first, that this ICE is driven by perpendicular NBI deuterons, freshly ionised near their injection point in the outer midplane edge of LHD. We find that these NBI deuterons undergo collective sub-Alfvénic relaxation, which we follow deep into the nonlinear phase of the magnetoacoustic cyclotron instability (MCI). The frequency and wavenumber dependence of the saturated amplitudes of the excited fields determine our simulated ICE spectra, and these spectra are obtained for different local densities corresponding to the different LHD ICE-emitting plasmas. The variation with density of the spectral character of the simulated ICE corresponds well with that of the observed ICE from LHD. These results from heliotron–stellarator plasmas complement recent studies of density-dependent ICE from tokamak plasmas in KSTAR Thatipamula S.G. et al (2016 Plasma Phys. Control. Fusion 58 065003); Chapman B. et al (2017 Nucl. Fusion 57 124004), where the spectra vary on sub-microsecond timescales after an ELM crash. Taken together, these results confirm the strongly spatially localised character of ICE physics, and reinforce the potential of ICE as a diagnostic of energetic ion populations and of the ambient plasma.
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A theoretical approach to structural change of a polymer induced by beta decays of substituted tritium based on the linear response theory
Get PDFMolecular dynamics simulations of the hydrogen-removed polyethylene are carried out to study the structural change of polyethylene induced by beta decays of substituted tritium. Our simulations show that the folded structure of the hydrogen-removed polyethylene becomes more disordered as the number of removed hydrogen atoms becomes larger. We also propose a theoretical approach to explaining and predicting our molecular dynamics simulation results of hydrogen-removed polyethylene on the basis of the linear response theory. We derive the time derivative of the dynamical quantity, which is conjugate to the force applied as perturbation in the framework of the linear response theory, required to calculate the response function. The dynamical quantity in this study is the total potential energy difference of polyethylene before and after removal of hydrogen. Preliminary results of the response function for the total potential energy of polyethylene after removal of hydrogen are presented.journal articl
Global kinetic-MHD simulations of downward-sweeping reversed shear Alfvén eigenmodes in tokamak plasmas
Get PDFHybrid simulations are performed to investigate the dependence of downward-sweeping reversed shear Alfvén eigenmodes (RSAEs) on key parameters in tokamaks. The investigation mainly focuses on downsweeping RSAEs with toroidal number n = 3. It is found that the occurrence of downward-sweeping RSAEs strongly depends on the bulk plasma profiles, including the pressure profile and the safety factor profile. The influence of bulk plasma pressure value and pressure gradient at qmin on downsweeping RSAEs is studied, respectively. It is shown that the excitation of downsweeping RSAEs is more associated with the plasma pressure gradient at qmin than with the pressure value. The enhancement of the pressure gradient at qmin improves the likelihood of the destabilization of downsweeping RSAEs. Moreover, the downsweeping RSAEs only occur in a weak magnetic shear configuration in our simulations. By increasing the magnetic shear strength of central plasmas, it is numerically verified that the downsweeping RSAE activities are suppressed and eventually replaced by toroidal Alfvén eigenmodes (TAEs). Following this process, the transition region from TAEs to RSAEs is shifted to a region with lower qmin values. In this work, the downsweeping RSAE, for which it has been confirmed that the mode is in the nonperturbative regime, has a twisted mode structure in the poloidal plane, while the upsweeping RSAE, which is weakly affected by the energetic ion effects, has a nearly up–down symmetry mode structure. Further, downward-sweeping RSAEs, showing a relatively narrow mode spatial profile, are generally more stable than the upward-sweeping ones, which can be a reason why the downsweeping RSAEs are rare in experiments. Finally, it is found that the kinetic effect of the energetic ion central pitch angle does not affect the excitation of downward-sweeping RSAEs, but affects the mode stability.journal articl
Effect of the Pitch Modulation of Helical Coils on the Core Plasma Performance of the LHD-Type Helical Fusion Reactor
Get PDF0000-0001-6028-8980The effect of the pitch modulation of the helical coils on the core plasma performance of the LHD-type helical fusion reactor has been examined. The analysis of the MHD stability and neoclassical transport for the pitch modulation α = 0.0 and 0.1 has been conducted based on the finite-beta equilibrium calculated by the HINT code. It was found that the MHD stability is clearly improved without deteriorating the energy transport property by changing the pitch modulation α from 0.1 to 0.0. The reachable operation region expands to the higher density and the expected fusion gain can increase from ∼10 to ∼20. Because the change of the pitch modulation α from 0.1 to 0.0 requires only a slight change in the shape of the helical coils, the engineering design including the maintenance method that has been examined for the reactor with α = 0.1 can be applied without a major modification.journal articl
Study on impurity hole plasmas by global neoclassical simulation
Get PDF0000-0003-0969-1170An impurity hole observed in the large helical device (LHD) is a hollow density profile of an impurity ion species formed in the core plasma where the negative (inward-pointing) ambipolar radial electric field (Er) exists. Although local neoclassical models have predicted that the sign of Er in impurity hole plasmas is negative for the entire minor radius, an experimental measurement of an impurity hole plasma has shown that the Er changes the sign from negative to positive along the minor radius. In the present work, we investigate neoclassical impurity transport in an impurity hole plasma using a global neoclassical simulation code FORTEC-3D. The variation of electrostatic potential on each flux surface (Φ1) is evaluated from the quasi-neutrality condition in multi-ion-species plasma by the global simulation. The ambipolar Er and neoclassical fluxes are determined in solving a global drift-kinetic equation including the effect of Φ1. By the global simulation, we show that an Er which changes the sign along the radius is obtained as a solution of the ambipolar condition
and with such an Er, impurity carbon flux can be outwardly directed even where Er < 0 and the carbon density profile is hollow around the magnetic axis. Furthermore, it is found that the outward carbon flux is only a factor 2–3 from balancing the modeled inward turbulent flux. Our result indicates that we have moved one step closer to reproducing the impurity transport in impurity hole plasmas by kinetic simulation.journal articl
Operating scenario of 3GWth class FFHR power plant with bypass controlled supercritical CO2 gas turbine power generation system
Get PDFIn order to achieve high power generation efficiency, facility compactness, high safety, and high coexistence with fuel tritium in the 3 G Wth class FFHR(Force Free Helical Reactor) power plant, optimization, performance and operation scenario of the power generation system of the power plant model using the axial flow type uniaxial supercritical CO2 gas turbine power generation system were examined. As a result, the following conclusions were obtained.
(1) The most efficient and compact axial flow uniaxial design supercritical CO2 gas turbine power generation system suitable for 3 G WthFFHR power plant has a maximum heat capacity of 1.5GWth×2, and the gas turbine conversion rate at that time was evaluated as 46.9 %(= 1,267MWe/2,700MWth). Therefore, split connection with two power generation systems is optimal for a 3 G Wth class FFHR power plant. Here, the operating speed of the turbo equipment was set to 3600 rpm in order to adapt this plant model to the power supply system in Japan.
(2) Simultaneous supply of 818MWe/300 s electricity and 300MWth/30days heat is required at startup of the FFHR.
(3) The in-house power during steady operation under the self-ignition condition was reduced to about 31.7MWe, and the amount of power generated at the transmission end was evaluated as 1,234MWe.journal articl
Study of ion cyclotron range of frequencies heating characteristics in deuterium plasma in the Large Helical Device
Get PDF0000-0003-0755-2433The characteristics of ion cyclotron range of frequencies (ICRF) minority ion heating with a hydrogen minority and deuterium majority plasma were studied by ICRF modulation injection experiments in the Large Helical Device (LHD). In recent experiments with deuterium plasma, no significant increase in the neutron emission rate due to ICRF second harmonic deuteron heating was observed. Therefore, in this study, the neutron emission rate was used to refer to the information regarding the thermal ion component. Like the results of the observations of the heating efficiencies at various minority proton ratios, the experimental results showed good agreement with the simple model simulation of ICRF wave absorption. During these experiments, the accelerated minority hydrogen ions were observed by neutral particle analyzers. The counting rates of the energetic particles were higher in the lines of sight passing through the helical ripple than across the magnetic axis, and the counting rate decreased as the minority hydrogen ion ratio increased. The dependence of the minority hydrogen ion ratio on the density of the energetic ions was consistent with the experimentally observed heating efficiencies and simulations. The heating efficiency of ICRF minority ion heating could be well explained by simple model simulation in the LHD deuterium experiment.journal articl