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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

A comparative study of internal kink stability in EU DEMO designs with negative and positive triangularity

Abstract Internal kink (IK) instability is investigated for European demonstration fusion reactor (EU DEMO) plasmas in both negative triangularity (NT) and positive triangularity (PT) configurations. For NT plasmas, the IK becomes more unstable as an ideal conformal wall moves away from the plasma boundary, with the mode growth rate saturating at the wall radial location of about b / a = 1.5 , where a is the plasma minor radius and b the wall radial location. The plasma resistivity destabilizes the IK mode. The effect of sub-sonic toroidal plasma flow is sufficiently weak and can thus be ignored for these EU DEMO equilibria. These results are consistent with those for PT plasmas, albeit with larger mode growth rate in the NT configuration. Both perturbative and self-consistent magneto-hydrodynamic (MHD)-kinetic hybrid calculations predict (partial) stabilization of the IK modes in both NT and PT configurations, with inclusion of various kinetic contributions. Precessional drift motion of trapped fusion-born alphas in EU DEMO produces weak stabilization to the IK mode. Stronger stabilization occurs with the toroidal precession of trapped thermal particles (ions and electrons) and the bounce-transit motion of thermal ions. The stabilization is similar between the NT and PT configurations, due to the similarity of the mode eigenfunction (occupying a nearly circular region in the plasma core) despite the sign difference in the triangularity. The non-perturbative MHD-kinetic hybrid model predicts much less stabilization of the mode than the perturbative model, primarily due to the self-consistent determination of the mode eigenvalue in the former. Generally, no significant difference in the IK mode stability is found between the NT and PT plasmas in EU DEMO.

Physics↗
DOE OSTI2024

Comparative study of boron and neon injections on divertor heat fluxes using SOLPS-ITER simulations

Here, based on the EAST equilibrium, the effects of boron (B) and neon (Ne) injected at different locations on the target heat load, and the distributions of B and Ne particles were investigated by transport code SOLPS-ITER. It was found that the B injection was more sensitive to the injection location for heat flux control than impurity Ne. The high electron and ion densities near the inner target in the discharge with impurity B injected from over X-point (R 1 ) led to plasma detachment only at the inner target, and the localized B ions in the cases with injection from outer target location (R 2 ) and upstream location (R 3 ) led to far-SOL detachment at the outer target, but not at the inner target. In contrast, for Ne, the spatial distributions of Ne ions and electrons were found to be similar in all the cases at the three injection locations, and the detached plasma was achieved at the inner target and the electron temperature was reduced at the outer target. For locations R 2 and R 3 , impurity B showed a more pronounced effect on the heat flux at the far-SOL of the outer target. Further analysis indicated that Ne atoms came mainly from the recycling sources, whereas B atoms came mainly from injection, and that their distinct atomic distributions resulted from the difference in the ionization threshold and ionization mean free path. In addition, the radiation proportion of B in the divertor region was larger than that of Ne when the total radiation power was similar, which suggests that B has less influence on the core region.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Comparative study of the electronic structure, phonon spectra, and electron-phonon interaction of ZrB2 and TiB2

The electronic structure, optical and x-ray absorption spectra, angledependence of the cyclotron masses and extremal cross sections of the Fermisurface, phonon spectra, electron-phonon Eliashberg and transport spectralfunctions, temperature dependence of electrical resistivity of the MB2 (M=Tiand Zr) diborides were investigated from first principles using the fullpotential linear muffin-tin orbital method. The calculations of the dynamicmatrix were carried out within the framework of the linear response theory. Agood agreement with experimental data of optical and x-ray absorption spectra,phonon spectra, electron-phonon spectral functions, electrical resistivity,cyclotron masses and extremal cross sections of the Fermi surface was achieved.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comment on “Comparative study of ab initio nonradiative recombination rate calculations under different formalisms”

In a recent article [Phys. Rev. B 91, 205315 (2015)] Shi, Xu, and Wang presented a comparison of several formalisms to calculate nonradiative recombination rates and concluded the “one-dimensional (1D) quantum formula” that was used by Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] is insufficient to accurately describe nonradiative capture rates. Our analysis of the results of Shi, Xu, and Wang indicates that their conclusions about the 1D quantum formula are unfounded and stem from an error in their calculations. Further, our own calculations demonstrate that the 1D quantum formula approach yields reliable and accurate results for nonradiative recombination rates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Reply to “Comment on ‘Comparative study of ab initio nonradiative recombination rate calculations under different formalisms’ ”

As pointed out by the comments of Wickramaratne et al., there was indeed an algebra error in our original publication [Phys. Rev. B 91, 205315 (2015)] in calculating the 1D model. We have corrected this error, and did a renewed calculation and comparison of the 1D model and other approximated models. The revised comparison provides some insights for the origin of the difference between the full phonon spectrum electron-phonon coupling static formula results and other approximated results, especially for how to evaluate the amplitude of electron-phonon coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Comparative study of the pressure dependence of optical-phonon transverse-effective charges and linewidths in wurtzite InN

In this work, we investigate the hydrostatic pressure dependence of the zone center optical phonons of c -plane and a -plane wurtzite InN epilayers grown on GaN substrates. The longitudinal to transverse mode splitting for the A 1 and E 1 modes was found to increase with increasing pressure, whereas the associated transverse effective charge decreases for both modes as e T * ( A 1 ) = 2.93 – 9.9 × 10 - 3 P and e T * ( E 1 ) = 2.80 – 10.6 × 10 - 3 P (in units of elementary charge and P in GPa). These observations are well in line with results for other II–VI, III–V, and group-IV semiconductor compounds as far as the relation between the magnitude and sign of the pressure derivative of e T * and the bond ionicity is concerned. As the latter increases so does | ∂ e T * / ∂ P | with a sign change from positive to negative for bond ionicities around f i = 0.46 for compounds with anions belonging to the first row of the Periodic Table. A comparison of the results for InN and other nine tetrahedrally bonded compounds indicate that the pressure behavior of the transverse effective charge is mainly determined by the strength of the Pauli repulsion between cation valence electrons and those of the anion core. We also perform ab initio calculations in order to address the origin of the observed increase in linewidth of the E 2 high mode which is found to arise from a pressure-induced increase in the rate of two-phonon decay processes. In conclusion, this broadening is associated with tuning into resonance of a steep edge in the two-phonon density of states around 460 cm - 1 with the frequency of the E 2 high mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Comparative study of He 6 β -decay based on different similarity-renormalization-group evolved chiral interactions

We report on a study of the Gamow-Teller matrix element contributing to ${}^6$He $\beta$-decay with similarity renormalization group (SRG) versions of momentum- and configuration-space two-nucleon interactions. These interactions are derived from two different formulations of chiral effective field theory ($\chi$EFT) -- without and with the explicit inclusion of $\Delta$-isobars. We consider evolution parameters $\Lambda_{\rm SRG}$ in the range between 1.2 and 2.0 fm$^{-1}$ and, for the $\Delta$-less case, also the unevolved (bare) interaction. The axial current contains one- and two-body terms, consistently derived at tree level (no loops) in the two distinct $\chi$EFT formulations we have adopted here. The ${}^6$He and ${}^6$Li ground-state wave functions are obtained from hyperspherical-harmonics (HH) solutions of the nuclear many-body problem. In $A\,$=$\,6$ systems, the HH method is limited at present to treat only two-body interactions and non-SRG evolved currents. Our results exhibit a significant dependence on $\Lambda_{\text{SRG}}$ of the contributions associated with two-body currents, suggesting that a consistent SRG-evolution of these is needed in order to obtain reliable estimates. We also show that the contributions from one-pion-exchange currents depend strongly on the model (chiral) interactions and on the momentum- or configuration-space cutoffs used to regularize them. These results might prove helpful in clarifying the origin of the sign difference recently found in No-Core-Shell-Model and Quantum Monte Carlo calculations of the ${}^6$He Gamow-Teller matrix element.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Comparative study of Minnesota functionals performance on ferroelectric BaTiO 3 and PbTiO 3

Density functional theory based simulations are the leading tool for the computational investigation of ferroelectrics and the parametrization of their classical potentials. However, the predictions often depend strongly on the exchange correlation functional. The most popular choices, LDA and GGA, tend to underestimate or overestimate some structural, electric, and energy properties. These impede development of highly accurate classical potentials that extend the reach of first-principles simulations to finite temperatures and realistic sizes. In this work, we investigate the performance of recently developed Minnesota exchange correlation functionals on prototypical ferroelectrics BaTiO 3 , and PbTiO 3 , in comparison with some popular ones. We find that there exists strong correlation between predictions for some properties (tetragonality, phase energy difference, and polarization) by different functionals. Along the correlation line, we find a range of functionals (including some from the Minnesota suite) whose predictions fall in between those of LDA and GGA and, therefore, offer an improvement. A way to relatively rank functional performance with respect to chosen benchmarks is proposed and applied to identify the top performers for BaTiO 3 and PbTiO 3 . Here, the performance is found to be material dependent. Therefore, we propose that the performance assessment carried out in this work is employed to "screen" functionals prior to their use on ferroelectrics. Quick and computationally inexpensive, it is likely to lead to improved descriptions, especially for classical potential parametrization. The Minnesota functional suite is found to be suitable for this task.

36 MATERIALS SCIENCE↗