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Chen, L.

Publications and source records attributed to Chen, L..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Precise branching ratio measurement for the superallowed β+ decay of 26 Si: Completion of a second mirror pair

As part of a continuing effort to test the unitarity of the Cabbibo-Kobayashi-Maskawa matrix, we have measured the branching ratio for the superallowed 0 + → 0 + β + -decay transition from 26 Si to be 0.7569(14), a result with 0.18% relative precision. With this result, the ft value for the transition is now established with comparable precision to the well-known superallowed transitions currently used in the determination of V ud . Furthermore, it also completes a second pair of mirror superallowed transitions, 26 Si → 26m Al and 26m Al → 26 Mg, the ratio of whose measured ft values provides a sensitive test of the method used to calculate the isospin-symmetry-breaking correction δ C needed to determine V ud . Like the previously measured ratio for the mass-38 mirror pair, the new result agrees well with the calculation based on Woods-Saxon radial wave functions and strengthens the case for rejecting the calculation that uses Hartree-Fock radial functions.

20 ≤ A ≤ 38↗
DOE OSTI2019

Advances in plasma–wall interaction control for H-mode operation over 100 s with ITER-like tungsten divertor on EAST

A total power injection up to 0.3 GJ has been achieved in EAST long pulse H-mode operation of 101.2 s with an ITER-like water-cooled tungsten (W) mono-block divertor, which has steady-state power exhaust capability of 10 MWm –2 . The peak temperature of W target saturated at 12 s to the value T ~ 500 °C with a heat flux ~3.3 MW m –2 being maintained during the discharge. By tailoring the 3D divertor plasma footprint through edge magnetic topology change, the heat load was broadly dispersed and thus peak heat flux and W sputtering were well controlled. Active feedback control of H-mode detachment with D 2 fuelling or divertor impurity seeding has been achieved successfully, with excellent compatibility with the core plasma performance. Active feedback control of radiative power utilizing neon seeding was achieved with f rad = 18%–41% in H-mode operation, exhibiting potential for heat flux reduction with divertor and edge radiation. This has been further demonstrated in DIII-D high β P H-mode scenario within the joint DIII-D/EAST experiment using impurity seeding from the divertor volume. Steady-state particle control and impurity exhaust has been achieved for long pulse H-mode operation over 100 s with the W divertor by leveraging the effect of drifts and optimized divertor configuration, coupled with strong pumping and extensive wall conditioning. Methods toward the reduction of divertor W sourcing, which is of crucial importance for a metal-wall tokamak, are also explored. These advances provide vital experimental information on favourable core-edge integration for high power, long-pulse H-mode operation in EAST, ITER and CFETR.

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

Unconventional thermal metallic state of charge-neutral fermions in an insulator

Quantum oscillations in transport and thermodynamic parameters at high magnetic fields are an unambiguous signature of the Fermi surface, the defining characteristic of a metal. Recent observations of quantum oscillations in insulating SmB 6 and YbB 12 , therefore, have been a big surprise—despite the large charge gap inferred from the insulating behaviour of the resistivity, these compounds seemingly host a Fermi surface at high magnetic fields. However, the nature of the ground state in zero field has been little explored. Here, we report the use of low-temperature heat-transport measurements to discover gapless, itinerant, charge-neutral excitations in the ground state of YbB 12 . At zero field, sizeable linear temperature-dependent terms in the heat capacity and thermal conductivity are clearly resolved in the zero-temperature limit, indicating the presence of gapless fermionic excitations with an itinerant character. Remarkably, linear temperature-dependent thermal conductivity leads to a spectacular violation of the Wiedemann–Franz law: the Lorenz ratio is 10 4 –10 5 times larger than that expected in conventional metals, indicating that YbB 12 is a charge insulator and a thermal metal. Furthermore, we find that these fermions couple to magnetic fields, despite their charge neutrality. Our findings expose novel quasiparticles in this unconventional quantum state.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A new method for calculating normal boiling point of liquids

A method for predicting normal boiling points (NBPs) of pure liquids based on linear free energy relationships (LFERs) and thermodynamics formulas is proposed. For 15 classes of chemical compounds (a total of 683 substances), correlation equations relating NBPs to molecular descriptors were obtained using stepwise regression. For all equations, the mean relative deviations (MD) lie between 0.89% and 2.83%, being mostly less than 2%. The correlation coefficients of 13 equations are larger than 0.98. The correlation equations obtained are simple, have clear physical sense, wide applicability range and provide rather high predictive power and accuracy of calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

LAMOST DR1: Stellar Parameters and Chemical Abundances with SP-Ace

We present a new analysis of the LAMOST DR1 survey spectral database performed with the code SP-Ace, which provides the derived stellar parameters T{sub eff}, logg, [Fe/H], and [α/H] for 1,097,231 stellar objects. We tested the reliability of our results by comparing them to reference results from high spectral resolution surveys. The expected errors can be summarized as ∼120 K in T{sub eff}, ∼0.2 in logg, ∼0.15 dex in [Fe/H], and ∼0.1 dex in [α/Fe] for spectra with S/N > 40, with some differences between dwarf and giant stars. SP-Ace provides error estimations consistent with the discrepancies observed between derived and reference parameters. Some systematic errors are identified and discussed. The resulting catalog is publicly available at the LAMOST and CDS websites.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Experimental study of heating scheme effect on the inner divertor power footprint widths in EAST lower single null discharges

A comprehensive work of the effects of plasma current and heating schemes on divertor power footprint widths is carried out in the experimental advanced superconducting tokamak (EAST). The divertor power footprint widths, i.e., the scrape-off layer heat flux decay length λ q and the heat spreading S, are crucial physical and engineering parameters for fusion reactors. Strong inverse scaling of λ q and S with plasma current have been demonstrated for both neutral beam (NB) and lower hybrid wave (LHW) heated L-mode and H-mode plasmas at the inner divertor target. For plasmas heated by the combination of the two kinds of auxiliary heating schemes (NB and LHW), the divertor power widths tend to be larger in plasmas with higher ratio of LHW power. Comparison between experimental heat flux profiles at outer mid-plane (OMP) and divertor target for NB heated and LHW heated L-mode plasmas reveals that the magnetic topology changes induced by LHW may be the main reason to the wider divertor power widths in LHW heated discharges. The effect of heating schemes on divertor peak heat flux has also been investigated, and it is found that LHW heated discharges tend to have a lower divertor peak heat flux compared with NB heated discharges under similar input power. All these findings seem to suggest that plasmas with LHW auxiliary heating scheme are better heat exhaust scenarios for fusion reactors and should be the priorities for the design of next-step fusion reactors like China Fusion Engineering Test Reactor.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Excitation of a global plasma mode by an intense electron beam in a dc discharge

The interaction of an intense electron beam with a finite-length, inhomogeneous plasma is investigated numerically. The plasma density profile is maximal in the middle and decays towards the plasma edges. Two regimes of the two-stream instability are observed. In one regime, the frequency of the instability is the plasma frequency at the density maximum and plasma waves are excited in the middle of the plasma. In the other regime, the frequency of the instability matches the local plasma frequency near the edges of the plasma and the intense plasma oscillations occur near plasma boundaries. The latter regime appears sporadically and only for strong electron beam currents. This instability generates a copious amount of suprathermal electrons. Finally, the energy transfer to suprathermal electrons is the saturation mechanism of the instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Analysis of the Duration of Rising Tone Chorus Elements

Abstract The duration of chorus elements is an important parameter to understand chorus excitation and to quantify the effects of nonlinear wave‐particle interactions on energetic electron dynamics. In this work, we analyze the duration of rising tone chorus elements statistically using Van Allen Probes data. We present the distribution of chorus element duration ( τ ) as a function of magnetic local time (MLT) and the geomagnetic activity level characterized by auroral electrojet ( A E ) index. We show that the typical value of τ for nightside and dawnside is about 0.12 s, smaller than that for dayside and duskside by about a factor of 2 to 4. Using a previously developed hybrid code, DAWN, we suggest that the background magnetic field inhomogeneity might be an important factor in controlling the chorus element duration. We also report that τ is larger during quiet times and shorter during moderate and active periods; this result is consistent with the MLT dependence of τ and the occurrence pattern of chorus waves at different levels of geomagnetic activity. We then investigate the correlation between τ and the frequency chirping rate (Γ). We show that, from observation, τ scales with Γ as , suggesting that statistically the frequency range of chorus elements ( τ Γ) should be roughly the same for different elements. These findings should be useful to the further development of a theoretical model of chorus excitation and to the quantification of nonlinear wave‐particle interactions on energetic electron dynamics.

Teng, S.↗
DOE OSTI2017

Evolution of the Design and Fabrication of Astrophysics Targets for Turbulent Dynamo (TDYNO) Experiments on OMEGA

Although the overall function of a campaign’s primary target design may remain unchanged, the components and structure often evolve from one shot day to the next to better meet experimental goals. The target fabrication engineer’s involvement in this evolution can be important for advising modifications in order to improve and simplify assembly at the same time. Highly complex targets are constructed by General Atomics (GA) for astrophysics experiments conducted by the University of Chicago at the OMEGA laser facility. Several novel target components are fabricated, precision-assembled, and extensively measured in support of this campaign, and have evolved over the last three years to improve both the science and assembly. Examples include unique laser machined polyimide grids to enhance plasma mixing at target center, precision micromachined cylindrical shields that also act as component spacers, drawn glass target supports to suspend physics packages at critical distances, and tilted pinholes for collimated proton radiography. Target component fabrication and evolution details for this turbulent dynamics (TDYNO) campaign are presented, along with precision-assembly techniques, metrology methods, and considerations for future TDYNO experiments on OMEGA.

42 ENGINEERING↗
DOE OSTI2017

Structure, luminescence and thermal quenching properties of Eu doped Sr{sub 2−x}Ba{sub x}Si{sub 5}N{sub 8} red phosphors

Eu{sup 2+} doped Sr{sub 2-x}Ba{sub x}Si{sub 5}N{sub 8} phosphors were synthesized at 1610 ℃ for 4 h via the solid-state reaction method. The XRD results confirm that the complete solid solutions are formed. With the increase of x, the emission spectra show an obvious blue-shift from 610 nm to 585 nm under the excitation of 460 nm. The color tone can be tuned from yellow to red. The corresponding mechanism for the blue-shift of peak-wavelength is studied in detail. The results of decomposed Gaussian spectra and fluorescence lifetime show that the local coordination structure surrounding activator ions changes with increasing x value. It is found that the probability of Eu occupying Sr1 and Sr2 site is dependent on Ba/Sr ratio. The variation of thermal quenching properties and the corresponding mechanism is discussed in detail. The results indicate that Eu{sup 2+} doped Sr{sub 2-x}Ba{sub x}Si{sub 5}N{sub 8} is a promising orange red-emitting phosphor for near UV or blue light-pumped white light-emitting-diodes (wLEDs). - Graphical abstract: Eu{sup 2+} doped Sr{sub 2-x}Ba{sub x}Si{sub 5}N{sub 8} solid solutions were prepared by the solid-state reaction method. The structure, luminescence and thermal quenching properties with varying Ba/Sr ratio were investigated in detail. - Highlights: • The stucture and luminescence properties of Eu doped Sr{sub 2-x}Ba{sub x}Si{sub 5}N{sub 8} phosphors were investigated. • The samples with the intermediate compositions(x=1.0,1.5) show better stability than the end members of both Sr{sub 2}Si{sub 5}N{sub 8}:Eu{sup 2+} and Ba{sub 2}Si{sub 5}N{sub 8}:Eu{sup 2+}. • The possible mechanism for the improvement of thermal quenching properties was proposed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗