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Chen, S

Publications and source records attributed to Chen, S.

41 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Generation of tunable, 100-800 MeV quasi-monoenergetic electron beams from a laser-wakefield accelerator in the blowout regime

In this paper, we present results on a scalable high-energy electron source based on laser wakefield acceleration. The electron accelerator using 30-80 TW, 30 fs laser pulses, operates in the blowout regime, and produces high-quality, quasi-monoenergetic electron beams in the range 100-800 MeV. These beams have angular divergence of 1-4 mrad, and 5%-25% energy spread, with a resulting brightness 10{sup 11} electrons mm{sup -2} MeV{sup -1} mrad{sup -2}. The beam parameters can be tuned by varying the laser and plasma conditions. The use of a high-quality laser pulse and appropriate target conditions enables optimization of beam quality, concentrating a significant fraction of the accelerated charge into the quasi-monoenergetic component.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

IS THE COSMIC MICROWAVE BACKGROUND ASYMMETRY DUE TO THE KINEMATIC DIPOLE?

Parity violation found in the cosmic microwave background (CMB) radiation is a crucial clue for the non-standard cosmological model or the possible contamination of various foreground residuals and/or calibration of the CMB data sets. In this paper, we study the directional properties of the CMB parity asymmetry by excluding the m = 0 modes in the definition of parity parameters. We find that the preferred directions of the parity parameters coincide with the CMB kinematic dipole, which implies that the CMB parity asymmetry may be connected with the possible contamination of the residual dipole component. We also find that such tendency is not only localized at l = 2, 3, but in the extended multipole ranges up to l {approx} 22.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Diameter-Dependent Modulus and Melting Behavior in Electrospun Semicrystalline Polymer Fibers

Confinement of the semicrystalline polymers, poly(ethylene-co-vinyl acetate) (PEVA) and low-density polyethylene (LDPE), produced by electrospinning has been observed to produce fibers with large protrusions, which have not been previously observed in fibers of comparable diameters produced by other methods. SAXS spectra confirmed the crystalline structure and determined that the lamellar spacing was almost unchanged from the bulk. Measurement of the mechanical properties of these fibers, by both shear modulation force microscopy (SMFM) and atomic force acoustic microscopy (AFAM), indicates that the modulii of these fibers increases with decreasing diameter, with the onset at {approx}10 {micro}m, which is an order of magnitude larger than previously reported. Melting point measurements indicate a decrease of more than 7% in T{sub m}/T{sub 0} (where T{sub m} is the melting point of semicrystalline polymer fibers and T{sub 0} is the melting point of the bulk polymer) for fibers ranging from 4 to 10 {micro}m in diameter. The functional form of the decrease followed a universal curve for PEVA, when scaled with T{sub 0}.

36 MATERIALS SCIENCE↗
DOE OSTI2011

The Influence of Heat-Treatment Temperature on the Cation Distribution of LiNi0.5Mn0.5O2 and Its Rate Capability in Lithium Rechargeable Batteries

LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} samples were prepared from NiMnO{sub 3} and Li{sub 2}CO{sub 3} in a range of temperatures from 900 to 1050 C. Synchrotron X-ray diffraction analysis combined with X-ray absorption spectroscopy showed that LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} segregated into one major Ni{sup 2+}O-enriched phase and one minor Li{sub 2}Mn{sup 4+}O{sub 3}-enriched phase, where the extent of segregation decreased with increasing synthesis temperature from 900 to 1050 C. Scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy revealed that the segregated domains exist in individual particles. Although all of the LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} samples showed comparable specific capacity ({approx}200 mAh/g) and capacity retention at low current densities, the rate capability of LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} of 900 C is lower than that of LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} of 1000 C. As X-ray photoelectron spectroscopy analysis showed that all of the LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} samples had comparable surface chemistry, the higher rate capability of LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} of 1000 C can be attributed to reduced cation segregation of Ni{sup 2+}O-enriched domains in the layered structure of the major phase, having potentially faster lithium diffusion than that of LiNi{sub 0.5}Mn{sub 0.5}O{sub 2} of 900 C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗