1,569 research outputs found
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Development of ferritic steels for fusion reactor applications
Chromium-molybdenum ferritic (martensitic) steels are leading candidates for the structural components for future fusion reactors. However, irradiation of such steels in a fusion environment will produce long-lived radioactive isotopes that will lead to difficult waste-disposal problems. Such problems could be reduced by replacing the elements in the steels (i.e., Mo, Nb, Ni, N, and Cu) that lead to long-lived radioactive isotopes. We have proposed the development of ferritic steels analogous to conventional Cr-Mo steels, which contain molybdenum and niobium. It is proposed that molybdenum be replaced by tungsten and niobium be replaced by tantalum. Eight experimental steels were produced. Chromium concentrations of 2.25, 5, 9, and 12% were used (all concentrations are in wt %). Steels with these chromium compositions, each containing 2% W and 0.25% V, were produced. To determine the effect of tungsten and vanadium, 2.25 Cr steels were produced with 2% W and no vanadium and with 0.25% V and O and 1% W. A 9Cr steel containing 2% W, 0.25 V, and 0.07% Ta was also studied. For all alloys, carbon was maintained at 0.1%. Tempering studies on the normalized steels indicated that the tempering behavior of the new Cr-W steels was similar to that of the analogous Cr-Mo steels. Microscopy studies indicated that 2% tungsten was required in the 2.25 Cr steels to produce 100% bainite in 15.9-mm-thick plate during normalization. The 5Cr and 9Cr steels were 100% martensite, but the 12 Cr steel contained about 75% martensite with the balance delta-ferrite. 33 refs., 35 figs., 5 tabs
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Comparisons of irradiation-induced shifts in fracture toughness, crack arrest toughness, and Charpy impact energy in high-copper welds
The Heavy-Section Steel Irradiation (HSSI) Program is examining relative shifts and changes in shape of fracture and crack-arrest toughness versus temperature behavior for two high-copper welds. Fracture toughness 100-MPa{radical}m temperature shifts are greater than Charpy 41-J shifts for both welds. Mean curve fits to the fracture toughness data provide mixed results regarding curve shape changes, but curves constructed as lower boundaries indicate lower slopes. Preliminary crack-arrest toughness results indicate that shifts of lower-bound curves are approximately the same as CVN 41-J shifts with no shape changes
Cooling atoms in an optical trap by selective parametric excitation
We demonstrate the possibility of energy-selective removal of cold atoms from
a tight optical trap by means of parametric excitation of the trap vibrational
modes. Taking advantage of the anharmonicity of the trap potential, we
selectively remove the most energetic trapped atoms or excite those at the
bottom of the trap by tuning the parametric modulation frequency. This process,
which had been previously identified as a possible source of heating, also
appears to be a robust way for forcing evaporative cooling in anharmonic traps.Comment: 5 pages, 5 figure
A multi-color and Fourier study of RR Lyrae variables in the globular cluster NGC 5272 (M3)
We have performed a detailed study of the pulsational and evolutionary
characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3)
using highly accurate BVI data taken on 5 separate epochs. M3 seems to contain
no less than ~32% of Blazhko stars, and the occurrence and characteristics of
the Blazhko effect have been analyzed in detail. We have identified a good
number (~ 14%) of overluminous RR Lyrae stars that are likely in a more
advanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physical
parameters (i.e. temperature, luminosity, mass) have been derived from (B--V)
colors and accurate color-temperature calibration, and compared with Horizontal
Branch evolutionary models and with the requirements of stellar pulsation
theory. Additional analysis by means of Fourier decomposition of the V light
curves confirms, as expected, that no metallicity spread is present in M3.
Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko
stars at low amplitude phase do affect [Fe/H] distributions as they appear more
metal-rich. Absolute magnitudes derived from Fourier coefficients might provide
useful average estimates for groups of stars, if applicable, but do not give
reliable {\em individual} values. Intrinsic colors derived from Fourier
coefficients show significant discrepancies with the observed ones, hence the
resulting temperatures and temperature-related parameters are unreliable.Comment: 86 pages, 19 figures, 13 tables, in press A
Observation of p-wave Threshold Law Using Evaporatively Cooled Fermionic Atoms
We have measured independently both s-wave and p-wave cross-dimensional
thermalization rates for ultracold potassium-40 atoms held in a magnetic trap.
These measurements reveal that this fermionic isotope has a large positive
s-wave triplet scattering length in addition to a low temperature p-wave shape
resonance. We have observed directly the p-wave threshold law which, combined
with the Fermi statistics, dramatically suppresses elastic collision rates at
low temperatures. In addition, we present initial evaporative cooling results
that make possible these collision measurements and are a precursor to
achieving quantum degeneracy in this neutral, low-density Fermi system.Comment: 5 pages, 3 figures, 1 tabl
Theory of an optical dipole trap for cold atoms
The theory of an atom dipole trap composed of a focused, far red-detuned, trapping laser beam, and a pair of red-detuned, counterpropagating, cooling beams is developed for the simplest realistic multilevel dipole interaction scheme based on a model of a (3+5)-level atom. The description of atomic motion in the trap is based on the quantum kinetic equations for the atomic density matrix and the reduced quasiclassical kinetic equation for atomic distribution function. It is shown that when the detuning of the trapping field is much larger than the detuning of the cooling field, and with low saturation, the one-photon absorption (emission) processes responsible for the trapping potential can be well separated from the two-photon processes responsible for sub-Doppler cooling atoms in the trap. Two conditions are derived that are necessary and sufficient for stable atomic trapping. The conditions show that stable atomic trapping in the optical dipole trap can be achieved when the trapping field has no effect on the two-photon cooling process and when the cooling field does not change the structure of the trapping potential but changes only the numerical value of the trapping potential well. It is concluded that the separation of the trapping and cooling processes in a pure optical dipole trap allows one to cool trapped atoms down to a minimum temperature close to the recoil temperature, keeping simultaneously a deep potential well
Direction Of Hepatitis Supported Research At Namru-2
Titik berat penelitian hepatitis di NAMRU-2 pada saat ini adalah: I) penelitian binatang untuk mengetahui populasi reservoar HEV; 2) penggunaan model hewan untuk lebih diketahuinya transmisi HEV pada populasi reservoar yang dicurigai; 3) diketahuinya akurasi alat diagnostik yang digunakan (untuk semua marker); 4) diketahuinya lama pengeluaran virus hepatitis E pada kotoran manusia yang menderita hepatitis akut maupun dari model hewan; 5) diketahuinya insidens hepatitis E akut melalui pencarian kasus dengan menggunakan metode penelitian lapangan di masyarakat; dan 6) serokonversi infeksi hepatitis E pada anak-anak. Hubungan erat dengan berbagai universitas dan instansi pemerintah yang terkait dengan penanganan kasus-kasus hepatitis, telah dan masih menjadi bagian penting bagi suksesnya penelitian hepatitis di NAMRU-2. Hasil yang dicapai berdasarkan program penelitian di atas antara lain: 1) seminar hepatitis E di Kalimantan Barat; 2) tersedianya kemampuan diagnostik pada laboratorium setempat; 3) pembinaan peneliti di Indonesia dan Asia Tenggara dalam hal epidemiologi; serta 4) alih teknologi dalam pelaksanaan penelitian yang dapat melibatkan minat para peneliti pada penyakit hepatitis di Indonesia
The Globular Cluster M15. I. Identification, Discovery, and Period Determination of Variable Stars
We present new CCD photometry for variables in the globular cluster
M15. Our photometry was obtained using both the image subtraction package ISIS
and DAOPHOT/ALLFRAME. The data were acquired in 2001 on two observing runs on
11 observing nights using the 2-m telescope of the Bulgarian National
Astronomical Observatory ``Rozhen'' with a Photometrics CCD camera. For 40
previously known variables, we present a period for the first time, and
improved periods were obtained for many previously known variables. Fourteen
new variables are reported. We present updated Bailey diagrams for the cluster
and discuss its Oosterhoff classification. Although many of M15's RRab
pulsators fall at an intermediate locus between Oosterhoff types I and II in
the Bailey diagram, we argue that M15 is indeed a bona-fide Oosterhoff type IIComment: 48 pages, 7 figures, 3 table
Sub-Doppler spectroscopy of Rb atoms in a sub-micron vapor cell in the presence of a magnetic field
We report the first use of an extremely thin vapor cell (thickness ~ 400 nm)
to study the magnetic-field dependence of laser-induced-fluorescence excitation
spectra of alkali atoms. This thin cell allows for sub-Doppler resolution
without the complexity of atomic beam or laser cooling techniques. This
technique is used to study the laser-induced-fluorescence excitation spectra of
Rb in a 50 G magnetic field. At this field strength the electronic angular
momentum J and nuclear angular momentum I are only partially decoupled. As a
result of the mixing of wavefunctions of different hyperfine states, we observe
a nonlinear Zeeman effect for each sublevel, a substantial modification of the
transition probabilities between different magnetic sublevels, and the
appearance of transitions that are strictly forbidden in the absence of the
magnetic field. For the case of right- and left- handed circularly polarized
laser excitation, the fluorescence spectra differs qualitatively. Well
pronounced magnetic field induced circular dichroism is observed. These
observations are explained with a standard approach that describes the partial
decoupling of I and J states
Cooper Pairing in Ultracold K-40 Using Feshbach Resonances
We point out that the fermionic isotope K-40 is a likely candidate for the
formation of Cooper pairs in an ultracold atomic gas. Specifically, in an
optical trap that simultaneously traps the spin states |9/2,-9/2> and
|9/2,-7/2>, there exists a broad magnetic field Feshbach resonance at B = 196
gauss that can provide the required strong attractive interaction between
atoms. An additional resonance, at B = 191 gauss, could generate p-wave pairing
between identical |9/2,-7/2> atoms. A Cooper-paired degenerate Fermi gas could
thus be constructed with existing ultracold atom technology.Comment: 4 pages, 2 figs, submitted to Phys. Rev.
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