SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Quantification of the intensity and linewidth of the ν (CN) IR band in a series of neutral and anionic nitrile-functionalized oligophenylenes reveals that the CN vibration is coupled to nuclear and electronic structural changes.
Studenikin et al. recently demonstrated a significant enhancement of the fringe contrast of coherent Landau-Zener-Stückelberg (LZS) oscillations between singlet S and triplet T+ two-spin states using a modified charge detection technique called enhanced charge detection (ECD). In this paper we explain the amplitude phase reversal and confirm the magnitude of the effect is consistent with our calibrations. We also show that the enhancement cannot be explained by a T{sub 1} effect.
Direct measurements of the vector turbulent emf in a driven two-vortex flow of liquid sodium were performed in the Madison Dynamo Experiment [K. Rahbarnia et al., Astrophys. J. 759, 80 (2012)]. The measured turbulent emf is anti-parallel with the mean current and is almost entirely described by an enhanced resistivity, which increases the threshold for a kinematic dynamo. We have demonstrated that this enhanced resistivity can be mitigated by eliminating the largest-scale eddies through the introduction of baffles. By tailoring the flow to reduce large-scale components and control the helical pitch, we have reduced the power required to drive the impellers, doubled the magnetic flux generated by differential rotation, and increased the decay time of externally applied magnetic fields. Despite these improvements, the flows remain sub-critical to the dynamo instability due to the reemergence of turbulent fluctuations at high flow speeds.
Er{sup 3+}:Yb{sup 3+} co-doped Sodium telluroborate glasses were prepared with the chemical composition (49.5–x)B{sub 2}O{sub 3}+25TeO{sub 2}+5Li{sub 2}CO{sub 3}+10ZnO+10NaF+0.5Er{sub 2}O{sub 3}+xYb{sub 2}O{sub 3} (where x= 0.1, 0.5, 1.0 and 2.0 in mol %) following the melt quenching technique. With the addition of Yb{sup 3+} ions into Er{sup 3+} ions in the prepared glasses, the absorption cross-section values were found to increase due to the effective energy transfer from {sup 2}F{sub 5/2} level of Yb{sup 3+} ions to the {sup 4}I{sub 11/2} level of Er{sup 3+} ions. The fluorescence around 1550 nm correspond to the {sup 4}I{sub 13/2}→{sup 4}I{sub 15/2} transition was observed under 980 nm pumping. Among the present glasses, integrated intensity was found to be higher for 1.0 mol% Yb{sup 3+} ion glass. The parameters such as stimulated emission cross- section, Gain bandwidth and quantum efficiency of the {sup 4}I{sub 13/2}→{sup 4}I{sub 15/2} transition was found to be higher for the NTBE1.0Y glass and the same is suggested for potential NIR lasers and optical amplifier applications.
We investigate the effect of an electrical current on the attenuation length of a 900 nm wavelength spin-wave in a permalloy/Pt bilayer using propagating spin-wave spectroscopy. The modification of the spin-wave relaxation rate is linear in current density, reaching up to 14% for a current density of 2.3 × 10{sup 11} A/m{sup 2} in Pt. This change is attributed to the spin transfer torque induced by the spin Hall effect and corresponds to an effective spin Hall angle of 0.13, which is among the highest values reported so far. The spin Hall effect thus appears as an efficient way of amplifying/attenuating propagating spin waves.
This article was originally published online on 18 December 2017. Due to a production error, the initially-published version contained several typographical errors not corrected during proof revisions, none of which affected the substance of the article. AIP Publishing apologizes for this error. All online versions of the article were corrected on 21 December 2017, and it now appears correctly in print.
Propagation of charged particle beams in background plasma as a method of space charge neutralization has been shown to achieve a high degree of charge and current neutralization and therefore enables nearly ballistic propagation and focusing of charged particle beams. Correspondingly, the use of plasmas for propagation of charged particle beams has important applications for transport and focusing of intense particle beams in inertial fusion and high energy density laboratory plasma physics. However, the streaming of beam ions through a background plasma can lead to the development of two-stream instability between the beam ions and the plasma electrons. The beam electric and magnetic fields enhanced by the two-stream instability can lead to defocusing of the ion beam. Using particle-in-cell simulations, we study the scaling of the instability-driven self-electromagnetic fields and consequent defocusing forces with the background plasma density and beam ion mass. In conclusion, we identify plasma parameters where the defocusing forces can be reduced.
Here, we demonstrate the use of custom high electron mobility transistors (HEMTs) fabricated in GaAs/AlGaAs heterostructures to amplify current from quantum dot devices. The amplifier circuit is located adjacent to the quantum dot device, at sub-Kelvin temperatures, in order to reduce the impact of cable capacitance and environmental noise. Using this circuit, we show a current gain of 380 for 0.56 μW of power dissipation, with a bandwidth of 2.7 MHz and current noise referred to the input of 24 fA/Hz 1/2 for frequencies of 0.1–1 MHz. The power consumption required for similar gain is reduced by more than a factor of 20 compared to a previous demonstration using a commercial off-the-shelf HEMT. Lastly, we also demonstrate integration of a HEMT amplifier circuit on-chip with a quantum dot device, which has the potential to reduce parasitics and should allow for more complex circuits with reduced footprints.
We report that plasmas may be used as gain media for amplifying intense lasers, and external magnetic fields may be applied to improve the performance. For midinfrared lasers, the requisite magnetic field is on the megagauss scale, which can already be provided by current technologies. Designing the laser amplifier requires knowing the magnetized three-wave coupling coefficient, which is mapped out systematically in this paper. By numerically evaluating its formula, we demonstrate how the coupling coefficient depends on the angle of wave propagation, laser polarization, magnetic field strength, plasma temperature, and plasma density in the backscattering geometry. In conclusion, since the mediation is now provided by magnetized plasma waves, the coupling can differ significantly from unmagnetized Raman and Brillouin scatterings.
The gain characteristics of the medium of a pulsed DF – CO{sub 2} laser in the ten-micron region at the working gas pressures from 1 to 2.5 atm, which were experimentally determined in [4], are numerically simulated using a scheme that includes the main chemical and relaxation processes. It is shown that the chosen scheme of processes makes it possible (i) to numerically describe the experimental data on the temporal behaviour of gains; (ii) to explain the reason for early degradation of gains (at the degree of D{sub 2} transformation of about 20%); (iii) from comparison of experimental and calculated temporal gain profiles, to determine the degree of photodissociation of F{sub 2} molecules, which is an important parameter determining the operation of pulsed chemical DF – CO{sub 2} lasers; and (iv) to predict the gain characteristics of working mixtures depending on their composition and pressure and on the initiation parameters. The predicted gains in the mixtures of the optimal composition at a pressure of 2.5 atm, a degree of dissociation of F{sub 2} molecules per flash ∼1%, and a flash duration at half-width of ∼3 μs are ∼7 m{sup -1}. (lasers)
The scheme generating UV pulses with a duration 15 ps and output energy up to 11.5 mJ is implemented in the system consisting of a picosecond Nd{sup 3+}:YAG laser with multistage nonlinearoptical conversion of fundamental frequency radiation into radiation with a wavelength 193 nm followed by an excimer ArF amplifier. The temporal characteristics and the contrast of the amplified pulses are measured. (lasers)
Numerical simulation methods have been used to study the effect of frequency modulation of input laser radiation with a pulse duration of 250 ps on the gain characteristics in a gas XeF (C – A) amplifier of the hybrid THL-100 laser system at a pump energy of 270 – 400 J. Using the Wigner distribution function, local radiation spectra are calculated at various points of the input laser beam with positive and negative chirps. The effect of the pump energy of the amplifier on the energy, intensity, and duration of the output laser radiation pulses is investigated. It is shown that with an insignificant change in energy, the pulse duration of the output radiation turns out to be longer with a negative chirp of the input radiation. (lasers)
Not Available
The avalanche of runaway electrons is thought to pose a significant obstacle to the success of reactor scale devices such as ITER. As a result, a significant effort has been devoted toward quantifying both the threshold for the initiation of the avalanche of runaway electrons and the efficiency of the avalanche mechanism. In this work, these two quantities are computed utilizing a guiding-center formulation with large-angle collision operators of varying physics fidelity. The use of a guiding-center formulation, while computationally more costly compared to bounce-averaged approaches, provides a conceptually straightforward means of incorporating tokamak geometry. It is found that while the avalanche threshold is only weakly impacted by toroidal geometry for fully ionized low- Z plasmas, it can be significantly impacted if high- Z impurities are present. Furthermore, it is shown that the efficiency of the avalanche mechanism depends sensitively on the impurity content, the charge state of the underlying impurities, and the radial profile of the seed electron population. Lastly, the commonly employed Møller secondary source term used to model the generation of secondary electrons is shown to yield avalanche growth rates and thresholds in good agreement with a more complete conservative large-angle collision operator.
The enhanced vegetation growth by climate warming plays a pivotal role in amplifying the seasonal cycle of atmospheric CO 2 at northern lands (>50° N) since 1960s. However, the correlation between vegetation growth, temperature and seasonal amplitude of atmospheric CO 2 concentration have become elusive with the slowed increasing trend of vegetation growth and weakened temperature control on CO 2 uptake since late 1990s. Here, based on in situ atmospheric CO 2 concentration records from the Barrow observatory site, we found a slowdown in the increasing trend of the atmospheric CO 2 amplitude from 1990s to mid-2000s. This phenomenon was associated with the paused decrease in the minimum CO 2 concentration ([CO 2 ]min), which was significantly correlated with the slowdown of vegetation greening and growing-season length extension. We then showed that both the vegetation greenness and growing-season length were positively correlated with spring but not autumn temperature over the northern lands. Furthermore, such asymmetric dependences of vegetation growth upon spring and autumn temperature cannot be captured by the state-of-art terrestrial biosphere models. These findings indicate that the responses of vegetation growth to spring and autumn warming are asymmetric, and highlight the need of improving autumn phenology in the models for predicting seasonal cycle of atmospheric CO 2 concentration.
Explore the source record for details and available documents.