7,355 research outputs found
Advanced propulsion for LEO-Moon transport. 1: A method for evaluating advanced propulsion performance
This report describes a study to evaluate the benefits of advanced propulsion technologies for transporting materials between low Earth orbit and the Moon. A relatively conventional reference transportation system, and several other systems, each of which includes one advanced technology component, are compared in terms of how well they perform a chosen mission objective. The evaluation method is based on a pairwise life-cycle cost comparison of each of the advanced systems with the reference system. Somewhat novel and economically important features of the procedure are the inclusion not only of mass payback ratios based on Earth launch costs, but also of repair and capital acquisition costs, and of adjustments in the latter to reflect the technological maturity of the advanced technologies. The required input information is developed by panels of experts. The overall scope and approach of the study are presented in the introduction. The bulk of the paper describes the evaluation method; the reference system and an advanced transportation system, including a spinning tether in an eccentric Earth orbit, are used to illustrate it
Stringent upper limit on the direct 3α decay of the Hoyle state in 12C
We investigate an implication of the most recent observation of a second Jπ=2+ state in 12C, which was measured using the 12C(γ,α)8Be(g.s.) reaction. In addition to the dissociation of 12C to an α-particle and 8Be in its ground state, a small fraction of events (2%) were identified as direct decays and decays to excited states in 8Be. This allowed a limit on the direct 3α partial decay width to be determined as Γ3α<32(4) keV. Since this 2+ state is predicted by all theoretical models to be a collective excitation of the Hoyle state, the 3α partial width of the Hoyle state is calculable from the ratio of 3α decay penetrabilities of the Hoyle and 2+ states. This was calculated by using the semiclassical Wenzel-Kramers-Brillouin approach and we deduce a stringent upper limit for the direct decay branching ratio of the Hoyle state of
Γ3α
Γ
<5.7×10−6, over an order of magnitude lower than previously reported. This result places the direct measurement of this rare decay mode beyond current experimental capabilities
Solving the Jitter Problem in Microwave Compressed Ultrafast Electron Diffraction Instruments: Robust Sub-50 fs Cavity-Laser Phase Stabilization
We demonstrate the compression of electron pulses in a high-brightness
ultrafast electron diffraction (UED) instrument using phase-locked microwave
signals directly generated from a mode-locked femtosecond oscillator.
Additionally, a continuous-wave phase stabilization system that accurately
corrects for phase fluctuations arising in the compression cavity from both
power amplification and thermal drift induced detuning was designed and
implemented. An improvement in the microwave timing stability from 100 fs to 5
fs RMS is measured electronically and the long-term arrival time stability
(10 hours) of the electron pulses improves to below our measurement
resolution of 50 fs. These results demonstrate sub-relativistic ultrafast
electron diffraction with compressed pulses that is no longer limited by
laser-microwave synchronization.Comment: Accepted for publication in Structural Dynamic
The spectrum of lattice QCD with staggered fermions at strong coupling
Using 4 flavors of staggered fermions at infinite gauge coupling, we compare
various analytic results for the hadron spectrum with exact Monte Carlo
simulations. Agreement with Ref. \cite{Martin_etal} is very good, at the level
of a few percent.
Our results give credence to a discrepancy between the baryon mass and the
critical chemical potential, for which baryons fill the lattice at zero
temperature and infinite gauge coupling. Independent determinations of the
latter set it at about 30% less than the baryon mass. One possible explanation
is that the nuclear attraction becomes strong at infinite gauge coupling.Comment: 11 pages, 3 figure
Some Experimental Non-Constant-Sum Games Revisited. Perception of Opponent\u27s Payoffs. Part III
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