29,082 research outputs found
Gradient-index Solar Sail and its Optimal Orbital Control
Solar sails with the capability of generating a tangential radiation pressure
at the sun-pointing attitude, such as refractive sails can provide more
efficient methods for attitude and orbital control of sailcraft. This paper
presents the concept of gradient-index sail as an advanced class of refractive
sail, which operates by guiding the solar radiation through a structure made of
graded refractive index material. The design of the sail's refractive index
distribution is performed by transformation optics, and the resultant index
realized by the effective refractive index of non-resonant bulk metamaterials
made of silica. The performance of the sail was evaluated by using ray tracing
for a broad spectrum of solar radiation under the normal incidence angle, which
showed an efficiency of 90.5% for generation of a tangential radiation
pressure. We also studied the orbital control of the
tangential-radiation-pressure-generating sails, and showed that the full
orbital control, including the modification of orbital axes, eccentricity, and
inclination can be applied by changing the attitude of the sail merely around
the sun-sail axis, while the sail keeps the sun-pointing attitude at every
point of the orbit
Towards a warped inflationary brane scanning
We present a detailed systematics for comparing warped brane inflation with
the observations, incorporating the effects of both moduli stabilization and
ultraviolet bulk physics. We explicitly construct an example of the inflaton
potential governing the motion of a mobile D3 brane in the entire warped
deformed conifold. This allows us to precisely identify the corresponding
scales of the cosmic microwave background. The effects due to bulk fluxes or
localized sources are parametrized using gauge/string duality. We next perform
some sample scannings to explore the parameter space of the complete potential,
and first demonstrate that without the bulk effects there can be large
degenerate sets of parameters with observationally consistent predictions. When
the bulk perturbations are included, however, the observational predictions are
generally spoiled. For them to remain consistent, the magnitudes of the bulk
effects need to be highly suppressed via fine tuning.Comment: (v1) 11 pages, 2 figures, 2 tables; (v2) more clarifications and
references added; (v3) 12 pages, more discussions, to appear in Physical
Review
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