10,758 research outputs found
Star scanner
A star scanner on a spin stabilized spacecraft is described which includes a reticle with a pair of slits having different separations as a function of the spacecraft vertical plane, to form a V slit. The time between a star image crossing one of the slits relative to a reference telemetry time provides an indication of azimuth angle. The time between the image crossing the two slits provides an indication of elevation angle of the star. If a star cluster is detected such that two stars pass the slits in less time than normally required for a single star to cross the two slits, an indication of the cluster occurrence is derived. Means are provided to prevent effective detection of large celestial bodies, such as the sun or moon
Active Brownian Motion Tunable by Light
Active Brownian particles are capable of taking up energy from their
environment and converting it into directed motion; examples range from
chemotactic cells and bacteria to artificial micro-swimmers. We have recently
demonstrated that Janus particles, i.e. gold-capped colloidal spheres,
suspended in a critical binary liquid mixture perform active Brownian motion
when illuminated by light. In this article, we investigate in some more details
their swimming mechanism leading to active Brownian motion. We show that the
illumination-borne heating induces a local asymmetric demixing of the binary
mixture generating a spatial chemical concentration gradient, which is
responsible for the particle's self-diffusiophoretic motion. We study this
effect as a function of the functionalization of the gold cap, the particle
size and the illumination intensity: the functionalization determines what
component of the binary mixture is preferentially adsorbed at the cap and the
swimming direction (towards or away from the cap); the particle size determines
the rotational diffusion and, therefore, the random reorientation of the
particle; and the intensity tunes the strength of the heating and, therefore,
of the motion. Finally, we harness this dependence of the swimming strength on
the illumination intensity to investigate the behaviour of a micro-swimmer in a
spatial light gradient, where its swimming properties are space-dependent
An electromagnetically levitated two-axis gimbaless pointing mechanism
A brief description for a proposed new pointing mechanism which requires no mechanical gimbals, is virtually friction free, and is vibration isolated from a ground support system or vehicle is presented. The device uses electromagnetic forces for support levitation and pointing, both being accomplished from a ground reference thereby leaving the payload virtually free from a remotely located command center. Solid pointing angles of almost 2(pi) steradians are achievable, limited only by structural interference. A third degree-of-freedom tilt axis can be added at will, but will not be elaborated. Although the system is primarily intended for space vehicles in a micro-gravity environment, earth-ground support is possible with superconducting electromagnets
Low energy neutrino scattering measurements at future Spallation Source facilities
In the future several Spallation Source facilities will be available
worldwide. Spallation Sources produce large amount of neutrinos from
decay-at-rest muons and thus can be well adapted to accommodate
state-of-the-art neutrino experiments. In this paper low energy neutrino
scattering experiments that can be performed at such facilities are reviewed.
Estimation of expected event rates are given for several nuclei, electrons and
protons at a detector located close to the source. A neutrino program at
Spallation Sources comprises neutrino-nucleus cross section measurements
relevant for neutrino and core-collapse supernova physics, electroweak tests
and lepton-flavor violation searches.Comment: 12 pages, 4 figures, 5 table
Experimental and numerical study of error fields in the CNT stellarator
Sources of error fields were indirectly inferred in a stellarator by
reconciling computed and numerical flux surfaces. Sources considered so far
include the displacements and tilts (but not the deformations, yet) of the four
circular coils featured in the simple CNT stellarator. The flux surfaces were
measured by means of an electron beam and phosphor rod, and were computed by
means of a Biot-Savart field-line tracing code. If the ideal coil locations and
orientations are used in the computation, agreement with measurements is poor.
Discrepancies are ascribed to errors in the positioning and orientation of the
in-vessel interlocked coils. To that end, an iterative numerical method was
developed. A Newton-Raphson algorithm searches for the coils' displacements and
tilts that minimize the discrepancy between the measured and computed flux
surfaces. This method was verified by misplacing and tilting the coils in a
numerical model of CNT, calculating the flux surfaces that they generated, and
testing the algorithm's ability to deduce the coils' displacements and tilts.
Subsequently, the numerical method was applied to the experimental data,
arriving at a set of coil displacements whose resulting field errors exhibited
significantly improved quantitative and qualitative agreement with experimental
results.Comment: Special Issue on the 20th International Stellarator-Heliotron
Worksho
Neutrino-nucleus interaction rates at a low-energy beta-beam facility
We compute the neutrino detection rates to be expected at a low-energy
beta-beam facility. We consider various nuclei as neutrino detectors and
compare the case of a small versus large storage ring.Comment: 6 pages, 3 figure
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