16,134 research outputs found
Pair plasma cushions in the hole-boring scenario
Pulses from a 10 PW laser are predicted to produce large numbers of
gamma-rays and electron-positron pairs on hitting a solid target. However, a
pair plasma, if it accumulates in front of the target, may partially shield it
from the pulse. Using stationary, one-dimensional solutions of the two-fluid
(electron-positron) and Maxwell equations, including a classical radiation
reaction term, we examine this effect in the hole-boring scenario. We find the
collective effects of a pair plasma "cushion" substantially reduce the
reflectivity, converting the absorbed flux into high-energy gamma-rays. There
is also a modest increase in the laser intensity needed to achieve threshold
for a non-linear pair cascade.Comment: 17 pages, 5 figures. Accepted for publication in Plasma Physics and
Controlled Fusion. Typos corrected, reference update
Rayleigh scattering in the transmission spectrum of HAT-P-18b
We have performed ground-based transmission spectroscopy of the hot Jupiter
HAT-P-18b using the ACAM instrument on the William Herschel Telescope (WHT).
Differential spectroscopy over an entire night was carried out at a resolution
of using a nearby comparison star. We detect a bluewards slope
extending across our optical transmission spectrum which runs from 4750 to
9250\AA. The slope is consistent with Rayleigh scattering at the equilibrium
temperature of the planet (852K). We do not detect enhanced sodium absorption,
which indicates that a high-altitude haze is masking the feature and giving
rise to the Rayleigh slope. This is only the second discovery of a Rayleigh
scattering slope in a hot Jupiter atmosphere from the ground, and our study
illustrates how ground-based observations can provide transmission spectra with
precision comparable to the Hubble Space Telescope.Comment: 11 pages, 9 figures, accepted for publication in MNRA
LRG-BEASTS III: Ground-based transmission spectrum of the gas giant orbiting the cool dwarf WASP-80
We have performed ground-based transmission spectroscopy of the hot Jupiter
orbiting the cool dwarf WASP-80 using the ACAM instrument on the William
Herschel Telescope (WHT) as part of the LRG-BEASTS programme. This is the third
paper of a ground-based transmission spectroscopy survey of hot Jupiters using
low-resolution grism spectrographs. We observed two transits of the planet and
have constructed transmission spectra spanning a wavelength range of
4640-8840A. Our transmission spectrum is inconsistent with a previously claimed
detection of potassium in WASP-80b's atmosphere, and is instead most consistent
with a haze. We also do not see evidence for sodium absorption at a resolution
of 100A.Comment: 11 pages, 9 figures. Accepted for publication in MNRA
Mechanical capacitor
A new energy storage system (the mechanical capacitor), using a spokeless magnetically levitated composite ring rotor, is described and design formulas for sizing the components are presented. This new system is configured around a permanent magnet (flux biased) suspension which has active servo control in the radial direction and passive control in the axial direction. The storage ring is used as a moving rotor and electronic commutation of the stationary armature coils is proposed. There is no mechanical contact with the rotating spokeless ring; therefore, long life and near zero rundown losses are projected. A 7-kW h system is sized to demonstrate feasibility. A literature review of flywheel energy storage systems is also presented and general formulas are developed for comparing rotor geometries
Geological and geothermal investigations for HCMM-derived data
An attempt was made to match HCMM- and U2HCMR-derived temperature data over two test sites of very local size to similar data collected in the field at nearly the same times. Results indicate that HCMM investigations using resolutions cells of 500 m or so are best conducted with areally-extensive sites, rather than point observations. The excellent quality day-VIS imagery is particularly useful for lineament studies, as is the DELTA-T imagery. Attempts to register the ground observed temperatures (even for 0.5 sq mile targets) were unsuccessful due to excessive pixel-to-pixel noise on the HCMM data. Several computer models were explored and related to thermal parameter value changes with observed data. Unless quite complex models, with many parameters which can be observed (perhaps not even measured (perhaps not even measured) only under remote sensing conditions (e.g., roughness, wind shear, etc) are used, the model outputs do not match the observed data. Empirical relationship may be most readily studied
Growth rates of the Weibel and tearing mode instabilities in a relativistic pair plasma
We present an algorithm for solving the linear dispersion relation in an
inhomogeneous, magnetised, relativistic plasma. The method is a generalisation
of a previously reported algorithm that was limited to the homogeneous case.
The extension involves projecting the spatial dependence of the perturbations
onto a set of basis functions that satisfy the boundary conditions (spectral
Galerkin method). To test this algorithm in the homogeneous case, we derive an
analytical expression for the growth rate of the Weibel instability for a
relativistic Maxwellian distribution and compare it with the numerical results.
In the inhomogeneous case, we present solutions of the dispersion relation for
the relativistic tearing mode, making no assumption about the thickness of the
current sheet, and check the numerical method against the analytical
expression.Comment: Accepted by PPC
Aerodynamics of lift fan V/STOL aircraft
Aerodynamic characteristics of lift fan installation for direct lift V/STOL aircraf
Self-consistent models for Coulomb heated X-ray pulsar atmospheres
Calculations of accreting magnetized neutron star atmospheres heated by the gradual deceleration of protons via Coulomb collisions are presented. Self consistent determinations of the temperature and density structure for different accretion rates are made by assuming hydrostatic equilibrium and energy balance, coupled with radiative transfer. The full radiative transfer in two polarizations, using magnetic cross sections but with cyclotron resonance effects treated approximately, is carried out in the inhomogeneous atmospheres
- …