80 research outputs found
Properties of the background of EPIC-pn onboard XMM-Newton
We have investigated the background properties of EPIC-pn onboard XMM-Newton
to establish the background subtraction method. Count rates of the background
vary violently by two orders of magnitude at the maximum, while during the most
quiet period, these are stable within 8 % at a 1 level. The overall
spectrum is dominated by particle events above 5 keV, and its spatial variation
is also found. The long-term variation of the background is also investigated
with CAL CLOSED data, where the filter wheel was in closed position with the
internal calibration source illuminating the sensitive area. The average
background count rate decreased by 20 % from March 2000 to January 2001, but it
regained in February 2001. For the modeling of the background spectrum, we
investigate relations between the 2.0--7.0 keV count rate and some
characteristic parameters. The 2.0--7.0 keV background count rate shows a good
correlation with the count rate of events outside the field of view. This
correlation is usable for the modeling of the background.Comment: 9 pages, submitted to A&
General critical states in type-II superconductors
The magnetic flux dynamics of type-II superconductors within the critical
state regime is posed in a generalized framework, by using a variational theory
supported by well established physical principles. The equivalence between the
variational statement and more conventional treatments, based on the solution
of the differential Maxwell equations together with appropriate conductivity
laws is shown. Advantages of the variational method are emphasized, focusing on
its numerical performance, that allows to explore new physical scenarios. In
particular, we present the extension of the so-called double critical state
model to three dimensional configurations in which only flux transport
(T-states), cutting (C-states) or both mechanisms (CT-states) occur. The theory
is applied to several problems. First, we show the features of the transition
from T to CT states. Second, we give a generalized expression for the flux
cutting threshold in 3-D and show its relevance in the slab geometry. In
addition, several models that allow to treat flux depinning and cutting
mechanisms are compared. Finally, the longitudinal transport problem (current
is applied parallel to the external magnetic field) is analyzed both under T
and CT conditions. The complex interaction between shielding and transport is
solved.Comment: 21 figures, submitted for publicatio
Scientific, Back-Illuminated CCD Development for the Transiting Exoplanet Survey Satellite
We describe the development of the fully depleted, back illuminated charge coupled devices for the Transiting Exoplanet Survey Satellite, which includes a set of four wide angle telescopes, each having a 2x2 array of CCDs. The devices are fabricated on the newly upgraded 200-mm wafer line at Lincoln Laboratory. We discuss methods used to produce the devices and present early performance results from the 100- micron thick, 15x15-microns, 2k x 4k pixel frame transfer CCDs
Critical state theory for nonparallel flux line lattices in type-II superconductors
Coarse-grained flux density profiles in type-II superconductors with
non-parallel vortex configurations are obtained by a proposed phenomenological
least action principle. We introduce a functional , which is minimized
under a constraint of the kind belongs to for the current density
vector, where is a bounded set. This generalizes the concept of
critical current density introduced by C. P. Bean for parallel vortex
configurations. In particular, we choose the isotropic case ( is a
circle), for which the field penetration profiles are derived when a
changing external excitation is applied. Faraday's law, and the principle of
minimum entropy production rate for stationary thermodynamic processes dictate
the evolution of the system. Calculations based on the model can reproduce the
physical phenomena of flux transport and consumption, and the striking effect
of magnetization collapse in crossed field measurements.Comment: The compiled TeX document length is 10 pages. Two figures (one page
each) are also included The paper is accepted for publication in Phys. Rev.
Let
Analytic Solution for the Critical State in Superconducting Elliptic Films
A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.Comment: Revtex file and 13 postscript figures, gives 10 pages of text with
figures built i
Critical State in Thin Anisotropic Superconductors of Arbitrary Shape
A thin flat superconductor of arbitrary shape and with arbitrary in-plane and
out-of-plane anisotropy of flux-line pinning is considered, in an external
magnetic field normal to its plane.
It is shown that the general three-dimensional critical state problem for
this superconductor reduces to the two-dimensional problem of an infinitely
thin sample of the same shape but with a modified induction dependence of the
critical sheet current. The methods of solving the latter problem are well
known. This finding thus enables one to study the critical states in realistic
samples of high-Tc superconductors with various types of anisotropic flux-line
pinning. As examples, we investigate the critical states of long strips and
rectangular platelets of high-Tc superconductors with pinning either by the
ab-planes or by extended defects aligned with the c-axis.Comment: 13 pages including 13 figure files in the tex
Continuum theory of partially fluidized granular flows
A continuum theory of partially fluidized granular flows is developed. The
theory is based on a combination of the equations for the flow velocity and
shear stresses coupled with the order parameter equation which describes the
transition between flowing and static components of the granular system. We
apply this theory to several important granular problems: avalanche flow in
deep and shallow inclined layers, rotating drums and shear granular flows
between two plates. We carry out quantitative comparisons between the theory
and experiment.Comment: 28 pages, 23 figures, submitted to Phys. Rev.
Stresses and strains in the surface of an infinite cylindrical working loaded uniformly and with axial symmetry along part of its length
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