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 $\sigma$ 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 $C[H(x)]$, which is minimized under a constraint of the kind $J$ belongs to $Delta$ for the current density vector, where $Delta$ 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 ($Delta$ is a circle), for which the field penetration profiles $H(x,t)$ 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.