12,405 research outputs found
Time-domain sensitivity enhancement in pulsed Pb-TDL gas monitors
A Pb-salt tunable diode laser (TDL) has found many applications in the field of atmospheric gas analysis. Its continuous tunablility and fine spectral purity in the mid infrared region are outstanding from other lasers. The only shortcoming is that it requires cryogenic operating temperatures, though, it is improved year by year towards the room temperature operation. A repeated pulse operation of Pb salt diode lasers is possible with a thermoelectric cooling device, which allows an instrument a portable geometry disusing a heavy, bulky and power consuming mechanical refrigerator. A derivative spectrometry was exploiting the quick tunability of Pb salt diode lasers, though they are continuous wave (cw) operated with refrigerator or liquid nitrogen so far. A new system for derivative spectrometry with a pulsed diode laser will extend its field of applications because of reduced weights and size of measuring instruments. A preliminary results is shown that demonstrates the feasibility of an attempt to implement the derivative spectrmetry with repeatedly pulse driven diode lasers. Atmospheric methane was measured with 8 ppm/m sensitivity. Further results of parametric optimization for the best signal to noise ratios under any given device characteristics as well as for available real devices is given
Noise Limited Computational Speed
In modern transistor based logic gates, the impact of noise on computation
has become increasingly relevant since the voltage scaling strategy, aimed at
decreasing the dissipated power, has increased the probability of error due to
the reduced switching threshold voltages. In this paper we discuss the role of
noise in a two state model that mimic the dynamics of standard logic gates and
show that the presence of the noise sets a fundamental limit to the computing
speed. An optimal idle time interval that minimizes the error probability, is
derived
The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: II. Saturation Level and Critical Magnetic Reynolds Number
The nonlinear evolution of the magnetorotational instability (MRI) in weakly
ionized accretion disks, including the effect of the Hall term and ohmic
dissipation, is investigated using local three-dimensional MHD simulations and
various initial magnetic field geometries. When the magnetic Reynolds number,
Re_M \equiv v_A^2 / \eta \Omega (where v_A is the Alfven speed, \eta the
magnetic diffusivity, and \Omega the angular frequency), is initially larger
than a critical value Re_{M, crit}, the MRI evolves into MHD turbulence in
which angular momentum is transported efficiently by the Maxwell stress. If
Re_M < Re_{M, crit}, however, ohmic dissipation suppresses the MRI, and the
stress is reduced by several orders of magnitude. The critical value is in the
range of 1 - 30 depending on the initial field configuration. The Hall effect
does not modify the critical magnetic Reynolds number by much, but enhances the
saturation level of the Maxwell stress by a factor of a few. We show that the
saturation level of the MRI is characterized by v_{Az}^2 / \eta \Omega, where
v_{Az} is the Alfven speed in the nonlinear regime along the vertical component
of the field. The condition for turbulence and significant transport is given
by v_{Az}^2 / \eta \Omega \gtrsim 1, and this critical value is independent of
the strength and geometry of the magnetic field or the size of the Hall term.
If the magnetic field strength in an accretion disk can be estimated
observationally, and the magnetic Reynolds number v_A^2 / \eta \Omega is larger
than about 30, this would imply the MRI is operating in the disk.Comment: 43 pages, 8 tables, 20 figures, accepted for publication in ApJ,
postscript version also available from
http://www.astro.umd.edu/~sano/publications
Combined Analysis of Numerical Diagonalization and Renormalization Group methods for the One-Dimensional - Model at Quarter filling
The one-dimensional extended Hubbard model with both the on-site and the
nearest neighbor interactions at quarter filling is studied by using a
novel finite size scaling. We diagonalize finite size systems numerically and
calculate the Luttinger-liquid parameter which is substituted into
the renormalization group equation as an initial condition. It leads
in the infinite size system and the result agrees very well with the available
exact result with . This approach also yields the charge gap in the
insulating state near the metal-insulator transition where the characteristic
energy becomes exponentially small and the usual finite size scaling is not
applicable.Comment: 7 pages, 8 figures,submitted to PR
Modification of Angular Velocity by Inhomogeneous MRI Growth in Protoplanetary Disks
We have investigated evolution of magneto-rotational instability (MRI) in
protoplanetary disks that have radially non-uniform magnetic field such that
stable and unstable regions coexist initially, and found that a zone in which
the disk gas rotates with a super-Keplerian velocity emerges as a result of the
non-uniformly growing MRI turbulence. We have carried out two-dimensional
resistive MHD simulations with a shearing box model. We found that if the
spatially averaged magnetic Reynolds number, which is determined by widths of
the stable and unstable regions in the initial conditions and values of the
resistivity, is smaller than unity, the original Keplerian shear flow is
transformed to the quasi-steady flow such that more flattened (rigid-rotation
in extreme cases) velocity profile emerges locally and the outer part of the
profile tends to be super-Keplerian. Angular momentum and mass transfer due to
temporally generated MRI turbulence in the initially unstable region is
responsible for the transformation. In the local super-Keplerian region,
migrations due to aerodynamic gas drag and tidal interaction with disk gas are
reversed. The simulation setting corresponds to the regions near the outer and
inner edges of a global MRI dead zone in a disk. Therefore, the outer edge of
dead zone, as well as the inner edge, would be a favorable site to accumulate
dust particles to form planetesimals and retain planetary embryos against type
I migration.Comment: 28 pages, 11figures, 1 table, accepted by Ap
Dynamics of a deformable self-propelled domain
We investigate the dynamical coupling between the motion and the deformation
of a single self-propelled domain based on two different model systems in two
dimensions. One is represented by the set of ordinary differential equations
for the center of gravity and two tensor variables characterizing deformations.
The other is an active cell model which has an internal mechanism of motility
and is represented by the partial differential equation for deformations.
Numerical simulations show a rich variety of dynamics, some of which are common
to the two model systems. The origin of the similarity and the difference is
discussed.Comment: 6 pages, 6 figure
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