507 research outputs found
The Lewis Research Center geomagnetic substorm simulation facility
A simulation facility was established to determine the response of typical spacecraft materials to the geomagnetic substorm environment and to evaluate instrumentation that will be used to monitor spacecraft system response to this environment. Space environment conditions simulated include the thermal-vacuum conditions of space, solar simulation, geomagnetic substorm electron fluxes and energies, and the low energy plasma environment. Measurements for spacecraft material tests include sample currents, sample surface potentials, and the cumulative number of discharges. Discharge transients are measured by means of current probes and oscilloscopes and are verified by a photomultiplier. Details of this facility and typical operating procedures are presented
Development of environmental charging effect monitors for operational satellites
Design details and design goals are given of an instrumentation package to monitor the effects of the environmental charging of spacecraft surfaces on the systems of operational spacecraft
Transport and diffusion in the embedding map
We study the transport properties of passive inertial particles in a
incompressible flows. Here the particle dynamics is represented by the
dissipative embedding map of area-preserving standard map which models
the incompressible flow. The system is a model for impurity dynamics in a fluid
and is characterized by two parameters, the inertia parameter , and the
dissipation parameter . We obtain the statistical characterisers of
transport for this system in these dynamical regimes. These are, the recurrence
time statistics, the diffusion constant, and the distribution of jump lengths.
The recurrence time distribution shows a power law tail in the dynamical
regimes where there is preferential concentration of particles in sticky
regions of the phase space, and an exponential decay in mixing regimes. The
diffusion constant shows behaviour of three types - normal, subdiffusive and
superdiffusive, depending on the parameter regimes. Phase diagrams of the
system are constructed to differentiate different types of diffusion behaviour,
as well as the behaviour of the absolute drift. We correlate the dynamical
regimes seen for the system at different parameter values with the transport
properties observed at these regimes, and in the behaviour of the transients.
This system also shows the existence of a crisis and unstable dimension
variability at certain parameter values. The signature of the unstable
dimension variability is seen in the statistical characterisers of transport.
We discuss the implications of our results for realistic systems.Comment: 28 pages, 14 figures, To Appear in Phys. Rev. E; Vol. 79 (2009
Electrometer system measures nanoamps at high voltage
Floating electrometer eliminates major source of error since any leakage from electrometer case, which is at high voltage, appears only as load on high voltage supply and not as part of current being measured. Commands to and data from floating electrometer are transferred across high voltage interface by means of optical channels
Maxwell-Drude-Bloch dissipative few-cycle optical solitons
We study the propagation of few-cycle pulses in two-component medium
consisting of nonlinear amplifying and absorbing two-level centers embedded
into a linear and conductive host material. First we present a linear theory of
propagation of short pulses in a purely conductive material, and demonstrate
the diffusive behavior for the evolution of the low-frequency components of the
magnetic field in the case of relatively strong conductivity. Then, numerical
simulations carried out in the frame of the full nonlinear theory involving the
Maxwell-Drude-Bloch model reveal the stable creation and propagation of
few-cycle dissipative solitons under excitation by incident femtosecond optical
pulses of relatively high energies. The broadband losses that are introduced by
the medium conductivity represent the main stabilization mechanism for the
dissipative few-cycle solitons.Comment: 38 pages, 10 figures. submitted to Physical Review
Stochastic Energetics of Quantum Transport
We examine the stochastic energetics of directed quantum transport due to
rectification of non-equilibrium thermal fluctuations. We calculate the quantum
efficiency of a ratchet device both in presence and absence of an external load
to characterize two quantifiers of efficiency. It has been shown that the
quantum current as well as efficiency in absence of load (Stokes efficiency) is
higher as compared to classical current and efficiency, respectively, at low
temperature. The conventional efficiency of the device in presence of load on
the other hand is higher for a classical system in contrast to its classical
counterpart. The maximum conventional efficiency being independent of the
nature of the bath and the potential remains the same for classical and quantum
systems.Comment: To be published in Phys. Rev.
Opto-Electronic Characterization of Three Dimensional Topological Insulators
We demonstrate that the terahertz/infrared radiation induced photogalvanic
effect, which is sensitive to the surface symmetry and scattering details, can
be applied to study the high frequency conductivity of the surface states in
(Bi1-xSbx)2Te3 based three dimensional (3D) topological insulators (TI). In
particular, measuring the polarization dependence of the photogalvanic current
and scanning with a micrometre sized beam spot across the sample, provides
access to (i) topographical inhomogeneity's in the electronic properties of the
surface states and (ii) the local domain orientation. An important advantage of
the proposed method is that it can be applied to study TIs at room temperature
and even in materials with a high electron density of bulk carriers.Comment: 6 pages, 4 figure
Quantum kinetic theory of shift current electron pumping in semiconductors
We develop a theory of laser beam generation of shift currents in
non-centrosymmetric semiconductors. The currents originate when the excited
electrons transfer between different bands or scatter inside these bands, and
asymmetrically shift their centers of mass in elementary cells. Quantum kinetic
equations for hot-carrier distributions and expressions for the induced
currents are derived by nonequilibrium Green functions. In applications, we
simplify the approach to the Boltzmann limit and use it to model laser-excited
GaAs in the presence of LO phonon scattering. The shift currents are calculated
in a steady-state regime.Comment: 23 pages, 5 figures (Latex
Excitonic effects on the two-color coherent control of interband transitions in bulk semiconductors
Quantum interference between one- and two-photon absorption pathways allows
coherent control of interband transitions in unbiased bulk semiconductors;
carrier population, carrier spin polarization, photocurrent injection, and spin
current injection may all be controlled. We extend the theory of these
processes to include the electron-hole interaction. Our focus is on photon
energies that excite carriers above the band edge, but close enough to it so
that transition amplitudes based on low order expansions in are
applicable; both allowed-allowed and allowed-forbidden two-photon transition
amplitudes are included. Analytic solutions are obtained using the effective
mass theory of Wannier excitons; degenerate bands are accounted for, but
envelope-hole coupling is neglected. We find a Coulomb enhancement of two-color
coherent control process, and relate it to the Coulomb enhancements of one- and
two-photon absorption. In addition, we find a frequency dependent phase shift
in the dependence of photocurrent and spin current on the optical phases. The
phase shift decreases monotonically from at the band edge to 0 over an
energy range governed by the exciton binding energy. It is the difference
between the partial wave phase shifts of the electron-hole envelope function
reached by one- and two-photon pathways.Comment: 31 pages, 4 figures, to be published in Phys. Rev.
Spin photocurrents and circular photon drag effect in (110)-grown quantum well structures
We report on the study of spin photocurrents in (110)-grown quantum well
structures. Investigated effects comprise the circular photogalvanic effect and
so far not observed circular photon drag effect. The experimental data can be
described by an analytical expression derived from a phenomenological theory. A
microscopic model of the circular photon drag effect is developed demonstrating
that the generated current has spin dependent origin.Comment: 6 pages, 3 figure
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