9,511 research outputs found
Nonlinear Coherent Modes of Trapped Bose-Einstein Condensates
Nonlinear coherent modes are the collective states of trapped Bose atoms,
corresponding to different energy levels. These modes can be created starting
from the ground state condensate that can be excited by means of a resonant
alternating field. A thorough theory for the resonant excitation of the
coherent modes is presented. The necessary and sufficient conditions for the
feasibility of this process are found. Temporal behaviour of fractional
populations and of relative phases exhibits dynamic critical phenomena on a
critical line of the parametric manifold. The origin of these critical
phenomena is elucidated by analyzing the structure of the phase space. An
atomic cloud, containing the coherent modes, possesses several interesting
features, such as interference patterns, interference current, spin squeezing,
and massive entanglement. The developed theory suggests a generalization of
resonant effects in optics to nonlinear systems of Bose-condensed atoms.Comment: 26 pages, Revtex, no figure
Servo-Stabilization of Low-Frequency Oscillations in a Liquid Bipropellant Rocket Motor
The recent work of H. S. Tsien concerning the servostabilization of rocket motors is extended to the liquid bipropellant rocket motor. It is shown that by use of a feedback system containing a device to sense the combustion chamber pressure, a suitably designed amplifier, and a servomechanism which governs the propellant How, the low-frequency oscillations which occur in the rocket configuration may be stabilized for any value of combustion time lag. A method is given for determining a transfer function of the feedback loop which will assure stable operation. The technique of the Satche diagram is employed in stability analysis
Design of a fractional control using performance contours. Application to an electromechanical system
The article proposes a frequency-based method to design a controller ensuring dynamic behavior of a closed-loop control: the first overshoot of the step response in the tracking mode or in the regulation mode, the damping ratio and the natural frequency of its dominant oscillatory mode. This method uses two contours called “performance contours” and constructed on the Nichols diagram. The first contour is the common Nichols magnitude contour which can be considered as an iso-overshoot contour. The second contour, whose construction and analytic expression are given in this article, is a new contour defined on the Nichols diagram and parameterized by the damping ratio. The proposed method uses complex non-integer (or fractional) differentiation to compute a transfer function whose open-loop Nichols locus tangents both performance contours, thus ensuring stability margins (or stability degree). The method is applied to a DC motor whose speed is controlled
The physics of dipolar bosonic quantum gases
This article reviews the recent theoretical and experimental advances in the
study of ultracold gases made of bosonic particles interacting via the
long-range, anisotropic dipole-dipole interaction, in addition to the
short-range and isotropic contact interaction usually at work in ultracold
gases. The specific properties emerging from the dipolar interaction are
emphasized, from the mean-field regime valid for dilute Bose-Einstein
condensates, to the strongly correlated regimes reached for dipolar bosons in
optical lattices.Comment: Review article, 71 pages, 35 figures, 350 references. Submitted to
Reports on Progress in Physic
- …