1,541 research outputs found
Nonlinear dynamics in superlattices driven by high frequency ac-fields
We investigate the dynamical processes taking place in nanodevices driven by
high-frequency electromagnetic fields. We want to elucidate the role of
different mechanisms that could lead to loss of quantum coherence. Our results
show how the dephasing effects of disorder that destroy after some periods
coherent oscillations, such as Rabi oscillations, can be overestimated if we do
not consider the electron-electron interactions that can reduce dramatically
the decoherence effects of the structural imperfections. Experimental
conditions for the observation of the predicted effects are discussed.Comment: REVTEX (8 pages) and 4 figures (Postscript
Are the Secrecy Order Compensation Provisions of the Patent Act Constitutional Under the Fifth Amendment?
The secrecy order provisions of the Patent Act\u27 raise a number of issues under the U.S. Constitution. The primary focus of this note is on the Fifth Amendment issues raised by the Invention Secrecy Act
Heat pipes for wing leading edges of hypersonic vehicles
Wing leading edge heat pipes were conceptually designed for three types of vehicle: an entry research vehicle, aero-space plane, and advanced shuttle. A full scale, internally instrumented sodium/Hastelloy X heat pipe was successfully designed and fabricated for the advanced shuttle application. The 69.4 inch long heat pipe reduces peak leading edge temperatures from 3500 F to 1800 F. It is internally instrumented with thermocouples and pressure transducers to measure sodium vapor qualities. Large thermal gradients and consequently large thermal stresses, which have the potential of limiting heat pipe life, were predicted to occur during startup. A test stand and test plan were developed for subsequent testing of this heat pipe. Heat pipe manufacturing technology was advanced during this program, including the development of an innovative technique for wick installation
Improved flux-surface parameterization through constrained nonlinear optimization
Parameterization of magnetic flux-surfaces is often used for magnetohydrodynamic stability analysis and microturbulence modeling in tokamaks. Shape parameters for such local parameterization of a (numerical) equilibrium are traditionally computed analytically using geometrically derived quantities. However, often the shape is approximated by the average of values for different sections of the flux-surface contour or a truncated series, which does not guarantee an optimal fit. Here, instead nonlinear least squares optimization is used to compute these parameters, with a weighted sum of squared error cost function that is robust to outliers. This method results in a lower total absolute error for both the parameterization of the flux-surface contour and the poloidal magnetic field density than current methods for several parameterizations based on the well-known "Miller geometry."Furthermore, rapid convergence of shape parameters is achieved, no approximate geometric measurements of the contour are needed, and the method is applicable to any analytical shape parameterization. Validation with local, linear gyrokinetic simulations using these optimized shape parameters showed reduced root mean square errors in both the growth rate and frequency spectra when compared with simulations based on numerical equilibria. In particular, the popular Turnbull-Miller parameterization benefits from this approach, extending its usability closer toward the last-closed flux-surface for cases with minor up-down asymmetry.</p
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