Parabolic Field Research Articles

Dune and Beach Morphodynamics at Cabo Falso, Baja California Sur, Mexico: Response to Natural, Hurricane Juliette (2001) and Anthropogenic Influence

Abstract Cabo Falso is located on the southern coast of the Baja California peninsula. The beach is broad and wide with a steep face and tall foredunes on the backshore. Landwards, an active, predominantly parabolic dune field has developed with average heights of 30 m. Part of the field is used by recreational all-terrain vehicles. The main goal of this study was to identify the principal morphodynamic characteristics of the deposits influenced by these recreational activities and affected by Hurricane Juliette in 2001. The morphology of the study area was monitored using seasonal topographic profiles, examination of aerial photographs from 1972 and 1993, and on-site photographs (November 1999 to April 2002). The topographic profiles indicated that the erosion period for both the beach and the dunes was June to December. Hurricane Juliette was the responsible for most of the observed erosion. The greatest sediment accumulation occurred between December 2001 and February 2002, associated with the onset of...

Solar–geothermal hybrid system

The Cerro Prieto Geothermal Power Plant is located in the northwest of Mexico, lat. 32°39′, long. 115°21′ in the northern hemisphere. A solar–geothermal hybrid system is proposed in order to increase the steam flow during the present geothermal cycle, adding a solar field of parabolic trough concentrators. Energy is supplied to the geothermal flow from wells in order to increase the steam generation rate. This configuration will increase the capacity factor of the system by generating additional steam during the peak demand hours. The parabolic trough solar field is evaluated in North–South and East–West orientation collector alignments. A proposal to obtain an increase of 10% in steam flow is evaluated, as the increase in flow is limited by the content of dissolved salts, so as to avoid a liquid phase with high salt concentrations. The size of the parabolic troughs field was obtained.

Control scheme for direct steam generation in parabolic troughs under recirculation operation mode

Electricity production using solar thermal energy is one of the main research areas at present in the field of renewable energies, these systems being characterised by the need of reliable control systems aimed at maintaining desired operating conditions in the face of changes in solar radiation, which is the main source of energy. A new prototype of solar system with parabolic trough collectors was implemented at the Plataforma Solar de Almería (PSA, South-East Spain) to investigate the direct steam generation process under real solar conditions in the parabolic solar collector field of a thermal power plant prototype. This paper presents details and some results of the application of a control scheme designed and tested for the recirculation operation mode, for which the main objective is to obtain steam at constant temperature and pressure at the outlet of the solar field, so that changes produced in the inlet water conditions and/or solar radiation will only affect the amount of steam produced by the solar field. The steam quality and consequently the nominal efficiency of the plant are thus maintained.

Inferring nonlinear parabolic field equations from modulus data

We give a means for measuring the equation of evolution of a complex scalar field that is known to obey an otherwise unspecified ( 2 + 1 ) -dimensional dissipative nonlinear parabolic differential equation, given field moduli over three closely-spaced planes. The formalism is tested by recovering nonlinear interactions and the associated equation of motion from simulated data for a range of ( 2 + 1 ) -dimensional nonlinear systems, including those which exhibit spontaneous symmetry breaking. The technique is of broad applicability, being able to infer a wide class of partial differential equations, which govern systems ranging from nonlinear optics to quantum fluids.

Novel strategies in feedforward adaptation to a position-dependent perturbation

To investigate the control mechanisms used in adapting to position-dependent forces, subjects performed 150 horizontal reaching movements over 25 cm in the presence of a position-dependent parabolic force field (PF). The PF acted only over the first 10 cm of the movement. On every fifth trial, a virtual mechanical guide (double wall) constrained subjects to move along a straight-line path between the start and target positions. Its purpose was to register lateral force to track formation of an internal model of the force field, and to look for evidence of possible alternative adaptive strategies. The force field produced a force to the right, which initially caused subjects to deviate in that direction. They reacted by producing deviations to the left, "into" the force field, as early as the second trial. Further adaptation resulted in rapid exponential reduction of kinematic error in the latter portion of the movement, where the greatest perturbation to the handpath was initially observed, whereas there was little modification of the handpath in the region where the PF was active. Significant force directed to counteract the PF was measured on the first guided trial, and was modified during the first half of the learning set. The total force impulse in the region of the PF increased throughout the learning trials, but it always remained less than that produced by the PF. The force profile did not resemble a mirror image of the PF in that it tended to be more trapezoidal than parabolic in shape. As in previous studies of force-field adaptation, we found that changes in muscle activation involved a general increase in the activity of all muscles, which increased arm stiffness, and selectively-greater increases in the activation of muscles which counteracted the PF. With training, activation was exponentially reduced, albeit more slowly than kinematic error. Progressive changes in kinematics and EMG occurred predominantly in the region of the workspace beyond the force field. We suggest that constraints on muscle mechanics limit the ability of the central nervous system to employ an inverse dynamics model to nullify impulse-like forces by generating mirror-image forces. Consequently, subjects adopted a strategy of slightly overcompensating for the first half of the force field, then allowing the force field to push them in the opposite direction. Muscle activity patterns in the region beyond the boundary of the force field were subsequently adjusted because of the relatively-slow response of the second-order mechanics of muscle impedance to the force impulse.

Quantum-state transfer via the ferromagnetic chain in a spatially modulated field

We show that a perfect quantum state transmission can be realized through a spin chain possessing a commensurate structure of energy spectrum, which is matched with the corresponding parity. As an exposition of the mirror inversion symmetry discovered by Albanese et. al (quant-ph/0405029), the parity matched the commensurability of energy spectra help us to present the novel pre-engineered spin systems for quantum information transmission. Based on the these theoretical analysis, we propose a protocol of near-perfect quantum state transfer by using a ferromagnetic Heisenberg chain with uniform coupling constant, but an external parabolic magnetic field. The numerical results shows that the initial Gaussian wave packet in this system with optimal field distribution can be reshaped near-perfectly over a longer distance.

Translational free random walk of spins in the presence of a parabolic magnetic field

The free random walk approach has been used to analyze the attenuation of the NMR signal due to spin dephasing in the presence of a constant and pulsed parabolic magnetic field. The spin echo sequence was chosen to examine the attenuation of the NMR signal resulting from self-diffusion. In the framework of the gaussian approach, the long-time limit calculations predict more pronounced diffusion weighting for the parabolic field than for linear magnetic field. Analytical results were obtained and compared with those from other approaches based on a variety of different of approximations.

Active and passive Brownian motion of charged particles in two-dimensional plasma models

The dynamics of charged Coulomb grains in a plasma is numerically and analytically investigated. Analogous to recent experiments, it is assumed that the grains are trapped in an external parabolic field. Our simulations are based on a Langevin model, where the grain-plasma interaction is realized by a velocity-dependent friction coefficient and a velocity-independent diffusion coefficient. In addition to the ordinary case of positive (passive) friction between grains and plasma, we also discuss the effects of negative (active) friction. The latter case seems particularly interesting, since recent analytical calculations have shown that friction coefficients with negative parts may appear in some models of ion absorption by grains as well as in models of ion-grain scattering. Such negative friction may cause active Brownian motions of the grains. As our computer simulations show, the influence of negative friction leads to the formation of various stationary modes (rotations, oscillations), which, to some extent, can also be estimated analytically.

Dynamics of charged particles in bifurcated current sheets: The κ ≈ 1 regime

We examine the nonlinear dynamics of charged particles in the double‐hump current sheets that develop during the late growth phase of substorms. We focus on particles for which the adiabaticity parameter κ (defined as the square root of the minimum curvature radius to maximum Larmor radius ratio) is of the order of unity. We show that as in the case of a simple parabolic field reversal, the magnetic moment scattering experienced by these particles may be described as the result of perturbation of the gyromotion by an impulsive centrifugal force. Here however the double‐hump structure of the current sheet leads to two successive centrifugal perturbations, which has a significant impact on the net change of magnetic moment. In a single‐hump current sheet, three distinct regimes of magnetic moment variations are obtained for a given value of κ, namely, systematic enhancement at small pitch angles, negligible change at large pitch angles, and in between either damping or enhancement depending upon gyration phase. In contrast, in a double‐hump current sheet, repeated application of this three‐branch pattern can lead to magnetic moment damping for particles that previously experience magnetic moment enhancement and vice versa. The gyrophase gain both during and between the centrifugal impulses is found to play an essential role in the net magnetic moment change. In particular, in contrast to single‐hump current sheets, the κ ≈ 1 limit may be characterized by quasi‐adiabatic behavior with negligible variations of the magnetic moment.

Ideal Magnetohydrodynamic Solution to the σ Problem in Crab‐like Pulsar Winds and General Asymptotic Analysis of Magnetized Outflows

Using relativistic, steady, axisymmetric, ideal magnetohydrodynamics (MHD), we analyze the super-Alfvénic regime of a pulsar wind by solving the momentum equation along the flow, as well as in the transfield direction. Employing a self-similar model, we demonstrate that ideal MHD can account for the full acceleration from high (≫1) to low (≪1) values of σ, the Poynting-to-matter energy flux ratio. The solutions also show a transition from a current-carrying to a return-current regime, partly satisfying the current-closure condition. We discuss the kind of boundary conditions near the base of the ideal MHD regime that are necessary in order to have the required transition from high to low σ in realistic distances and argue that this is a likely case for an equatorial wind. Examining the MHD asymptotics in general, we extend the analysis of Heyvaerts & Norman and Chiueh, Li, & Begelman by including two new elements: classes of quasi-conical and parabolic field line shapes that do not preclude an efficient and much faster than logarithmic acceleration, and the transition σ = σc after which the centrifugal forces (poloidal and azimuthal) are the dominant terms in the transfield force-balance equation.

Corrections to the strong-stretching theory of polymer brushes due to the proximal layer

Proximal-layer corrections to the strong-stretching theory (SST) for polymer brushes are tested against self-consistent field theory (SCFT). The corrections qualitatively account for a number of significant differences between SST and SCFT occurring next to the substrate. This includes severe deviations away from the parabolic field normally predicted by SST, and nonzero slopes in the single-chain segment distributions. When SST is also combined with corrections for the end-segment entropy, the quantitative agreement with SCFT becomes impressive, particularly in regards to the self-consistent field and the stretching energy of individual chains.

Influence of Direct Normal Irradiance Variation on the Optimal Parabolic Trough Field Size: A Problem Solved with Technical and Economical Simulations

One of the main problems when designing a solar thermal power plant is finding the optimal parabolic trough field size. Errors made in this context can easily lead to financial disaster. Simulation tools that handle all aspects of a power plant (technical as well as economical) treat such economical problems as a whole and can be very helpful during the design process. However, even the smartest simulation tool depends significantly on the input parameters, such as the solar irradiance. As a result of the given considerations, a new method for estimating the optimized solar field size as a function of the solar irradiance is proposed. Additionally, this paper demonstrates a path to simulate the complexity of a parabolic trough power plant.

Ion dynamics in the turbulent magnetotail-hidden influence of average closed field line topology

We model ion motion in the magnetotail with parabolic field lines in the presence of magnetic turbulence and cross tail constant electric field. It appeared that the average normal component and fluctuations have opposite influence on the ion dynamics. The higher is the level of magnetic fluctuations the larger is the width of the current layer and more pronounced is current splitting into double hump structure, larger is maximum temperature in the central plane and harder is the spectrum of non-Maxwellian tail of ion distribution function. At the same time average normal component causes suppression of the splitting effect and ion maximum temperature is inversely proportional to its value. Even when the amplitude of fluctuations is much higher than the average constant normal component value, ion dynamics is still controlled by the average magnetic field in the current sheet midplane.

Measurements of local magnetic field gradient and diffusion in polymers in the solid state

This paper describes two topics concerned with anisotropic molecular diffusion in a single crystal of stearic acid (SA) and measurements of local magnetic field gradient (MFG) in a powdered film of regio-regulated poly(4-methylthiazole-2,5-diyl)(HH-P4MeTz) by 1H transverse relaxation time measurements with two pulse Hahn-echo (TPHE) and Carr–Purcell Meiboom–Gill (CPMG) nuclear magnetic resonance (NMR) spectroscopy. The plots of the relaxation rate ( T 2 −1) with respect to τ −1, were fitted by a numerical Green function solution of Torrey's diffusion equation, with a parabolic magnetic field gradient and thereby, the internal MFGs were estimated for SA and HH-P4MeTz. The local MFGs obtained are thought to be characteristic of each system, and would be closely related to the molecular and electronic structure.

Implementation of low-energy surface-induced dissociation (eV SID) and high-energy collision-induced dissociation (keV CID) in a linear sector-TOF hybrid tandem mass spectrometer

In the work reported here, an in-line sector-TOF (time-of-flight) tandem mass spectrometer with a coaxial detector was developed. An advantage of the present design is that the instrument implements both high-energy collision-induced dissociation (keV CID) and low-energy surface-induced dissociation (eV SID) with no hardware manipulations required to switch between the two activation methods. Surface characterization by secondary-ion mass spectrometry is also possible with this design. Gas phase collision-induced dissociation in this sector-TOF requires ion gating of the continuous ion beam and an ion buncher to bring the ions into spatial focus at the focal-point “source” of the offset parabolic field reflectron. The offset parabolic field reflectron provides good resolution over virtually the entire product ion mass range, with no need to step the reflector voltages. For surface-induced dissociation experiments, accomplished with a surface placed after the last reflectron electrode, the electrodes that comprise the CID reflectron are used for deceleration and acceleration of the ion beam before and after it collides with the surface, respectively. The results obtained show successful implementation of keV CID and eV SID in a JEOL HX110A/TOF mass spectrometer. Product ion mass accuracy is better than ±0.4 u for keV CID and ±0.1 u for eV SID. Typical resolutions achieved are 8000–12,000 (for selected ions), 1000 (for keV CID fragments), and 300–400 (for eV SID fragments). This difference in the CID and SID resolution is partially the result of the different total flight times in CID and SID experiments. On the basis of a direct comparison of keV CID and eV SID of protonated leucine enkephalin, SID experiments have a lower detection limit than CID experiments by at least an order of magnitude.

Electron-TOF-analyser for complete momentum analysis in photoemission from surfaces

We present a new method for momentum-selective imaging by means of a time-of-flight (TOF) technique. The instrument employs a time- and space-resolving delayline detector in combination with a parabolic electrostatic field and a drift space. We use this kind of spectrometer, to raise the efficiency of experiments, which are total momentum resolved. The main difference to conventional photoemission experiments using a rotatable spectrometer is the simultaneous detection of all emitted photoelectrons. In addition to this feature, the angular distribution should be directly visible, to observe solid state symmetries. In order to facilitate these requirements, we use a delayline detector as a position and time sensitive device with a versatile user interface. First results obtained at BESSY I are presented.

Detailed structure of fs pulses passing through straight and tapered optical waveguides

The detailed structure of the field distribution in an arbitrary femtosecond pulse passing through straight and tapered optical waveguides is found using the scalar diffraction theory and the method of images. It is shown that the wave propagates down the guide as the superposition of many pulses that diffract in the off-axis direction and interfere with each other. For the straight multimode guide the usual result is recovered; the spatial profile of the wave is undistorted along its path of propagation provided the input pulse is mode matched. In the case of the tapered guide excited by the 10-fs pulse, which propagates in the off-axis direction and has the parabolic field distribution and spherical wavefront, the new phenomenon is manifested by the appearance of the sub-fs pulse inside the guide.

Optical and electrical properties of flash-evaporated amorphous CuInSe 2 films

A combined concentrating solar power system and a geothermal binary plant based on an Organic Rankine Cycle (ORC) is analyzed. Given a supercritical ORC, designed for the optimal utilization of an intermediate enthalpy geothermal source, a solar parabolic trough field was included in the plant, introducing an additional high temperature heat source for the cycle and increasing power production. The off-design performance analysis of the power cycle was performed first. An hour-by-hour simulation was then carried out to estimate the yearly production using a detailed solar field model. Finally, a differential economic analysis was performed to determine the cost of the additional electricity generated with the solar source. On the basis of the current cost of solar collectors, levelized costs of electricity of 145–280 €/MWh were obtained depending on the location of the plant: a competitive value with respect to large, stand-alone concentrating solar power plants.

Spin–oscillatory coupling effect in parabolic magnetic field

We consider eigenvalue problem of the coupled spin–oscillator system in the external parabolic magnetic field B =(B 0+G′x+G″x 2) z ̂ . In result we obtain the energy spectrum exhibiting highly non-standard structure, due to the second gradient term. For some values of the latter and oscillatory frequency there is large energy difference between some magnetic sublevels. The spectrum allows, in principle, to identify frequency of the oscillator via the spectrum of spin sublevels coming only from one oscillatory level.

Relaxation of nuclear magnetization in a nonuniform magnetic field gradient and in a restricted geometry

We study the influence of restriction on Carr-Purcell-Meiboom-Gill spin echo response of magnetization of spins diffusing in a bounded region in the presence of a nonuniform magnetic field gradient. We consider two fields in detail-a parabolic field which, like the uniform-gradient field, scales with the system size, and a cosine field which remains bounded. Corresponding to three main length scales, the pore size, L(S), the dephasing length, L(G), and the diffusion length during half-echo time, L(D), we identify three main regimes of decay of the total magnetization: motionally averaged, localization, and short-time. In the short-time regime (L(D) << L(S), L(G)), we confirm that the leading order behavior is controlled by the average of the square of the gradient, (nablaB(z))(2), and in the motionally averaged regime (MAv), where L(S) << L(D), L(G), by (integral dxB(z))(2). We verify numerically that two different fields for which those two averages are identical result in very similar decay profiles not only in the limits of short and long times but also in the intermediate times, with important practical implications. In the motionally averaged regime we found that previous estimates of the decay exponent for the parabolic field, based on a soft-boundary condition, are significantly altered in the presence of a more realistic, hard wall. We find the scaling of the decay exponent in the MAv regime with pore size to be L(2)(S) for the cosine field and L(6)(S) for the parabolic field, as contrasted with the linear gradient scaling of L(4)(S). In the localization regime, for both the cosine and the parabolic fields, the decay exponent depends on a fractional power of the gradient, implying a breakdown of the second cumulant or the Gaussian phase approximation. We also examined the validity of time-evolving the total magnetization according to a distribution of effective local gradients and found that such approximation works well only in the short-time regime and breaks down strongly for long times. Copyright 2000 Academic Press.