Search Results (19 total)

We present a simple model to explain the transition from oscillatory to smooth crack propagation in brittle metallic glasses. We demonstrate that the smooth fracture propagation that is characteristic for higher temperature or higher crack opening velocities (for type 1 crack propagation) becomes unstable and oscillatory behavior is being observed. The characteristic feature size of the crack propagation may be at the nanometer scale and grows as the opening velocity decreases.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H^- beam for the Spallation Neutron Source (SNS) ring. First, H^- atoms are converted to H⁰ by a magnetic field, then H⁰ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H^- beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

This paper presents a scheme for three-step laser-based stripping of an H^- beam for charge exchange injection into a high-intensity proton ring. First, H^- atoms are converted to H⁰ by Lorentz stripping in a strong magnetic field, then neutral hydrogen atoms are excited from the ground state to upper levels by a laser, and the remaining electron, now more weakly bound, is stripped in a strong magnetic field. The energy spread of the beam particles gives rise to a Doppler broadened absorption linewidth, which makes for an inefficient population of the upper state by a narrow-band laser. We propose to overcome this limitation with a “frequency sweeping” arrangement, which populates the upper state with almost 100% efficiency. We present estimates of peak laser power and describe a method to reduce the power by tailoring the dispersion function at the laser-particle beam interaction point. We present a scheme for reducing the average power requirements by using an optical ring resonator. Finally, we discuss an experimental setup to demonstrate this approach in a proof-of-principle experiment.

We apply optimal control theory to substantially reduce transient times for transitions between in-phase and out-of-phase states in coupled solid-state lasers. The control is a time-varying optical field that is injected into the cavities of each laser. We have analytically derived the optimal control and numerically solved the optimality system. Numerical simulations indicate that transient times can be significantly reduced upon increasing the injection strength very briefly.

We propose a new algorithm to control frictional dynamics of a small array of particles towards preassigned values of the average sliding velocity. The control is based on the concepts of non-Lipschitzian dynamics and terminal attractor. Extensive numerical simulations illustrate the robustness, efficiency, and convenience of the algorithm.

We study the entrainment of coupled solid-state lasers by an external injected field. We show that the total output intensity exhibits unexpected nonmonotonic behavior as a function of the injected field and find the critical amplitude marking the transition to the low-intensity branch. In addition, we also show that substantial partial entrainment can be achieved for injected fields much weaker than that required for full entrainment.

When a discrete nonlinear array is driven across a periodic surface spatially coherent modes of motion can coexist associated with different average velocities due to resonant parametric forcing of the particle fluctuations by the center of mass motion. Depending on the coupling strength κ and size of the array N, jumps in the minimum friction (maximum velocity) exhibited by the array occur at κ_m(N)∼(N/m)² as new modes stabilize and are selected by the dynamics. The existence of such coherent modes allows both an effective low dimensional description of the dynamics to exist and the possibility for control of friction close to these instabilities.

We present numerical and experimental evidence which demonstrates that under certain conditions friction can be reduced by spatial disorder and/or thermal noise. We discuss possible mechanisms for this behavior.

We show that chaotic diffusion of a single particle moving on a one-dimensional rough surface is enhanced by a small amount of spatial quenched disorder. In addition to enhanced diffusion we also find that there is a crossover from expanding to bounded motion. The crossover time to bounded motion decreases with increasing disorder, and there exists a threshold value of disorder above which chaotic motion is completely suppressed.

We suggest that coupling-induced orbit hopping is one possible mechanism for stick-slip dynamics. This mechanism is dominant in the highly nonlinear regime. Our example is a one-dimensional array of nonlinearly coupled oscillators subject to a strong periodic potential. The nonlinear dynamics leads to a fundamentally different friction law, in particular when the driving force is barely larger than the minimal force needed to start motion. We find a dramatic increase in the friction coefficient of the array compared to that of a single uncoupled oscillator, even though the same constant force f is applied to each oscillator in the array. The sliding friction coefficient η is found to diverge as η ∝ (κ-κ_c )^-1/2 , where κ_c is the critical value of the coupling constant κ. The coefficient η also grows linearly with the number of elements in the array N and shows dynamical transitions as the external force f applied to each of the oscillators is increased.

We study the nonlinear contribution to stick-slip motion in a weakly coupled discrete one-dimensional array of oscillators subject to a periodic potential. We find a dramatic increase in the friction coefficient of the array compared to that of a single uncoupled oscillator, even though the same constant force f is applied to each oscillator in the array. The sliding friction coefficient is found to diverge as η∝(κ-κ_c)^{-1 / 2}, where κ_c is the critical value of the coupling constant κ, and shows two dynamical transitions as we increase the external force f applied to each of the oscillators.

We present a numerical study of control of chaos in a model of a multimode neodymium-doped yttrium aluminum garnet laser using periodic perturbations of accessible control parameters. We show that a small modulation of either the losses or the pump can eliminate chaos or make the system even more chaotic. Depending on the parameters of the perturbation, two qualitatively different periodic outputs can be achieved. © 1996 The American Physical Society.

We study the effects of quenched disorder on the dynamics of two-dimensional arrays of overdamped Josephson junctions. Disorder in both the junction critical currents and resistances is considered. Analytical results for small arrays are used to identify a physical mechanism which promotes frequency locking across each row of the array, and to show that no such locking mechanism exists between rows. The intrarow locking mechanism is surprisingly strong, so that a row can tolerate large amounts of disorder before frequency locking is destroyed.

We find that the natural antiphasing tendency in linear solid-state laser arrays can be overcome by an injected field, even if the N elements are not identical. We derive a condition for full entrainment that agrees well with numerical simulations using experimentally accessible parameters. The resulting output intensity saturates near the maximum coherent value of N² times that of a single laser. We find that the entrained output can be modulated in a prescribed manner by a suitable choice of the injected field.

The dynamics of a linear chain of atoms with a central heavy atom is shown to be significantly affected by the presence of an external periodic potential representing a substrate. The system exhibits several dynamical phases, some of which are consequences of the existence of the substrate. The phases differ in the degree of energy localization and in other dynamical properties. It is shown that, unlike in a chain that is not subject to an external potential, energy localization is possible for heavy to light mass ratios that are of order one. Possible chemical applications are briefly mentioned.

We show that the addition of a weak periodic drive can stabilize the in-phase state of a current-biased series array of N Josephson junctions. The stabilization can be predicted by a local analysis of an effective one-junction problem regardless of N. In the parameter regime considered, the stabilization appears to be global.

We investigate the symmetry properties of finite Frenkel-Kontorova chains with free-end boundary conditions. A symmetry-breaking transition is found as one varies η, the relative strength of the harmonic and substrate interactions. This transition, from a symmetric configuration to a configuration with broken symmetry at η=η_c, is characterized by a gap in the phonon spectrum and by disorder and reflection-symmetry parameters, all of which display the scaling behavior (η_c-η)^0.5. The chains are shown to be pinned both below and above the symmetry-breaking transition.

The possibility of eliminating chaos in a dynamical system or of decreasing the leading Liapunov exponent by applying a weak periodic external forcing to the system is demonstrated through the example of a periodically driven pendulum. The applications of the external forcing also results in other striking changes in the dynamics such as a stabilization of narrow subharmonic steps and the achievement of very low winding numbers.

In this Letter we investigate a symmetry-breaking transition in the Frenkel-Kontorova model for finite chains with free-end boundary conditions. We present a detailed study of the behavior in the vicinity of the transition. It is shown that the gap in the phonon spectrum, the disorder parameter, and the reflection-symmetry parameter display scaling properties close to the transition. Their associated critical exponents are discussed and related to the displacements of the particles in the chains.