Strength of Higher-Order Spin-Orbit Resonances

When polarized particles are accelerated in a synchrotron, the spin precession can be periodically driven by Fourier components of the electromagnetic fields through which the particles travel. This leads to resonant perturbations when the spin-precession frequency is close to a linear combination of the orbital frequencies. When such resonance conditions are crossed, partial depolarization or spin flip can occur. The amount of polarization that survives after resonance crossing is a function of the resonance strength and the crossing speed. This function is commonly called the Froissart-Stora formula. It is very useful for predicting the amount of polarization after an acceleration cycle of a synchrotron or for computing the required speed of the acceleration cycle to maintain a required amount of polarization. However, the resonance strength could in general only be computed for first-order resonances and for synchrotron sidebands. When Siberian Snakes adjust the spin tune to be 1/2, as is required for high energy accelerators, first-order resonances do not appear and higher-order resonances become dominant. Here we will introduce the strength of a higher-order spin-orbit resonance, and also present an efficient method of computing it. Several tracking examples will show that the so computed resonance strength can indeed be used in the Froissart-Stora formula. HERA-p is used for these examples which demonstrate that our results are very relevant for existing accelerators.Comment: 10 pages, 6 figure

Parametrically forced sine-Gordon equation and domain walls dynamics in ferromagnets

A parametrically forced sine-Gordon equation with a fast periodic {\em mean-zero} forcing is considered. It is shown that $\pi$-kinks represent a class of solitary-wave solutions of the equation. This result is applied to quasi-one-dimensional ferromagnets with an easy plane anisotropy, in a rapidly oscillating magnetic field. In this case the $\pi$-kink solution we have introduced corresponds to the uniform ``true'' domain wall motion, since the magnetization directions on opposite sides of the wall are anti-parallel. In contrast to previous work, no additional anisotropy is required to obtain a true domain wall. Numerical simulations showed good qualitative agreement with the theory.Comment: 3 pages, 1 figure, revte

Carotid Baroreflex Activation: Past, Present, and Future

Electrical activation of the carotid baroreceptor system is an attractive therapy for the treatment of resistant hypertension. In the past, several attempts were made to directly activate the baroreceptor system in humans, but the method had to be restricted to a few selected patients. Adverse effects, the need for better electrical devices and better surgical techniques, and the lack of knowledge about long-term effects has greatly hampered developments in this area for many years. Recently, a new and promising device was evaluated in a multicenter feasibility trial, which showed a clinically and statistically significant reduction in office systolic blood pressure (>20 mm Hg). This reduction could be sustained for at least 2 years with an acceptable safety profile. In the future, this new device may stimulate further application of electrical activation of the carotid baroreflex in treatment-resistant hypertension

Remarkable charged particle dynamics near magnetic field null lines

The study of charged-particle motion in electromagnetic fields is a rich source of problems, models, and new phenomena for nonlinear dynamics. The case of a strong magnetic field is well studied in the framework of a guiding center theory, which is based on conservation of an adiabatic invariant—the magnetic moment. This theory ceases to work near a line on which the magnetic field vanishes—the magnetic field null line. In this paper, we show that the existence of these lines leads to remarkable phenomena which are new both for nonlinear dynamics in general and for the theory of charged-particle motion. We consider the planar motion of a charged particle in a strong stationary perpendicular magnetic field with a null line and a strong electric field. We show that particle dynamics switch between a slow guiding center motion and the fast traverse along a segment of the magnetic field null line. This segment is the same (in the principal approximation) for all particles with the same total energy. During the phase of a guiding center motion, the magnetic moment of particle’s Larmor rotation stays approximately constant, i.e., it is an adiabatic invariant. However, upon each traversing of the null line, the magnetic moment changes in a random fashion, causing the particle to choose a new trajectory of the guiding center motion. This results in a stationary distribution of the magnetic moment, which only depends on the particle’s total energy. The jumps in the adiabatic invariant are described by Painlevé II equation. The existence of adiabatic invariants—approximate conservation laws for systems with slow and fast motions—plays an important role in different physical theories. One of such theories is guiding center theory of motion of charged particles in a strong magnetic field. This theory is based on the adiabatic invariance of magnetic moment for the particle motion. Basic assumption of the guiding center approach is that the magnetic field is strong and vanishes nowhere. We show that, for a planar motion in strong perpendicular magnetic fields, if the magnetic field vanishes on some line (magnetic field null line) and the strong electric field is present, then the particle gets involved in a peculiar process of capture and release by the null line, which leads to large chaotic oscillations of the particle magnetic moment. Such a behaviour has not been previously reported in charged particles dynamics or in nonlinear dynamics in general

Functional cooking skills and neuropsychological functioning in patients with stroke: An ecological validity study

Efforts to relate neuropsychological performance to real-world task functioning have predominantly yielded lackluster results, typically with neuropsychological performance accounting for modest amounts of variance in function. Nonetheless, the ecological validity of neuropsychological measures for predicting functional abilities remains a strong research interest and clinical necessity. This study relates neuropsychological performance to performance on a standardized cooking task (Rabideau Kitchen Evaluation-Revised; RKE-R) in persons with stroke. Results showed that while the composite score of mean neuropsychological performance had the largest association with meal preparation, several neuropsychological measures were significantly related to the RKE-R. Groups of left and right hemisphere stroke patients were not significantly different in terms of RKE-R performance. These results suggest that functional cooking task performance is related to intact cognitive abilities in delayed verbal memory, simple auditory attention, and visuospatial skills, as well as overall cognitive performance. Implications for neuropsychologists are discussed