12,726 research outputs found
Optimal Axes of Siberian Snakes for Polarized Proton Acceleration
Accelerating polarized proton beams and storing them for many turns can lead
to a loss of polarization when accelerating through energies where a spin
rotation frequency is in resonance with orbit oscillation frequencies.
First-order resonance effects can be avoided by installing Siberian Snakes in
the ring, devices which rotate the spin by 180 degrees around the snake axis
while not changing the beam's orbit significantly. For large rings, several
Siberian Snakes are required.
Here a criterion will be derived that allows to find an optimal choice of the
snake axes. Rings with super-period four are analyzed in detail, and the HERA
proton ring is used as an example for approximate four-fold symmetry. The
proposed arrangement of Siberian Snakes matches their effects so that all
spin-orbit coupling integrals vanish at all energies and therefore there is no
first-order spin-orbit coupling at all for this choice, which I call snakes
matching. It will be shown that in general at least eight Siberian Snakes are
needed and that there are exactly four possibilities to arrange their axes.
When the betatron phase advance between snakes is chosen suitably, four
Siberian Snakes can be sufficient.
To show that favorable choice of snakes have been found, polarized protons
are tracked for part of HERA-p's acceleration cycle which shows that
polarization is preserved best for the here proposed arrangement of Siberian
Snakes.Comment: 14 pages, 16 figure
Approximate Newton Methods for Policy Search in Markov Decision Processes
Approximate Newton methods are standard optimization tools which aim to maintain the benefits of Newton's method, such as a fast rate of convergence, while alleviating its drawbacks, such as computationally expensive calculation or estimation of the inverse Hessian. In this work we investigate approximate Newton methods for policy optimization in Markov decision processes (MDPs). We first analyse the structure of the Hessian of the total expected reward, which is a standard objective function for MDPs. We show that, like the gradient, the Hessian exhibits useful structure in the context of MDPs and we use this analysis to motivate two Gauss-Newton methods for MDPs. Like the Gauss- Newton method for non-linear least squares, these methods drop certain terms in the Hessian. The approximate Hessians possess desirable properties, such as negative definiteness, and we demonstrate several important performance guarantees including guaranteed ascent directions, invariance to affine transformation of the parameter space and convergence guarantees. We finally provide a unifying perspective of key policy search algorithms, demonstrating that our second Gauss- Newton algorithm is closely related to both the EM-algorithm and natural gradient ascent applied to MDPs, but performs significantly better in practice on a range of challenging domains
COMPARISON OF PELVIS KINEMATICS DURING THE BASEBALL PITCH: FATIGUED AND NON-FATIGUED CONDITIONS
The purpose of this study was to identify changes that occur in pelvis kinematics as baseball pitchers fatigue during extended performances. Kinematic data describing the actions of the pelvis were collected using electromagnetic tracking techniques and calculated using the ISB recommendations. There were significant differences between non-fatigued and fatigued conditions in the angle of lateral pelvis flexion at maximum external rotation and release (
Non-universal dynamics of dimer growing interfaces
A finite temperature version of body-centered solid-on-solid growth models
involving attachment and detachment of dimers is discussed in 1+1 dimensions.
The dynamic exponent of the growing interface is studied numerically via the
spectrum gap of the underlying evolution operator. The finite size scaling of
the latter is found to be affected by a standard surface tension term on which
the growth rates depend. This non-universal aspect is also corroborated by the
growth behavior observed in large scale simulations. By contrast, the
roughening exponent remains robust over wide temperature ranges.Comment: 11 pages, 7 figures. v2 with some slight correction
Research and development of high-performance light-weight fuel cell electrodes final report, nov. 1, 1963 - oct. 31, 1964
High performance light weight fuel cell electrode developmen
Bubble Shape Oscillations and the Onset of Sonoluminescence
An air bubble trapped in water by an oscillating acoustic field undergoes
either radial or nonspherical pulsations depending on the strength of the
forcing pressure. Two different instability mechanisms (the Rayleigh--Taylor
instability and parametric instability) cause deviations from sphericity.
Distinguishing these mechanisms allows explanation of many features of recent
experiments on sonoluminescence, and suggests methods for finding
sonoluminescence in different parameter regimes.Comment: Phys. Rev. Lett., in pres
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