Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session Q13: Computational Materials and Accelerator Physics |
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Sponsoring Units: DCOMP Room: Plaza Court 3 |
Monday, May 4, 2009 10:45AM - 10:57AM |
Q13.00001: Energetics and transport coherence of a Brownian motor driven by position dependent temperature Ronald Benjamin We investigate the energetics and transport coherence of a Brownian motor driven by spatially inhomogeneous temperature. The heat dissipation by kinetic energy severely limits the generalized efficiency of the motor. It is found that coupling many Brownian particles reduces the efficiency of the motor. The transport coherence of the Brownian motor is reduced by the thermal gradient. It is also found that the heat transfer between the reservoirs and the entropy production obey the fluctuation theorem. [Preview Abstract] |
Monday, May 4, 2009 10:57AM - 11:09AM |
Q13.00002: Band structure calculations by modal analysis Mahmoud Hussein In this paper, we present reduced Bloch mode expansion for fast lattice band structure calculations. The expansion employs a natural basis composed of a selected reduced set of Bloch eigenfunctions. The reduced basis is selected within the irreducible Brillouin zone at high symmetry points determined by the medium's crystal structure and group theory. At each of the reciprocal lattice selection points, a number of Bloch eigenfunctions are selected up to the frequency/energy range of interest for the band structure calculations. Being in line with the well known concept of modal analysis, the proposed approach maintains accuracy while reducing the computation time by up to two orders of magnitudes or more depending on the size and extent of the calculations. Results are presented for phononic, photonic and electronic band structures. [Preview Abstract] |
Monday, May 4, 2009 11:09AM - 11:21AM |
Q13.00003: Topological defects in thin films during simulation of magnetization reversal Adebanjo Oriade Topological defects arise during the magnetization reversal of a NiFe ferromagnetic thin film. The reversal process is described by accounting for topological defects (singularities in the order-parameter space) and by using aspects of the theory of homotopy groups. The spin configuration data consists of planar spins on a two dimensional lattice, and it is shown that a singularity can be found at a point not on the lattice, by inspecting the winding number $n$. Singularities could either be topologically unstable ($n=0$) or topologically stable ($n\ne 0$). Some topological defects (such as an anti-vortex, winding number $n=-1$) are not obvious in a plot of the spin configurations of the film. The conservation of winding number for the entire sample in the course of reversal can be used to check that all topological defects are accounted for. One reason these defects are {\it hidden} is the inevitably coarse grained representation of the film in the finite-temperature Monte-Carlo simulation. Typical dimensions of the film are $0.2\mu m \times 1\mu m \times 5nm$ and the grain size is about $8nm \times 8nm \times 5nm$. A method for locating topological defects hidden within the simulation lattice will be discussed. [Preview Abstract] |
Monday, May 4, 2009 11:21AM - 11:33AM |
Q13.00004: Determine the Dispersion Relation of an A6 Magnetron Using Conformal Finite Difference Time Domain Method M.C. Lin, C. Nieter, P.H. Stoltz, D.N. Smithe This work introduces a conformal finite difference time domain (CFDTD) method to accurately determine the dispersion relation of an A6 relativistic magnetron. The accuracy is measured by comparing with accurate SUPERFISH calculations based on finite element method. The results show that an accuracy of 99.4{\%} can be achieved by using only 10,000 mesh points with Dey-Mittra algorithm. By comparison, a mesh number of 360,000 is needed to preserve 99{\%} accuracy using conventional FDTD method. This suggests one can efficiently and accurately study the hot tests of microwave tubes using CFDTD particle-in-cell method instead of conventional FDTD one. [Preview Abstract] |
Monday, May 4, 2009 11:33AM - 11:45AM |
Q13.00005: Space-Charge and Fringe-Field Effects in Simulations of Non-Scaling FFAGs Dan Abell, George Bell, Andrey Sobol, Alessandro Ruggiero, Dejan Trbojevic, Etienne Forest Recent simulations of non-scaling fixed-field alternating gradient accelerators (FFAGs) suggest that magnet fringe-field effects are of signal importance. In addition, non-scaling FFAGs are sensitive to a slew of resonances during the acceleration ramp. We present simulations of possible non-scaling FFAG designs, focusing especially on the effects fringe-fields and space-charge, using newly developed capabilities in the code PTC% \footnote{E. Forest, Y. Nogiwa, F. Schmidt, ``The FPP and PTC Libraries'', Proc. ICAP'2006.}. In particular, we study how fringe extent and other parameters affect important measures of machine performance. An important consideration---because it affects the amount of rf power required---will be the speed at which resonances must be crossed. [Preview Abstract] |
Monday, May 4, 2009 11:45AM - 11:57AM |
Q13.00006: Compton based Polarized positron source for ILC, CLIC and SuperB Vitaly Yakimenko, Igor Pogorelsky, Michail Polyansky We propose a polarized gamma source suitable for generating a polarized positron beam for the next generation of electron-positron colliders, such as the International Linear Collider (ILC), and the Compact Linear Collider (CLIC). This 60-MeV polarized gamma source is based on Compton scattering inside a picosecond CO2 laser cavity generated from electron bunches produced by a 6-GeV linac. We identified and experimentally verified the optimum conditions for obtaining at least one gamma photon per electron. After multiplication at several consecutive interaction points, the circularly polarized gamma rays are stopped on a target, thereby creating copious numbers of polarized positrons. We address the practicality of having an intracavity Compton-polarized positron source as the injector for these new colliders. This approach resolves many technical challenges of the baseline wiggler based approach and is particularly attractive as an upgrade option in case of the minimum ILC configuration. [Preview Abstract] |
Monday, May 4, 2009 11:57AM - 12:09PM |
Q13.00007: Experimental test of new technique to overcome spin-depolarizing resonances V.S. Morozov, A.W. Chao, A.D. Krisch, M.A. Leonova, R.S. Raymond, D.W. Sivers, V.K. Wong, A. Garishvili, R. Gebel, A. Lehrach, B. Lorentz, R. Maier, D. Prasuhn, H. Stockhorst, D. Welsch, F. Hinterberger, A.M. Kondratenko We recently tested a new spin resonance crossing technique, Kondratenko Crossing (KC), by sweeping an rf solenoid's frequency through an rf-induced spin resonance with both the KC and traditional Fast Crossing (FC) patterns. Using both rf bunched and unbunched 1.85 GeV/$c$ polarized deuterons stored in COSY, we varied the parameters of both crossing patterns. Compared to FC with the same crossing speed, KC reduced the depolarization by measured factors of 4$.$7 \textit{$\pm $} 0$.$3 and 19 \textit{$\pm $} $_{5}^{12}$ for unbunched and bunched beams, respectively. This showed the large potential benefit of KC over FC. [Preview Abstract] |
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