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An efficient approximate expression for unsteady pipe flow with high- viscosity fluid

Description: An approximate first-order expression for modeling frequency-dependent friction of unsteady pipe flow with high-viscosity fluid has been deveoped with the method of nonlinear square integral optimum in the frequency domain. This simple expression of first-order lag elements is more accurate and efficient than others in both the frequency and domain domains and can be applied to calculations of both frequency and transient response of unsteady pipe flow for oil hydraulic systems.
Date: June 1996
Creator: Cai, Y.
Partner: UNT Libraries Government Documents Department

Beam-Beam Study on the Upgrade of Beijing Electron Positron Collider

Description: It is an important issue to study the beam-beam interaction in the design and performance of such a high luminosity collider as BEPCII, the upgrade of Beijing Electron Positron Collider. The weak-strong simulation is generally used during the design of a collider. For performance a large scale tune scan, the weak-strong simulation studies on beam-beam interaction were done, and the geometry effects were taken into account. The strong-strong simulation studies were done for investigating the luminosity goal and the dependence of the luminosity on the beam parameters.
Date: February 10, 2006
Creator: Wang, S. & Cai, Y.
Partner: UNT Libraries Government Documents Department

Theory of Microwave Instability and Coherent Synchrotron Radiation in Electron Storage Rings

Description: Bursting of coherent synchrotron radiation has been observed and in fact used to generate THz radiation in many electron storage rings. In order to understand and control the bursting, we return to the study of the microwave instability. In this paper, we will report on the theoretical understanding, including recent developments, of the microwave instability in electron storage rings. The historical progress of the theories will be surveyed, starting from the dispersion relation of coasting beams, to the work of Sacherer on a bunched beam, and ending with the Oide and Yokoya method of discretization. This theoretical survey will be supplemented with key experimental results over the years. Finally, we will describe the recent theoretical development of utilizing the Laguerre polynomials in the presence of potential-well distortion. This self-consistent method will be applied to study the microwave instability driven the impedances due to the coherent synchrotron radiation. Over the past quarter century, there has been steady progress toward smaller transverse emittances in electron storage rings used for synchrotron light sources, from tens of nm decades ago to the nm range recently. In contrast, there is not much progress made in the longitudinal plane. For an electron bunch in a typical ring, its relative energy spread {sigma}{sub {delta}} remains about 10{sup -3} and its length {sigma}{sub z} is still in between 5 mm to 10 mm. Now the longitudinal emittance ({sigma}{sub {delta}}{sigma}{sub z}) becomes a factor of thousand larger than those in the transverse dimensions. In this paper, we will address questions of: How short a bunch can be? What is the fundamental limit? If there is a limit, is there any mitigation method? Since the synchrotron radiation is so fundamental in electron storage rings, let us start with the coherent synchrotron radiation (CSR).
Date: December 9, 2011
Creator: Cai, Y.
Partner: UNT Libraries Government Documents Department

A Multi-Bunch, Three-Dimensional, Strong-Strong Beam-Beam Simulation Code for Parallel Computers

Description: For simulating the strong-strong beam-beam effect, using Particle-In-Cell codes has become one of the methods of choice. While the two-dimensional problem is readily treatable using PC-class machines, the three-dimensional problem, i.e., a problem encompassing hourglass and phase-averaging effects, requires the use of parallel processors. In this paper, we introduce a strong-strong code NIMZOVICH, which was specifically designed for parallel processors and which is optimally used for many bunches and parasitic crossings. We describe the parallelization scheme and give some benchmarking results.
Date: May 11, 2005
Creator: Cai, Y. & Kabel, A. C.
Partner: UNT Libraries Government Documents Department

Dynamical Effects Due to Fringe Field of the Magnets in Circular Accelerators

Description: The leading Lie generators, including the chromatic effects, due to hard-edge fringe field of single multipole and solenoid are derived from the vector potentials within a Hamiltonian system. These nonlinear generators are applied to the interaction region of PEP-II to analyze the linear errors due to the feed-down from the off-centered quadrupoles and solenoid. The nonlinear effects of tune shifts at large amplitude, the synchro-betatron sidebands near half integer and their impacts on the dynamic aperture are studied in the paper.
Date: May 16, 2005
Creator: Cai, Y. & Nosochkov, Yu
Partner: UNT Libraries Government Documents Department

Vibration and stability of a group of tubes in crossflow

Description: This paper presents an unsteady flow theory for flow-induced vibration and instability of tube arrays in crossflow. It includes measurements of motion-dependent fluid forces, mathematical model, and experiments on nonlinear response of tube arrays. The unsteady flow theory can be used to provide answers to complex vibration problems in steam generators.
Date: December 31, 1995
Creator: Chen, S.S. & Cai, Y.
Partner: UNT Libraries Government Documents Department

A review of dynamic characteristics of magnetically levitated vehicle systems

Description: The dynamic response of magnetically levitated (maglev) ground transportation systems has important consequences for safety and ride quality, guideway design, and system costs. Ride quality is determined by vehicle response and by environmental factors such as humidity and noise. The dynamic response of the vehicles is the key element in determining ride quality, while vehicle stability is an important safety-related element. To design a guideway that provides acceptable ride quality in the stable region, vehicle dynamics must be understood. Furthermore, the trade-off between guideway smoothness and levitation and control systems must be considered if maglev systems are to be economically feasible. The link between the guideway and the other maglev components is vehicle dynamics. For a commercial maglev system, vehicle dynamics must be analyzed and tested in detail. This report, which reviews various aspects of the dynamic characteristics, experiments and analysis, and design guidelines for maglev systems, discusses vehicle stability, motion dependent magnetic force components, guideway characteristics, vehicle/ guideway interaction, ride quality, suspension control laws, aerodynamic loads and other excitations, and research needs.
Date: November 1, 1995
Creator: Cai, Y. & Chen, S.S.
Partner: UNT Libraries Government Documents Department

Nonlinear dynamics of a stack/cable system subjected to vortex-induced vibration

Description: A model of a stack/wire system, wind-induced vibration of the stack based on an unsteady-flow theory, and nonlinear dynamics of the stack`s heavy elastic suspended cables was developed in this study. The response characteristics of the stack and cables are presented for different conditions. The dominant excitation mechanisms are lock-in resonance of the stack by vortex shedding and parametric resonance of suspended cables by stack motion at their support ends.
Date: December 31, 1995
Creator: Cai, Y. & Chen, S.S.
Partner: UNT Libraries Government Documents Department

Nonlinear dynamics of a stack/cable system

Description: In this study, we developed a coupled model of wind-induced vibration of a stack, based on an unsteady-flow theory and nonlinear dynamics of the stack`s heavy elastic suspended cables. Numerical analysis was performed to identify excitation mechanisms. The stack was found to be excited by vortex shedding. Once lock-in resonance occurred, the cables were excited by the transverse motion of the stack. Large-amplitude oscillations of the cables were due to parametric resonance. Appropriate techniques have been proposed to alleviate the vibration problem.
Date: July 1, 1995
Creator: Cai, Y. & Chen, S.S.
Partner: UNT Libraries Government Documents Department

Chaotic dynamics of loosely supported tubes in crossflow

Description: By means of the unsteady-flow theory and a bilinear mathematical model, a theoretical study was conducted of the chaotic dynamics associated with the fluidelastic instability of loosely supported tubes. Calculations were performed for the RMS of tube displacement, bifurcation diagram, phase portrait, power spectral density, and Poincare map. Analytical results show the existence of chaotic, quasiperiodic, and periodic regions when flow velocity exceeds a threshold value. 38 refs., 15 figs., 2 tabs.
Date: July 1, 1991
Creator: Cai, Y. & Chen, S.S.
Partner: UNT Libraries Government Documents Department

Chaotic vibrations of nonlinearly supported tubes in crossflow

Description: By means of the unsteady-flow theory and a bilinear mathematical model, a theoretical study is presented for chaotic vibrations associated with the fluidelastic instability of nonlinearly supported tubes in a crossflow. Effective tools, including phase portraits, power spectral density, Poincare maps, Lyapunov exponent, fractal dimension, and bifurcation diagrams, are utilized to distinguish periodic and chaotic motions when the tubes vibrate in the instability region. The results show periodic and chaotic motions in the region corresponding to fluid-damping-controlled instability. Nonlinear supports, with symmetric or asymmetric gaps, significantly affect the distribution of periodic, quasiperiodic, and chaotic motions of a tube exposed to various flow velocities in the instability region of the tube-support-plate-inactive mode.
Date: February 1, 1992
Creator: Cai, Y. & Chen, S.S.
Partner: UNT Libraries Government Documents Department

Chaotic vibrations of tubes with nonlinear supports in crossflow

Description: By means of the unsteady flow theory and a bilinear mathematical model, a theoretical study is presented for chaotic vibrations associated with the fluidelastic instability of nonlinearly supported tubes in a crossflow. A series of effective tools, including phase portraits, power spectral density, Poincar'e maps, Lyapunov exponent, fractal dimension, and bifurcation diagrams, are utilized to distinguish periodic and chaotic motions when the tubes vibrate in the instability region. Results show periodic and chaotic motions in the region corresponding to the fluid damping controlled instability. Nonlinear supports, with symmetric or asymmetric gaps, significantly affect the distributions of periodic, quasiperiodic and chaotic motions of the tube with various flow velocity in the instability region of the TSP(tube-support-plate)-inactive mode.
Date: January 1, 1992
Creator: Cai, Y. & Chen, S.S.
Partner: UNT Libraries Government Documents Department

Wind induced vibration of a stack

Description: A stack supported by guy wires at four levels is subjected to large-amplitude oscillations when the wind speed is over 15 m/s. The excitation mechanisms are identified based on scoping calculations, analytical prediction using a finite element code, and observation of the stack/wire response. The stack is determined to be excited by vortex shedding. Once lock-in resonance occurs, the guy wires are excited by the transverse motion of the stack. Large-amplitude oscillations of the guy wires are due to parametric resonance. Several methods are recommended to alleviate vibrational problem for short-term and long-term solutions. A new stack which is modified based on the results of this study is not subjected to any unacceptable oscillations.
Date: January 1, 1992
Creator: Chen, S.S. & Cai, Y.
Partner: UNT Libraries Government Documents Department

Chaotic vibrations of tubes with nonlinear supports in crossflow

Description: By means of the unsteady flow theory and a bilinear mathematical model, a theoretical study is presented for chaotic vibrations associated with the fluidelastic instability of nonlinearly supported tubes in a crossflow. A series of effective tools, including phase portraits, power spectral density, Poincar`e maps, Lyapunov exponent, fractal dimension, and bifurcation diagrams, are utilized to distinguish periodic and chaotic motions when the tubes vibrate in the instability region. Results show periodic and chaotic motions in the region corresponding to the fluid damping controlled instability. Nonlinear supports, with symmetric or asymmetric gaps, significantly affect the distributions of periodic, quasiperiodic and chaotic motions of the tube with various flow velocity in the instability region of the TSP(tube-support-plate)-inactive mode.
Date: December 1, 1992
Creator: Cai, Y. & Chen, S. S.
Partner: UNT Libraries Government Documents Department

Wind induced vibration of a stack

Description: A stack supported by guy wires at four levels is subjected to large-amplitude oscillations when the wind speed is over 15 m/s. The excitation mechanisms are identified based on scoping calculations, analytical prediction using a finite element code, and observation of the stack/wire response. The stack is determined to be excited by vortex shedding. Once lock-in resonance occurs, the guy wires are excited by the transverse motion of the stack. Large-amplitude oscillations of the guy wires are due to parametric resonance. Several methods are recommended to alleviate vibrational problem for short-term and long-term solutions. A new stack which is modified based on the results of this study is not subjected to any unacceptable oscillations.
Date: December 1, 1992
Creator: Chen, S. S. & Cai, Y.
Partner: UNT Libraries Government Documents Department

Distortion of Crabbed Bunch Due to Electron Cloud and Global Crabbing

Description: Crab cavities may be used improve the luminosity in colliding beam colliders with crab crossing. In a global crab crossing correction, only one crab cavity is installed in each ring and the crab cavities generate a horizontally titled bunch oscillating around the ring. The electron cloud in positively charged rings may distort the crabbed bunch and cause the luminosity drop. This paper briefly estimates the distortion of positron bunch due to the electron cloud with global crab and estimates the effect in the KEKB and possible LHC upgrades.
Date: August 1, 2008
Creator: Wang, L.; Raubenheimer, T.O.; Cai, Y. & /SLAC
Partner: UNT Libraries Government Documents Department

Achieving Large Dynamic Aperture in the ILC Damping Rings

Description: The Damping Rings for the International Linear Collider have challenging requirements for the acceptance, because of the high average injected beam power and the large beam produced from the positron source. At the same time, the luminosity goals mean that the natural emittance must be very small, and this makes it particularly difficult to achieve a good dynamic aperture. We describe design approaches and lattice designs that meet the emittance specification and have very promising dynamic aperture.
Date: May 27, 2005
Creator: Wolski, A.; Cai, Y & /LBL, Berkeley /SLAC
Partner: UNT Libraries Government Documents Department

Suppression of Beam-Ion Instability in Electron Rings with Multi-Bunch Train Beam Fillings

Description: The ion-caused beam instability in the future light sources and electron damping rings can be serious due to the high beam current and ultra-small emittance of picometer level. One simple and effective mitigation of the instability is a multi-bunch train beam filling pattern which can significantly reduce the ion density near the beam, and therefore reduce the instability growth rate up to two orders of magnitude. The suppression is more effective for high intensity beams with low emittance. The distribution and the field of trapped ions are benchmarked to validate the model used in the paper. The wake field of ion-cloud and the beam-ion instability is investigated both analytically and numerically. We derived a simple formula for the build-up of ion-cloud and instability growth rate with the multi-bunch-train filling pattern. The ion instabilities in ILC damping ring, SuperKEKB and SPEAR3 are used to compare with our analyses. The analyses in this paper agree well with simulations.
Date: August 18, 2011
Creator: Wang, L.; Cai, Y.; Raubenheimer, T.O.; /SLAC; Fukuma, H. & /KEK, Tsukuba
Partner: UNT Libraries Government Documents Department

Long-Term Simulation of Beam-Beam Effects in the Tevatron at Collision Energy

Description: The beam-beam effect is a significant source of nonlinearities in the Tevatron. We have developed a code which allows us to estimate its contribution to the finite lifetime of the anti-proton beam, both at collision and injection energy, by tracking realistic particle distribution for a high number of terms and extrapolating from the particle loss rate. We describe the physical modeling underlying the code and give benchmarking results.
Date: February 10, 2006
Creator: Kabel, A.C.; Cai, Y.; /SLAC; Sen, T. & /Fermilab
Partner: UNT Libraries Government Documents Department

Simulation of the Beam-Ion Instability in the Electron Damping Ring of the International Linear Collider

Description: Ion induced beam instability is one critical issue for the electron damping ring of the International Linear Collider (ILC) due to its ultra small emittance of 2pm. Bunch train filling pattern is proposed to mitigate the instability and bunch-by-bunch feedback is applied to suppress it. Multibunch train fill pattern is introduced in the electron beam to reduce the number of trapped ions. Our study shows that the ion effects can be significantly mitigated by using multiple gaps. However, the beam can still suffer from the beam-ion instability driven by the accumulated ions that cannot escape from the beam during the gaps. The effects of beam fill pattern, emittance, vacuum and various damping mechanism are studied using self-consistent program, which includes the optics of the ring.
Date: July 6, 2007
Creator: Wang, L.; Cai, Y.; Raubenheimer, T. & /SLAC
Partner: UNT Libraries Government Documents Department

Doubling the PEP-II Luminosity in Simulation

Description: Simulations show that luminosity of the PEP-II B-factory can be doubled from its present peak value of 1 x 10{sup 34}cm{sup -2}s{sup -1}. The particle simulation code BBI developed for studying beam-beam interaction was used to perform the simulations. It was first found that the parasitic collisions significantly degrade the simulated luminosity as the beam currents are increased from 3A and 1.7A to 4A and 2.2A in the low and high energy rings, respectively. The effect of changes in various accelerator parameters on luminosity was then studied in detail from a rough starting point based on analytic estimates and in the process we systematically optimized the luminosity and showed that a luminosity of over 2 x 10{sup 34}cm{sup -2}s{sup -1} is achievable within feasible limits.
Date: June 23, 2006
Creator: Cai, Y.; Seeman, J.; Sonnad, K.; Wienands, U. & /SLAC
Partner: UNT Libraries Government Documents Department

A Parallel Code for Lifetime Simulations in Hadron Storage Rings in the Presence of Parasitic Beam-Beam Interactions

Description: The usual approach to predict particle loss in storage rings in the presence of nonlinearities consists in the determination of the dynamic aperture of the machine. This method, however, will not directly predict the lifetimes of beams. We have developed a code which can, by parallelization and careful speed optimization, predict lifetimes in the presence of 100 parasitic beam-beam crossings by tracking > 10{sup 10} particles-turns. An application of this code to the anti-proton lifetime in the Tevatron at injection is discussed.
Date: March 17, 2008
Creator: Kabel, A. C.; Cai, Y.; Erdelyi, B.; Sen, T. & Xiao, M.
Partner: UNT Libraries Government Documents Department

Experimental Study of Crossing-Angle and Parasitic Effects at the PEP-II e+e- Collider

Description: In a series of dedicated accelerator experiments, we measure the dependence of the PEP-II luminosity performance on small horizontal crossing angles and on the horizontal separation at the first parasitic crossing. The experiment is carried out by varying the IP angle of one of the beams in two different bunch patterns, one with and one without parasitic crossings. The measurements show satisfactory agreement with three-dimensional beam-beam simulations.
Date: October 7, 2005
Creator: Kozanecki, W.; Narsky, Ilya V.; Cai, Y.; Seeman, J. T. & Sullivan, M.
Partner: UNT Libraries Government Documents Department