University of Tokyo, Tokyo
KEK, Ibaraki
Recently, calculation of wake field and impedance has become more important. In many cases they are usually calculated numerically by using a mesh. It will be shown here that the mesh calculation based on the paraxial approximation can be much faster than ordinary methods when the bunch is very short. There are two advantages. One is to be able to choose the longitudinal mesh size independent of the bunch length. The other is that the problem can be solved as an initial-value problem in spite of frequency domain calculation.
KEK, Ibaraki
Two methods were investigated to study microwave instability in LER of KEKB and SuperKEKB. One is macroparticle tracking code based on PIC. The other one solves the VFP equation directly. First we compare the two methods using a resonator impedance model of KEKB LER. Then we use the calculated impedance including CSR to study the beam instability of LER of KEKB and SuperKEKB. Convergence properties of these two methods due to numerical noise are discussed.
IHEP Beijing, Beijing
Electron Cloud Instability (ECI) may take place in any positively charged particle circular accelerator especially in positron and proton storage ring. This instability has been confirmed to be a serious restriction to the beam stabilities. The physical model on the formation of electron cloud in various kinds of magnetic fields was introduced in the first section of the paper. The transverse and longitudinal wake field model to present the interaction between electron cloud and beam were introduced in another section of the paper. As an example, in positron storage of BEPCII and RCS of CSNS, the densities of electron cloud and beam instabilities caused by the accumulated electrons were simulated.
LLNL, Livermore, California
The Finite Difference Time Domain (FDTD) method and the related Time Domain Finite Element Method (TDFEM) are routinely used for simulation of RF and microwave structures. In traditional FDTD and TDFEM algorithms the electric field E is associated with the mesh edges, and the magnetic flux density B is associated with mesh faces. It can be shown that when using this traditional discretization , projection of an arbitrary current density J(x,t) onto the computational mesh can be problematic. We developed and tested a new discretization that uses electric flux density D and magnetic field H as the fundamental quantities, with the D-field on mesh faces and the H-field on mesh edges. The electric current density J is associated with mesh faces, and charge is associated with mesh elements. When combined with the Particle In Cell (PIC) approach of representing J(x,t) by discrete macroparticles that transport through the mesh, the resulting algorithm conserves charge in the discrete sense, exactly, independent of the mesh resolution h. This new algorithm has been applied to unstructured mesh simulations of charged particle transport in laser target chambers with great success.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock
Demanding applications such as heavy ion fusion, high energy colliders and free electron lasers require the study of beam phenomena like space-charge induced instabilities, emittance growth and halo formation. Numerical simulations for instance with GPT (General Particle Tracer, Pulsar Physics) calculate the mutual Coulomb interactions of the tracked particles *. The direct summation of the forces is rather costly and scales with O(N2). In this paper we investigate a new approach for the efficient calculation of particle-particle interactions: the fast summation by Nonequispaced Fast Fourier Transform (NFFT) **, whereas the NFFT is a generalization of the well known Fast Fourier Transformation (FFT). We describe the algorithm and discuss the performance and accuracy of this method for several particle distributions.
SLAC, Menlo Park, California
High current B-Factory BPM designs incorporate a button type electrode which introduces a small gap between the button and the beam chamber. For achievable currents and bunch lengths, simulations indicate that potentials can be induced in this gap which are comparable to the breakdown voltage. This study characterizes beam induced voltages in the existing PEP-II storage ring collider BPM as a function of bunch length and beam current.