Paper  Title  Page 

TUPMN091  Planned Use of Pulsed Crab Cavities for Short Xray Pulse Generation at the Advanced Photon Source  1127 


Funding: Work supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DEAC0206CH11357.
In recent years, we have explored application to the Advanced Photon Source (APS) of Zholents'* crabcavitybased scheme for production of short xray pulses. Work concentrated on using superconducting (SC) cavities in order to have a continuous stream of crabbed bunches and flexibility of operating modes. The challenges of the SC approach are related to the size, cost, and development time of the cavities and associated systems. A good case can be made for a pulsed system** using roomtemperature cavities. APS has elected to pursue such a system in the near term, with the SCbased system planned for a later date. This paper describes the motivation for the pulsed system and gives an overview of the planned implementation and issues. Among these are overall configuration options and constraints, cavity design options, frequency choice, cavity design challenges, tolerances, instability issues, and diagnostics plans.
*A. Zholents et al., NIM A 425, 385 (1999).**P. Anfinrud, private communication. 

WEXC02  The Impedance Database Computation and Prediction of Single Bunch Instabilities  1996 


Funding: Work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC0206CH11357. The Impedance Database is a standardized 3D computation of the wake potential generated by a highintensity beam. The database concept is described and compared to analytical and modelbased approaches. The talk will address the computational challenges introduced by tapers, collimators, and very short bunches. Finally, singlebunch instabilities are predicted through tracking and compared to measurements at the Advanced Photon Source and other accelerators. 

Slides  
THPAN090  Fourier Spectral Simulation for Wake Field in Conducting Cavities  3432 


Recent demand of shortbunch beams poses highorder computational tools for investigating beam dynamics in order to improve the beam quality. We have studied a new computational approach with spectrally accurate highorder approximation for wake field calculations. The technique employs the standard Fourier basis combined with a postprocessing procedure for noise reduction by Gegenbauer reconstruction. We integrate this scheme into the existing 2D wake field calculation code ABCI and investigate possible enhancemance of its performance on the same grid base. We will demontrate 2D wake potential simulations for various cylindrically symmetric structures with the quality improvement in comparison to the conventional lowerorder method.  
THPAN091  SpectralElement Discontinuous Galerkin Simulations for Wake Potential Calculations: NEKCEM  3435 


The demand for short bunches of 1 ps or less poses not only technical challenges in order to deliver the beams for leadingedge research but also poses computational challenges when it comes to investigating bunched multiparticle beam dynamics in order to improve the beam quality. We introduce a powerful highorder numerical tool based on spetralelement discretizations with discontinuous Galerkin approximation approach, which includes spectral element time domain solver for Maxwell's equation and electrostatic Poisson solver. We will demonstrate 3D simulations for wakefield and wake potential calculations in conducting cavity structures, as well as meshing and visualization components. We will discuss the overcome of the computational bottleneck by widelyused loworder finite difference programs for calculating wake field excited by 1ps bunches, provided with performance and accuracy comparison.  
FRPMN103  SingleBunch Instability Estimates for the 1nm APS Storage Ring Upgrade with a Smaller Vacuum Chamber  4330 


Funding: Work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC0206CH11357.
We recently studied a lattice achieving 1nm emittance at the APS storage ring*. The successful design required very strong sextupoles in order to tune the machine to the desired positive chromaticity. A preliminary design of such magnets indicated saturation in the poles unless the vacuum chamber gets smaller by a factor of two compared to the existing APS chamber. Since the resistive wall impedance scales as 1/b3, where b is the radius of the chamber, we questioned how much current we can store in a single bunch at the 1nm storage ring. In order to answer this question quantitatively, we calculated all wake potentials of impedance elements of the existing APS storage ring with the transverse dimension properly scaled but with the longitudinal dimension kept unchanged. With the newly calculated impedance of a smaller chamber, we estimated the singlebunch current limit. It turned out that the ring with a smaller chamber would not diminish the singlebunch current limit substantially. We present both wake potentials of 1nm and the existing rings followed by the simulation results carried out for determining the accumulation limit to the ring.
* A. Xiao, "A 1nm Lattice for the APS Storage Ring" these proceedings. 

FRPMN104  Impedance Database II for the Advanced Photon Source Storage Ring  4336 


Funding: Work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC0206CH11357.
The first Impedance Database* constructed at the Advanced Photon Source was successfully used in reproducing the main characteristics of singlebunch instabilities observed in the storage ring. However, the finite bandwidth of the corresponding impedance model was limited to 25 GHz, which happens to be the resolution limit of the density modulation observed in the microwave instability simulation. In order to resolve simulation results never verified in the experiments, we decided to extend the calculated bandwidth of impedance to 50 GHz by recalculating the wake potentials excited by a shorter bunch. Since loworder electromagnetic code requires 2040 grid points per wavelength, reducing the bunch length required a large number of grids for the 3D structure. We used bunch lengths of 1 and 2mm in the Gaussian distribution in the Impedance Database II project. For the largescale computation we used the 3D electromagnetic code GdfidL ** for wake potential calculation at the cluster equipped with 240 GB of memory. The resultant wake potential excited by the short bunch together with application to the storage ring for collective effects is presented in the paper.
* Y.C. Chae, "The Impedance Database and Its Application to the APS Storage Ring" Proc. 2003 PAC, p. 3017.** http://www.gdfidl.de 

FRPMN105  The Wakefield Effects of Pulsed Crab Cavities at the Advanced Photon Source for ShortXray Pulse Generation  4339 


Funding: Work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC0206CH11357.
In recent years we have explored the application to the Advanced Photon Source (APS) of Zholents' crabcavitybased scheme for production of short xray pulses. As a nearterm project, the APS has elected to pursue a pulsed system using roomtemperature cavities*. The cavity design has been optimized to heavily damp parasitic modes while maintaining large shunt impedance for the deflecting dipole mode**. We evaluated a system consisting of three crab cavities as an impedance source and determined their effect on the single and multibunch instabilities. In the singlebunch instability we used the APS impedance model as the reference system in order to predict the overall performance of the ring when the crab cavities are installed in the future. For multibunch instabilities we used a realistic fill pattern, including hybridfill, and tracked multiple bunches where each bunch was treated as soft in distribution. To verify the electrical design, the realistic wake potential of the 3D structure was calculated using GdfidL and this wake potential was used in the multibunch simulations.
* M. Borland et al., "Planned Use of Pulsed Crab Cavities at the APS for Short Xray Pulse Generation," these proceedings.** V. Dolgashev et al., "RF Design of Normal Conducting Deflecting Structures for the APS," these proceedings. 

FRPMN109  200mA Studies in the APS Storage Ring  4354 


Funding: Work supported by U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC0206CH11357. The Advanced Photon Source storage ring is normally operated with 100 mA of beam current. A number of highcurrent studies were carried out to determine the multibunch instability limits. The longitudinal multibunch instability is dominated by the rf cavity higherorder modes (HOMs), and the coupledbunch instability (CBI) threshold is bunchpattern dependent. We can stably store 200 mA with 324 bunches, and the CBI threshold is 245 mA. With 24 bunches, several components are approaching temperature limits above 160 mA, including the HOM dampers. We do not see any CBI at this current. The transverse multibunch instabilities are most likely driven by the resistive wall impedance; there is little evidence that the dipole HOMs contribute. Presently, we rely on the chromaticity to stabilize the transverse multibunch instabilities. When we stored beam up to 245 mA, we used high chromaticity, and the beam was transversely stable. The stabilizing chromaticity was studied as a function of current. We can use these experimental results to predict multibunch instability thresholds for various upgrade options, such as smallergap or longer ID chambers and the associated increased impedance. 