Electron beam energy chirp is an important parameter that affects the bandwidth and performance of a linac-based, free-electron laser. In this paper we study the wakefields generated by a beam passing between flat metallic plates with small corrugations, and then apply such a device as a passive dechirper for the Linac Coherent Light Source (LCLS) energy chirp control with a multi-GeV and femtosecond electron beam. Similar devices have been tested in several places at relatively low energies (∼100 MeV) and with relatively long bunches (> 1 ps). In the parameter regime of the LCLS dechirper, with the corrugation size similar to the gap between the plates, the analytical solutions of the wakefields are no longer applicable, and we resort to a field matching program to obtain the wakes. Based on the numerical calculations, we fit the short-range, longitudinal wakes to simple formulas, valid over a large, useful parameter range. Finally, since the transverse wakefields-both dipole and quadrupole-are strong, we compute and include them in beam dynamics simulations to investigate the error tolerances when this device is introduced in the LCLS.

This paper analyzes the longitudinal space charge impedances of a round uniform beam inside a rectangular and parallel plate chambers using the image charge method. This analysis is valid for arbitrary wavelengths, and the calculations converge rapidly. The research shows that only a few of the image beams are needed to obtain a relative error less than 0.1%. The beam offset effect is also discussed in the analysis.

This is a more comprehensive report of the accelerator physics in the white paper "PEP-X Light Source at SLAC". A new light source called "PEP-X" would reside in the 2.2-km PEP-II tunnel. It has a hybrid lattice where two of its six arcs contain DBA cells that provide a total of 30 straight sections for insertion device beam lines and the remaining arcs contain TME cells for an extremely low emittance. Using 90 meter damping wigglers the horizontal emittance at 4.5 GeV is further reduced to 0.1 nm-rad. Many collective effects including intra-beam scattering, Touschek lifetime, and fast ion instability are studied. We expect that PEP-X will produce photon beams having brightnesses near 10 22 (ph/s/mm 2 /mrad 2 /0.1% BW) at 10 keV and 10 21 at 35 keV.

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9This paper studies the space charge impedances of a rectangular beam inside a rectangular chamber, and the limiting 10 case, e.g., a rectangular beam between parallel plates, respectively. The charged beam has uniform density in vertical 11 direction and arbitrary distribution in horizontal direction. The method of separation of variables is used to calculate the 12 space charge potentials, fields, and impedances which are valid in the whole perturbation wavelength spectrum. 13Comparisons between the theoretical calculations and the numerical simulations are also provided and they match quite 14 well. It is shown that the rectangular beam shape may help to reduce the longitudinal space charge impedances. 19 chamber; line charge density modulations; separation of variables; parallel plates; finite element method.20 1. Introduction 21 In order to study the longitudinal beam instabilities due to the interactions between the perturbed beam 22 and the conducting vacuum chamber surrounding it, various space charge field models with different cross-23 sections of the beam and chamber have been investigated. Ref. [1] derived the longitudinal space charge 24 impedances of a round beam inside a rectangular chamber in the long-wavelength limits. Ref. [2] and Ref. 25 [3] studied the longitudinal resistive-wall instability and the space-charge driven microwave instability, 26 respectively, using a model consisting of a round beam inside a round vacuum chamber. In the research for 27 future linear colliders, some merits of flat (planar or rectangular) electromagnetic structures have been 28 found and aroused the interests of beam physicists, such as the reduced space charge forces [4] [5]. Ref. [6] 29 explored the properties of a planar beam between a pair of perfectly conducting plates. It concluded that, 30 comparing with the conventional axially symmetric configurations, the flat geometries of both the beam 31 and the chamber may help to reduce the longitudinal space charge fields. The two-dimensional (2D) 32 electrostatic space charge field of a rectangular beam inside a rectangular chamber was solved by Ref. [7] 33 using the method of separation of variables. While in this model, the field was induced only by the 34 unperturbed (constant) beam intensity without longitudinal modulations. The results are only valid when 35 the perturbation wavelengths of the longitudinal charge density are much larger than the transverse 36 dimensions of vacuum chamber, hence cannot be used directly in the study of short-wavelength instabilities, 37 such as microwave instability and micro-bunching instability. Another model of rectangular beam inside 38 rectangular chamber in Ref. [8] assumed the beam perturbation took place in the vertical direction, and 39 hence this model was devoted to study the transverse resistive wall instability instead of the longitudinal 40 one. 41 This paper introduced a three-dimensional (3D) space charge field model consisting of a rectangular 42 beam with sinusoidal longitudinal density modulations inside a r...