Equivalent Circuit Analysis Of The SLAC Damped Detuned Structure*

An accelerating structure designed as described previously is nearing completion. An equivalent circuit analysis, elaborated to take account of both the lower two dipole bands and the nonuniform properties of the damping manifolds, has been carried out. The equivalent circuit has nine parameters per cell, determined by matching the dispersion curves of the three lowest modes (two dipole modes plus the manifold mode) as computed by MAFIA. This procedure is carried out for eleven selected cells, after which interpolation is used to determine the parameters for the remaining 195 cells. Because the manifold-cell coupling is strong, a numerically challenging non-perturbative treatment is required. Wakefield and other results are presented. Presented at the 5th European Particle Accelerator Conference (EPAC 96), Sitges (Barcelona), Spain, June 10-14,1996 *Work supported by the Department of Energy, contracts DE-AC03-76SF00515 and DE-FG03-93ER40759. Equivalent Circuit Analysis Of The SLAC Damped Detuned Structure* R.M. Jones†‡, K. Ko†, N.M. Kroll†‡, R.H. Miller† & K.A. Thompson† †Stanford Linear Accelerator Center, M/S 26, P.O Box 4349, Stanford, CA 94309 ‡University of California, San Diego, La Jolla, CA 92093-0319. 1. ABSTRACT AND IN T R O D U C T I O N The Damped Detuned Structures (DDS) currently under construction to serve as accelerating cavities for the NLC Test Accelerator (NLCTA) incorporate both damping and detuning as a means of suppressing the transverse wakefield. Detuning is accomplished by systematic variation of cell parameters so as to produce a spread in the frequencies of the dipole modes excited by the beam, and damping is accomplished by coupling the individual cells to four waveguide-like structures (called damping manifolds) that run parallel to the cavity and propagate dipole mode energy to loads coupled to the ends of these manifolds. The details of the DDS as well as the rationale underlying its design are discussed in [1]. It differs from previously considered designs [2] in that the manifolds have only one propagating mode in the frequency range of interest and also in that, like the cells, their parameters vary along the structure. The previously reported equivalent circuit analysis [2] has been elaborated in the following respects: (1) The treatment of the manifolds has been modified so as to take account of the effect of its coupling to the cells on its propagation characteristics and also to include the effect of their cell-to-cell variation. (2) The manifold-to-cell coupling network has been modified to take account of the TE,,, like character of the manifold propagating mode. (3) The chain of resonant circuits intended to represent the cells has been doubled (following Bane-Gluckstern [3]) to take account of the mixed TE-TM character of the dipole modes. 2. EQUIVALENT CIRCUIT ANALYSIS The equivalent circuit which we use to represent the structure is shown in Fig. 1. The LC circuits represent the TE and TM components of the dipole field of the individual cells. Each component is magnetically coupled to both components of the adjacent cells. The electron beam excitation of the cavity is modeled by the input currents to each of the TM cells. The manifold structure is modeled by the uppermost sequence of transmission line sections each carrying a TElo waveguide mode and shunted by an LC circuit at the junction of adjacent transmission lines. Coupling of the accelerator cells to the manifolds is represented by a coupling between the shunt capacitance of the manifold and the capacitance of the TE component of the corresponding accelerator cell: