SCATTERING MATRIX ANALYSIS OF THE NLC ACCELERATING STRUCTURE
In the Next Linear Collider (NLC)[1], long trains of short, intense bunches are accelerated through the linac on their way to the collision point. One serious problem that needs to be addressed is the multi-bunch, beam break-up instability in the linac. To counteract this instability the accelerating structures are designed so that the dipole mode wakefields are detuned and weakly damped. Detuning is accomplished by gradually varying the dimensions of the 206 cavity cells in each structure, and weak damping by surrounding the structure with four manifolds, which allow the dipole modes to weakly couple out of the cells. In order to design and predict the performance of such a structure in the NLC, it is necessary to be able to calculate accurately the strength of its long-range wakefields. NLC detuned structures were designed first using an equivalent circuit approach, for example, the double band model of Ref. [2]. With the introduction of weak damping, a more elaborate equivalent circuit approach was required[3]. Other methods that have been used for detuned structures are the open-mode, field expansion method[4], and a finite element calculation employing 206 parallel processors[5]. A scattering matrix method can also be applied to cavities that consist of a series of waveguide sections[6], such as the NLC structure, and such a method has also been applied to the problem of detuned accelerating [7],[8]. Among its natural strengths when applied to structure problems are the ability to model the effects of complicated 3 dimensional couplers and to investigate the behavior of higher passband modes. In this report we will employ a computer program that uses such an approach [9], to demonstrate that the scattering matrix method can yield accurate results concerning the wakefields of damped, detuned accelerating structures. We begin by calculating the dipole mode impedance and of a slightly simplified version of a detuned (DT) structure, results which we compare with those of the double circuit model. An important related problem is wakefield effect caused by internal structure misalignments, a problem which has the special difficulty of no longer being cylindrically symmetric; and we next study the transverse kicks to the beam caused by misaligning just one cell in a DT structure, and compare results with those of a perturbation calculation based on the circuit model. Finally, a future goal is to be able to use the scattering matrix formalism to model the more complicated NLC damped, detuned structure (DDS), and we present some preliminary results toward this goal.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- AC03-76SF00515
- OSTI ID:
- 10045
- Report Number(s):
- SLAC-PUB-8105; TRN: US0103321
- Resource Relation:
- Other Information: PBD: 12 Apr 1999
- Country of Publication:
- United States
- Language:
- English
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