Studies of Beam Induced Dipole-Mode Signals in Accelerating Structures at the SLC

Beam emittance dilution by self induced transverse elds (wake elds) in accelerating structures is a key problem in linear accelerators. To minimize the wake eld e ects the beam trajectory must be precisely centered within the structures. An e cient way to achieve this is to detect beam induced microwave signals in the lowest dipole mode band and to steer the beam by minimizing these signals. This paper brie y covers some experiences from SLC S-band structures, but mainly concentrates on results of a wake eld instrumentation scheme applied to a NLC prototype X-band structure and tested with beam in the SLC linac. A beam based in-situ structure straightness measurement is shown as well as results of beam steering experiments based on phase and amplitude detection of two separated modes in the structure. After centering the beam the reduction of the wake eld was demonstrated independently by probing it with a test bunch that is de ected by the residual wake eld at a short distance behind the drive bunch. Invited talk presented at the 1997 Particle Accelerator Conference Vancouver, B.C., Canada, May 12-16, 1997 Work supported by Department of Energy contract DE{AC03{76SF00515. Studies of Beam Induced Dipole-Mode Signals in Accelerating Structures at the SLC M. Seidel Stanford Linear Accelerator Center, Stanford University, CA 94309 Abstract Beam emittance dilution by self induced transverse fields (wakefields) in accelerating structures is a key problem in linear accelerators. To minimize the wakefield effects the beam trajectory must be precisely centered within the structures. An efficient way to achieve this is to detect beam induced microwave signals in the lowest dipole mode band and to steer the beam by minimizing these signals. This paper briefly covers some experiences from SLC S-band structures, but mainly concentrates on results of a wakefield instrumentation scheme applied to a NLC prototype X-band structure and tested with beam in the SLC linac. A beam based in-situ structure straightness measurement is shown as well as results of beam steering experiments based on phase and amplitude detection of two separated modes in the structure. After centering the beam the reduction of the wakefield was demonstrated independently by probing it with a test bunch that is deflected by the residual wakefield at a short distance behind the drive bunch.Beam emittance dilution by self induced transverse fields (wakefields) in accelerating structures is a key problem in linear accelerators. To minimize the wakefield effects the beam trajectory must be precisely centered within the structures. An efficient way to achieve this is to detect beam induced microwave signals in the lowest dipole mode band and to steer the beam by minimizing these signals. This paper briefly covers some experiences from SLC S-band structures, but mainly concentrates on results of a wakefield instrumentation scheme applied to a NLC prototype X-band structure and tested with beam in the SLC linac. A beam based in-situ structure straightness measurement is shown as well as results of beam steering experiments based on phase and amplitude detection of two separated modes in the structure. After centering the beam the reduction of the wakefield was demonstrated independently by probing it with a test bunch that is deflected by the residual wakefield at a short distance behind the drive bunch.