Direct measurement of transverse wakefields in the SLC linac

The transverse wakefields that generate tails in the beam distribution of an off-axis bunch can be viewed as resonant dipole modes in the structure. These same dipole modes were responsible for beam break up of long bunch trains during the early operation of the SLAC linac. This paper describes the excitation of these modes by the single, high-intensity bunches used in SLC operation. Both time and frequency domain measurements were made of the beam induced signals to identify the dipole modes. The amplitude of the measured dipole mode signal is found to be proportional to the bunch´s offset in the structure. The utility of this technique for obtaining alignment information of the linac structure is discussed for both the SLC and future linear colliders