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Direct observation of the multibunch instability caused by a transverse wake field.

The multibunch instability caused by a transverse wake field is relevant to future linear colliders. This effect was directly observed at the primary electron section of the KEK positron generator linac. About five bunches, each with a charge of several nC, were accelerated to 250 MeV by a rf electric field at a frequency of 2856 MHz. A blowup of the last bunch was observed by increasing the transverse offset of the beam position at the entrance to the first accelerating cavity. Several tests were performed to confirm that the observed effect is due to a transverse wake field.