The S-band 36-cell experiment

In a multibunch collider scheme deterioration of the beam due to long range wakefields is a severe problem to be overcome. While, in principle, it is possible to couple selectively to dangerous Higher Order Modes (HOMs) a different way is to prevent coherent excitation of these wakes by means of modifying a constant gradient accelerator tube such that a structure detuned with respect to HOM modes is obtained. Since the phenomenon of longer interacting dipole modes which are trapped completely in an inner part of the accelerating structure has, to our knowledge, never been studied experimentally-even theoretically it was not notorious-it was decided to design and build a test structure with the characteristics mentioned above. First results of measurements and numerical predictions are presented.