Modeling a 30 GHz Waveguide Loaded Detuned Structure for the Compact Linear Collider (CLIC)

Previous research on damped as well as detuned structures for CLIC has led to the following general conclusions. Damped structures can in principle achieve the required wakefield performance for the CLIC multi bunch option, if ideally matched broad band loads are assumed. In order to get a broad band damping of the dipole spectrum over several bands, a Tshaped waveguide cross section has to be used. The ability to make a sufficiently good broad-band load, and the fabrication difficulties associated with the reduced rigidity of the discs are both however major concerns. Detuning, on the other hand, has the advantage of affecting all dipole bands and of being able to create a fast roll-off of the transverse wake with much reduced mechanical difficulties. The critical point of a purely detuned design is recoherence leading to high long range wake levels. The complementarity of these two methods has lead to the development of hybrid structures, employing both methods, as in the SLAC damped detuned structure DDS. The SLAC DDS uses a fully-fledged Gaussian detuning to obtain an optimum roll off behavior, combined with a slight damping to suppress recoherence effects. For the first few bunches, the wake function is largely determined by detuning, it is only in the long range, that the damping becomes effective. The drawback in this case is the high precision required: Due to the fact, that all cells are coupled to one set of manifolds, the cell to manifold coupling influences also the cell to cell coupling. Similar to weak damping, an exaggerated damping also leads to ripples in the transverse impedance and a deteriorated performance. The optimum amount of damping is therefore closely related to the detuning, so that certain mechanical imprecisions are felt twice, giving a non ideal detuning and over/undercoupling to the manifold. For a CLIC multi bunch structure, the following requirements would therefore be