Application of the Mode Matching Technique to the analysis of Waveguide Arrays

In this contribution the mode matching technique is applied to compute the absorption characteristics of a twodimensional array of rectangular waveguides. This analysis is motivated by a proposal of a broadband absorber for TESLA. Outside the waveguide array the so-called Rayleigh expansion is used which represents the electromagnetic field in terms of space harmonics whereas the complete modal spectrum of eigenmodes is taken into account inside the waveguides. In the case of normal incidence the validity of the presented method is confirmed by MAFIA computations. The absorption characteristics for various angles of incidence are calculated for a broad frequency range. The accuracy of the results is checked by a study of convergence. It is shown that the Rayleigh expansion has to be modified if the waveguide array is excited by an ultra-relativistic beam instead of an incoming plane wave. Numerical results for the beam parameters are presented for an array of parallel-plate waveguides and compared with those obtained by other methods

Design of a HOM Broadband Absorber for TESLA

For the TESLA FEL operation very short intense bunches of electrons have to be accelerated. These bunches excite a broad spectrum of HOM (Higher Order Modes) up to frequencies of some THz. Two HOM couplers per cavity are foreseen in the present design proposal in order to extract some of the low frequency HOM from the superconducting accelerating structure. In this contribution an additional HOM broadband absorber is suggested which is to be installed between two cryogenic modules at a temperature of 70 K. Its task is to prevent that the really high frequency HOM are absorbed in the accelerator structure at the 2 K level. The absorption characteristics and the short range wake of four structures which make use of SiC as absorbing material are investigated using the MAFIA computer code. The proposed structures are easier to manufacture and better suited for the operation under vacuum conditions than the recently suggested waveguide array absorber