Design of low level RF control for the TESLA superstructure

The superstructure is a viable option for the TESLA linear collider because of a high effective gradient and a reduced number of RF components. However, the high number and close proximity of passband modes impose challenging demands on the RF control system. The control problem is complicated by the fact that the cavity probe signal does not exactly reflect the actual accelerating voltage experienced by the beam due to the different coupling of the generator, pickup probe and beam to the FM passband modes. The digital control system developed for the standard 9-cell cavity is not adequate for operation of the superstructure. As discussed in this paper, an additional filter is needed to guarantee robustness and stability. Based on RF control simulations the filter characteristic is optimized