An alternative layout of the main accelerator for the 800 GeV upgrade of the TESLA collider is based on weakly coupled pairs of 9-cell superconducting cavities - the superstructure. The concept of the superstructure layout and results of the first beam test in the TTF linac (finished Nov. 18, 2002) will be presented. A very good damping of parasitic modes in the weakly coupled structures makes a new application of superstructures feasible for FEL linacs when the accelerated current is in the range of 100 mA. Recent HOM damping tests on a copper model of two coupled modified CEBAF cavities are encouraging. Further studies of this layout should answer a question, of the superstructure impact on the performance of the FEL linacs and on the investment costs.


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(Coffee before the seminar starting 9:30 AM)