  A new cavity-chain layout has been proposed for the main linac of the TESLA
linear collider. This superstructure-layout is based upon four 7-cell
superconducting standing-wave cavities, coupled by short beam pipes. The main
advantages of the superstructure are an increase in the active accelerating
length in TESLA and a saving in rf components, especially power couplers, as
compared to the present 9-cell cavities. The proposed scheme allows to handle
the field-flatness tuning and the HOM damping at sub-unit level, in contrast to
standard multi-cell cavities. The superstructure-layout is extensively studied
at DESY since 1999. Computations have been performed for the rf properties of
the cavity-chain, the bunch-to-bunch energy spread and multibunch dynamics. A
copper model of the superstructure has been built in order to compare with the
simulations and for testing the field-profile tuning and the HOM damping
scheme. A "proof of principle" niobium prototype of the superstructure is now
under construction and will be tested with beam at the TESLA Test Facility in
2001. In this paper we present latest results of these investigations.
