  Recent experiments at SLAC [1,2] and CERN [3] have revealed evidence of
significant deformation in the form of "pitting" of the cells of the 1.8m
series of structures DDS/RDDS (Damped Detuned Structure/Rounded Damped Detuned
Structure). This pitting occurs in the high group velocity (vg /c = 0.012) end
of the accelerating structure and little evidence of breakdown has been found
in the lower group velocity end of the structure. Additional, albeit
preliminary experimental evidence, suggests that shorter and lower group
velocity structures have reduced breakdown events with increasing accelerating
field strengths. Two designs are presented here, firstly a 90cm structure
consisting of 83 cells with an initial vg/c = 0.0506 (known as H90VG5) and
secondly, an even shorter structure of length 60cm consisting of 55 cells with
an initial vg /c = 0.03 (known as H60VG3). The feasibility of using these
structures to accelerate a charged beam over 10km is investigated. The
particular issue focussed upon is suppression of the dipole wakefields via
detuning of the cell frequencies and by locally damping individual cells in
order to avoid BBU (Beam Break Up). Results are presented on beam-induced
dipole wakefields and on the beam dynamics encountered on tracking the progress
of the beam through several thousand accelerating structures.
  [1] C. Adolphsen, ROAA003, this conf. [2] R.H. Miller et al, FPAH062, this
conf. [3] L. Groening et al, MPPH039, this conf
