  Early tests of short low group velocity and standing wave structures
indicated the viability of operating X-band linacs with accelerating gradients
in excess of 100 MeV/m. Conventional scaling of traveling wave traveling wave
linacs with frequency scales the cell dimensions with l. Because Q scales as
l1/2, the length of the structures scale not linearly but as l3/2 in order to
preserve the attenuation through each structure. For NLC we chose not to follow
this scaling from the SLAC S-band linac to its fourth harmonic at X-band. We
wanted to increase the length of the structures to reduce the number of
couplers and waveguide drives which can be a significant part of the cost of a
microwave linac. Furthermore, scaling the iris size of the disk-loaded
structures gave unacceptably high short range dipole wakefields. Consequently,
we chose to go up a factor of about 5 in average group velocity and length of
the structures, which increases the power fed to each structure by the same
factor and decreases the short range dipole wakes by a similar factor.
Unfortunately, these longer (1.8 m) structures have not performed nearly as
well in high gradient tests as the short structures. We believe we have at
least a partial understanding of the reason and will discuss it below. We are
now studying two types of short structures with large apertures with moderately
good efficiency including: 1) traveling wave structures with the group velocity
lowered by going to large phase advance per period with bulges on the iris, 2)
pi mode standing wave structures
