Conference Paper

Room temperature accelerator structures for linear colliders

Stanford Linear Accelerator Center, Menlo Park, CA
DOI: 10.1109/PAC.2001.988264 Conference: Particle Accelerator Conference, 2001. PAC 2001. Proceedings of the 2001, Volume: 5
Source: IEEE Xplore

ABSTRACT 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 λ. Because Q scales as λ1/2,
the length of the structures scale not linearly but as λ3/2
in order to preserve the attenuation through each structure. For
the NLC we chose not to follow this scaling from the SLAC S-band linac
to its fourth harmonic at the 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) π mode standing wave
structures

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