  The millimeter wave all-metallic accelerating structure, aimed to provide
more than 100 MeV/m gradient and fed by feeding RF pulses of 20-30 ns duration,
is proposed. The structure is based on a waveguide with small helical
corrugation. Each section of 10-20 wavelengths long has big circular
cross-section aperture comparable with wavelength. Because short wavelength
structures are expected to be critical to wakefields excitation and emittance
growth, we suggest to combine in one structure properties of a linear
accelerator and a cooling damping ring simultaneously. It provides acceleration
of straight on-axis beam as well as cooling of this beam due to the synchrotron
radiation of particles in strong non-synchronous transverse fields. These
properties are provided by specific slow eigen mode which consists of two
partial waves, TM01 and TM11. Simulations show that shunt impedance can be as
high as 100 MOhm/m. Results of the first low-power tests with 30 GHz
accelerating section are analyzed.
