  When a high-contrast ultra-relativistic laser beam enters a micro-sized
plasma waveguide, the pulse energy is coupled into waveguide modes, which
remarkably modifies the interaction of electrons and electromagnetic wave. The
electrons that pulled out of walls form a dense helical bunch inside the
channel are efficiently accelerated by the transverse magnetic modes to
hundreds of MeVs. In the mean time, the asymmetry in the transverse electric
and magnetic fields provides significant wiggling that leads to a bright,
well-collimated emission of hard X-rays. In this paper, we present our study on
the underlying physics in the aforementioned process using 3D particle-in-cell
simulations. The mechanism of electron acceleration and the dependence of
radiation properties on different laser plasma parameters are addressed. A
theoretical analysis model and basic scalings for X-ray emission are also
presented by considering the lowest optical modes in the waveguide, which is
adequate to describe the basic observed phenomenon. In addition, the effects of
high order modes as well as laser polarization are also qualitatively
discussed. The considered X-ray source have promising features that might serve
as a competitive candidate for future tabletop synchrotron source.
