  Owing to the rapid progress in laser technology, very high-contrast
femtosecond laser pulses of relativistic intensities become available. These
pulses allow for interaction with micro-structured solid-density plasma without
destroying the structure by parasitic pre-pulses. This opens a new realm of
possibilities for laser interaction with micro- and nano-scales photonic
materials at the relativistic intensities. Here we demonstrate, for the first
time, that when coupling with a readily available 1.8 Joule laser, a
micro-plasma-waveguide (MPW) may serve as a novel compact x-ray source.
Electrons are extracted from the walls and form a dense self-organized helical
bunch inside the channel. These electrons are efficiently accelerated and
wiggled by the waveguide modes in the MPW, which results in a bright,
well-collimated emission of hard x-rays in the range of 1~100 keV.
