  The channeling of laser pulses in waveguides filled with a rare plasma is one
of promising techniques of laser wakefield acceleration. A solid-state
capillary can precisely guide tightly focused pulses. Regardless of the
material of the capillary, its walls behave like a plasma under the influence
of a high-intensity laser pulse. Therefore, the waveguide modes in the
capillaries have a universal structure, which depends only on the shape of the
cross-section. Due to the large ratio of the capillary radius to the laser
wavelength, the modes in circular capillaries differ from the classical TE and
TM modes. The attenuation length for such modes is two orders of magnitude
longer than that obtained from the classical formula, and the incident pulse of
the proper radius can transfer up to 98% of its initial energy to the
fundamental mode. However, finding eigenmodes in capillaries of arbitrary
cross-section is a complex mathematical problem that remains to be solved.
