Attenuation of waveguide modes in narrow metal capillaries

The channeling of laser pulses in waveguides filled with a rare plasma is one of the 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 classical TE and TM modes. We consider the structure of capillary modes in a circular capillary, calculate the attenuation rates, discuss the mode expansion of the incident pulse using minimal simplifications, and analyze the accuracy of commonly used approximations. 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-sections is a complex mathematical problem that remains to be solved.