We present the results of an analytical study and of a two-dimensional particle-in-cell simulation of a relativistically strong laser pulse propagating in a narrow channel which eliminates the pulse spreading due to diffraction. In an empty channel with sharp boundaries, the main absorption mechanism is ''vacuum heating'' of the electrons expelled from the walls. These electrons fill the channel and form a charged cloud which moves at a relativistic velocity behind the pulse and produces a longitudinal electric field that can be used to accelerate charged particles. This cloud can also act as a mirror and, interacting with electromagnetic radiation, upshift its frequency. The interaction with the channel wall depends on the pulse intensity and polarization. TM-polarized pulses undergo greater losses than TE-pulses, and cause the formation of a charged cloud, of high harmonics and of a quasi-static magnetic field. In the case of a channel filled by an underdense plasma, an ultrashort pulse excites a strong wake wave with a longitudinal electric wake field which can accelerate charged particles.
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