Extending plasma accelerators: Guiding with capillary tubes

In order to extend plasma accelerators, the laser beam has to be guided inside gas or plasma over a distance of the order of the dephasing length, which is typically much larger than the diffraction length z/sub R/ of the laser. A capillary tube can be used as a waveguide for high-intensity laser pulses over distances well in excess of z/sub R/. Experimental demonstration of monomode guiding over 100 z/sub R/ of 10/sup 16/ W/cm/sup 2/ pulses has been obtained in evacuated capillary tubes (45-70-/spl mu/m inner diameter). A drop of transmission has been observed when the intensity of the amplified spontaneous emission (ASE) is high enough to ionize the capillary tube entrance. Propagation in helium gas-filled (10-40 mbar) capillary tubes has been studied at intensities up to 10/sup 16/ W/cm/sup 2/; a plasma column with on-axis density of the order of 10/sup 17/ cm/sup -3/ has been created on a length of the order of 4 cm. The use of a capillary tube for an extended accelerator is discussed for the ease of linear, resonant excitation of plasma waves by laser wakefield.