Extending plasma accelerators: Guiding with capillary tubes

In order to extend plasma accelerators, the laser beam has to be guided ins ide gas or plasma over a distance of the order of the dephasing length, whi ch is typically much larger than the diffraction length z(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(R). Experimental demonstration of monom ode guiding over 100 z(R) of 10(16) W/cm(2) pulses has been obtained in eva cuated capillary tubes (45-70-mum inner diameter). A drop of transmission h as been observed when the intensity of the amplified spontaneous emission ( ASE) is high enough to ionize the capillary tube entrance. Propagation in h elium gas-filled (10-40 mbar) capillary tubes has been studied at intensiti es up to 10(16) W/cm(2): a plasma column with on-axis density of the order of 10(17) cm(-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 case of linear, resonant excitation of plasma waves by laser wakefield.