Channel Formation in Long Laser Pulse Interaction with a Helium Gas Jet

Experimental realization of an electron density channel created by a low intensity laser in a helium gas jet is presented. The long (2.5 mm) plasma channel is fully ionized and thus prevents undesirable refraction effects for propagation and guiding of a subsequent high intensity laser pulse. The channel parameters are easily controlled and well suited for laser guiding. The radial plasma expansion and the temperature evolution have been measured and compared to hydrodynamic simulations which show that the plasma expansion is governed by a thermal wave during the laser pulse.