Guiding of terawatt laser pulses in performed plasma channels

Experiments were performed on the guiding of intense sub-100 femtosecond laser pulses in preformed plasma channels. The plasma channel was produced from the hydrodynamic expansion of a laser-produced spark, induced by a longer, 100 ps pulse focused with an axicon into a gas target. The channel serves as a waveguide for the high intensity 100-fs pulse over the distance of ∼1--1.5 cm. The waveguide generation and injection lasers are synchronized to each other with an accuracy better than 100 ps. This time is shorter than the fastest channel hydrodynamic time scale of ∼100 ps.;At guided intensities of 5 x 1015 W/cm2, 30% energy throughput in the lowest-order mode was achieved, limited by ionization-induced refraction of the injected beam before it entered the plasma channel. The ionization-induced refraction changes both the intensity envelope and the phase of the pulse and was studied in greater detail by using a Time-domain Interferometric Frequency Resolved Optical Gating technique. The experimental results were found to be in good qualitative agreement with the results of the numerical simulations.;Ionization-induced effects at the entrance of the plasma channels were significantly reduced by producing the plasma channel in an elongated gas jet rather than in the ambient gas. Coupling of up to 52% for 50 mJ∼110 fs pulses was measured in 1.5 cm long preformed plasma channels generated in a high repetition rate argon gas jet, giving guided intensities of up to ∼7 x 1016 W/cm2.;Shadowgrams of the coupling of injected pulses into the channel show that there is ∼0.5 min of less ionized gas remaining at the waveguide entrance after channel generation, even for the case of overfilling of the density profile by the axicon focus. This is due to the gas density fall-off at the jet ends, which results in reduced heating and ionization there by the waveguide-generation pulse. At short delays, the remaining neutral gas results in pulse shortening due to ionization-induced effects, however these effects can be further reduced by injecting a prepulse into the channel.;The experimental results reported here demonstrate that plasma channels are promising means for propagation of high intensity pulses suitable for laser driven plasma accelerators.