CO2 laser-driven absorption waves in underdense hydrogen

A 600-MW CO2 laser oscillator has been used to induce breakdown in hydrogen with filling pressures in the range 10–20 Torr. The breakdown region is observed to undergo a transition from an optically thin symmetric expansion at laser fluxes in excess of ∼ 108 W/cm2 to an optically thick undirectional expansion at laser intensities below this.