clamped intensity*

In this paper  we demonstrate for the first time self-guiding of a 10.2 μm ~1TW CO₂ laser pulse in the atmosphere over at least 30 meters (~20 ZR). We observe that when the peak power of such pulses exceeds ~870 GW, a centimeter-diameter single filament is formed in air. We call such a single filament a megafilament because it confines an ~1TW laser pulse with several Joules of energy and its cross-section is 10⁴ times larger than a typical near-IR single filament. The clamped intensity of ~1 TW/cm² for the long-wave infrared light  confined in this megafilament was measured to be much smaller than that required for the tunnel ionization of O₂/N₂.  By anchoring the experimental data with numerical simulations based on the concept of many-body interactions in the atmospheric pressure gas, we find that Kerr self-focusing at such a low laser intensity is arrested by a new and different ionization mechanism due to many-body Coulomb ionization . The amount of free carriers produced inside of the filament is rather small (≤10¹³cm^-3) but sufficient  to effectively decrease the molecular polarizability during the laser pulse.