Recurrent femtosecond pulse collapse in air due to plasma generation: Numerical results

In this report we present numerical simulations of nonlinear pulse propagation in air to elucidate the physical mechanism underlying the experimentally observed long distance propagation of filaments. Simulations of the nonlinear Schrödinger equation for the electromagnetic field coupled to the electron plasma generated via multiphoton ionization yield a very dynamic picture of long distance propagation in which pulses form, are absorbed, and subsequently are replenished by new pulses, thereby creating the illusion of one pulse, of energy much less than the input, which is self-guided. Moreover, the evolution of the field and plasma display rich spatio-temporal structures with strong gradients, eventually leading to the breakdown of the numerics. Adaptive mesh refinement methods are explored to overcome these difficulties and to address the onset and recurrence of multiple light filaments during the long distance propagation of intense femtosecond infrared pulses in air and point out the features which are common to strong turbulence in other physical systems. The space-time collapse events drive the turbulence here, and plasma defocusing, not dissipation, is the dominant mechanism regularizing the collapse.