Modeling of femtosecond filament lifetime extension via CW laser coupling

Abstract This study investigates the applicability of a CW laser for a femtosecond filament lifetime extension. Initially the formation of the filament is modeled using a zero-dimensional semi-analytical collisionless model that accounts for the generation of photoelectrons in the high-intensity field of the fs-pulse. The filament post-discharge phase and the subsequent CW addition are modeled using a comprehensive (radial) 1-D axisymmetric model, incorporating both plasma dynamics and chemical kinetics. The model self-consistently integrates the Navier–Stokes fluid flow equations with the conservation of electron energy and vibrational energy within the plasma to account for non-equilibrium flow effects. The results show that the femtosecond filament ( I fs = 5 × 10 13 W/cm −2 ) leads to strong dissociation of air molecules during the post-discharge but very little gas heating ( T gas ∼ 450 K). The filament decays quite rapidly at 30 bar, with the electron density decaying by more than four orders of magnitude during the first 100 nanoseconds after the pulse. Both results are in good agreement with past experimental observations. Interestingly, overlapping a CW laser beam ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>I</mml:mi> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mi>C</mml:mi> <mml:mi>W</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> W/cm −2 ) on the fs-pulse at 30 bar is shown to slow down the electron number density decay and stabilize the electron temperature to ∼ 0.7 eV. The ability to extend the femtosecond filament lifetime using a CW laser could be applied to develop an electrodeless continuous optical discharge for various practical applications in spectroscopy, material processing, plasma medicine or plasma-assisted combustion.