Experimental investigation of recoil momentum generation efficiency under nir femtosecond laser ablation of refractory metals in vacuum

Summary form only given. Irradiation by ultrashort laser pulses is a way to increase recoil momentum under laser ablation of materials, because, in this case, the energy deposition per unit volume of the target material is substantially higher due to reduced heat dissipation. Using methods of combined interferometry, we estimated the specific impulse (~200-900 s), momentum coupling coefficient (~2×10^-5-3×10^-4 Ns/J), laser energy conversion efficiency to kinetic energy of the gas-plasma flow (~0.05-0.82), and degree of the gas-plasma flow monochromaticity (~0.72-0.92) under femtosecond (τ ~ 45 fs, λ ~ 800 nm) ablation of refractory metals (Ti, Zr, Mo, and Nb) in vacuum. The obtained values of specific mechanical recoil momentum (C_m, max~3×10^-4 Ns/J) and laser to plasma flow kinetic energy conversion efficiency (η_max~0.82) exceed those obtained for microsecond and nanosecond pulses in air and are close to the values of these parameters achieved under irradiation of polymer materials. For the first time, the peculiar threshold C_m~10^-4 Ns/J determining the expediency of using any material as a fuel in laser rocket engines was exceeded for refractory metals. The degree of gas-plasma flows monochromaticity (μ_max~0.92) induced by femtosecond laser pulses ensures a high degree of transformation of particles kinetic energy to the recoil momentum. The proximity of interaction regimes ensuring peak values of specific mechanical recoil momentum and laser energy conversion efficiency to kinetic energy of gas-plasma flow in the case of laser irradiation of metals substantially simplifies the task of laser thrusters operation regimes optimization. For the first time, we demonstrated that momentum coupling coefficient, as a function of laser radiation energy density, can have two peaks, in the low- and high-energy regimes of interaction, w- ile the presence of an inflection point between them substantially broadens the range of acceptable (from the point of view of efficiency) regimes of laser irradiation.