Generation of plasma Eields in the interaction between two oppositely propagating short laser pulses in an underdense plasma

Abstract

A study is made of the interaction (“collision”) between two identical laser pulses with lengths much shorter than the diffraction length, propagating in a plasma toward one another. It is shown that the plasma response to the pulses depends essentially on the value of the parameter ω_pτ, where ω_p is the plasma frequency and τ is the pulse duration. Short laser pulses (such that $\omega _p \tau \leqslant \sqrt 2$) efficiently generate plasma waves on two characteristic scale lengths. Large-scale wake waves with a wavelength of about c/ω_p are generated over the entire path of the pulses and form a two-dimensional standing plasma wave in the region between the pulses after their interaction. In the interaction region, the pulses excite small-scale plasma oscillations with a wavelength equal to half the laser wavelength, which remain in the plasma after the interaction. Long laser pulses (such that $\omega _p \tau \leqslant \sqrt 2$) also generate quasistatic plasma perturbations on two scale lengths. Perturbations generated on large scales of about the pulse length accompany the propagating pulses and are somewhat amplified in the interaction between them. Small-scale plasma fields are generated only during the interaction between the pulses, and they disappear after the interaction. The influence of the generation of plasma fields on the energy of the laser pulses and on their shape, as well as the possible applications of the effects under consideration, is discussed.