Electron energy enhancement by frequency chirp of a radially polarized laser pulse during ionization of low-density gases

A scheme is proposed to enhance the energy of the electrons generated during the ionization of low-density krypton ions Kr³²⁺ and argon ions Ar¹⁶⁺ by a radially polarized laser pulse using a negative frequency chirp. If a suitable frequency chirp is introduced then the energy of the electrons increases significantly and scattering decreases. The optimum value of the frequency chirp decreases with laser intensity and as well as spot size. The laser spot size also has an optimum value. The electron energy shows strong initial phase dependence. The scheme can be used to obtain quasi-monoenergetic collimated MeV/GeV electrons using the right choice of parameters. The chirped radially polarized laser pulse is more efficient than a chirped circularly polarized laser pulse to enhance energy and obtain quasi-monoenergetic electron beams.