The multifocus structure of radiation upon femtosecond filamentation of an optical vortex in a medium with an anomalous group velocity dispersion

We analyze numerically the femtosecond filamentation of a vortex beam with the topological charge $m$ = 1 at wavelength $\lambda_0$ = 1800 nm in fused silica under anomalous group velocity dispersion conditions. We show that, as a pulse with a fivefold excess of its power over the critical power propagates through the medium, a multifocus ring structure of radiation with a peak intensity of higher than 10$^{13}$ W/cm$^2$ and surface energy density of about 0.2 J/cm$^2$ is formed. The self-action of the optical vortex is accompanied by the transformation of the spectral energy mainly into the Stokes region. A comparison is made with the case of a normal dispersion of the group velocity upon the self-action of a vortex beam at the wavelength of $\lambda_0$ = 400 nm.