Evolution of finite-size phonon beams in superfluid helium

The nonlinear stage of the evolution of anisotropic beams of high-energy and low-energy phonons, which are practically one-dimensional interacting systems in phase space, is considered in the work. The analysis of the results of observations and theoreti-cal consideration of the spatial and temporal evolution of these systems in the general case, when they are limited in the longitu-dinal and transverse directions, are carried out. As a result of the work, the system of equations for the beam parameters is reduced to a single nonlinear equation that describes the longitudinal and transverse evolution of the beam. A numerical solution of this equation made it possible to explain the characteristic features of the motion and deformation of the beams that are observed exper-imentally, as well as to determine the main parameters and stages of the evolution of the beams. © 2020 Institute for Low Temperature Physics and Engineering. All rights reserved.