The properties of a high energy short pulse electron beam in a magnetic mirror field are studied numerically by the Particle-in-Cell method. The complex nature of processes studied, and also the need for the calculation of the positions, velocities, and trajectories of billions of particles, required the use of a scalable parallel algorithm. The use of modern supercomputers has allowed us to calculate the plasma dynamics, to determine the electron beam in the magnetic mirror field. Mathematical modeling of non-neutral plasma dynamics at the trap is carried out. The axially integrated radial density profiles, electric field distribution, and phase space distribution of velocity are measured. The axial and radial oscillations of a confined non-neutral plasma are also observed.