<p>We study numerically the dynamics of space charge accumulation in a trap between the magnetic and electrostatic mirrors during formation of a paraxial helical electron beam (PHEB) in an electron-optical system with the cusp magnetic field. The time dependences of the value of the trapped charge of the electrons reflected from the magnetic mirror, the potential in different cross sections of the beam, the currents of the cathode and anode bombardment by reflected particles, and the distribution of the bombardment current along the cathode surface are calculated. The current passing into the cavity is found taking into account the reflection of some electrons from the magnetic mirror. The evolution of the function of electron distribution in the rotational velocities is investigated for the case of an increase in the cold pitch factor of the PHEB. The results of the non-stationary analysis of PHEB formation open up the possibility of decreasing the value of the trapped charge in an electron-optical system with the cusp magnetic field.</p>