I supose you are looking/asking for ciclotron frequency and magnitude of the magnetic field needed to close and trap the electron trayectories.

Actually, I am asking how the presence of the magnetic field or the variation of its intensity influences electron trapping.

Thanks

Actually the variation in magnetic field strength acts as magnetic mirror and causes trapping of charged particle.

When a magnetic field is applied, the moving plasma electrons are trapped to move in a helical direction along the magnetic field lines due to Lorentz's force. If the magnetic field intensity is increased, the trapping of electrons is enhanced and can be used in applications like plasma acceleration.

Thank you all for your answers,

but the trapping, in this case, is due to the electrostatic potential not to the magnetic field, because the electrons are non-isothermal, i.e., represented by a vortex-like distribution function. In other words, there are free and trapped electrons, and the last ones are trapped in the electrostatic potential trough.