Assume that the magnetic field is uniform in a cubical region and is zero outside. Can you project a charged particle from outside into the field so that the particle describes a complete circle in the field?
Magnetic field is uniform(assumption). If the charged particle is perpendicular to the magnetic field the particle will exhibits a circular trajectory. Let us consider that the charged particle is coming into the field from outside along the positive y-axis. And Let us take the field is in x-axis. X-axis is perpendicular to the y-axis. So magnetic field will be perpendicular to the changed particle. Then the particle will make a circular path.