Radial Confinement
A charged particle moving in a constant magnetic magnetic field will execute cyclotron motion if the particle has any momentum in the plane perpendicular to the magnetic field. So the magnetic field confines the particle in the radial plane, but it is free to move along the magnetic field lines.
Ion Motion in a Penning Trap
The resulting ion motion in a Penning trap consists of three independent eigenmotions, two in the radial plane and one in the axial. The standard cyclotron motion of a particle in a magnetic field is modified by the presence of the applied electric field. The result is the addition of a slow magnetron motion in the radial plane and a (reduced) cyclotron motion.
Radial + Axial Confinement
By placing an electrode structure consisting of two end cap electrodes (on the top and bottom), and a ring electrode in the magnetic field it is possible to achieve full 3D confinement. The electrode structure is used to generate a quadrupolar electric field that confines a charged particle in the axial direction along the magnetic field lines. The combination of the magnetic field and electrode structure is called a Penning trap.