Working principles of a Penning trap

A Penning trap consists of a set of electrodes and a static magnetic field B = B u_z. These allow for capturing electrons or, in general, any charged particles. The working principle is straightforward: the magnetic field forces the particle to follow circular orbits around its axes, thus trapping it in the radial direction u_r, perpendicular to B. Trapping in the axial direction, u_z, along the magnetic field axes, is accomplished by an electrostatic harmonic potential. 

There are different implementations of Penning traps; one widely used ist the cylindrical Penning trap, which consists of a stack of five electrodes: the central one, called the “ring”, two adjacent compensation electrodes and two end-caps. A sketch is shown in the figure below.