The systems in which the Stern–Gerlach force is most prominent are those with a high electromagnetic field gradient. Section 2 considers the implications of the coupling between the spin of a classical electron and the rapidly varying electromagnetic field produced by a laser-driven plasma wave. Sufficiently short, high-intensity laser pulses can form longitudinal waves within the electron density of a plasma. These density waves propagate with speed comparable to the group speed of the laser pulse. Not all plasma electrons form this wave, however; some of the electrons are caught up in the wave and accelerated by its high fields. The wave eventually collapses as these electrons damp the wave (the wave ‘breaks’). The extremely high electric field gradient of a plasma wave near wavebreaking provides an excellent theoretical testing ground for the effects of Stern–Gerlach-type contributions to the trajectory of a test electron.
