They use electromagnetic fields to accelerate and direct charged particles along a predetermined path, and a magnetic lens system to focus these streams on a target.
The particles' kinetic energy is imparted to matter in the target, inducing near-instantaneous and catastrophic superheating at the surface, and when penetrating deeper, ionization effects can upset or destroy electronics.
However, many accelerators used for high-energy nuclear physics are quite large (sometimes on the order of kilometers in length, such as the LHC), with highly constrained construction, operation, and maintenance requirements.
[1] This creates an electrically neutral beam of high-energy hydrogen atoms, that can proceed in a straight line at near the speed of light to hit the target.
The speed of a beam approaching that of light in combination with the energy deposited in the target was thought to negate any realistic defense.
[6] It reached a maximum altitude of over 200 km, and successfully operated autonomously in space before returning to earth intact.