Refraction of a particle beam: Boundary effects

The refraction of light at an interface is familiar as a rainbow or the 'bending' of a pencil in a glass of water. Here we show that particles can also be refracted and even totally internally reflected, as evidenced by an electron beam of 28.5 × 10 9 electron volts being deflected by more than a milli-radian upon exiting a passive boundary between a plasma and a gas — the electron beam is bent away from the normal to the interface, just like light leaving a medium of higher refractive index. This phenomenon could lead to the replacement of magnetic kickers by fast optical kickers in particle accelerators, for example, or to compact magnet-less storage rings in which beams are guided by plasma fibre optics.