Particle beams that bend like light
May 2, 2001
An electron beam that is powerful enough to pierce several millimetres of steel can - remarkably - be reflected by a layer of gas that is a million times thinner than air. Thomas Katsouleas of the University of Southern California in Los Angeles and colleagues demonstrated the phenomenon - which is similar to the refraction of light at a boundary - at the Stanford Linear Accelerator Center in the US. The team believes the technique could be used to control beams of particles inside particle accelerators more efficiently than existing methods based on magnets (P Muggli et al 2001 Nature 411 43).
When a beam of energetic electrons travels through a plasma, the space charge at the tip of the beam knocks electrons from the ions in the plasma, leaving the ions with a larger positive charge. If the plasma ions are massive, they do not move, and the tip of the electron beam effectively bores a positively charged channel through the plasma. The symmetrical array of positive charges focuses the remainder of the beam into the channel as it passes through.
Now imagine that the plasma has a boundary with a less dense gas. If the electron beam approaches this interface at an angle, the end of the charged tunnel at the interface would be slanted. Particle physicists had predicted previously that this asymmetry would deflect the beam. Katsouleas and co-workers found that the beam is indeed skewed towards the normal by a twentieth of a degree - in the same way that light bends as it enters a less dense medium. The team also found that the particle beam can be totally internally reflected - that is, reflected back into the plasma - at very shallow angles of incidence.
In contrast to the refraction of light at an interface, the electron beam briefly splits into two because the plasma-gas boundary takes a split second to react to the head of the particle beam. This means that the first pulse of electrons is undeflected, while the tail of the beam is bent towards the normal.