Experiments using a high-energy electron beam at the Stanford Linear Accelerator Center (SLAC) have shown the way in which such a beam can be bent as it crosses the boundary between a plasma and a gas.
Electrons in the gas are expelled by the electrostatic pressure at the head of the high-energy electron beam, leaving a positively charged channel that steers the remainder of the beam. This force is asymmetric at the plasma boundary, there deflecting the beam in the same way that light is refracted.
These results confirm earlier simulations. According to the authors of the report, the results together show that “it is possible to refract and even reflect a particle beam from a dilute plasma gas. Remarkably, for a 28.5 GeV beam that can bore through several millimetres of steel, the collective effects of a plasma are strong enough to ‘bounce’ the beam off an interface that is one million times less dense than air.”
According to the SLAC-Southern California-UCLA team, the effects also suggest that beam refraction could lead to the replacement of bulky magnetic kickers in particle accelerators by fast optical kickers, or even the use of plasma fibre optics to make storage rings without magnets.
Particles can also be channelled by the arrangement of atoms in crystals, and specially bent crystals are already used in accelerator laboratories to steer high-energy beams. For example, a bent crystal is used at CERN’s SPS synchrotron to split off a small fraction of protons to generate neutral kaons for the NA48 CP violation experiment. The small crystal, which is only a few centimetres long, bends 450 GeV protons through 7.2 mrad, which would otherwise require a magnet 5 m long.
The deflection angles achieved by gas refraction are comparable to those provided by channelling in crystals. Unlike in crystal channelling, however, there are no losses from surface transmission. In addition the plasma, being much more dilute than a crystal, does not suffer from the same degree of loss due to scattering. Channelling expert Soeren Pape Moeller of Aarhus, Denmark, said: “The only serious difficulty is that it is without doubt a complicated device, 1.4 m long. Whether it can be turned into more modest and practical device, only time can show. But I think both the idea and the demonstration deserve attention.”
