normablue writes: http://physicsweb.org/articles/news/9/9/15/1?rss=2.0
27 September 2005
Physicists
in the US have developed a new type of laser-based particle
accelerator. Tomas Plettner and co-workers at Stanford University and
the Stanford Linear Accelerator (SLAC) used a commercial laser
operating at a wavelength of 800 nm to modulate the energy of electrons
travelling in a vacuum. The modulation they achieved was equivalent to
an electric field gradient of 40 million volts per metre. The technique
could be used to accelerate particles into the TeV energy range in the
future (Phys. Rev. Lett. 95 134801).
Conventional
accelerators have to be hundreds of metres in length, or longer, to
accelerate particles to energies of interest to particle physicists. In
recent years, scientists have developed a variety of techniques, mostly
based on laser-produced plasmas, that are able to achieve much higher
acceleration gradients than conventional accelerators. This opens the
possibility of significantly reducing the length of these machines.
However, some of these techniques suffer from synchrotron radiation
losses or poor beam quality, which will limit their appeal to particle
physicists.
The
new method demonstrated by the Stanford team involves using a laser
beam with a longitudinal electric field component -- that is, an
electric field component in the direction that the laser beam is
travelling -- to accelerate electrons that are travelling in the same
direction. The energy gained by the electron corresponds to the
integral of the longitudinal electric field component over the distance
along which the electron beam and the laser beam interact with each
other. The device relies on accelerating the electrons in a vacuum
rather than in the much more complicated environment of a plasma.
In
free space, the phase velocity of the laser -- the speed at which light
of a single wavelength would move -- does not match the velocity of the
electrons, so there is no acceleration. However, Plettner and
co-workers have now overcome this problem by placing a "boundary", made
of gold-coated polymer tape, at the point where the beams interact (see
figure). This limits the interaction between the beams and allows for a
non-zero energy exchange between the two, which leads to electron
acceleration.
"The initial and main motivation for this
work is the possibility for developing particle accelerator technology
that could reduce the length of existing linear accelerators by an
order of magnitude," says Plettner. "This will lead to a compact
high-luminosity lepton collider with the potential for collision
energies of 1 TeV (1012 electron volts) and beyond." The new approach could also lead to the development of very compact coherent X-ray sources.
