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When an intense laser pulse propagates through a plasma, the ponderomotive force pushes electrons away from the front and back of the pulse thereby forming a trailing longitudinal density wave. The longitudinal electric field in the plasma wave can be as high as 100 GV m-1, more than three orders of magnitude larger than that found in conventional RF accelerators such as those used at CERN. Particles injected into the correct phase of the plasma wave can be accelerated to energies of order 1 GeV in only a few tens of millimetres. This 'laser wakefield accelerator' is particularly promising for generating beams of short pulse, high-energy electrons for applications in femtosecond electron diffraction, medical imaging, and miniature free-electron X-ray lasers.
One factor which limits the energy to which particles can be accelerated is the distance over which the intensity of the driving laser can be maintained. Over the last few years our group has developed a technique for channelling laser pulses with peak intensities of up to 1018 Wcm-2 over distances which are much longer than the limit set by diffraction. In collaboration with a group at Lawrence Berkeley National Laboratory (LBNL), we have used this technique to extend the length over which acceleration can be maintained by an order of magnitude and thereby generated electrons with energies of 1 GeV. This energy is of the order of that used in many synchrotrons around the world, but the plasma accelerator is only 33 mm long instead of 150 m! For further details, see the further reading suggested below.