Plasma Wakefields Accelerate Positrons

An experiment conducted at SLAC features a number of firsts: the first time positrons have been accelerated by the plasma wakefield method (see PNU #385 for background information); the first time wakefield acceleration has been achieved with meter-size plasmas (previous efforts have taken place in 10 cm cells); and the first to operate under realistic accelerator conditions (in this case a 30-GeV beam of positrons). In this UCLA/SLAC/USC collaboration, positron bursts are sent into a 1.4-meter-long chamber filled with a lithium plasma. The first two-thirds of the burst sets up powerful electric fields in the plasma which then serve to accelerate the trailing one-third of the burst to higher energy. The boosted positrons increased their energy by about 80 MeV over a length of 1.4 m, for an acceleration gradient of about 50 MeV/m. This is comparable to the best acceleration that can be accomplished with conventional RF techniques in which electrons or positrons are taken up to higher energies by soaking up radio energy coupled into the beam pipe. But the wakefield researchers expect that the gradient can be enhanced a hundredfold to 5 GeV/m if the size of the beam pulses can be shrunk by a factor of 10. According to Chan Joshi of UCLA (contact Chan Joshi, 310-825-7279) the wakefield approach may not be fully mature by the time the next electron-positron collider is built, but its benefit could be tested by installing two plasma accelerator sections, one for positrons and one for electrons, just before the interaction point for some final energy boosting in an existing collider. (M. J. Hogan et al., Physical Review Letters, 23 May 2003)