SLAC demonstrates plasma ‘afterburner’

A team of researchers at SLAC has shown that plasma acceleration can dramatically boost the energy of particles over a short distance. The breakthrough is the culmination of almost a decade of work, led by Chan Joshi from University of California, Los Angeles, Thomas Katsouleas from the University of Southern California and Robert Siemann from SLAC.

The technique uses the plasma-wakefield effect – the high electric fields generated in the wake of an intense beam of either photons or charged particles passing through a plasma. In 2006, Wim Leemans and colleagues from the Lawrence Berkeley National Laboratory and Oxford University accelerated electrons to 1 GeV in laser-driven wakefields over 3.3 cm. Now Ian Blumenfeld and colleagues have used the intense, ultrarelativistic electron beam from the 3 km linac at SLAC to create the wakefields.

In the experiment at SLAC, the team directed the 42 GeV beam from the linac into lithium gas in an 85 cm long plasma chamber. The electrons ionize the gas at the front of the beam pulse, creating a plasma, and also push out the plasma electrons to leave a column of ions. The plasma electrons are attracted back to the ions, but overshoot, setting up space–charge oscillations at the rear of the pulse, forming the wake. While most of the electrons in the beam pulse lose energy as they create the wakefield, those near the back of each pulse are accelerated in the high field created there. The measurements showed that some electrons more than doubled their energy, up to a maximum of 85 ± 7 GeV (see figure), implying a peak accelerating field of around 53 GV/m. In 800 events, 30% showed an energy gain of more than 30 GeV.

In tests with a 113 cm lithium-gas column, the team measured a maximum energy of just 71 ±11 GeV, and only 3% of 8000 consecutive events showed an energy gain of more than 30 GeV. This apparent saturation in the energy gain appears to be due to an expansion of the front of the beam, which could be reduced with a lower-emittance beam.