The high electric-field gradients that can be set up in plasma have offered the promise of compact particle accelerators since the late 1970s. The basic idea is to use the space-charge separation that arises in the wake of either an intense laser pulse or a pulse of ultra-relativistic charged particles. Towards the end of 2014, groups working on both approaches reached important milestones. One team, working at the Facility for Advanced Accelerator Experimental Tests (FACET) at SLAC, demonstrated plasma-wakefield acceleration with both a high gradient and a high energy-transfer efficiency – a crucial combination not previously achieved. At Lawrence Berkeley National Laboratory, a team working at the Berkeley Lab Laser Accelerator (BELLA) facility boosted electrons to the highest energies ever recorded for the laser-wakefield technique.
Several years ago, a team at SLAC successfully accelerated electrons in the tail of a long electron bunch from 42 GeV to 85 GeV in less than 1 m of plasma. In that experiment, the particles leading the bunch created the wakefield to accelerate those in the tail, and the total charge accelerated was small. Since then, FACET has come on line. Using the first 2 km of the SLAC linac to deliver an electron beam of 20 GeV, the facility is designed to produce pairs of high-current bunches with a small enough separation to allow the trailing bunch to be accelerated in the plasma wakefield of the drive bunch.
Using the pairs of bunches at FACET, some of the earlier team members together with new colleagues have carried out an experiment in the so-called “blow-out” regime of plasma-wakefield acceleration, where maximum energy gains at maximum efficiencies are to be found. The team succeeded in accelerating some 74 pC of charge in the core of the trailing bunch of electrons to about 1.6 GeV per particle in a gradient of about 4.4 GeV/m (Litos et al. 2014). The final energy spread for the core particles was as low as 0.7%, and the maxiumum efficiency of energy transfer from the wake to the trailing bunch was in excess of 30%.
Meanwhile, a team at Berkeley has been successfully pursuing laser-wakefield acceleration for more than a decade. This research was boosted when the specially conceived BELLA facility recently came on line with its petawatt laser. In work published in December, the team at BELLA used laser pulses at 0.3 PW peak power to create a plasma channel in a 9-cm-long capillary discharge waveguide and accelerate electrons to the record energy of 4.2 GeV (Leemans et al. 2014). Importantly, the 16 J of laser energy used was significantly lower than in previous experiments – a result of using the preformed plasma waveguide set up by pulsing an electrical discharge through hydrogen in a capillary. The combination of increased electron-beam energy and lower laser energy bodes well for the group’s aim to reach the target of 10 GeV.
Further reading
W P Leemans et al. 2014 Phys. Rev. Lett. 113 245002.
M Litos et al. 2014 Nature 515 93.
