Researchers at the US Department of Energy are building a form of laser-driven particle accelerator, which may one day offer physicists an alternative to the vast conventional accelerators such as CERN’s Large Hadron Collider when carrying out high energy experiments. The tabletop accelerator under construction by the LOASIS programme at Berkeley National Labs is named BELLA (the Berkeley Lab Laser Accelerator). The system is a laser-plasma wakefield accelerator, capable of accelerating an electron beam to 10bn electron volts in a distance of just one metre; this, the researchers say, is one fifth of the energy achieved by the two-mile long linear accelerator at the SLAC National Accelerator Laboratory.
Development of a laser-plasma accelerator requires detailed simulations of its operation in three-dimensions, and until now such simulations have challenged or exceeded available computing capacity. A team of researchers led by Jean-Luc Vay of Berkeley Lab’s Accelerator and Fusion Research Division (AFRD) has perfected a 'boosted-frame' method, incorporating special relativity to predict what happens when a laser pulse interacts with the plasma within the accelerator. The technique has cut simulations down to just a few hours of supercomputer time, meaning that design of such an accelerator is viable for the first time.
Laser-plasma wakefield acceleration involves sending a short laser pulse through a plasma measuring a few centimetres or more - many orders of magnitude longer than the pulse itself (or the even-shorter wavelength of its light). The laser pulse creates alternating waves of positively and negatively charged particles within the plasma, thereby setting-up intense electric fields. Bunches of free electrons surf the waves in the plasma and are thereby accelerated to high energies.
The team's simulations have proceeded well: 'We have produced the first full multidimensional simulation of the 10‑billion-electron-volt design for BELLA,' says Vay. 'We even ran simulations all the way up to a trillion electron volts, which establishes our ability to model the behaviour of laser-plasma wakefield accelerator stages at varying energies. With this calculation we achieved the theoretical maximum speed-up of the boosted-frame method for such systems – a million times faster than similar calculations in the laboratory frame.'