BELLA will boost plasma accelerator research

Lawrence Berkeley National Laboratory is set to explore further the high-gradient acceleration of electron beams using ultra-short pulse lasers with the construction of a new facility – BELLA, the Berkeley Lab Laser Accelerator. The primary goal is to provide researchers within the laboratory’s Laser Optical Systems Integrated Studies (LOASIS) programme with a petawatt-class, ultra-short pulse laser system for experiments aimed at demonstrating a 10 GeV electron beam from a metre-long plasma channel.

A laser plasma accelerator (LPA) of this kind relies on creating an electron-density wave in an ionized medium (i.e., plasma) by displacing the electrons away from the ions with an intense laser pulse. The charge separation results in a strong electric field (up to 10¹⁰ V/m) that co-propagates with the laser pulse (like a wake behind a boat) and is capable of accelerating electrons to very high energies in a short distance. Electrons pulled out of the background plasma into the wake can then “surf” on it to reach high energies. Typical electric fields generated in an LPA can be more than a 1000 times larger than in conventional RF accelerators, enabling the acceleration of electrons to giga-electron-volt energies in distances of centimetres instead of tens of metres.

BELLA will build on previous results from the LOASIS programme, which is led by Wim Leemans, one of six recipients of the US Department of Energy’s Ernest Orlando Lawrence Award for 2009. In 2004, researchers with LOASIS showed that high-quality electron beams with an energy spread of a few per cent could be produced at energies of 100 MeV from a structure only 2 mm long. Two years later, the team demonstrated that beams of 1 GeV can be produced from a 3 cm-long plasma structure. One of the key elements of these experiments was the guiding of the laser beams in plasma channels over distances that are long compared with their natural diffraction distance, much as an optical fibre guides a low-power beam.

The aim with the BELLA facility is to scale up these experiments to produce electron beams with energies exceeding 10 GeV in a metre-scale plasma channel. Such devices could form the building blocks of a future-generation linear collider for particle physics, provided that technology is developed to cascade many of these modules and to produce high-quality electron beams with high efficiency. While it could take decades to match the output of the highest-energy RF-based machines, BELLA represents an essential step in investigating how more powerful accelerators of the future might become not only more compact and but much less expensive. Such systems also hold the promise of making possible a table-top accelerator operating in the range of tens of giga-electron-volts, which would be small and cheap enough for universities and hospitals.

The development of a compact linear accelerator with the BELLA project will have also several short-term applications. Among the unique features of LPA-produced electron beams are their duration of a few femtoseconds and their intrinsic synchronization to a conventional laser. A high-quality 10 GeV electron beam could be used to build a soft X-ray free-electron laser, which would be a valuable tool for biologists, chemists, materials scientists and biomedical researchers, allowing them to observe and time-resolve ultrashort (femtosecond) phenomena. A multigiga-electron-volt electron beam could also be used to produce highly collimated, mega-electron-volt photons that could penetrate cargo in a nondestructive way and be highly useful for remote detection of nuclear material. Such high-energy photon beams can be produced by scattering an intense (low-energy photon) laser pulse off the high-energy electron beam.

BELLA will be housed in an existing building at Berkeley. The space will be reconfigured and upgraded to include a clean room, new laser laboratory space and additional shielding. The project is funded largely by the American Recovery and Reinvestment Act (commonly known as economic stimulus funding), which is providing $20 million towards BELLA’s construction. The facility will be completed in about three and a half years.