A world record for laser-driven wakefield acceleration has been set by a team at the Berkeley Lab Laser Accelerator (BELLA) Center in the US. Physicists used a novel scheme to channel 850 TW laser pulses through a 20 cm-long plasma, allowing electron beams to be accelerated to an energy of 7.8 GeV – almost double the previous record set by the same group in 2014.
Proposed 40 years ago, plasma-wakefield acceleration can produce gradients hundreds of times higher than those achievable with conventional techniques based on radio-frequency cavities. It is often likened to surfing a wave. Relativistic laser pulses with a duration of the order of the plasma period generate large-amplitude electron plasma waves that displace electrons with respect to the background ions, allowing the plasma waves to accelerate charged particles to relativistic energies. Initial work showed that TeV energies could be reached in just a few hundred metres using multiple laser-plasma accelerator stages, each driven by petawatt laser pulses propagating through a plasma with a density of about 1017 cm–3. However, this requires the focused laser pulses to be guided over distances of tens of centimetres. While a capillary discharge is commonly used to create the necessary plasma channel, achieving a sufficiently deep channel at a plasma density of 1017 cm–3 is challenging.
In the latest BELLA demonstration, the plasma channel produced by the capillary discharge was modified by a nanosecond-long “heater” pulse that confined the focused laser pulses over the 20 cm distance. This allowed for the acceleration of electron beams with quasi-monoenergetic peaks up to 7.8 GeV. “This experiment demonstrates that lasers can be used to accelerate electrons to energies relevant to X-ray free-electron lasers, positron generation, and high-energy collider stages,” says lead author Tony Gonsalves. “However, the beam quality currently available from laser-wakefield accelerators is far from that required by future colliders.”
The quality of the accelerated electron beam is determined by how background plasma electrons are trapped in the accelerating and focusing “bucket” of the plasma wave. Several different methods of initiating electron trapping have been proposed to improve the beam emittance and brightness significantly beyond state-of-the-art particle sources, representing an important area of research. Another challenge, says Gonsalves, is to improve the stability and reproducibility of the accelerated electron beams, which are currently limited by fluctuations in the laser systems caused by air and ground motion.
In addition to laser-driven schemes, particle-driven plasma acceleration holds promise for high-energy physics applications. Experiments using electron-beam drivers are ongoing and planned at various facilities including FLASHForward at DESY and FACET-II at SLAC (CERN Courier January/February 2019 p10). The need for staging multiple plasma accelerators may even be circumvented by using energetic proton beams as drivers. Recent experiments at CERN’s Advanced Wakefield Experiment demonstrated electron acceleration gradients of around 200 MV/m using proton-beam-driven plasma wakefields (CERN Courier October 2018 p7).
Experiments at Berkeley in the next few years will focus on demonstrating the staging of laser-plasma accelerators with multi-GeV energy gains. “The field of plasma wakefield acceleration is picking up speed,” writes Florian Grüner of the University of Hamburg in an accompanying APS Viewpoint article. “If plasma wakefields can have gradients of 1 TV/m, one might imagine that a ‘table-top version of CERN’ is possible.”
A Gonsalves et al. 2019 Phys. Rev. Lett. 122 084801.