Particle accelerators powered by lasers rather than radio-frequency waves could help lower the cost of high-energy physics research.
Traditional particle accelerators, such as the Large Hadron Collider, use high-power radio-frequency waves to energize particles, but another method scientists have grappled with over the last two decades is blasting a powerful laser beam into a plasma to accelerate particles.
The process promises significantly smaller accelerators than those currently in existence or even under consideration as it can accelerate particles far more rapidly than current RF technology, but the method requires a petawatt laser that can deliver thousands of pulses per second.
The petwatt laser at Lawrence Berkeley National Laboratory's Berkeley Lab Laser Accelerator (BELLA) Center in the USA has the highest repetition rate of any petawatt laser in the world – just one pulse per second.
Since less powerful lasers recharge faster European researchers have suggested using an array of smaller lasers to produce one enormous pulse, but they worried that synchronising hundreds of lasers to pulse within less than a femtosecond of one another would present serious and expensive technical problems.
Today, in a theoretical paper published in journal Physics of Plasmas, researchers from the BELLA lab have questioned whether such precision is necessary, with head of the centre Wim Leemans comparing it pushing a swing.
“Instead of one big push, we would give it many smaller pushes at roughly the same time,” he said. “It's not quite perfect, but the swing doesn't really care. It averages over all these little pushes and up it goes."
According to the authors such an approach is theoretically possible and laxer timing would make the method practical, lowering the cost of building accelerators to the point where they would be cost effective for all kinds of desirable scientific, industrial and medical uses.
Laser-plasma accelerators work by blasting a plasma with a high-powered laser, causing the fast moving electrons to leave the heavy ions behind. As they separate, they create gigantic electric fields, 100 to 1,000 times larger than those in conventional accelerators.
While Stanford's Linear Accelerator Center takes two miles to drive an electron to 50 billion electron volts (GeV), Leemans' experimental laser-plasma accelerator takes electrons to more than 1GeV in roughly 3cm.
"The effect is like the wake of boat speeding down a lake. If the wake was big enough, a surfer could ride it," said Leemans. "Imagine that the plasma is the lake and the laser is the motorboat.
“When the laser ploughs through the plasma, the pressure created by its photons pushes the electrons out of the way. They wind up surfing the wake, or wakefield, created by the laser as it moves down the accelerator."
While the work is still very much at the theoretical stage Leeman believes it makes the idea of larger accelerators exploiting this technique potentially viable.
He hopes to power them with a new technology based on highly-efficient fibre lasers. The power that off-the-shelf welding lasers offer demonstrates multi-kW capabilities but much work is needed to pack the power into ultra-short pulses needed for laser plasma accelerators.