Electrons surf the plasma wave

Will the particle accelerator of the future fit into a briefcase??

For 25 years, physicists searched for ways to make electrons ride waves in plasma. So far, the results have been sparse, but now several research groups are simultaneously presenting their breakthroughs in the scientific journal nature.

Independently of each other, stuart mangles of imperial college london, cameron geddes of lawrence berkeley national laboratory, jerome faure of france’s ecole polytechnique ensta-cnrs and colleagues from other institutes have succeeded in accelerating charged particles in plasma. Plasma is the fourth aggregate state of matter, which arises after the solid, the liquid and the gaseous state with increasing temperature. In the plasma, the components of the atoms, the electrons and nuclei, separate from each other.

Alec thomas (left) and stuart mangles of imperial college london at the astra laser of the cclrc rutherford appleton laboratory

For a quarter of a century, scientists around the world have been working to create effects in a plasma by means of laser-driven waves at short distances similar to those in the crude particle accelerators hundreds of meters and kilometers long at the european laboratory for particle physics (cern) in geneva or at the fermi national accelerator laboratory (fermilab), where studies of the microcosm are carried out (has the higgs boson been discovered??)

Wakefield accelerator

The working principle of a wakefield laser was first proposed by toshi tajima and john dawson in 1979 (nuclear physics with lasers). Theoretically, it is possible to bundle extremely strong beams of electrically charged particles and accelerate them very strongly even in a small space with the help of waves. So far, it has been possible to create very strong electric fields in the plasma – up to ten thousand times stronger than in classical particle accelerators – but the effect worked only at the very short distance of the full intensity of a laser pulse, just a few hundred micrometers (millionths of a meter) away. The resulting particle beams were of poor quality with inadequate focusing. This has now changed, as the three research groups were able to make significant practical improvements to the principle of the wakefield accelerator. One of the authors, wim leemans explains the process:

Laser wakefield acceleration works on the principle that a laser pulse is sent through a gas to create a plasma – negatively charged electrons are separated from positively charged ions in gas – some of the free electrons are carried forward in the plasma wave created by the laser. Imagine the plasma is the ocean and the laser pulse is a ship moving through it. The electrons are then surfers riding the wave generated by the wake of the ship.

The three research teams relied on precise control of the laser and plasma parameters to accurately control the effect of wave breaking – comparable to wave breaking on a beach. The three experiments are similar. Lasers with a power of 10 to 33 terawatts were used in each case, sending their pulses of 30 to 55 femtoseconds long into an ionized gas of similar density. Out came a high-energy, focused electron beam (up to 109 electrons per beam).

In the plasma channel, which condenses at the edge, the laser pulse creates waves that roll from left to right.

Laser wakefield technology makes it possible to build compact, high-energy accelerators for exploring the subatomic world. And that at a distance of only a few millimeters. Such accelerators are important not only for physics, but also for materials science and medical imaging techniques. Stuart mangles of imperial college london, lead author of one of the articles, comments:

Scientists have known for a while that we need a completely new approach to producing the particle beams needed for the new generation of light sources and the study of high-energy physics. In this experiment, we have demonstrated that compact, high-power lasers can be a viable new technology.