They may be small, but they sure pack a punch.
When you hear the words “particle accelerator,” you likely think about the giant, 17-mile circular electron smasher located in Switzerland. But there is a whole other world of particle accelerators. They are called laser wakefield devices, and although they are small — about 12 inches — these machines can accelerate electrons to near the speed of light using a fraction of the distances required by conventional particle accelerators.
In fact, the acceleration is so powerful that wakefield devices can boost electrons to ultra-high-energy in just inches. The most advanced conventional accelerators need many yards.
However, their major drawback is that the electrons are not all uniformly accelerated. What this means is that the beams created are a mix of faster (higher energy) and slower (lower energy) particles, which results in an energy spread of electrons.
Now, a team of researchers from China, South Korea, and the US has proposed a new way to minimize this energy spread.
First, it is important to know exactly how laser wakefield accelerators work. They shoot an ultrafast laser pulse through a plasma of positively charged ions and free electrons, and as the laser travels through the plasma, it pushes the electrons out of the way, leaving behind a region of positively charged ions.
This positive charge then pulls the electrons behind the laser pulse in waves, which results in the generation of strong electric fields that trap electrons and accelerate them to energy levels on the order of one billion electron volts — or 99.99999 percent the speed of light.
But wakefield accelerators do have some pitfalls. First, electrons can enter the plasma at different times, so the electrons that enter first are accelerated for longer. Second, the acceleration is not uniform, meaning electrons at different locations receive different energy boosts. Both of these contribute to an energy spread for the accelerated electrons.
However, Liu and his colleagues have proposed a new way to minimize the energy spread. After the electrons enter the plasma wave, but before they are accelerated, they suggest inserting a plasma compressor. The compressor squeezes the electrons together, but also flips their order, so that the fast electrons that were at the front of the pulse are now at the back.
"Along the axis that the laser propagates, the longitudinal electric field resembles a very steep ocean wave about to break, which will cause electrons trapped near the rear to feel a very strong forward acceleration," said Jiansheng Liu, a physicist with the Chinese Academy of Sciences, in a press release.
New method to improve wakefield accelerators by compressing the electron beam. Images show two-dimensional simulation of electron density distribution for the injector stage (A), compressor stage (B) and accelerator stage (C), where the target electron beam is circled (in red). Photo credit: Jiansheng Liu/Chinese Academy of Sciences
When the shortened pulse is accelerated, the fast electrons at the back catch up to the slow electrons at the front, resulting in a final pulse with a very small energy spread.
This is not the first attempt made to reduce wakefield accelerator energy spreads. However, this new method will be more than 10 times better. Liu and his team are currently working to test their method by building a device in the lab.
This new method has been published in the journal Physics of Plasmas.
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