Plasma acceleration under the microscope

1 March 2023
Light from helium plasma
Wakefield insight Light from the 1 mm-long helium plasma in which the electrons were accelerated. Credit: S Bohlen/DESY

A team led by DESY researchers has used a noninvasive technique to measure the energy evolution of an electron bunch inside a laser-plasma accelerator for the first time, opening new possibilities to understand the fundamental mechanisms behind this next-generation accelerator technology.

Laser-driven plasma-wakefield acceleration, which is under study at DESY, SLAC and several other labs worldwide, promises to significantly reduce the size of particle accelerators. The idea is to use a high-power laser to create a plasma in a gas, in which charge displacements generate electric fields of the order 100 GV/m. Such fields can accelerate electron bunches to highly relativistic energies over short distances, outperforming conventional radio-frequency technologies by orders of magnitude. The AWAKE experiment at CERN, meanwhile, is a unique facility for the investigation of proton-driven plasma acceleration, which could enable even higher energies to be reached. Turning the concept of wakefield acceleration into a practical device, on the other hand, is a major challenge. 

Turning the concept of wakefield acceleration into a practical device is a major challenge

In order to understand and thus improve the process of laser-plasma acceleration, which lasts for a period of femtoseconds to picoseconds, it is essential to observe as precisely as possible how the properties of the accelerated particles change in the plasma. Publishing their results in December, a team led by DESY’s Simon Bohlen and Kristjan Põder tracked the evolution of the electron beam energy inside a laser-plasma accelerator with high spatial resolution. The feat was performed within a project called PLASMED X, which aims to develop a compact, narrowband and tunable X-ray source for medical imaging. 

The team began by splitting the laser beam into two parts: one was used for electron acceleration, while the other was superimposed so that the light could be scattered by the electrons. Using an X-ray detector to measure the energy of Thomson-scattered photons at 20 points over a 400 μm section of the plasma, the team was able to reconstruct the energy evolution of the electrons over most of the accelerator length without disturbing either the electron beam or the acceleration process itself. 

“We were able to show in our measurements that the acceleration gradient can change significantly over very short distances,” says Bohlen. “With the new measurement method, we now have direct insight into a plasma acceleration process and can thus investigate the direct influence of different laser parameters or geometries of plasma cells on the acceleration process.”

Further reading

S Bohlen et al. 2022 Phys. Rev. Lett. 129 244801.

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