Paving the Way for Next-Generation X-Ray Sources

To build powerful but small particle accelerators, one requires both strong accelerating fields and particles in the right place at the right time.

Previously, it was shown that it is possible to inject resting electrons into a plasma structure smaller than the width of a hair travelling at the speed of light. Now, a team of researchers led by Prof Peter Norreys from the University of Oxford, have demonstrated how to switch off this injection process in a controlled manner enabling clean electron beams which may be accelerated over greater distances than previously feasible.

X-Rays have served humanity greatly allowing to penetrate materials and image them – be it in medical, industrial or security applications. Detecting foreign objects in human tissue, micro-cracks in metals or sharp objects in suitcases is part of our everyday lives. However, there is a limitation to the detail that can be achieved, which greatly corresponds to the quality of the X-Rays.

Generating even brighter bursts of high-energy X-Rays in a narrow energy band requires precise control over the acceleration of charged particles which are the source of the radiation. Laser-driven plasma accelerators offer a promising route to unprecedented electric fields, dramatically reducing the cost and footprint of accelerators.

In an article published in Physical Review Letters, Norreys and collaborators show that the particles to be accelerated can be trapped in the accelerating wave in a controlled way. They do this by using two ultra-short co-propagating laser pulses of different diffraction lengths, separating the trapping process from the creation of an accelerating micro-structure. This allows for generating electron bunches with narrow energy spread that can be guided over great distances enabling next-generation accelerators.

Schematic of the experimental setup in the target chamber. The drive beam path is depicted as a dashed black line. The annular injector beam path is depicted as a green line. Both pulses travel through the gas cell from the right-hand side to the left-hand side. The red area in the gas cell indicates the volume of injection. The blue dashed lines represent possible electron trajectories. (Image reproduced from the original article under Creative Commons Attribution 4.0 International license)

Full article:

M.W. von der Leyen et al., “Observation of Monoenergetic Electrons from Two-Pulse Ionization Injection in Quasilinear Laser Wakefields”, PHYSICAL REVIEW LETTERS 130, 105002 (2023)