The principle of laser wakefield accelerators: A high-power laser excites a charge wave in a plasma, which propagates at the speed of the laser pulse and pulls electrons behind it in its "wake", thus accelerating them.

Electron energies in the GeV range have been achievable with this technique for some time. However, the electron bunches produced in this way have so far been too small and too poorly focused to use the synchrotron radiation they emit, an intense, coherent light that is used for research in many different disciplines.

For the further development of the technology, a method is therefore needed to precisely measure and control the cross-section and quality of the electron bunches individually. PTB's storage ring, the Metrology Light Source (MLS), allows the generation of small electron bunches in flexible research operation, which are very similar to those of laser wakefield accelerators, but whose properties can be set and varied very reproducibly and precisely.

A team at HZB and PTB has now developed a method to measure the lateral expansion of the electron beam of a laser plasma accelerator with a resolution in the micrometre range.

"To do this, we use a technique that is successfully employed at the Bessy II storage ring," explains Thorsten Kamps, co-author of the study.

First author Ji-Gwang Hwang had the idea of using the coherent radiation of electron pulses in the visible range via the phenomenon of interference (double slit) and determining the beam cross-section as a deviation from a perfect point source.

With the help of a highly sensitive camera and complex algorithms, the team succeeded in measuring the lateral beam size in the range of a few micrometres. Katharina Albrecht carried out the measurements as part of her bachelor's thesis in physics.

"We worked very closely with our colleagues from the (MLS) at PTB for this project," Kamps emphasises. "There, it is possible to imitate the electron beam from a plasma accelerator on a beamline and thus test the method under realistic conditions," says Kamps.

Research Report: "Monitoring the size of low-intensity beams at plasma-wakefield accelerators using high-resolution interferometry"

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