Helmholtz Center Dresden develops new measure method for laser-plasma acceleration

03 Sep 2024

LPAs are promising research tools and take up less space than conventional facilities.

Helmholtz Zentrum Dresden-Rossendorf (HZDR), which conducts long-term research in the fields of energy, health and matter, has announced the development of a new diagnostic tool specified for a laser-plasma accelerator.

The center stated that these accelerators “are seen as highly promising research tools: laser-plasma accelerators take up less space than conventional facilities, which are sometimes kilometers long. Such compact particle sources can accelerate electron bunches efficiently, enabling x-ray lasers that fit in the basement of a university institute.”

But there are a few challenges to be met: in order to produce UV or x-ray light, the electron bunches generated by a laser-plasma accelerator must be very finely bundled and have defined properties. To date, it has been difficult to even measure these bunches precisely. The team from HZDR has developed a novel measuring method which should help to drive the development of laser-plasma acceleration, as the group describes in Nature Photonics.

In laser-plasma acceleration, a laser shoots intense light pulses into a gas. The pulse is so strong that it ionizes the gas and generates a plasma – a mixture of electrons and ions. As the laser pulse forces the lighter electrons out of the way more quickly than the heavier ions, an electrically positively charged “bubble” forms behind it. If some electrons are injected into this bubble, the strength of the electro-magnetic field can virtually catapult them forward. This process only requires a few centimeters but can accelerate the electrons bundled in bunches as much as a conventional set-up that measures dozens or even hundreds of meters and uses radio waves to get the particles moving.

The free electron laser (FEL) is an interesting application for state-of-the-art laser-plasma accelerators. In this, electron bunches fly through a so-called undulator at almost the speed of light. This array of magnets forces the particles onto slalom paths, making them emit strong, laser-like x-ray or UV flashes, which can be utilized to track extremely fast processes such as chemical reactions.

Compact and cost-effective

There are now several of these research machines, including the European XFEL in Hamburg, Germany. They are based on conventional linear accelerators, some of which are several kilometers long. But, so far, these facilities are rare and thus the available beamtime is limited.

If FELs could be constructed on the basis of laser-plasma accelerators, the facilities could be built so compactly and cost-effectively that a university institute, for instance, would be able to afford one. The technology would thus become available to far more research teams than at present.

Initial successes have already been achieved: since 2021, three research groups have managed to demonstrate that an FEL based on plasma accelerators can be implemented – a team in Shanghai, China, a group in Frascati, Italy, and a team working with Dr. Arie Irman at HZDR’s Institute for Radiation Physics.

“Among other things, we have to improve the quality and stability of the accelerated electron bunches and minimize the distribution of the electrons’ energy within the bunches,” said Irman, one of the authors of the paper. “But it is also important to develop new diagnostic methods in order to investigate the processes in a laser-plasma accelerator more precisely.”

Electron bunches

This is where the new HZDR project comes in: Dr. Maxwell LaBerge, a postdoc in Irman’s Team, has developed a measuring procedure that allows scientists to analyze in detail extremely short electron bunches measuring just a few micrometers. LaBerge explained the principle: “We shoot the electron bunches, almost at the speed of light, from the plasma accelerator onto a thin metal foil. This sets the electrons on the surface of the foil in motion.”

As a result, these electrons send a signal that can be detected by sensors. “Using this signal, we can precisely reconstruct what the electron bunches that have traversed the foil look like,” said LaBerge, describing the process, known as Coherent Optical Transition Radiation (COTR).

HZDR’s experts have used their new measuring method to explore different ways of injecting the electrons into the plasma bubble. Arie Irman described the result: “We’ve been able to ascertain that different injection methods produce quite different forms of electron bunches, which shows that the new method can help to control the form and structure of the electron bunches much more precisely.”