Pinch of nitrogen and artificial intelligence move laser plasma acceleration one step closer to practical applications

DESY’s LUX team celebrates not just one but two milestones in the development of innovative plasma accelerators. Scientists at the University of Hamburg and DESY used their accelerator to test a technique that keeps the energy distribution of the produced electron beams particularly narrow. They also used artificial intelligence to allow the accelerator to optimize its own operation. Scientists report their experiments in two articles published shortly thereafter in the journal Physical examination letters. “It’s fantastic to see how quickly the new plasma acceleration technology is reaching a level of maturity where it can be used in a wide range of applications,” congratulates Wim Leemans, Director of DESY’s Accelerators Division.

Plasma acceleration is an innovative technology that gives birth to a new generation of particle accelerators that are not only remarkably compact but also extremely versatile. The aim is to make accelerated electrons available for applications in different fields of industry, science and medicine.

The acceleration takes place in a tiny channel, a few millimeters long, filled with an ionized gas called plasma. An intense laser pulse generates a wave in the channel, which can capture and accelerate the electrons in the plasma. “Like a surfer, the electrons are carried away by the plasma wave, which accelerates them to high energies,” explains Manuel Kirchen, lead author of one of the articles. “Thanks to this technique, plasma accelerators are able to achieve accelerations up to a thousand times greater than those of the most powerful machines currently in use,” adds Sören Jalas, author of the second article.

However, this compactness is both a curse and a blessing: as the acceleration process is concentrated in a tiny space up to 1000 times smaller than conventional large-scale machines, acceleration takes place under truly conditions. extremes. Therefore, a number of challenges still need to be overcome before the new technology is ready to enter series production.

The research team led by Andreas Maier, accelerator physicist at DESY, has now taken two critical steps at the LUX test facility – jointly operated by DESY and the University of Hamburg: they have found a way to significantly reduce the energy distribution of accelerated electron bundles – one of the most essential properties for many potential applications. To do this, they programmed a self-learning autopilot for the throttle, which automatically optimizes LUX for maximum performance.

The group carried out their experiments using a new type of plasma cell, specially developed for this purpose, whose plasma channel is divided into two regions. Plasma is generated from a mixture of hydrogen and nitrogen in the front part of the cell, which is about 10 millimeters long, while the area behind is filled with pure hydrogen. As a result, the researchers were able to obtain the electrons for their group of particles from the front part of the plasma cell, which were then accelerated across the entire back section of the cell. “Being more closely related, the electrons in the nitrogen are released a little later, making them ideal for being accelerated by the plasma wave,” says Manuel Kirchen. The electron beam also absorbs energy from the plasma wave, changing the shape of the wave. “We were able to take advantage of this effect and adjust the shape of the wave so that the electrons reach the same energy regardless of their position along the wave,” adds Kirchen.

Based on this recipe for achieving high electron beam quality, the team went on to achieve a second research success: Sören Jalas and his colleagues were able to use artificial intelligence (AI) to modify an algorithm that controls and optimizes the complex plasma accelerator system. To do this, the scientists provided the algorithm with a functional model of the plasma accelerator and a set of adjustable parameters, which the algorithm then optimized on its own. Essentially, the system changed five main parameters, including the concentration and density of gases and the energy and focus of the laser, and used the resulting measurements to find an operating point at which the electron beam is suitable. optimal. “During its balancing exercise in 5-dimensional space, the algorithm constantly learned and very quickly refined the accelerator model more and more,” explains Jalas. “The AI ​​takes about an hour to find a stable optimal operating point for the accelerator; by comparison, we estimate that humans would need more than a week.”

Another advantage is that all parameters and measured variables continue to train the accelerator AI model, making the optimization process faster, more systematic and more focused. “The latest progress from LUX means that we are on the right track to test initial applications for testing,” says Andreas Maier, who is in charge of laser development for plasma accelerators at DESY. “Ultimately, we also want to use plasma accelerated electron packets to operate a free electron laser.”

The experiments were carried out by researchers at the Center for Free-Electron Laser Science (CFEL), a collaboration between DESY, the University of Hamburg and the Max Planck Society, as well as a colleague from the Lawrence Berkeley Laboratory in California.

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