To the speed of light at the click of a mouse

DESY not only operates various accelerator facilities, it is also working on promising concepts for the future: “Laser plasma acceleration should one day make it possible to build machines that are significantly more compact, space-saving and cost-effective than today’s large-scale accelerators,” says Wim Leemans, director of the Accelerator Division at DESY. An elaborate visual animation now demonstrates how this innovative technique works: Viewers can watch interactively and in detail how powerful laser pulses are transformed into electron beams that travel at nearly the speed of light and are tremendously useful in research.

The new website, the principle of laser plasma acceleration can be interactively explored. Credit: Science Communication Lab/Manuel Kirchen and Sören Jalas for DESY

Particle accelerators belong to the most important tools in science. Among other things, they generate extremely intense X-rays that can be used to examine and analyze a wide variety of samples and materials in detail – from biomolecules and semiconductors to rock samples and archaeological finds. In today’s accelerators, radio waves are applied to metal tubes called cavities. The particles to be accelerated – in most cases electrons – can ride these waves like surfers on an ocean wave and are propelled forward by them. However, the maximum accelerating voltages that can be achieved are limited. For this reason, a large number of cavities have to be connected in series to raise the electrons to high energy levels. This makes the machines complex and long – sometimes several kilometers long.


Significantly higher accelerating voltages can be achieved using a different method, one that is still in its infancy: laser plasma acceleration. This would make it possible to build much smaller machines in the future and thus complement our existing accelerator technology portfolio: An accelerator that was a hundred meters long could now, in principle, fit inside a basement laboratory in the future. “We have been working on this new technology for several years at DESY,” explains Andreas Maier, lead scientist at DESY. “Powerful and efficient lasers form the starting point.” The idea is to fire short laser pulses into a plasma, which is an electrically charged gas. As the high-energy laser pulse passes through, it leaves a kind of wake behind it, much like the wake that forms behind a boat. This so-called plasma wave can accelerate electrons enormously over a distance of just a few millimeters.


Although the principle works, it is not yet ready to be used in practical applications. This is where a new DESY project called KALDERA, on which Maier’s team is working, should achieve some important breakthroughs. “In the prototypes built so far, the lasers only manage to fire about one shot per second. This not only makes the laser plasma accelerators vulnerable to external interferences, but is also far too low for concrete applications,” explains the physicist. “With KALDERA, we are developing a new laser that will fire about 1,000 pulses per second and turn them into electron pulses, which would make the technology competitive.” The facility is currently under construction and should produce its first pulses in three years.


The new superlaser is already up and running on the Internet, though – in the form of a website providing a realistic 3D simulation of KALDERA. To create this, Maier’s team joined forces with the Science Communication Lab. This agency based in Kiel specialises in elaborate digital visualisations of complex scientific systems. “The simulation shows the entire process, from the production of the laser pulses, through the acceleration of the electrons to the generation of high-intensity X-rays by the high-speed electrons,” explains Konrad Rappaport, co-founder and art director of the Science Communication Lab. “The events can be viewed in succession, like a film, but you can also use a slider to go to the individual steps and look at these in detail.”


As a special highlight, it offers two different levels. The overview displays an extremely realistic 3D representation of the intended experimental setup: The highly complex laser fires its pulses through lenses and mirrors into a plasma cell. There, it accelerates the electrons, which then fly into a special magnetic apparatus where they generate a powerful X-ray beam.


Switching over to the “zoom” opens the door to the micro-world: You can now see in detail how the laser pulse is generated, amplified and shaped by special optics, as if you were looking through a super-microscope. The electrons then appear in the plasma cell, bundled up as a bunch by the extremely strong alternating electric field generated by the laser pulse and catapulted to almost the speed of light. Shortly afterwards, a magnetic lens squeezes the electron bunch together as it starts to drift apart. This allows it to be guided into a special magnetic structure where it is forced to follow a precise slalom course – causing the tiny electrons to emit a dazzlingly bright pulse of X-rays.


“With this website, we are hoping to make the fascination we feel for our research tangible for others,” Maier explains. “We hope to appeal to people who have no prior knowledge of the subject, especially thanks to the fantastic visualization. And those interested in a more in-depth look at our technology will find a range of additional information.” For example, a graph illustrates which parts of the plasma cell are filled with which gases in order to achieve the most efficient acceleration. Another detail illustrates how a sophisticated control mechanism manoeuvres one of the mirrors back and forth so that the pulses always end up in the ideal position inside the plasma cell.


“The animation of the laser plasma accelerator is not based on special effects, but on substantiated scientific data,” Rappaport emphasizes. “In order to display them correctly and at the same time visually appealing, we worked closely with the DESY physicists Manuel Kirchen and Sören Jalas, which proved to be excellent and highly productive.” In the future, the cooperation could even bear further fruit, because the team already has new ideas: “My dream would be to follow the events wearing VR glasses and to fly through the machine alongside the light pulses and electron beams,” Rappaport tells us. “I imagine that would be absolutely incredible!”


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