Particle acceleration: Accelerating to the top

There are many disciplines where Germany would like to be a pioneer. In particle acceleration, however, it already is. “Germany is really playing a leading role worldwide,” says Dr. Arik Willner. He should know: the physicist is the CTO of Deutsches Elektronen-Synchrotron DESY (German Electron Synchrotron), which operates particle accelerators and conducts research on how they work in Hamburg and Zeuthen (near Berlin). A synchrotron is a particle accelerator of a relatively old, circular design. The most famous example is the Large Hadron Collider at the CERN research institute in Geneva (featured in the novel Angels & Demons by Dan Brown). The first accelerator at DESY was also a synchrotron. Today, there are different types of particle accelerators, which are now known as storage rings. The basic principle remains the same, however: charged particles, such as electrons, are accelerated using powerful electromagnetic fields. The electrons are guided through arrangements of magnets known as undulators, where they emit X-ray radiation. Researchers use this intense X-ray light to conduct experiments. “This has unbelievable potential for a whole host of research disciplines,” says Prof. Dr. Wim Leemans, Director of the Accelerator Division at DESY.

27,000 laser flashes per second

If scientists need particularly bright, laser-like X-ray radiation, they use linear accelerators that fire electrons toward the undulators in a straight line. DESY is home to the starting point of a 3.4-kilometer-long X-ray laser, the European XFEL. It generates extremely intense X-ray laser flashes for use in science and industry at a rate of 27,000 per second. The XFEL runs in a straight line from the DESY campus in Hamburg-Bahrenfeld to the research campus in Schenefeld. It passes beneath houses, streets and sports fields. The intensity of the light generated is billions of times higher than the kind used in radiotherapy or medical diagnostics, and it's used by researchers from all around the world.

“The idea to build the X-ray laser came from DESY, and there are very close links between our institutions,” says Dr. Thomas Tschentscher, scientific director at the European XFEL. That said, DESY and the European XFEL research different applications and run different experiments. The X-ray light sources and their particle accelerators are scientific tools with an enormous number of possible uses. Around 100 experiments take place at the European XFEL alone each year.

Along with physics research, there are also medical applications. DESY even played its part in fighting the COVID pandemic. “BioNTech used our X-ray light source back in 2019,” affirms Willner. The aim was to optimize the nanocarriers used to transport mRNA. “This work led to patents that have now been incorporated into the latest version of the vaccines.”

Radiation for health

Particle accelerators are used in other fields of medicine, too. “The data generated by the experiments is hugely important in developing medications,” says Willner. Radiotherapy is another area, and many hospitals already have their own particle accelerators for this purpose, albeit much smaller ones than the facility in Hamburg. They accelerate electrons and fire them at tumors to destroy cancer cells. This technique is already being used successfully, but more research still needs to be done. Most particle accelerators are unwieldy and take up large amounts of space in clinical settings. Leemans has therefore proposed units small enough to fit into a small van. A research project by the Technical University of Darmstadt and several partners has demonstrated the potential feasibility. Accelerator physicists at DESY have been developing a laser-driven electron accelerator smaller than a fingernail that could fit on a silicon chip. “When it comes to compact particle accelerators, there are certainly promising applications in medical technology,” says Prof. Oliver Boine-Frankenheim, head of the Accelerator Physics department at TU Darmstadt. Fiberglass cables could introduce the chip into the body and enable targeted irradiation of a tumor from close proximity. Other potential uses are not yet clear according to Prof. Boine-Frankenheim, who adds that the compact nature of such an accelerator would result in very small radiation beams. He says that as a rule, it would only be possible to accelerate electrons – not ions or protons. Nevertheless, work on miniature accelerators continues. In fall 2023, the Friedrich-Alexander University Erlangen-Nuremberg, a partner to the Darmstadt researchers, reported a breakthrough in accelerating electrons in a structure smaller than a one-cent coin.

Mega acceleration in Darmstadt

Darmstadt, alongside Hamburg, is another major location for accelerators. That makes Germany an “important center for accelerator physics,” Prof. Boine-Frankenheim says. From his Darmstadt office, he can watch the daily progress at the the construction site of the next big accelerator center. The Facility for Antiproton and Ion Research (FAIR) will be one of the largest research projects of this type anywhere in the world. The German shareholder is the Darmstadt-based GSI Helmholtzzentrum für Schwerionenforschung, where Prof. Boine-Frankenheim heads the Accelerator Physics department in addition to his work at TU Darmstadt. “We're designing the accelerator for maximum performance,” he reports. FAIR will be one of the most modern accelerator facilities in the world. €3.3 billion is being invested in Darmstadt, and the site will require 600,000 cubic meters of concrete and 65,000 metric tons of steel. It's an enormous expenditure that matches the project's ambition: FAIR aims to study a question no less profound than the origin of the universe. The installation will enable scientists to imitate the extreme conditions to which matter is exposed in giant planets and stars, and also during stellar explosions and collisions. The accelerator will bombard small samples of matter with ions. At the tiny impact points, the collisions will, for a brief moment, create cosmic matter. The scale of the facility is vast: the ring accelerator alone will have a circumference of 1,100 meters, not including the experimentation rings and stations. The existing GSI facility will serve as a pre-accelerator.

Uncovering the secrets of the universe itself has always fascinated the scientists working on particle accelerators. “The research on exoplanets has given a lot of momentum to astrophysics applications in recent years,” says European XFEL director Dr. Tschentscher. The Hamburg linear accelerator is making its own contribution to the research. Measurements with the X-ray laser are being used to deduce thermodynamic variables such as conductivity, density variation and thermal capacity. These parameters, which can help us better understand planets and their origins, were previously impossible to determine on Earth without X-ray lasers and accelerators.

Standardization for safety

Alongside scientific uses, industrial applications of particle accelerators are also growing. Safety is a particularly pertinent topic here, even more so than in other applications. Companies are using particle accelerators to examine materials at the molecular level, for example. This provides valuable information on material compositions, thicknesses and structures, particularly in the semiconductor and automotive industries.