The European XFEL in Hamburg is a superlative research facility. The abbreviation XFEL stands for X-Ray Free-Electron-Laser, i.e. an X-ray light laser that works with free electrons. In the first half of the XFEL, electrons are accelerated almost up to the speed of light over a 1.7 km straight stretch. The energy gain is enormous: the electrons that leave the accelerator every second have as much energy as an elephant at full speed. Then the electrons are periodically deflected from their trajectory, releasing short X-ray pulses – similar to a laser – with which one can “see” the movement and structure of individual molecules, as it were in a microscope.
Scientists from the international research program ACHIP (Accelerator on a Chip) of the Gordon and Betty Moore Foundation are investigating the other extreme and trying to build miniature accelerators on microchips. The goal is a cheap and easy-to-manufacture particle accelerator that fits into a shoebox with all the necessary components. The team of Uwe Niedermayer and Oliver Boine-Frankenheim from the Technical University of Darmstadt is also involved in the project. You recently presented a concept in the journal “Physical Review Letters” (DOI: 10.1103 / PhysRevLett.125.164801) with which electron accelerators could be reduced to millimeter size.
The miniature accelerators work on the same principle as classic high-frequency linear accelerators: electrons are negatively charged particles and are accelerated in electrical fields. That is why they are exposed to alternating electrical fields that change direction at a high frequency. The oscillating field alone would, however, alternately accelerate and decelerate the electrons. Thanks to a clever design, the electrons are shielded from the braking part, so that only the accelerating part of the field remains. The length of the accelerator depends directly on the frequency of the alternating voltage: the higher the frequency, the smaller the accelerator.
The trick to making particle accelerators smaller is to use fields with a higher frequency. Back in 2013, researchers at Stanford University carried out the first experiment to accelerate electrons with an infrared laser. The light from the laser consists of electric and magnetic fields that oscillate around ten thousand times faster than the fields in the XFEL. In order to contain the braking part of the radiation, the electrons are guided through two rows of tiny columns made of quartz glass – like through a tree-lined avenue. The glass strengthens the electric field of the laser between the pillars, it remains unchanged in the gaps. If the electrons pass the columns during the accelerating phase and the gaps in the braking phase, they are accelerated more than decelerated on their way through the avenue.
However, the tiny dimensions of the accelerator chips presented the team at Stanford with new challenges. The electrons fly bundled through the avenue, and so that the bundles do not smear along the trajectory, they have to be shorter than the width of the columns. In addition, the avenue is a hundred times narrower than a hair, and the electron beam must be precisely focused so that the particles do not hit the pillars.
The TU Darmstadt team tries to solve both problems with an improved architecture. Two years ago they showed that precisely set gaps in the row of columns prevent the particle beam from diverging and bundle the electron bunches. The basic idea is to focus the electron beam alternately along the direction of flight and across it. However, they could only partially focus the electrons up and down using a magnet.
In their recently introduced concept, they manage without external components. Instead, they change the structure of the pillars. They are now made up of layers of glass and silicon. The two materials amplify the laser light to different degrees and thereby focus the electron beam in a similar way to a lens. The pillars can be manufactured easily and inexpensively with the usual lithography technology as with computer chips. “This means that every university laboratory could afford its own electron accelerator,” Niedermayer is quoted in the magazine “hoch³FORSCHEN” of the TU Darmstadt.
The energy of the electrons from the laser accelerator is a good thousand times less than that of the giant XFEL. This is not necessarily a disadvantage, as different applications require different beam properties. In radiation therapy, electrons with this lower energy are used to fight tumors. One day, the tiny laser accelerators could be attached directly to the end of an endoscope to irradiate tumor cells at close range without damaging the surrounding tissue.
The miniature accelerators will not replace the conventional devices in the long term, says Thorsten Kamps from Humboldt University and the Helmholtz Center Berlin. “I think that these new accelerators are complementary to the conventional approaches and open up new possibilities,” he says. »The many applications of accelerators ask for many different beam properties. We are now at the point where we are developing special accelerators for these properties – the tool box is getting fuller and the tools are more special. “