How are electromagnets used in particle accelerators

Colossi that control tiny particles

Some things are easier to work with when you roll them up. A poster, for example, becomes transportable - you just have to wrap a rubber band around it. At PSI, on the other hand, protons and electrons are to be made to race at almost the speed of light using particle accelerators. The equivalent of a rubber band here are magnets: Without them, the particles would simply fly straight ahead and would be lost in a fraction of a second. The magnets direct the particles in the accelerator onto a circular path. If the particles are fast enough, they can be used to generate neutrons or X-ray light, with which one can see inside the matter.

, explains J├╝rgen Duppich, Head of Technology / Coordination and Operations at PSI. In short: every magnet is surrounded by a magnetic field. If an electron or a proton now flies through this magnetic field, it is deflected. In which direction and how strong is the magnet. This must therefore be precisely manufactured and precisely positioned so that the particles move on the desired path.

Magnets from a few kilograms up to the size of an elephant

It is precisely this precision that makes the magnets at PSI so special. The Synchrotron Light Source Switzerland SLS, for example, is circular and roughly the size of a football field. The electrons here turn around a million rounds per second. Nevertheless, the magnets keep you on course - accurate to a thousandth of a millimeter. In addition, the magnets continuously shape the particle beam. Mostly it is a matter of keeping the charged particles close together.

St├ęphane Sanfilippo and his team of ten people are working to ensure that the magnets achieve this accuracy and that it is constantly improving. You look after the thousand magnets at PSI. Their shape and size vary enormously: the elephant-sized colossi in the proton ring cyclotron weigh 240 tons. Other magnets that are used to fine-tune the particle beam weigh only a few kilograms.

Almost all magnets at PSI have in common that they are electromagnets. In contrast to the magnets on the refrigerator door at home, electromagnets are powered by electricity and can therefore be switched on and off. They essentially consist of a wire that is wound around an iron core to form a coil. The magnet is only active as long as current flows through the wire. Hardly a magnet is a little bigger than a soccer ball and it needs high-voltage current and water cooling, because the wire through which it flows becomes hot.

At the interface

The magnet specialist group sits at the interface of many other working groups. explains Sanfilippo. The typical magnet at PSI needs a power supply that supplies the electricity. He needs a water connection for cooling. And last but not least, the dimensions of the magnet must fit into the system. It must not be too big, but it must have enough space inside so that the vacuum tube, in which the charged particles race, can fit through. and that is the measurement of the field of the finished magnet.

In industry, the extreme precision that the researchers at PSI need is exceptional. No matter how good the theoretical model is, the unavoidable manufacturing tolerances mean that the coils are not placed exactly and the result is a minimally distorted magnetic field, which in turn has far-reaching effects on the particle beam. But Sanfilippo and his colleagues also have experience with this: They already plan a reserve in the model for the most important parameters, which could iron out the production deviations again - if this were necessary due to the measurements.

Well-tried means for field measurement are used here, for example a so-called Hall probe. This is slowly moved by the magnetic field and continuously records the location and strength of the field. At the end you get a three-dimensional field map with several thousand measuring points.

The trend is towards accuracy

But new developments also come into play, because the trend, explains Sanfilippo, has not been about building magnets as strong as possible for about twenty years, but rather as precisely as possible. In the workshop of the magnet specialist group, for example, there is an air-conditioned room in which the magnets are measured with a vibrating wire behind a pane of glass. says Sanfilippo, not without pride.

As simple as the name sounds, this measuring system is just as sensitive. The heart consists of a tensioned and vibrated wire through which current flows. If this lies exactly in the central axis along which the magnetic field of a quadrupole magnet has a zero line, the wire stops vibrating. However, since this area is theoretically infinitely small, it is correspondingly difficult to identify.

And the vibrating wire may not be the end of the flagpole. The more precise the magnets become, the more precise the measuring equipment must also be. So when Sanfilippo and his colleagues are not designing new magnets, they are working on further developing the methods for measuring magnetic fields.

Persistent

Finally, the new magnet is installed in the intended location within the accelerator. Here, too, the greatest mechanical precision is required. The magnet can be compared to an optical lens - it also only deforms a light beam as planned if it is perfectly aligned. The magnet therefore already has measurement marks on its outer surfaces in the planning phase. Within the system, these are targeted with a laser tracker so that the magnet can be precisely positioned.

Then finally the electromagnet goes into operation. And hopefully it will stay that way for a long time. Some magnets at PSI have been reliably in service for over 40 years. , explains Duppich. Once perfectly installed, they are good-natured and persistent. The team of technicians thanks them and treats them with respect. Also because the special iron that forms the core of the electromagnets is an expensive material: If a magnet is taken out of service, its parts are recycled and made into new magnets. - Duppich puts it so solemnly. The magnets are actually the lifeline of the accelerator.

Text: Laura Hennemann