Large-scale research facilities: Particles in energy-saving mode | nd-aktuell.de

Large particle accelerators consume a lot of energy. In the future, part of it will be recovered and electricity saved. A team at TU Darmstadt has already demonstrated two-stage energy recovery.

Particle accelerators have become an integral part of many areas of research. They are indispensable tools – not only in nuclear and particle physics, but increasingly also in materials research and medicine. However, they usually also come with a high electricity bill because they require a lot of energy to operate. This is expected to change in the future, at least for certain types of particle accelerators. With the help of intelligently designed energy recovery, power consumption should be reduced – or significantly greater acceleration performance should be possible with the same power consumption.

Particle accelerators bring charged particles – usually electrons or protons – to extremely high energies and speeds close to the speed of light. Many experiments are designed in such a way that the particles are first accelerated in packets by an electric field and then brought into collision. However, often only a very small proportion of the high-energy particles collide. The remaining particles are guided into a beam catcher. There they are stopped and their energy is lost.

“The idea behind energy recovery is not to stop the particles remaining in the beam somewhere, but to let them run through the particle accelerator again,” explains Norbert Pietralla, managing director of the Institute for Nuclear Physics at the Technical University of Darmstadt. “But now they are slowed down and release their kinetic energy back into the electromagnetic field in the accelerator.” This energy can then be used to accelerate the next particle beam. Therefore, less energy has to be supplied to the accelerator system from outside and the particle accelerator runs “in energy-saving mode”.

There are various ways to accelerate particles. Nowadays, strong alternating electromagnetic fields are usually used in specially shaped cavities through which the particles fly. There is a vacuum throughout the entire system so that the particles can move freely. The alternating fields oscillate at high frequencies, in the microwave range, similar to those in kitchen appliances – but with much more energy.

The wave pushes the surfer

If you want to accelerate electrons to high energies, you feed them into the cavities in small packets. “You do this exactly when the alternating field oscillates in such a way that the electrons in it experience a force directed forward,” explains Pietralla. When the electromagnetic field swings back again, the electron bunch has already left the cavity. The next bunch of electrons can only be accelerated when the wave swings forward again. The size of the cavities and the frequency of the oscillations determine the distance at which the electron bunches can follow one another.

The electron bunches pass through several such acceleration paths one after the other, with the fields in the subsequent cavities being coordinated with one another in time. The particles are accelerated like a surfer on a water wave and leave the acceleration path with high energy.

The necessary energy must be supplied to the accelerator system from outside using suitable microwave transmitters. It becomes even more efficient if you send the beam through the entire accelerator several times. To do this, the beam is guided back to the beginning of the accelerator using magnetic fields and injected into the cavities again at exactly the right time. This causes the electron bunches to continue to accelerate. “At the superconducting linear accelerator S-DALINAC in Darmstadt, where I work with my team, we recirculate the beam up to three times before the particles collide,” says Pietralla.

The surfer pushes the wave

The trick in energy recovery is to send the remaining electrons not with the wave, but against it, at the end of the acceleration process – on the S-DALINAC after up to three rounds. They pass through the same cavities three more times as during acceleration. “But instead of letting the wave accelerate them, the electron bunches in turn push the wave and release energy into the alternating field,” says the researcher. In order to achieve this effect, the path length of the electrons for re-injection into the accelerator must be changed – in such a way that they enter the cavities filled with microwaves exactly when the wave is swinging back. The electron bunches are then slowed down and part of their kinetic energy is transferred to the microwave field.

The scientists were able to come up with convincing results Results to reach. “With an acceleration process with just a single cycle, the recovery is already very good and can reach 99 percent,” says Pietralla. “But we usually use several circulations until the accelerated electrons collide with each other – up to three in the S-DALINAC.” The more often the electrons have circulated in the accelerator, the more difficult it becomes to recover the total energy of the remaining electrons. This is because after several laps of braking, they rush through the accelerator in an increasingly disorderly manner.

“In our most recent experiments, we were able to recover 87 percent of the kinetic energy in two rounds of a weak beam,” says the scientist. With a very intense beam with many particles, the electrons become somewhat more disordered, so that only around 60 percent of the energy can be recovered. The efficiency therefore depends not only on the number of revolutions, but also on the beam strength. “But you have to say that the S-DALINAC was not originally built and optimized for energy recovery,” says Pietralla. “We had to modify it slightly for this feasibility study.” Future accelerators in which this is implemented from the start should be able to recycle the energy of the particle beams much more efficiently.

In principle, two effects can be achieved with the process. Firstly, energy can be saved because less electricity is required to operate the alternating electromagnetic fields. The power of the microwave generators can range from a few kilowatts – such as the Darmstadt S-DALINAC – to many megawatts in large systems. If you achieve a recovery of over 90 percent, this not only saves energy, but also significantly reduces operating costs for large systems. And secondly, the recovered energy can also be used to generate stronger particle beams than was previously technically possible. The microwave generators can only feed energy into the alternating fields within certain limits. This total energy can be increased by recycling the particle energy within the system.

Need also in industry

This technology now opens up new possibilities – both in particle physics and in industry. Concepts for extremely strong electron colliders are currently being discussed in basic research. “But if such a system has such a high power consumption that several large power plants are needed to supply energy, then these plans would not be financially feasible and would not be justifiable in today’s time of energy shortages,” explains Pietralla. Energy recovery could reduce the electricity consumption of such systems to an acceptable level.

There is also a need for powerful electron accelerators in industry – for example as a source of gamma radiation, such as that used to produce silicon chips. Electricity consumption could also be reduced here. The development of such systems is therefore progressing rapidly – and German institutes are also at the forefront. A particle accelerator, the MESA accelerator, is currently being built in Mainz. It was designed from the outset to recover energy and will serve as a prototype for further models. “I am sure that the concept will become established in many new particle accelerators,” concludes Pietralla.