Innovative concepts for better accelerators

Accelerators and detectors are among the key tools of particle, hadron and nuclear physics. Without them, it would be impossible to penetrate ever deeper into the microcosm, for example to bring to light novel details about the strong interaction. In the design of accelerators and detectors, the experts push the boundaries of what’s technologically feasible. Instead of being able to rely on standard components, they often have to plan, develop and realize completely new components.

At the Helmholtz Institute Mainz, the ACID (Accelerator Design and Integrated Detectors) section is working on such innovative accelerator technologies. In close cooperation with other sections of the HIM, the ACID section devises concepts that will enable significantly better and more precise experiments. The focus is on two projects: One team is developing a novel superconducting accelerator that will enable superheavy chemical elements to be produced much more effectively than before. Another group is working on an expansion stage of the future FAIR accelerator HESR in Darmstadt. Its aim is to ensure that the new facility will one day generate significantly more particle collisions—and thus allow more measurement data to be recorded.

Ion accelerator in continuous operation

In order to produce superheavy chemical elements, accelerators fire medium-heavy ions of calcium, titanium or chromium onto a target. On impact, some of the accelerated ions can fuse with atomic nuclei of the target into extremely heavy elements, which can then be analyzed with special detectors. So far, the accelerators of GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt provide pulsed beams, i.e. the facilities fire series of bursts. The ACID section is working on a new accelerator technology that provides a continuous, i.e. uninterrupted ion beam. This should make the future production of superheavy elements much more efficient than before.

One difficulty in producing superheavy elements in the laboratory is that the reactions between the fast ions and the atomic nuclei of the target happen only very rarely. To nevertheless achieve a measurable yield, the physicists must accelerate as many ions as possible and deliver them to the target with the necessary reaction condition. With the accelerator technology used so far, the ions are bundled into short pulses. This has a disadvantage for the target material, as the pulses put a heavy load on it in a short time, in extreme cases even threatening to destroy it.

More favorable would be a load in which the ions do not pounce on the target in violent bursts, but hit it steadily and continuously in the so-called continuous-wave mode. In a way, the situation is similar to a glass of water that you want to fill to the brim: if you pour the water with too much momentum, you will probably spill part of it. A more promising approach is to pour the water into the glass in a smooth, not too strong jet.

However, such a continuous-wave mode is difficult to achieve with conventional accelerator technologies. That’s why the HIM physicists are working on a new approach: they are developing radio-frequency cavities (accelerating tubes) based on superconductivity. In superconducting materials, electric current can flow without loss and without resistance, provided the conductors are cooled with liquid helium to temperatures near absolute zero at minus 273 degrees Celsius.

On the one hand, such superconducting cavities enable very high accelerating gradients, i.e. the ions are accelerated very strongly over short distances. On the other hand, the cavities exhibit little loss, so use less energy than conventional ones. These two advantages more than outweigh the disadvantage of the elaborate helium cooling.

Although there are already superconducting accelerators in the world, such as the electron accelerator of the European XFEL X-ray laser in Hamburg, the cavities for the planned continuous-wave accelerator for heavy ions in Darmstadt must have a much more complex shape than those in existing facilities: in contrast to the relatively simply formed European XFEL cavities, the heavy-ion cavities from Mainz, which are made of the metal niobium, need a complex inner structure consisting of several cross-shaped arrangements with integrated drift tubes.

In addition, the cavities are not all identical. Each of them is manufactured individually. The reason for the differences is that the facility will not accelerate lightweight electrons, all of which move at almost the speed of light, but much heavier ions. At the beginning of the accelerator, these are very slow and gain momentum only gradually. As a consequence, each cavity must be adapted individually to a specific speed range and thus differs from the others. Twelve cavities in total are planned for the new accelerator. It is to have an overall length of about 15 meters and to accelerate the ions up to almost ten percent of the speed of light. Since experiments are planned at different energies, the accelerator must feature a high degree of flexibility.

The development challenges are huge: the HIM experts have to manufacture the complex mechanical structure with high accuracy – the positioning of the elements should be accurate to a few micrometers. In addition, how superconductivity behaves in such complex structures under high radio-frequency power during beam operation still needs to be investigated.

Together with a specialized company, the HIM researchers built the first cavity of the accelerator. The subsequent testing on a test bench was successful: the targeted accelerating gradients were not only achieved, but significantly exceeded. This can be considered when designing the other cavities – with the result that the accelerator can be built even more compact than originally planned. In 2017, the cavity passed the first “real” beam test at GSI in Darmstadt.

Next, the physicists will equip a complete cryomodule, consisting of three cavities and a device for focusing the ion beams. To this end, a complete assembly and preparation line including a high-pressure rinse for surface treatment of the cavities is available in the HIM cleanroom. The module should be complete and ready for testing in 2019. The work is supported by cooperation partners in particular from the University of Frankfurt, but also by research teams from Russia, Australia and the USA.

After successful testing of the cryomodule, construction of the entire accelerator consisting of four modules could begin. The facility in Darmstadt, which could also serve some of the upcoming FAIR experiments, could be ready in 2024. At the HIM in Mainz, one section in particular would benefit from the new continuous-wave accelerator: with its help, the researchers of the SHE (SuperHeavy Elements) section could produce and investigate superheavy chemical elements much more effectively than before.

Electron cooling for more accurate data with PANDA

The HESR high-energy storage ring will be one of the core elements of the FAIR accelerator complex under construction in Darmstadt: it will accelerate and store ions, in particular antiprotons. The fast antiparticles enable novel, highly promising experiments with the planned PANDA detector, which will be inserted into the beam path of the HESR storage ring. In order for the experiments to run as effectively as possible in the long term, some technical hurdles need to be overcome. The ACID section is working on one of these challenges: it is developing a new, highly efficient cooling technology for the ion beam.

In the PANDA experiments, at each revolution around the HESR ring the stored antiproton beam hits a target consisting of a beam of tiny frozen hydrogen particles called pellets. The ensuing collisions create new, exotic particle states, which PANDA will measure in detail. The beam traverses the target about one million times per second. Each time, only a few antiprotons in the beam collide with the protons in the target. Most ions only cross the target, remain in the ring and continue on their laps.

The problem is that, in these frequent crossings, the antiprotons are deflected a bit by the target—some to the right or left, others up or down. As a consequence, the cross section of the beam increases over time, it is literally spread out. The ion beam thus gradually heats up by interacting with the target, making it harder to control. In the long run, the laboriously produced antiproton beam will be lost, which makes the experiment extremely difficult.

To prevent this, the HIM physicists intend to cool the beam and thereby limit it to a narrow cross section. To this end, they will superimpose the “hot” antiproton beam with a much cooler medium—a high-intensity electron beam. It should dissipate the heat from the ion beam and reduce its diameter to a fraction.

This is how it should work in detail: in one of the two straight sections of the racetrack-shaped HESR ring, the physicists will lead the two beams side by side parallel to each other for a certain distance, steering the electrons in and out of the antiproton beam with the help of magnetic fields. On their parallel flight, the cold electrons and the warm ions will inevitably collide with each other. In these collisions, the antiprotons transfer some of their kinetic energy to the electrons—the former slow down, the latter accelerate.

The concept poses several challenges. Among other things, the electron beam has to be continuous and have a high intensity: around 6 × 10¹⁸ electrons per second should fly parallel to the ion beam, corresponding to a current in the ampere range. The electrons need to have a kinetic energy of eight megaelectronvolts, corresponding to a power of eight megawatts. The generation of such a power at a voltage of eight megavolts will be outside the technological possibilities for the foreseeable future and would also be questionable from an ecological point of view due to the enormous energy requirement. Therefore, more than 99 percent of the kinetic energy is to be recovered by decelerating the electrons as they pass through the accelerating field in the opposite direction. This energy recovery requires extremely precise control of the electron beam—after all, the power levels are so great that a misdirected beam could cause damage to the accelerator. The HIM researchers are currently working on this precise steering.

Another challenge is the focusing of the electron beam. This will be done using special magnetic lenses housed in the electrostatic acceleration section. Supplying electric power to these lenses is a tricky undertaking, as supply via cables does not work in such a high-voltage environment. This is why the HIM scientists are working with experts from the Russian Budker Institute on an unusual solution: the energy for operating the magnetic lenses and the electron source will come from special turbine generators driven by compressed gas. The gas can be transported in insulated pipes, which are not affected by the high voltage.

Currently, the researchers are developing a system based on a commercial gas turbine with a power of five kilowatts. To test it, the experts have set up an electron cooler test bench at the HIM. They were already able to show that the energy recovery works with high efficiency at a voltage of 17,000 volts. The beam can be properly controlled, and only one in ten million electrons on average is lost.

The physicists are currently building a module that runs at a high voltage of 600,000 volts and has a power supply based on turbines. The experimental hall of the HIM is used for the construction and testing of these modules. Incidentally, the University of Mainz and GSI Helmholtzzentrum für Schwerionenforschung also use another section of this large experimental area for mounting superconducting accelerator modules, for example for MESA. This new electron accelerator for particle and hadron physics is currently being developed as part of the Excellence Initiative.

The HESR ring is to start up at FAIR in Darmstadt in 2025. Initially, the experiments will run without electron cooling. Later, the system could be retrofitted with the HIM electron cooler—thereby significantly increasing the performance of the accelerator and the experimental yield at PANDA.