12 March 2022

Photo: DESY/Christian Mrotzek

You need to pass many individual milestones if you want to learn more about dark matter with a new experiment. The team of the ALPS II experiment, which is being set up in an accelerator tunnel underneath Hamburg, has just reached one of them: the superconducting magnets, in which ultimately particles of light are supposed to transform into dark matter particles, ran at the full current of 5700 amperes for the first time at the beginning of March.

“It was very exciting, ” says ALPS II project manager Axel Lindner, summing up the last few weeks. "The magnets built for the HERA accelerator some 35 years ago had to be modified for ALPS II, including an almost complete redesign of the safety systems, for example. Seeing the 24 magnets run at target current for the first time was a great feeling,” he says. A current of 5700 amperes flowed through the superconducting niobium-titanium cables - an important step towards the upcoming continuous operation. The DESY experts also tested how the magnets behave when the magnetic field is cyclically switched on, off and on again, which also worked like clockwork.

Continuous operation is a way off yet: while the experiment is located in the underground accelerator tunnel, the power supply is located one floor up. However, there is no data network there yet. The team from DESY´s MPC (Machine Power Control) group, which also specialises in power supplies for magnets, therefore had to control the operational test practically by hand on site, instead of directly from the control room as is usually the case. “We’re working on getting a network there,” Lindner laughs.

Before the 5700-amperes current could flow, the 250-metre-long magnet string had to be cooled down to its operating temperature of minus 269 degrees. Only superconducting magnets can generate the strong magnetic field that ALPS II needs for its measurements, and these magnets achieve superconductivity – the state in which electrical conductors lose their electrical resistance – at an operating temperature of almost absolute zero. The team reached that milestone shortly before Christmas 2021. They had to proceed very carefully because components contract as they cool down. In total, the cooldown process takes around three weeks.

“In terms of operational tests, that´s it for now,” says Lindner. “We now have to warm up the magnets again for work in the refrigeration plant. The warm-up also takes three weeks, and following that we will continue to work with the international partners on the construction of the complex optics over the next few months.” In the summer, the ALPS magnets will again be cooled down to minus 269 degrees in preparation for the start of the experiment. In the meantime, the researchers will also be looking at their measurements from the magnets' test run, which they hope will provide information on how much background noise to expect in the experiment, “just to understand it,” Lindner says.

About ALPS II

The “light-through-the-wall experiment” ALPS II (Any Light Particle Search) will search for particularly light particles that could make up dark matter. With the help of twenty-four recycled HERA magnets, laser light and a highly sensitive detector system, the international team wants to search for these so-called axion-like particles. The idea is that in a strong magnetic field, photons – particles of light – could transform into these mysterious particles and back into light again. The first section of the HERA magnets, arranged in a straight line, encloses a vacuum tube in which high-intensity laser light is reflected back and forth. If a photon were to transform into an axion, it could pass through a wall located at the centre of the two magnet strings. Behind it, this axion could change back into light, again in an optical resonator, in the second magnetic string, which has almost exactly the same setup.