DESY scientist Carl Andreas Lindstrøm has been awarded the Gold Medal of His Majesty the Norwegian King Harald V for outstanding young researchers from the University of Oslo. The Norwegian scientist, who is today working at DESY on future linear accelerators, was honoured for his doctoral thesis about future particle accelerators. This includes the discovery of an innovative plasma lens for focussing particle beams and studies of the acceleration of positrons in a hollow plasma channel.

“For electrons, plasma wakefield acceleration achieves the two things we need from it to build the machines we would like to build: They accelerate quickly and maintain their quality,” Lindstrøm told Symmetry Magazine. “It’s just unlucky, really, that the same is not true for positrons, and that is the huge challenge we are facing.” At the SLAC National Accelerator Laboratory in the US, Lindstrøm and his colleagues investigated the options of accelerating positrons in a hollow plasma channel. “The most recent studies verify where we currently stand, with this large challenge in front of us,” Lindstrøm says. “But in the process of getting there, we learned a lot about how this technology works.” The scientists report their measurements in the journal Physical Review Letters.

At the European particle research lab CERN, a team from Oslo, the University of Oxford, CERN and DESY had investigated plasma lenses that are regarded as a promising alternative to the magnets used today to focus particle beams. In a so-called active plasma lens, a strong electric current passes through the plasma along the path of the particles, creating a magnetic field vortex. In contrast to classical quadrupole magnets, this field can simultaneously focus the height and the width of the particle beam. Along with their high focusing power, due to the strong magnetic fields, this makes such lenses extremely attractive for use in particle accelerators.

However, a key problem had so far prevented such a deployment of plasma lenses: an optical flaw in the lens destroys the focus of the particle beam as it passes through the plasma cell. This so-called aberration occurs because zones of different temperatures form within the plasma when the plasma is heated by the current flowing through it and at the same time cooled by the outer walls of the plasma cell. As a result, the strength of the focussing field varies in different regions of the plasma, leading to the optical flaw in the lens. Until now, this aberration has occurred so quickly after igniting the plasma that it was not possible to use the plasma lens to focus a particle bunch before the lens developed the flaw.

The effect of this distortion was what Lindstrøm and the other researchers were to measure. First, they set up an experiment with a plasma lens filled with helium gas. “We had designed the experiment in almost the same way as other research groups have done before,” says Lindstrøm, who was working at the University of Oslo back then. “But because we used a slightly different setup, it did not work.” After several failed tries, the team changed the set-up and replaced helium with argon – and with that, they got the experiment going and started to chart the aberration of the lens. But to their surprise there was none.

“We measured again and again, always with the same result: no distortion,” Lindstrøm recalls. “At first we thought that the other laboratories might have done something wrong, since we used a different method that should be more precise. In the end, we managed to carry out the experiment with helium as well, and then the distortion appeared again. Then we realized that we had stumbled upon something interesting!”

The explanation: The heavier argon slows down the conduction of heat within the gas for long enough to allow a bunch of particles to be focused immediately after the plasma has been formed and the current to the magnet has been switched on, without the quality of the beam suffering as a result. The plasma lens only begins to distort the beam after the bunch has passed through it. In other words, using argon instead of helium eliminated one of the biggest problems with active plasma lenses. Thus, an important component in future accelerators is one step closer, as the team reports in the journal Physical Review Letters.

The concept of active plasma lenses stems from the 1950s and was first conceived for ion beams. In 2015 at the Lawrence Berkeley National Laboratory (LBNL), the team of Wim Leemans, who is today director of DESY's Accelerator Division, re-discovered the concept and was the first to use active plasma lensing for electron beams. This triggered many follow on experiments in labs around the world.

 

Further reading: Hans Majestet Kongens gullmedalje (in Norwegian)