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discovered_01_2016

WWW.HZDR.DE 04 05 TITLE // THE HZDR RESEARCH MAGAZINE Hadron Collider) at CERN in Geneva sees hydrogen atoms impacting at record energies. Huge electron accelerators, such as PETRA III at DESY in Hamburg, generate extremely intense X-ray beams which make it possible to scrutinize materials and biomolecules. And in hospitals, accelerators are required for radiation therapy – one of the core methods for treating cancer. Plasma thrust instead of surfriding All these facilities are based on acceleration using radio waves: Strong transmitters feed intense radio waves into a resonator – a pipe-like vacuum tube providing ideal conditions in which the waves can spread out. If an electrically-charged particle flies through the resonator, it can ride the waves like a surfer and gather additional momentum. But the method has its limits: "Particles can only be accelerated up to a certain extent in this way," Arie Irman explains. "You can only feed a certain maximum intensity of radio waves into the resonator, otherwise the field breaks down." This means that in order to reach the energy desired, a series of resonators have to be connected, which can result in extremely large facilities: The accelerator at the European X-Ray Laser Facility (European XFEL), which is scheduled to be completed shortly, will be about 3.4 kilometers long. Irman and Zeil are working on an alternative that would take up much less space – laser-plasma acceleration. "It promises to be much more efficient," says Karl Zeil. "It can house acceleration fields that are stronger by orders of magnitude than today’s resonators." It works on the principle that ultra- strong laser pulses are fired at matter. The force of the light pulse drives the atoms out of the electrons, creating a plasma – an ionized state of matter that can get exceedingly hot. By applying a laser pulse, an extremely strong electric field can be generated in the plasma within a tiny fraction of a second. This field can accelerate electrons or ions to nearly the speed of light in next to no time. "The procedure is still new," Irman explains. "It was only in 2004 that experiments in the United States and Europe were able to show that a laser plasma can bring electron packages to energies of several hundred megaelectronvolts (MeV) over a distance of just a few millimeters!" Electron sprint record Today, the record stands at 4,000 MeV for an acceleration section of just under seven centimeters. By comparison: In order to reach the same energy, a traditional linear accelerator would have to be several hundred meters long. But the work being undertaken in Dresden is not trying to achieve new energy records. Irman and his team want to produce optimum beam quality – and also the first applications using laser-accelerated electrons. In his lab, Irman demonstrates the vacuum chamber, a room the size of an office filled with countless stainless steel components. The pulses from the high-power laser DRACO COSTLY: The Titan sapphire crystal in the center right of the picture has a diameter of 12 centimeters. It helps to significantly increase the energy of the 10-centimeter wide laser pulse. Photo: Oliver Killig

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