discovered_01_2016

WWW.HZDR.DE discovered 01.16 TITLE Utilizing irregularities In their simulations, Kluge and his colleagues try to capture these instabilities as precisely as possible. "Our aim is to minimize the instabilities," the physicist explains. "But in some cases, we actually want to utilize them." Because the researchers have discovered that there are advantages to working with micrometer-sized irregularities in the surface of the foil: It means the laser pulse is presented with a larger contact surface, so the material can absorb more laser energy, which makes the acceleration of the particles up to 50 percent more efficient. However, the simulations are complex and cannot be done on an ordinary office computer. They need supercomputers. Among others, Kluge and his colleagues use HZDR’s high- performance "Hypnos" cluster, but they have also worked in the US with one of the fastest machines in the world, the supercomputer "Titan". Some of the simulations run for several weeks – on thousands of processors simultaneously. Their findings, incidentally, are also of interest to astrophysicists because conditions inside the planets and stars may be similar to those created by the interaction of laser pulse and foil. "You may get comparable instabilities there, as well," says Thomas Kluge. "The outcomes of our computer simulations may help to develop more precise models of the evolution of planets and stars." One thing is clear: So far, computer simulations are the only way of looking directly into the plasma and the interplay of laser pulse and matter. All previous experimental methods are at best able to deliver indirect evidence: Detectors, for example, can measure the X-radiation that is generated as a by-product when the laser pulse hits the foil, but the signals can only achieve a resolution of a few micrometers at most. A resolution in the nanometer range is what is needed – this is the scale on which the crucial processes take place. Kluge and his colleagues are working on a new method that could bring some progress on this front: "We want to work with the extremely strong and short X-ray flashes which the European X-ray Laser is going to start producing in 2017." The European XFEL is currently under construction in Hamburg – a 3.4 km long acceleration facility which will generate the world’s strongest X-ray pulses. Profound insights into plasma HZDR researchers are planning a spectacular experiment on this giant: A standard laser fires short light pulses onto a sample, such as a foil. On a micrometer-sized spot, a plasma forms which can accelerate particles highly efficiently. As soon as the light pulse has hit the foil, a second pulse arrives – the X-ray flash from the European XFEL. It literally X-rays the events in much the same way as a doctor X-rays patients in hospital. "By varying the time between the laser pulse and the X-ray flash, we can virtually scan the process," Kluge explains. "We then want to combine the images into a film and observe what is really going on." This method is expected to achieve a resolution of just a few nanometers – accurate enough to detect significant instabilities in the plasma. Being a theoretician, Kluge has already simulated the experiment on the computer. But the decisive question can only be answered by an experiment: When you fire at the hot plasma with an X-ray flash, will the sample really scatter enough of the X-ray signals? The researchers have been able to test the procedure during a large-scale measuring experiment at what is currently the strongest light source worldwide, the LCLS (Linac Coherent Light Source) in California. The outcome? "We really did observe a scattering image that was just as we had expected," Kluge enthuses. "So we have proved that the procedure is viable." What is more, the physicists have even been able to gain initial physical insights. "Under certain circumstances, shock waves occur which travel across the foil," Kluge continues. "This causes fractures or phase transformations in the material." It emerged that these different areas are strictly demarcated – an important detail when it comes to understanding plasma processes. The road-map is clear: From 2018 onwards, the researchers want to regularly make their way to Hamburg to conduct experiments at the HIBEF measurement station, which is operated jointly by HZDR and DESY as part of an international collaboration. "Even though I’m a theoretical physicist, I’ll certainly be there sometimes, even if it means doing the odd night-shift," Kluge explains, "because this is the very first method that allows you to look directly into plasma, and for us theoreticians, that promises to be a great leap forward in our ability to make predictions." PUBLICATION: T. Kluge et al.: "Nanoscale femtosecond imaging of transient hot solid density plasmas with elemental and charge state sensitivity using resonant coherent diffraction", in Physics of Plasmas 2016, online http://arxiv.org/abs/1508.03988, DOI: 10.1063/1.4942786 CONTACT _Institute of Radiation Physics at HZDR Dr. Thomas Kluge t.kluge@hzdr.de www.hibef.de

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