discovered_02_2015 - Research: Fast calculations for plasma slow motion

WWW.HZDR.DE 24 25 RESEARCH // THE HZDR RESEARCH MAGAZINE A high-power laser shoots, a piece of metal foil crackles, and a bit of steam rises - that's just how simple a description of the laser particle acceleration process can be. But a glance at the physics of what is happening is much more impressive: The hundred trillion watt laser at the Helmholtz center in Dresden propels quadrillions of electrons forward, thus forming a kind of hot cloud composed of charged particles. In the same time it takes an electron to orbit its atomic nucleus, all matter is ripped apart, leaving just its fundamental constituents intact - a so-called high-energy plasma is produced. "For us as physicists that is a very interesting state of matter. The interactions between electrons and ions are so amazingly complex that our basic research requires exact, reviewable computer models from the very start," explains Michael Bussmann, head of the junior scientist group "Computational Radiation Physics" at the HZDR. The Dresden junior researchers have developed "PIConGPU" to enable them to calculate such models. This special simulation code is so powerful that it even gives the American super computer "Titan" a run for its money. With its 18,688 graphics processing units (GPU), Titan is not only currently the second fastest computer in the world, but also especially well-suited for such work: "GPUs are capable of simultaneously analyzing the movement of an amazing number of particles - and that's exactly what it comes down to when extremely complex processes of laser particle acceleration have to be modeled within a very short amount of time," says Michael Bussmann. Following one electron out of 100 billion particles Dresden scientists therefore don't have their procedures carried out by normal main processors as usual, but rather by graphics cards. These solve differential equations using the particle in cell method (PIC), which correlates the evolution of electric and magnetic fields with the movement of particles in plasma. At a record value of 7.1 quadrillion calculations per second (PFLOPs/s), this method is used to create physical 3D models of the ultra-fast particle chaos in the plasma. So the code can be used to view everything in a kind of slow motion: Even in a group of 100 billion particles, researchers can track a single electron and calculate its influence on the system as a whole. The real advantage is the high scalability of the code. This means that the computing power of the code increases proportionally to the number of graphics cards used. Something that can't be taken for granted, as Heiko Burau, a graduate student in Bussmann's group, emphasizes: "Even the smallest unused waiting time during data processing // Super-fast, scalable, and free – what started as a project for the German high-school student competition "Jugend forscht" is now one of the most powerful program codes for calculations in astro- and plasma physics. _TEXT . Christian Döring FAST CALCULATIONS FOR PLASMA SLOW MOTION IN ACTION: : A laser beam hits a spherical target. A plasma made up of electrons and ions is formed. Just like an explosion, the plasma expands and the electrons fly away from it, followed by the ions.

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