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Team led by PPPL’s Chang receives DOE 2015 INCITE award totaling 270 million core hours to study key problem in fusion

The U.S. Department of Energy has bestowed many hours of access to scientists at the Center for Edge Physics Simulation (EPSI), led by C.S. Chang, a physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory. The highly competitive award allows 270 million core hours on two powerful supercomputers that will enable researchers to continue staging complex simulations of how charged particles behave in the tokamak edge. The award was the second highest in the INCITE program.

Officials representing the DOE’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program announced the awards to EPSI along with 55 other cutting-edge research projects in the U.S. The scientists will share a total of 5.8 billion core hours on the two fastest computers in the U.S.

INCITE will give the EPSI team 170 million core hours on Titan, the Cray XK7 supercomputer at Oak Ridge National Laboratory. It is the world’s second-largest computer and can do over 20 quadrillion (one thousand million million) calculations per second. The program also gave EPSI 100 million core hours on Mira, an IBM Blue Gene/Q computer at Argonne National Laboratory, which can perform 10 quadrillion calculations per second. One day on Mira is equivalent to what the average person could accomplish in 20,000 years on a personal computer.

The award is the third in a three-year grant from INCITE. EPSI received 229 million core hours last year and 100 million core hours two years ago.

Understanding and controlling the edge of a plasma produced during experiments is a crucial issue in producing fusion energy in ITER, the international fusion experiment being constructed in Cadarache, France. The EPSI team built the edge gyrokinetic code XGC for this purpose.

The award will allow the EPSI team of physicists, mathematicians and computer scientists from 10 different institutions and PPPL to use the XGC code to study how charged particles generate turbulence and how the turbulence affects the charged particles.

Turbulence at the edge is nonlinear and intermittent, meaning it is high-density and “blobby,” Chang said. Charged particles bunch together at the edge and carry particles and energy away with them toward the material wall. This leads to a loss of mass and energy in the core of the plasma where extremely high heat of 100 million degrees Centigrade is needed to maintain the fusion reaction.

Chang believes the “blobby” turbulence is the most common type of turbulence at the edge of the plasma, as seen in the experiment for a long time. “We are studying how they are born and how they move around and how they die, how they carry mass and energy away from the plasma to the wall,” Chang said.

The additional time will be used to simulate how the blobby turbulence at the edge of the plasma will behave with electromagnetic fluctuations in ITER, Chang said. The simulations will also study how the “blobby” turbulence will affect the tokamak’s divertor. The divertor is composed of metal plates made of tungsten at the base of the tokamak that take heat and helium ash from the plasma to keep the core plasma stable and maintain the plasma reaction. The super-hot plasma can cause damage to the divertor plates if the heat load is not adequately distributed.

PPPL physicist Stewart Zweben, among others, first observed the “blobby” turbulence in edge plasmas in fusion experiments well over a decade ago, Chang said. Past studies used model equations but they were too simple to fully capture the physics of blobs. Researchers needed powerful computers to use a process called “gyrokinetic simulation,” that solves the complex kinetic interactions of the charged particles at the plasma’s edge.

With such an ambitious research agenda and the complex computer code, researchers could quickly use up the 270 million core hours, Chang said. He estimates that the XGC simulations would take about 10 million core hours in a full day, giving the EPSI team a total of approximately 17 full days on Titan and 10 full days on Mira. This amounts to fewer than three ITER physics studies if the full complexity of the XGC code is to be engaged, Chang said. For a routine ITER physics simulation, Chang said he will have to wait for more powerful computers. Such computers are expected to be operational in a few years at Oak Ridge National Laboratory, Argonne National Laboratory and the National Energy Research Scientific Computing Center at the Lawrence Berkeley National Laboratory and others.

PPPL physicists working on the simulations include Seung-Hoe Ku, Jianying Lang, Robert Hager, Daren Stotler, Stephane Ethier, Salomon Janhunen, and Michael Churchill.

Other national laboratories and universities working with the Center for Edge Physics Simulation include: Oak Ridge National Laboratory, MIT, Rutgers University, Lehigh University, the University of Colorado, the University of California at San Diego, the University of California, Irvine, the Lawrence Berkeley National Laboratory, Rensselaer Polytechnic Institute, and the University of Texas at Austin. Collaboration with these team members is invaluable, Chang said. 

U.S. Department of Energy
Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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