HPCwire: Researchers Conduct Breakthrough Fusion Simulation

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July 14, 2008

Researchers Conduct Breakthrough Fusion Simulation

GTC consumes 93 percent of 263TF Jaguar

July 14 -- A team of researchers from the University of California-Irvine (UCI), in conjunction with staff at Oak Ridge National Laboratory's National Center for Computational Sciences (NCCS), has just completed what it says is the largest run in fusion simulation history.

The team, led by Yong Xiao and Zhihong Lin of UCI, used 93 percent of the NCCS's flagship supercomputer Jaguar, a Cray XT4, with the classic fusion code GTC (Gyrokinetic Toroidal Code), the key production code of two fusion SciDAC projects (GPS-TTBP and GSEP).

The simulation primarily studied electron transport in ITER, a prototype fusion reactor now in development that is meant to test fusion's feasibility for commercial power production (because in ITER the fusion process will primarily heat electrons, electron transport will be more important compared to existing fusion devices). Fusion energy could one day provide the world with a cleaner, more abundant energy source with far fewer harmful emissions than fossil-fuel-burning power plants and fewer waste issues than current nuclear power production.

To successfully produce a fusion reaction, an extremely hot ionized gas known as a plasma must be confined magnetically for a sufficient period of time. Previous research has shown that heat transport for both the ions and the electrons in the plasma is far greater than theory predicts (the electron heat transport can be two orders of magnitude greater). This larger-than-expected heat transport could lead to confinement failure within one second, quickly rendering energy production impossible if reactor designs are not modified to accommodate it.

The researchers discovered, among other things, that for a device the size of ITER, the containment vessel will demonstrate GyroBohm scaling, meaning that the heat transport level is inversely proportional to the device size. In other words, the simulation supports the ITER design: a larger device will lead to more efficient confinement.

"The success of fusion research depends on good confinement of the burning plasma," said Xiao. "This simulation size is the one closest to ITER in terms of practical parameters and proper electron physics."

Because fusion simulation is such a complex process, today's leadership-class supercomputers such as Jaguar, with their fast CPU speeds and large memory, are an absolute necessity. GTC can effectively utilize large numbers of Jaguar's cores thanks to an efficient three-level parallelism (using both MPI and OpenMP) designed by Stephane Ethier (Princeton Plasma Physics Laboratory) and Zhihong Lin.

However, the huge amounts of data produced by fusion simulations can create I/O nightmares: in one GTC run, the team can produce terabytes of data (in this case 60TB). To address this potential bottleneck, the team used ADIOS, a set of library files that allows for easy and fast file input and output, developed mainly by the NCCS's Scott Klasky and Chen Jin and Georgia Tech's Jay Lofstead and Karsten Schwan.

"This huge amount of data needs fast and smooth file writing and reading," said Xiao. "With poor I/O, the file writing takes up precious computer time and the parallel file system on machines such as Jaguar can choke. With ADIOS, the I/O was vastly improved, consuming less than 3 percent of run time and allowing the researchers to write tens of terabytes of data smoothly without file system failure."

ADIOS is a good example of the symbiotic relationship between researchers and NCCS staff. "My experience with Cray and the NCCS has been very good," said Xiao. "The account and operation staffs are very accessible and responsible, which enabled us to run the GTC code with a total of 28,000 cores smoothly for two days. [The NCCS's] Scott Klasky provided an effective channel for technical communication between the science application team and the computational support team."

Despite the success of the recent simulation, there is still much work to be done. "Plasma turbulent transport is still an ongoing research area," said Xiao. However, each new simulation brings new data, which bring a revolutionary energy source one step closer to reality.

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Source: Scott Jones, National Center for Computational Sciences

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