National Center for Computational Sciences

By: Dawn Levy

Projects explore radio frequency waves and magnetohydrodynamics in fusion plasmas

Fusion codes must communicate to control plasma in reactors.

Image credit: Don Batchelor, Fred Jaeger, Sean Ahern, ORNL.

In the Bible, all people spoke the same language until God felled the Tower of Babel and produced diverse languages that prevented people from understanding each other. Today physicist Don Batchelor at Oak Ridge National Laboratory is trying to solve a Babel-esque problem by creating a common language and a computational framework that will allow diverse software codes to communicate with each other in simulations of plasma—the hot, ionized gas that fuels nuclear fusion reactors.

Right now only some codes can share data. Different data formats and data names confound communication between many codes. Also challenging is getting codes to couple, or provide information to other codes at specific times to solve equations about the evolving state of plasma. Coupling is a tough task because a factor described by one code depends on another factor described by some other code.

“We’ve developed basically a lingua franca whereby different physics codes can talk to each other,” Batchelor says. In 2007 he and a team of more than two dozen researchers at 10 institutions used resources at the National Center for Computational Sciences to make progress toward developing an integrated plasma simulator. The work was made possible with an allocation of 1.7 million processor hours on the center’s Cray Jaguar XT supercomputer through a Department of Energy (DOE) Office of Science program called INCITE (for Innovative and Novel Computational Impact on Theory and Experiment).

Getting codes to speak a common language is difficult but not impossible. Researchers in another field, climate science, have done it. They linked models describing different aspects of Earth’s climate system and got the codes to communicate with each other in a coupled simulation tool called the Community Climate System Model.

“The climate community developed sophisticated models of the atmosphere, ocean, land, biomass, and sea ice—but you can’t predict what the climate’s going to do until you know how all of those things interact,” Batchelor says. “We’re in a similar situation in fusion. Over the years we have developed highly sophisticated models in different areas of plasma physics that are necessary to understand fusion experiments.”

We need to talk

Controlling plasma is the key to getting cheap, clean energy from future commercial reactors. Construction is under way for ITER, an experimental fusion reactor expected to begin operations in Europe in 2016. The $13 billion, 30-year multinational megaproject will build and operate a full-scale experimental device to demonstrate the technical feasibility of fusion energy. Following ITER would be a full-scale commercial fusion plant by 2050 and large-scale fusion-power adoption over the ensuing 30 years.

Improved simulation capability is urgently needed to support ITER, Batchelor says. Simulations are critical to supporting both theory and experiment. Theorists use simulations to gain insight into complex equations that correctly describe the state of plasma but that are difficult to intuit without solving the equations and visualizing the data. Experimentalists use simulations to evaluate if the desired trial conditions are achievable with the available equipment, guide operation of equipment, and interpret measurements from experiments.

“Once you have a simulation which is validated against the experiment—that you have confidence in—then you can use it to predict the next device,” Batchelor says.

An Esperanto of fusion software codes would allow Batchelor and collaborators to couple diverse phenomena taking place at multiple scales. The phenomena that they study range in speed from the fast oscillations of the radio waves that heat plasma (one cycle every 100-millionth of a second) to the slow interval it takes plasma to heat (about a second). They range in size from the gyroradius of an electron spiraling around a magnetic field line (about one-tenth of a millimeter) to the height of a fusion reactor (about 6 meters).

Says Batchelor, “When we come to take the next step in going from ITER to some sort of a demo reactor, there are going to be a lot of design decisions that have to be made, and simulations that are validated against experiments will be the way that we have to make these design decisions. That will ride on supercomputing.”

Synchronized SWIMming

Scientists have developed theories and computer codes that deal with multiscale phenomena more or less in isolation. “Now the time has come to consider the interaction between these phenomena—which have been studied essentially as separate disciplines—because they feed back on each other,” Batchelor says.

Two of the most important fusion codes simulate radio frequency (RF) waves, which heat and control plasma, and magnetohydrodynamics (MHD), the behavior of “fluid” that has a magnetic field and carries current. Researchers developed the RF and MHD codes with support from the Scientific Discovery through Advanced Computing (SciDAC) program, funded by DOE’s Office of Advanced Scientific Computing Research (ASCR). Batchelor’s INCITE allocation supported three SciDAC projects. The first, the Center for Simulation of Wave Particle Interactions, developed RF codes including AORSA and TORIC, whereas the second, the Center for Extended MHD Modeling, developed MHD codes including M3D and NIMROD. The third, the Center for Simulation of Wave Interactions with Magnetohydrodynamics (SWIM), of which Batchelor is principal investigator, brought the RF and MHD codes from the two supporting projects into a framework to try to get the codes to talk to each other.

The goals of SWIM are to use coupled codes to improve our understanding of RF and MHD interactions in plasma, develop an integrated computational system for treating many physics phenomena, and serve as a prototype for the Fusion Simulation Project (FSP). The FSP aims to simulate the behavior of toroidal magnetic fusion devices called tokamaks on all important time and space scales and to account for the interactions of all relevant processes. It will contribute to ITER’s experimental planning and performance optimization and design of devices beyond ITER.

A major element of SWIM is the development of a tool called the Integrated Plasma Simulator (IPS), a computational framework that will link physical factors beyond RF and MHD to make available an expanded, coupled model.

“The Integrated Plasma Simulator has the ability, we claim, to couple any physics code,” Batchelor says. That’s a lot of coordination—fusion simulations can take tens of thousands of processors working in parallel to compute heat sources and diffusion coefficients.

SWIM employs almost as many computer scientists and mathematicians as physicists and is funded equally through the Office of Fusion Energy (OFES) and ASCR. The United States is in a strong position to field a fusion simulation project, Batchelor says. “We have two offices at DOE who are interested in working with each other—OFES and ASCR—and we have experience working with each other through the SciDACs.”

After ITER

Why study RF and MHD interactions? “The MHD stabilities put limits on the performance of the tokamak,” Batchelor says. “They can reduce the confinement time, can cause the plasma essentially to blow away, even cause damage to the device structure.” RF systems can control these instabilities.

Batchelor received a Director’s Discretion allocation of 300,000 hours on Jaguar in 2008 to make innovations to increase the codes’ flexibility and performance. To finish the INCITE simulations, Jaguar will simulate RF waves energizing ions inside Alcator C-Mod, an experimental tokamak at the Massachusetts Institute of Technology that is powered by RF only. One of Batchelor’s goals is to validate this aspect of simulations run using the IPS computational framework against experiments run on Alcator C-Mod.

If today’s simulations support experiments, tomorrow’s experiments may support simulations. “Looking ahead, you could consider that the purpose of the ITER device is to provide data to validate the FSP,” Batchelor says. “If the purpose of developing all this simulation is to take the next step—have a validated simulation that enables you to build a demo reactor or a commercial reactor—then all these experiments are to build a scientific basis for that, and a major factor for taking the next step beyond ITER is going to be simulation. On the one hand FSP will support ITER by helping it do its experiments. But the real value of ITER is going to be to validate the simulations so we can extrapolate with confidence to the next step.”