Plotting plasmas could influence fusion
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Collision course
Caflisch and fellow researchers from the Department of Energy’s Lawrence Livermore National Laboratory started with one important aspect of plasma simulation: Modeling collisions of ions – electrically charged atoms and molecules – and electrons.
“One of the most challenging problems in the simulation is to take those into account,” he adds. Simulating billions of collisions can slow computation to a crawl.
It’s faster to depict the particles as fluid elements, with each element containing millions of particles. “The trouble is the fluid equations aren’t quite accurate there,” Caflisch says. “We’re trying to blend those two together to get the speed of the fluid part and the accuracy of the particle part.”
To model particles, Caflisch uses the Monte Carlo method, so named because it relies on random numbers.
“We’re starting with a system that has maybe 1020 particles in it, which is a huge number” – a one followed by 20 zeroes, Caflisch says. “We’re trying to model it with 10,000 or 1 million particles.
“The way we do that is by randomizing the interactions so each numerical interaction represents millions of real interactions. That works amazingly well.”
Caflisch and his fellow researchers combine that with a fluid model.
“You represent the billions of particles by thinking of them as forming a collective unit that moves with a fluid speed and pressure and such,” Caflisch says.
The model evolves those properties through space and time as particles are selected out of the fluid distribution. The particles, in turn, influence the density, momentum or energy of the fluid.
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