Revised code could help improve efficiency of fusion experiments

Date:
August 25, 2020
Source:
DOE/Princeton Plasma Physics Laboratory
Summary:
Researchers have upgraded a key computer code for calculating
forces acting on magnetically confined plasma in fusion energy
experiments. The upgrade will help scientists further improve
the design of breakfast- cruller-shaped facilities known as
stellarators.



FULL STORY ==========================================================================
An international team of researchers led by the U.S. Department of
Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) has upgraded a
key computer code for calculating forces acting on magnetically confined
plasma in fusion energy experiments. The upgrade will be part of a
suite of computational tools that will allow scientists to further
improve the design of breakfast-cruller- shaped facilities known as stellarators. Together, the three codes in the suite could help scientists bring efficient fusion reactors closer to reality.


==========================================================================
The revised software lets researchers more easily determine the boundary
of plasma in stellarators. When used in concert with two other codes,
the code could help find a stellarator configuration that improves
the performance of the design. The two complementary codes determine
the optimal location for the plasma in a stellarator vacuum chamber
to maximize the efficiency of the fusion reactions, and determine the
shape that the external electromagnets must have to hold the plasma in
the proper position.

The revised software, called the "free-boundary stepped-pressure
equilibrium code (SPEC)," is one of a set of tools scientists can
use to tweak the performance of plasma to more easily create fusion
energy. "We want to optimize both the plasma position and the magnetic
coils to balance the force that makes the plasma expand while holding
it in place," said Stuart Hudson, physicist, deputy head of the Theory Department at PPPL and lead author of the paper reporting the results
in Plasma Physics and Controlled Fusion.

"That way we can create a stable plasma whose particles are more likely
to fuse. The updated SPEC code enables us to know where the plasma will
be for a given set of magnetic coils." Fusion combines light elements
in the form of plasma -- the hot, charged state of matter composed
of free electrons and atomic nuclei -- and in the process generates
massive amounts of energy in the sun and stars. Scientists are seeking
to replicate fusion in devices on Earth for a virtually inexhaustible
supply of safe and clean power to generate electricity.

Plasma stability is crucial for fusion. If plasma bounces around inside
a stellarator, it can escape, cool, and tamp down the fusion reactions,
in effect quenching the fusion fire. An earlier version of the code,
also developed by Hudson, could only calculate how forces were affecting a plasma if the researchers already knew the plasma's location. Researchers, however, typically don't have that information. "That's one of the
problems with plasmas," Hudson said. "They move all over the place."
The new version of the SPEC code helps solve the problem by allowing researchers to calculate the plasma's boundary without knowing its
position beforehand. Used in coordination with a coil-design code called
FOCUS and an optimization code called STELLOPT -- both of which were
also developed at PPPL -- SPEC lets physicists simultaneously ensure
that the plasma will have the best fusion performance and the magnets
will not be too complicated to build.

"There's no point optimizing the shape of the plasma and then later
finding out that the magnets would be incredibly difficult to construct," Hudson said.

One challenge that Hudson and colleagues faced was verifying that each
step of the code upgrade was done correctly. Their slow-and-steady
approach was crucial to making sure that the code makes accurate
calculations. "Let's say you are designing a component that will go on
a rocket to the moon," Hudson said. "It's very important that that part
works. So you test and test and test." Updating any computer code calls
for a number of interlocking steps:
* First, scientists must translate a set of mathematical equations
describing the plasma into a programming language that a computer
can understand;
* Next, scientists must determine the mathematical steps needed
to solve
the equations;
* Finally, the scientists must verify that the code produces correct
results, either by comparing the results with those produced by
a code that has already been verified or using the code to solve
simple equations whose answers are easy to check.

Hudson and colleagues performed the calculations with widely different
methods.

They used pencil and paper to determine the equations and solution steps,
and powerful PPPL computers to verify the results. "We demonstrated
that the code works," Hudson said. "Now it can be used to study current experiments and design new ones." Collaborators on the paper include researchers at the Max Planck Institute for Plasma Physics, the Australian National University, and the Swiss E'cole Polytechnique Fe'de'rale de
Lausanne. The research was supported by the DOE's Office of Science
(Fusion Energy Sciences), the Euratom research and training program,
the Australian Research Council, and the Simons Foundation.


========================================================================== Story Source: Materials provided by
DOE/Princeton_Plasma_Physics_Laboratory. Original written by Raphael
Rosen. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. S R Hudson, J Loizu, C Zhu, Z S Qu, C Nu"hrenberg, S Lazerson,
C B Smiet,
M J Hole. Free-boundary MRxMHD equilibrium calculations using the
stepped-pressure equilibrium code. Plasma Physics and Controlled
Fusion, 2020; 62 (8): 084002 DOI: 10.1088/1361-6587/ab9a61 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825165200.htm

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