Physicists improve method for designing fusion experiments

February 13, 2017, University of Maryland

Fusion experiments known as stellarators work by confining a mass of superheated plasma (orange horizontal mass) inside a magnetic field generated by external electromagnetic coils (multicolored vertical bands). A UMD physicist has made a revision to the software tools used to design these complex coil shapes, allowing researchers to create better designs with more room between the coils for repairs and instrumentation. The solid lines denote shapes made by the old software, while the dotted lines denote shapes made by the new software. Credit: Matt Landreman
"Measure twice, cut once" is an old carpenter's proverb—a reminder that careful planning can save time and materials in the long run.

The concept also applies to the design of stellarators, which are complex nuclear fusion experiments meant to explore fusion's potential as an energy source. Stellarators work by confining a ring of blazing-hot plasma inside a precisely shaped magnetic field generated by external electromagnetic coils. When the plasma gets to several million degrees—as hot as the interior of the sun—atomic nuclei begin to fuse together, releasing massive amounts of energy.

Before turning a single bolt to build one of these rare and expensive devices, engineers create exacting plans using a series of algorithms. However, a wide variety of coil shapes can all generate the same magnetic field, adding levels of complexity to the design process. Until now, few researchers have studied how to choose the best among all potential coil shapes for a specific stellarator.

University of Maryland physicist Matt Landreman has made an important revision to one of the most common software tools used to design stellarators. The new method is better at balancing tradeoffs between the ideal magnetic field shape and potential coil shapes, resulting in designs with more space between the coils. This extra space allows better access for repairs and more places to install sensors. Landreman's new method is described in a paper published February 13, 2017 in the journal Nuclear Fusion.

"Instead of optimizing only the magnetic field shape, this new method considers the complexity of the coil shapes simultaneously. So there is a bit of a tradeoff," said Landreman, an assistant research scientist at the UMD Institute for Research in Electronics and Applied Physics (IREAP) and sole author of the research paper. "It's a bit like buying a car. You might want the cheapest car, but you also want the safest car. Both features can be at odds with each other, so you have to find a way to meet in the middle."

Researchers used the previous method, called the Neumann Solver for Fields Produced by External Coils (NESCOIL) and first described in 1987, to design many of the stellarators in operation today—including the Wendelstein 7-X (W7-X). The largest stellarator in existence, W7-X began operation in 2015 at the Max Planck Institute of Plasma Physics in Germany.

"Most designs, including W7-X, started with a specifically shaped magnetic field to confine the plasma well. Then the designers shaped the coils to create this magnetic field," Landreman explained. "But this method typically required a lot of trial-and-error with the coil design tools to avoid coils coming too close together, making them infeasible to build, or leaving too little space to access the plasma chamber for maintenance."

Landreman's new method, which he calls Regularized NESCOIL—or REGCOIL for short—gets around this by tackling the coil spacing issue of stellarator design in tandem with the shaping of the itself. The result, Landreman said, is a fast, more robust process that yields better coil shapes on the first try.

Modeling tests performed by Landreman suggest that the designs produced by REGCOIL confine hot plasma in a desirable shape, while significantly increasing the minimum distances between coils.

"In mathematics, we'd call stellarator coil design an 'ill-posed problem,' meaning there are a lot of potential solutions. Finding the best solution is highly dependent on posing the problem in the right way," Landreman said. "REGCOIL does exactly that by simplifying coil shapes in a way that the problem can be solved very efficiently."

The development of as a viable energy source remains far off into the future. But innovations such as Landreman's new method will help bring down the cost and time investments needed to build new stellarators for research and—eventually—practical, energy-generating applications.

"This field is still in the basic research stage, and every new design is totally unique," Landreman said. "With these incompatible features to balance, there will always be different points where you can decide to strike a compromise. The REGCOIL method allows engineers to examine and model many different points along this spectrum."

The research paper, "An improved current potential method for fast computation of stellarator coil shapes," Matt Landreman, was published February 13, 2017 in the journal Nuclear Fusion.

Explore further: Physicists confirm the precision of magnetic fields in the most advanced stellarator in the world

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not rated yet Feb 14, 2017
I never really understood why the loops in a stellarator, but after seeing the picture of the plasma contained within the field coils it was instantly obvious how the the fields create a pinch point within which the conditions are correct for the conditions of fusion. A picture is worth .....
Now we need to figure out how to shrink it down so it fits in the palm of your hand. There I've dreampt it, make it happen.... I want to build a starship.
5 / 5 (1) Feb 14, 2017
This is kind of like "tuning" the the intake manifold and exhaust header on a old school carburetor performance engine. You want to shape the flow of the fuel during intake and exhaust on its way out of the combustion chamber, with the idea of minimizing turbulence. Wish I understood the math, would have loved to work on something like this!
5 / 5 (1) Feb 15, 2017
The energy you perceive are the wrinkles in the field of very fast moving charges, not fusion. The probability of any particular atom is stochastic. Fusion requires very little motion; thus, fusion will probably use energy and in what cases will it create energy? Energy is only the wrinkles in the field, the non-wrinkled fields are seen as a potential relative to some ground point. So all that energy is due to motion, fusion removes motion. You are trying to gain energy from a process that removes energy! Each charge has no mass and is an object, this object is a spherical field. It is updated at the speed of light relative to its center. We see these updates as energy!
not rated yet Feb 15, 2017
Anyway, no real control of the plasma states is defined other than some sort of overall shape. This is an observation and no science need apply! How do you manipulate each charge?
not rated yet Feb 15, 2017
Not for fusion, but magnetic controls, even motors: we are in the 21st century, our magnetics haven't changed since Tesla. Try using micro-chip current control per current line. The current line would be solving for the necessary and sufficient conditions to control the field of a defined location as a function of (i,t), where i is the current line under control. Simple! Max current per line?

Note: Total magnetic response is some function of i(x) omega t, pulse controls, sinusoidal, PWM, etc. I like sensing the current directly across the Mosfet with a fuzzy measure of i using Rd, just a secondary parallel sw biased the same with an Rsense, logic, and a diff amp. No real load on the network.
not rated yet Feb 15, 2017
Minimize i per line by maximizing the number of lines within the constraints of space. Communication and control maybe serial, even optical.
not rated yet Feb 16, 2017
I never really understood why the loops in a stellarator
-and you still dont
but after seeing the picture of the plasma contained within the field coils it was instantly obvious how the the fields create a pinch point within which the conditions are correct for the conditions of fusion
Theyre not creating a pinch point. Theyre configuring the fields to accomodate what the plasma wants to do.

"Stellarators, on the other hand, shape their magnetic field into a winding Möbius-like ribbon that completely confines the plasma..."

-What you see in the picture is a twisting ribbon-like plasma.
Da Schneib
not rated yet Feb 18, 2017
Alternative fusion is always interesting. Stellarators are one of the most promising paths.
Mar 08, 2017
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not rated yet Mar 08, 2017
With this concept, fusion within a laminar plasma, might work! However, it requires energy, a lot of energy, i.e. method. Fusion will not give any appreciable energy. Look at it as condensation. Charge bundles will only condense into another due to location, since the far field sees the net polarization within the sphere. The sphere for fusion is not a sphere but an opening in the field of superposition time and space, satisfying a set of conditions, i.e. it does not have enough energy to fly away or bounce off. Nothing more than tapping the 8 ball corner pocket. Now suppose all the balls are moving wildly. So laminar flow, meh.

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