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Plasma physics award won by PPPL postdoctoral fellow for novel approach to designing coils for fusion energy

Caoxiang Zhu, a postdoctoral researcher at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), has won the 2018 CAI Shidong Award for plasma physics in China. His research describes a computer code that Zhu developed with the help of PPPL physicists that takes a novel approach to designing the complex magnetic coils that confine plasma in fusion devices called stellarators, which are focused on one day creating energy using the process that powers the sun and the stars. 

Zhu’s code, called “Flexible Optimized Coils Using Space curves (FOCUS),” imagines a design for stellarator coils that dispenses with pre-conceived notions of the proper shape or placement of the coils. These are traditionally conceived as winding around a twisty doughnut-shaped surface called a winding surface, which is pre-defined. However, the FOCUS software designs coils that float freely in space. FOCUS also uses analytical formulas to help find the optimal coils for a given magnetic field. 

“I am thrilled that Dr. Zhu has received this recognition for his thesis work,” said David Gates, the head of Stellarators at PPPL and one of Zhu’s supervisors. “Caoxiang performed much of his thesis work here at PPPL, working with Stuart Hudson, deputy head of Theory and Computation, and we realized very soon that his work was very special. We are thrilled that he has accepted our offer to continue his stellarator optimization work here at PPPL.”

Award named for pioneering Chinese physicist

Zhu will receive the CAI Shidong award on Sept. 15 at the annual meeting of the Chinese Physical Society in Dalian China. The award is named after Chinese plasma physicist CAI Shidong (also known as Shih-Tung Tsai). Tsai graduated in 1969 from Princeton University’s Program in Plasma Physics in the Department of Astrophysical Sciences and was advised by pioneering plasma physicist Thomas H. Stix. The award is the only plasma physics award in China for graduate students and young scientists. Tsai’s widow will be one of the presenters of the award to Zhu and two other recipients, who received the award for research into high density plasma physics and for diagnostics research. “This really means a lot to me,” Zhu said. 

Zhu’s work focuses on stellarator coils, which create a three-dimensional steady-state magnetic field that confines plasma. Unlike more widely used fusion devices called tokamaks, stellarators do not need to induce an electric current in the plasma to complete the magnetic fields. This means stellarator plasmas are not subject to the risks that tokamaks face when instabilities in the plasma cause energy stored in the current to be released. This can produce disruptions that can halt fusion reactions and damage plasma-facing tokamak components.

The ability of stellarators to confine plasma in a steady and non-disruptive state makes their design promising for future fusion reactors. But designing and building the twisting complex coils can be a challenge. Zhu’s new approach has the potential to find simpler and more efficient stellarator designs. “We can design the optimal shape of the coils that has more effective control of the plasma, that’s our idea,” Zhu said. This method can be used to analyze distortions in the magnetic field caused by coil distortions prior to manufacturing and assembling. It can also be used to explore novel designs of certain coils on tokamaks. 

Zhu has tested FOCUS by designing and optimizing simulated coils in existing stellarators such as the Wendelstein 7-X in Germany, Japan’s Large Helical Device and the Helically Symmetric eXperiment at the University of Wisconsin-Madison. FOCUS proved itself “much faster and more robust” than previous codes, Zhu said.  

Coils move freely in space 

The new code produces coils that move freely in space. “All of the existing algorithms unnecessarily constrained the geometry of the coils,” Hudson said. “Why not introduce a representation of the coils that allows the coils to move freely without putting a constraint on their shape? We can access a wider variety of coil geometry because we’re not constraining them.” 

Hudson said Zhu has a rare combination of skills as a theoretical physicist. He has the ability to think about theoretical plasma physics, as well as the numerical skills to create the algorithms to carry out his ideas, and the grasp of experimental plasma physics to “illustrate how this tool can be used to come up with practical designs.”

Invited talks

Zhu received his undergraduate degree and his doctorate from the University of Science and Technology of China, a national university in Hefei that is considered the equivalent of an Ivy League university in China. Even before he received his graduate degree, he was invited to present his research on the FOCUS code at the International Stellarator-Heliotron Workshop in Kyoto, Japan, last year. He was also an invited speaker at the Sherwood Fusion Theory Conference at Auburn University in April. 

Zhu grew up in rural Anging, China, the son of a primary school teacher father and a homemaker mother. As a child he was always interested in science and enjoyed taking things apart and putting them back together again. He became interested in fusion research when a guest lecturer came to his physics class when he was an undergraduate student. “He told us this could solve the energy crisis. We could have totally clean energy for thousands of years,” Zhu recalled. “I thought that’s really impressive!” 

Zhu began working with Hudson as a visiting student at PPPL from 2015 to 2017. He returned to PPPL to finish his dissertation and received his Ph.D. in February. He became a PPPL postdoctoral researcher that same month. His work is supported by the Max Planck Princeton Research Center for Plasma Physics and by PPPL Laboratory Directed Research and Development Funds. 

Now he is looking into potential collaborators with national and international fusion programs. “I would like to use this to optimize plasma and fabricate and design the next generation of stellarators for PPPL or for the U.S.,” said Zhu.   

PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

 

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