PPPL physicist wins Early Career Research Program grant to develop tools to eliminate impurities in fusion plasmas
Physicist Luis Delgado-Aparicio, of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), has won a highly competitive Early Career Research award sponsored by the DOE’s Office of Science. The five-year grant of some $2.6 million will fund Delgado-Aparicio’s research aimed at eliminating a key barrier to developing fusion power as a safe, clean and abundant source of electric energy.
Delgado-Aparicio said he is delighted to receive the grant. “It’s a research and development process that will last for five years but it’s a program that will certainly have an impact in our field,” he said. “I’m very, very excited about it.”
Fusion occurs when a super-hot electrically charged gas called plasma is heated to temperatures hotter than the sun and becomes hot and dense enough to force atomic nuclei to fuse together and create a burst of energy. Delgado-Aparicio’s research focuses on the impurities that migrate from the interior walls and plasma-facing components of a fusion facility — or tokamak — into the plasma. These impurities are tiny particles that can cool the plasma and halt or slow the fusion reaction. Delgado-Aparicio is developing a process to enable researchers to pinpoint and analyze the impurities and quickly flush them out of the plasma.
Ridding plasmas of these impurities is becoming increasingly vital as experiments utilize longer pulses to produce more sustained fusion energy. “It’s a very important question: What’s going to happen when we try to confine a plasma for several minutes, hours, days or even months?” Delgado-Aparicio said. “We need to create a mechanism to mitigate the presence of these very troublesome impurities or to flush them from the core of the plasma.”
Tackling this problem will be crucial for ITER, the international fusion experiment under construction in the south of France, and the National Spherical Torus Experiment-Upgrade (NSTX-U), which is completing a nearly three-year upgrade at PPPL. Part of the ITER interior will be made of the heavy metal tungsten, a source of highly-charged impurities that can quickly radiate away heat from the core plasma. And researchers in Princeton will replace the carbon-coated walls of the NSTX-U with a full-metal wall made of tungsten or molybdenum in the next five years.
Delgado-Aparicio and David Gates, a principal research physicist at PPPL, recently proposed an answer to a mysterious problem in plasma physics called “the density limit” that also centers on impurities and prevents fusion reactors from operating at maximum efficiency. The researchers found that tiny, bubble-like magnetic islands form in the plasma and collect impurities. These islands not only cool the plasma but act like shields that block out added power that is pumped into the tokamak. If these islands grow too large, the electric current that heats and confines the plasma collapses and the plasma flies apart. One goal of Delgado-Aparicio’s research is to find methods to prevent these disruptions by minimizing the loss of energy caused by the impurities. These methods could achieve better performance in fusion reactors by ensuring that the maximum amount of energy goes into fusion reactions
Third winner at PPPL in the last three years
Delgado-Aparicio, of Montgomery Township, New Jersey, was one of 44 winners of an Early Career award nationwide and the third researcher at PPPL to win the honor in as many years. Physicist Brian Grierson won a grant last year for research into the mechanisms that govern the formation and maintenance of the hot edge of fusion plasmas. Ahmed Diallo, leader of the pedestal structure and control topical science group for the NSTX-U, won a grant in 2013 for research on the plasma edge.
News is bittersweet
Delgado-Aparicio said it was bittersweet when he first received news of the award. He learned of it in April while in the airport on his way to visit his ailing father in Lima, Peru. “It was difficult to be happy because half my brain was occupied by my dad,” he said. He phoned his parents about the award and his father, a former Peruvian congressman who was also named Luis Delgado-Aparicio, died half-an hour-later, shortly after his son boarded the airplane to visit him.
Delgado-Aparicio and his 10-year-old son Mateo were back in Lima for a memorial service for his father when DOE announced the Early Career recipients on May 6. He said he is now better able to both remember his father fondly and take pleasure in the award. “I begin to feel happier about his life” Delgado-Aparicio said. “He was an enthusiast of the work I was doing. He was a great supporter in my life. I feel very happy to have had him in my life and I’m sure he was ecstatic to hear about the prize.”
Delgado-Aparicio earned a bachelor’s degree in physics from the Pontificia Universidad Catolica del Peru, and a master’s degree in astrophysics from Princeton University in 2001. He earned a second master’s in physics from Johns Hopkins University and received his PhD in physics from Johns Hopkins in 2007. He joined PPPL in 2009 and spent three and a half years as a visiting scientist at MIT before returning to PPPL in the summer of 2013.
A complex diagnostic tool
Delgado-Aparicio plans to study how the impurities react with the plasma by using a complex x-ray diagnostic that will show exactly what happens to the plasma when the impurities are introduced. The device will reveal not only the size and location of the impurities, but also the kind of energy they radiate, which will pinpoint the sources as well as the properties of how impurities are transported in fusion plasmas. The diagnostic will also show how the impurities affect the energy and temperature of the plasma.
This information will provide a guide for removing the impurities, Delgado-Aparicio said. One solution could be to change how the impurities are transported into and through the plasma. For example, researchers might use a tangential neutral beam to heat the plasma to an extremely high temperature to change the way impurities are transported and steer them away from the core of the plasma. The neutral beam would also speed the movement of the plasma, creating a centrifugal effect to spin the impurities away from the plasma’s center. Another solution could be to create perturbations within the plasma to force the impurities out.
Delgado-Aparicio plans to test his diagnostic on NSTX-U. He also plans to conduct experiments on the Alcator C-Mod tokamak at MIT; the DIII-D tokamak at General Atomics in San Diego, and the Tokamak á Configuration Variable (TCV) in Lausanne, Switzerland.
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