Each second, the sun makes 100 million times more energy than the entire world population consumes in a year. Harnessing the source of the sun’s power — fusion — would ensure a safe, clean and virtually limitless way to meet global electricity needs. Discovering how to produce fusion on Earth is the primary mission of the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL).
To achieve this goal, scientists at PPPL have embarked on a $94 million upgrade of its main fusion experiment, the National Spherical Torus Experiment (NSTX). To be complete in 2014, the upgrade will make NSTX the most powerful experimental fusion device of its kind in the world. The upgrade “will provide a huge boost to all NSTX science missions and enhance U.S. fusion research capability,” said Stewart Prager, director of PPPL, which is managed by Princeton and located about three miles from the main campus.
The makeover will boost all the principal capabilities of the NSTX, which began operating in 1999 (see text box on page 41 for details on the NSTX upgrade). The revamped three-story-tall device, which is shaped like a cored apple and is known as a spherical tokamak, will heat charged gases, or plasma, to as high as 60 million degrees Celsius — six times hotter than the sun’s core — and confine the plasma in substantially strengthened magnetic fields. This will create conditions inside the machine that are similar to those required for fusion, which occurs when the atomic nuclei in plasma merge at extremely high temperatures and release a burst of energy. Producing a sustained fusion reaction that releases more energy than is needed to create the reaction is the ultimate goal of fusion research.
Work at PPPL also has moved ahead on many other fronts. In April 2012, PPPL physicists David Gates and Luis Delgado- Aparicio proposed a solution to a critical barrier to producing fusion known as the density limit, which can prevent fusion reactors from operating at maximum efficiency.
In a paper published in the journal Physical Review Letters, Gates and Delgado-Aparicio theorized that the problem was caused by tiny bubblelike islands that erupt in the superhot plasma. These islands grow, leak heat and block power as the plasma becomes denser, and when the limit is reached the plasma spirals break apart into a flash of light. The physicists hope to test their theory by injecting power directly into the islands to see if this will lead to higher density. If proven correct, the theory could bring researchers closer to developing fusion as a clean and abundant source of energy.
The laboratory has joined forces with other research institutions in collaborative ventures during the past year. In March 2012, Princeton teamed up with Germany’s Max Planck Society to create an institute designed to accelerate progress in plasma-based research, including harnessing fusion and understanding solar storms. The new Max Planck Princeton Research Center for Plasma Physics will combine the resources of PPPL and the Princeton Department of Astrophysical Sciences with the Max Planck Society’s institutes for plasma physics, astrophysics and solar-system research. Researchers will collaborate from their current locations on projects that are crucial to both fusion and to astrophysical plasmas, which make up 99 percent of the visible universe.
Working with Max Planck in a separate collaboration, PPPL scientists designed barn door-sized components called “trim coils” for the Wendelstein 7-X stellarator, an experimental fusion facility that the Max Planck Institute for Plasma Physics is building in Greifswald, Germany. The trim coils will help confine the plasma within a magnetic field shaped like a spiral wrapped around a circle. Researchers at PPPL also will conduct experiments on the stellarator, which, like the tokamak, is a major configuration for experimental fusion facilities.
In another collaborative effort, PPPL scientists led by physicist Erik Gilson last fall designed and delivered a crucial component for a linear accelerator at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory based at the University of California-Berkeley.
The copper-clad component resembles a spyglass and produces a plasma that focuses the accelerator beam down to a point that can heat a spot of foil to 30,000 degrees Celsius in less than a billionth of a second to create a substance called warm dense matter that researchers are eager to study. Gilson’s component completes the Berkeley Lab’s Neutralized Drift Compression Experiment- II, or NDCX-II, accelerator, and marks the third generation of plasma sources that Gilson has designed since 2001 for Berkeley Lab accelerators.
Scientists working under the leadership of PPPL also have developed a novel diagnostic instrument, called a reflectometer, for ITER, a 10-story tokamak that the United States, the European Union and five other countries are building in France as the next major step in harnessing fusion power. The goal of ITER is to produce a sustained fusion reaction by the late 2020s that will put out more energy than is needed to create it. ITER would serve as a bridge to such reactors, which energy experts expect to be in operation around 2050.
The PPPL-led team designed the reflectometer to measure electron density in plasma, which must be maintained at an optimal level for fusion to take place. The new design departs from that of standard reflectometers by using a space-saving single antenna system to measure electron density instead of the dual-antennae system commonly used today. Scientists at the University of California-Los Angeles and the U.S. Department of Energy’s Oak Ridge National Laboratory fabricated the novel system and it is being tested at the Department of Energy’s DIII-D National Fusion Facility at General Atomics in San Diego.
The results of these tests could bring researchers closer to creating and controlling the power of the sun and stars.
Probing new facets of fusion science
Scientists at PPPL will focus on the following areas when the upgrade of the National Spherical Torus Experiment (NSTX) is complete.
• Confinement under heat: Higher temperatures reduce the rate at which plasma particles collide with one another inside the machine, which means that less plasma energy escapes magnetic confinement. Jonathan Menard, a principal research physicist and program director for the NSTX, said that if the upgrade can effectively control the hotter plasma, “we could achieve high fusion power in a pretty compact machine, and that could make machines cheaper in the future.”
• Controlling electrical current: The upgrade will double the electric current that runs through the plasma and helps form the magnetic fields that confine it. Researchers will test new ways to sustain this current because future reactors will operate under conditions that could damage the coil, or solenoid, that delivers the current to the plasma. Eliminating the solenoid “is extremely important,” said Masayuki Ono, a principal research physicist who heads the NSTX department at PPPL. “If we can demonstrate that, we will have a very solid basis to design the next-step machine.”
• Preventing plasma escape: Hot plasma particles that escape confinement can damage the tokamak’s interior surfaces, pump impurities into the plasma and shut down the fusion reaction. PPPL researchers coated parts of the existing NSTX with lithium, a metal that turns liquid when struck by stray particles and sponges up the impurities. But the enhanced NSTX could result in more escaping particles and PPPL researchers are seeking solutions. How PPPL scientists handle the increased power flux could serve as a model for ITER.
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