Ask researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) to name one of the greatest science and engineering challenges ever undertaken and the answer comes easily: harnessing fusion energy.
Fusion happens naturally in the sun and other stars. The tremendous gravity from these massive stellar objects crushes together the nuclei of hydrogen atoms and releases vast amounts of energy. Bringing this process down to Earth could provide a safe, clean and virtually limitless supply of power for generating electricity.
But harnessing fusion energy is a supremely difficult task. The positively charged nuclei — or ions — inside atoms resist being squeezed together, and there is no solar gravity in laboratories to force the stubborn particles to merge.
Enter PPPL’s National Spherical Torus Experiment (NSTX-U). This device, called a “tokamak,” is in the midst of a $94 million upgrade that will make it the most powerful fusion facility of its kind in the world when the work is completed in 2014. Such facilities heat hydrogen to astronomical temperatures and trap it in magnetic fields to produce the superhot, electrically charged plasma gas that fuels fusion reactions.
Scientists then study the gas to learn how to use it to create a “burning plasma,” or sustained fusion reaction — the goal of global fusion research. “It’s as if we’re trying to create a state of matter on Earth that hasn’t existed before,” said PPPL Director Stewart Prager. “And that’s a hard thing to do.”
PPPL is a leader in this worldwide quest and a key source of public information and classroom instruction about the physics involved. The laboratory, which is managed by Princeton University and is located about three miles from campus, collaborates in major fusion experiments in Europe, Asia and the United States, and conducts educational programs for participants ranging from the general public to graduate students (see box and map).
The NSTX-U upgrade will enhance all capabilities of the machine. The temperature inside the three-story-tall tokamak could rise above 60 million degrees Celsius during experiments and reach six times the temperature at the core of the sun. The electric current that powers the machine’s huge magnets will double, as will the strength of the magnetic fields.
The sharply increased forces will quadruple the stress on all the NSTX-U components that support the magnets. This has required PPPL engineers to redesign and reinforce such structures throughout the machine. “It took a tremendous amount of analysis time to do this,” said engineer Ron Strykowsky, project manager for the upgrade.
Research on the powerful NSTX-U, whose spherical shape resembles a cored apple as compared with the donut-like shape of conventional tokamaks, will be followed by fusion researchers around the world. Experiments will show whether the streamlined, spherical design of the PPPL machine can serve as a model for the next major step in U.S. fusion research, and will produce vital data for ITER, the huge international fusion facility under construction in France.
PPPL has charted a five-year plan of action for the NSTX-U. The spherical device set records for efficient plasma confinement when it operated from 1999 to 2011 prior to the upgrade. Researchers now want to see if the enhanced machine can confine far hotter and harder-to-corral plasmas just as efficiently.
Plans also call for testing a system that will line the inner walls of the tokamak to protect them from the scorching plasma that escapes the magnetic field. Researchers will coat the walls with a thin layer of lithium, a silvery metal that turns liquid when struck by stray particles, to absorb the hot gas. “It works the way sweat moistens and protects the skin,” said Masayuki Ono, project director for the NSTX-U department at PPPL.
The escaping heat poses further challenges. The plasma could easily slice through a metal plate called a “divertor,” which serves as an exhaust system in tokamaks, unless the heat can be spread before it reaches the plate. Researchers will test an awardwinning device called a “snowflake divertor,” which PPPL helped develop and employed prior to the upgrade, to see how well it can spread the NSTX-U heat flux.
Likewise high on the PPPL agenda will be testing new ways to create and sustain the electric current that runs through tokamak plasmas. This current now is generated by a coil called a “solenoid” that will be unable to operate in the continuous fashion that future facilities will require. While the NSTX-U will still use a solenoid, researchers also will inject current through a pair of electrodes installed in the tokamak as a possible replacement for the coil.
Scientists will address all these issues in experiments called “shots” that will heat the plasma and run the NSTX-U magnets for up to five seconds — five times longer than previously possible. Preliminary plans call for some three shots an hour, eight hours a day, for 120 experiments a week.
These shots will determine if a spherically shaped tokamak could be a strong contender for the next key device in the U.S. fusion program. That envisioned device, called a Fusion Nuclear Science Facility (FNSF), would assemble and test all the components needed for a fusion power plant. This would pave the way for a demonstration fusion facility that would generate electricity on the grid and lead in turn to construction of a commercial fusion plant around the middle of the century.
The FNSF “would propel fusion forward fantastically,” said Prager. And the NSTX-U “will give us the physics information so the world can make a yes-or-no judgment about whether the spherical tokamak is a good candidate for that next step.”
Bringing plasma to the people
Plasma is everywhere, from the gas in neon light bulbs to the fuel that lights the stars. PPPL’s mission includes highlighting the properties of this fourth state of matter for the general public and inspiring and educating the next generation of scientists. “We want the public to know what we do and why we do it,” said John DeLooper, head of the best practices and outreach programs at PPPL. “And we want to excite young people to go into the world of science.”
The laboratory carries out this role through wide-ranging programs. PPPL has a variety of portable scientific demonstrations and experiments that staffers bring to public events and school classrooms. The laboratory also provides a 10-week summer internship in plasma physics for college undergraduates. Seventy-two percent of the physics and engineering students who have taken the course have entered doctoral programs in physics since 2000.
For students who go on with their studies, PPPL supports graduate education chiefly through the University’s Program in Plasma Physics in the Department of Astrophysical Sciences. The program has awarded more than 265 doctorate degrees, many to people who have become leaders in the field.
-By John Greenwald