FPN12-11

NSTX Upgrade Underway at PPPL

March 3, 2012

Despite the general reductions forecast in the President's FY 2013 budget request to Congress,the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) is getting to start on a $94 million, project to upgrade the National Spherical Torus Experiment (NSTX), though at a slower pace than previously envisaged. The completion date, originally planned for 2014, is now planned for September 2015. The upgrade will enhance the capabilities of NSTX by upgrading the magnet system to permit higher plasma currents and magnetic fields and by installing a second neutral beam heating system to enable better control of plasma instability.

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. Experiments done on NSTX, he said, "will establish the physics basis to determine next steps in fusion research and development.” PPPL physicists will use the upgraded NSTX facility to assess the role of more compact tokamak configurations in the development plans for fusion power plants.

A key issue for the NSTX upgrade is to see if it can improve on its record-high level of a measure called "beta" — the ratio of the pressure of a plasma to the strength of the magnetic field that confines it — as the plasma grows hotter. The higher the beta, the more cost-effective the confinement. The NSTX upgrade will furnish new tools for probing such issues and "provide ample research opportunities for years of productive research," said Michael Zarnstorff, deputy director for research at PPPL. "The whole NSTX group is quite excited by the research opportunities on this leading fusion facility."

The NSTX began operating in 1999. The upgrade will double the field strength to one tesla — or 20,000 times the strength of the Earth's magnetic field. The electric current flowing in the plasma will also double and reach 2 million amperes. Achieving these increases calls for widening a stack at the center of the device that puts current in the plasma and helps to complete the magnetic field. Widening the center stack also will increase the length of the electric pulse that drives the plasma current from one second to five seconds, giving researchers more time to study the plasma. The enhancements will help double the temperature at the core of the plasma to at least 20 million degrees Celsius.

The increased power will enable PPPL scientists to tackle these major questions:

Can the compact device continue to effectively contain plasma when the temperature rises, which could make the confinement more difficult? If the upgrade can effectively control the hotter plasma, "that means we could achieve high fusion power in a pretty compact machine, and that could make machines cheaper in the future," said Jonathan Menard, a principal research physicist and program director for the NSTX.
Can the researchers find new ways to start and sustain the electric current that creates the plasma? New methods are essential because future compact reactors will operate under conditions that would damage the spherical tokamak reactor's solenoid — a coil that winds around the center stack and delivers the current. PPPL scientists have been testing alternatives. 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."
Can the upgrade tame the hot plasma particles that escape the confinement and reach the reactor walls? This "power flux" can damage interior surfaces, drive impurities back into the plasma and shut down the reaction. Researchers have coated parts of the present NSTX torus with lithium, a metal that turns liquid when struck by stray particles and sponges up the impurities. But "the power flux that we expect in the upgrade will be very high compared to what we handle today," Ono said. "That is something we need to find attractive solutions for."

How PPPL scientists handle the increased flux could serve as a model for ITER, a major conventional test reactor that a consortium of countries including the United States is building in the south of France. ITER aims to produce a sustained fusion reaction -- or "burning plasma" -- by the late 2020s that will put out ten more energy than is needed to create it.

The NSTX upgrade could also help determine the path to a possible next-generation spherical torus that would produce a burning plasma to complement the output of ITER. Such a spherical torus would be roughly twice as powerful as the NSTX upgrade, said deputy PPPL director Zarnstorff, and could be used to test components for a commercial fusion reactor by around mid century.