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DOE Pulse
  • Number 325  |
  • November 22, 2010

Taming thermonuclear plasma with a snowflake

A “snowflake” divertor — a novel plasma-material interface is realized in the National Spherical Torus Experiment (NSTX). A drawing of NSTX is at top, with a circle around the snowflake divertor, and below the machine is the snowflake.

A “snowflake” divertor — a
novel plasma-material interface
is realized in the National
Spherical Torus Experiment
(NSTX). A drawing of NSTX is
at top, with a circle around the
snowflake divertor, and below
the machine is the snowflake.
(Click image for larger version.)

Physicists working on the National Spherical Torus Experiment (NSTX) at the DOE Princeton Plasma Physics Laboratory (PPPL) are now one step closer to solving one of the grand challenges of magnetic fusion research — how to reduce the effect that the hot plasma has on fusion machine walls (or how to tame the plasma-material interface). Plasma is a hot, electrically charged gas used as the fuel for fusion energy production. Some heat from the hot plasma core of a fusion energy device escapes the plasma and can interact with reactor vessel walls. This erodes the walls and other components, and contaminates the plasma — all challenges for practical fusion. One method to protect machine walls involves divertors, chambers outside the plasma into which the plasma heat exhaust, and impurities, flow. A new divertor concept, called the “snowflake,” has been shown to significantly reduce the interaction between hot plasma and the cold walls surrounding it.

Strong magnetic fields shape the hot plasma in the form of a donut in a magnetic fusion plasma reactor called a tokamak. As confined plasma particles move along magnetic field lines inside the tokamak, some particles and heat escape because of instabilities in the plasma. Surrounding the hot plasma is a colder plasma layer, the scrape-off layer, which forms the plasma-material interface. In this layer, escaped particles and heat flow along an “open” magnetic field line to a separate part of the vessel and enter a “divertor chamber.” If the plasma striking the divertor surface is too hot, melting of the plasma-facing components and loss of coolant can occur. Under such undesirable conditions, the plasma-facing component lifetime would also be an issue, as they would tend to wear off too quickly.

While the conventional magnetic X-point divertor concept has existed for three decades, a very recent theoretical idea and supporting calculations by Dr. D.D. Ryutov from the DOE Lawrence Livermore National Laboratory have indicated that a novel magnetic divertor — the “snowflake divertor” — would have much improved heat handling characteristics for the plasma-material interface. The name is derived from the appearance of magnetic field lines forming this novel magnetic interface, as shown in the figure.

This magnetic configuration was recently realized in NSTX and fully confirmed the theoretical predictions. The snowflake divertor configuration was created by using only two or three existing magnetic coils. This is an important result for future tokamak reactors that will operate with few magnetic coils. Because the snowflake divertor configuration flares the scrape-off layer at the divertor surface, the peak heat load is considerably reduced, as was confirmed by the divertor heat flux on NSTX. The plasma in the snowflake divertor, instead of heating the divertor surface on impact, radiated the heat away, cooled down and did not erode the plasma-facing components as much, thus extending their lifetime.

The snowflake divertor did not have an impact on the high performance and confinement of the high-temperature core plasma, and even reduced the impurity contamination level of the main plasma. These highly encouraging results provide further support for the snowflake divertor as a viable plasma-material interface for future tokamak devices and for fusion development applications.

[Patti Wieser, 609.243.2757,
pwieser@pppl.gov]