This is a "snowflake" divertor -- a novel plasma-material interface is realized in the National Spherical Torus Experiment.
Credit: V. Soukhanovskii, Lawrence Livermore National Laboratory
Physicists working on the National Spherical Torus Experiment (NSTX) at
the Princeton Plasma Physics Laboratory are now one step closer to
solving one of the grand challenges of magnetic fusion researchhow to
reduce the effect that the hot plasma has on fusion machine walls (or
how to tame the plasma-material interface). Some heat from the hot
plasma core of a fusion energy device escapes the plasma and can
interact with reactor vessel walls. This not only erodes the walls and
other components, but also contaminates the plasmaall 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 Lawrence Livermore National
Laboratory have indicated that a novel magnetic divertorthe "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.
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 achievement 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. Plasma TV images show
more divertor radiation in the snowflake divertor plasmas in comparison
with the standard plasmas. Importantly, 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.