Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

• exploration of high-temperature superconductors, leading to steady-state magnets with
relatively low operating costs, improved maintainability (demountability), and easier
fabrication compared with conventional superconductors. inherently steady-state devices
such as stellarators would benefit greatly from this emerging technology (see Thrust 7).
The goal of this set of modeling and development activities is the development of a practical magnet
system for a Pe-class Qs stellarator experiment with a predictable cost and schedule.
action 3: Divertors for 3-D configurations
magnetic field lines at the relatively cold edge of the plasma must be diverted to a region where
helium ash from the fusion reaction and other impurities can be removed. The plasma temperature
must be low enough to prevent rapid erosion of the plasma facing material in this divertor region.
The high-density capability of even moderate-field stellarators makes a radiatively cooled divertor
solution plausible, though the 3-d geometry makes the engineering design difficult. however,
the understanding of divertor behavior in stellarators is less well-developed, and the adaptation
of an effective divertor to 3-d geometry is more complex than in tokamaks. The island divertor
concept employed on the W7-as and lhd stellarators, and planned for W7-X, requires control
of the edge rotational transform. it also constrains the divertor to adjoin the main confinement
region. Three-dimensional divertor designs that require less edge plasma control and allow for
expanded exhaust with rapid pumping are highly desirable. such designs must also be integrated
with the optimization of the entire stellarator magnet system.
key research steps include:
• designing advanced divertors that handle the necessary power and particle exhaust, and
control neutral and impurity influx, while remaining compatible with Qs 3-d shaping.
• increasing participation on the large lhd and W7-X experiments in Japan and Germany
in the area of 3-d divertor physics. This activity includes the benchmarking of 3-d edge
physics transport modeling codes.
action 4: 3-D shaping for improved operation of other toroidal systems
There is no clear demarcation between tokamaks and Qa stellarators: the 3-d plasma shape can
be continuously varied while maintaining quasi-symmetry. This action pursues the potential benefits
of controllable levels of Qa 3-d shaping to be pursued on tokamaks and other toroidal confinement
systems:
• Providing sufficient poloidal magnetic field for sustainment of the magnetic configuration.
• ensuring robust stability to prevent uncontrolled vertical displacements and disruptions.
• minimizing the need for feedback systems.
• Understanding the empirical density limits of tokamaks by extrapolating from lowcurrent
3-d stellarators (in which they are understood) to higher-current axisymmetric
systems.
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