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

The plasma facing components in a demo reactor will face much more extreme boundary plasma
conditions and operating requirements than any present or planned experiment, including iteR
and the superconducting tokamaks abroad. The gap between current and planned tokamaks and
demo in the area of compatible core-edge solutions is very large in terms of steady-state handling
of demo-level power efflux on high-temperature material surfaces. This gap needs to be
bridged in a manner consistent with external control of plasma current, heating, and fueling to
maintain an optimized clean, hot and dense core plasma. The configurational flexibility and complete
diagnostic coverage that will be needed for testing solutions for this demanding environment
would be most readily accomplished in a limited activation facility. The knowledge gained
from this research program would accelerate the development of integrated core-boundary solutions
for high-fluence burning plasmas such as in a Fusion nuclear science Facility (FnsF), and a
demonstration (demo) fusion power plant.
solutions for individual challenges outlined should be addressed by several thrusts to inform the
design of the facility to support Thrust 12. The physics of the boundary layer plasma and methods
to disperse the heat flux over a greater surface area are to be investigated in Thrust 9. Physics of
the plasma surface interaction are to be investigated in Thrust 10. heat removal technologies and
components will be developed in Thrust 11. The effects of neutrons will be tested in Thrusts 11
and 13. techniques for limiting heat flux transients will be developed in Thrust 2, and methods
for core plasma sustainment and control will developed in Thrust 5. however, there is no present
or planned facility where these solutions can be combined and the complex interaction of these issues
studied and tested at demo-like conditions: this is the central goal of Thrust 12.
Primary Challenges for a DEMO boundary Configuration
The most basic of physical conditions that a demo fusion power plant must accommodate is
the very high heat efflux from the burning plasma core that arrives on PFc surfaces. For typical
demo designs, the total power flowing that must be exhausted onto material surfaces is approximately
four times that predicted for iteR, yet in a device of similar physical dimensions. since
iteR’s PFcs are already at the limits of engineering design, new techniques for spreading the exhaust
heat flux over as much of the vessel surface as possible must be developed for demo.
simultaneous with such high-power handling requirements, demo will have a much higher
duty factor (≥80%) than existing or planned devices, resulting in an annual energy and particle
throughput 100-200 times greater than for iteR. such constant high loading would lead to macroscopic
levels of erosion of PFc material, which will be thin (~ 5 mm) for reasons of heat removal
and tritium breeding. an associated concern is that the eroded material may redeposit with poor
thermal and mechanical properties.
While various aspects of demo heat flux solutions will be developed in Thrusts 9, 10 and 11, the
PFc surface designs and operational techniques must be shown to be compatible with a robust edge
pedestal that assures high core confinement and simultaneously allows efficient core control techniques.
steady-state operation will require external means of driving plasma current, fueling the
core plasma and, ideally, optimizing the core plasma profiles. an important core-edge issue is the
efficient coupling of the control carrier (e.g., waves) through a “thick” edge and collisionless ped-
326