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

estal into the core. We must also demonstrate that the required control launchers can be designed
and operated to accommodate a high heat flux environment with high first-wall temperatures.
another aspect of control developed in Thrust 2 that must be shown to be compatible with the
demo heat flux environment is the control, avoidance, and mitigation of heat flux transients
such as those from edge localized modes (elms) and disruptions. The energy released in an uncontrolled
demo elm is likely to be ~ 4 x that of iteR where control techniques are already required
to reduce the energy released in an elm by ~ 90%. disruptions as well must be essentially
eliminated in a demo. techniques for avoiding, or suppressing, these transients must be deployed
in a manner consistent with both a robust edge pedestal and control of the steady heat flux
to PFcs.
The high temperatures of PFcs that are envisioned in aRies designs (500-1000 o c) to make the
fusion power cycle more thermally efficient also bring new challenges and opportunities. The
challenge is that the PFc materials and technologies to be used at such high temperatures are
both at their heat load handling limits and have a narrow temperature operational window. The
opportunity associated with high-temperature PFcs is to study hydrogenic retention and control
the tritium fuel cycle; in a demo reactor less than 1 in 100,000 t ions striking the wall can be retained
in the material. That should be contrasted with current devices where this ratio is found to
typically be only ~1 in 100. The high hydrogenic diffusion and reaction rates at demo PFc temperatures
will greatly aid removal of fuel ions from PFc surfaces and allow demonstration of the
fuel control required for demo. The intense steady and transient loads on the PFcs, however, will
result in dust production. dust needs to be well-controlled for safety reasons in demo.
The flux of fusion neutrons also represents a great challenge for a demo reactor’s PFc materials.
While the neutron damage in the bulk PFc material may lead to increased tritium retention, the
main impact of neutron irradiation is on thermal conductivity and brittleness, which affect the
necessary thickness and structural integrity of the PFc components. For this reason the effects of
neutrons are largely separable from the plasma-wall processes and resulting interaction with core
plasma operation. efforts to address the effect of neutrons on PFcs are encompassed in Thrusts
11 and 13.
initial analysis of the Machine Characteristics Required for this Thrust
The first task for designing a facility to support the central goal of this Thrust, demonstration of
an integrated solution for the core and boundary, is to develop the requirements for such a facility’s
optimal scientific and technical capabilities. The first of these characteristics is the power
density. There are two metrics that the community often uses for comparing the power density
across devices (table 1). Their different scalings reflect different heat flux issues and the large uncertainty
in our present understanding. The first of these, P/s, is the average power through the
surface of the plasma at its edge. The second metric, P/R, which assumes a constant width of heat
flux in the boundary plasma, represents a more conservative projection to iteR and demo. a facility
to support this Thrust should be designed for power flows into the edge and to the PFcs approaching
demo levels (table 1). Further work on heat flux width scaling, such as that now ongoing,
and that proposed in Thrust 9, will lead to improved estimates of expected heat flux profiles
that can be taken into account for specification of the heating power requirement.
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