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

to demonstrate that steady and transient heat flux control and other PFc solutions will be compatible
with optimal core plasmas, the facility to support this Thrust must have the ability to obtain
a robust edge pedestal. an important parameter for obtaining demo-like pedestal behavior
is the edge bootstrap current profile that strongly affects edge stability and elm dynamics. to approach
demo conditions the edge collisionality must be low. study of pedestal issues, as well as
radiative mantle solutions, will add an additional requirement on the input power to be a significant
multiple of the power required to obtain high confinement mode (h-mode). taken together
these capabilities will produce a robust pedestal with demo-like elm characteristics as a test bed
for the most promising techniques and technologies for controlling both steady-state and transient
heat flux.
Plasma sustainment is also an important characteristic of the facility to support this Thrust in
achieving very long pulses. The facility should be designed to allow for testing a variety of heating
and current drive issues and options. design of heating and current drive systems employing ion
cyclotron resonance heating and lower-hybrid must include an examination of coupling efficiency
through the expected dense boundary plasma and the strong pressure gradients in the pedestal
into the core plasma. The launching structures must endure long periods of interaction with the
boundary plasma. The design of neutral beam, electron cyclotron current drive and electron cyclotron
heating systems will generally be less sensitive to edge issues, but here the focus will be
on their capabilities for controlling the core over a sufficient range of densities, due to accessibility
physics.
another demo characteristic that will not be addressed by current or planned facilities is that
of high temperatures for all components; this will be required in demo because higher operating
temperatures lead to higher thermal conversion efficiency to electricity. The capability to attain
such high temperatures (in the range 500 o c to 1000 o c), as well as explore their effects on
core and boundary characteristics, is central to the goals of this Thrust. he coolant technology
options are currently the most strongly considered for demo, but liquid metal coolants should
also be considered. tungsten PFcs will likely have an operational temperature window above the
ductile to brittle transition temperature, but below the limit due to recrystallization. liquid-surface
PFcs are likely to have a lower maximum operating temperature due to their higher evaporation
and sputtering rates at high temperature. techniques to monitor and control dust production
from solid PFc surfaces and evaporated material from liquid PFc surfaces will need to be qualified.
Fuel retention and permeation into PFcs will also have a strong temperature dependence,
and thus control of temperature will be needed to allow exploration of the effect on retention in
PFcs and on the overall fuel cycle. Given the history of tokamaks showing a strong dependence
of core energy and particle confinement on recycling and surface conditions, active control of surface
temperatures could also be important for maximizing the performance of the core plasma.
While the ultimate goal for a steady-state demo is pulse lengths of order 10 7 sec with >80%
duty factor, design and science trade-offs (e.g., costs of pulse length vs. gains in physics explored)
should be considered in determining the appropriate specifications for a facility to support Thrust
12. in general, the requirement is that the cumulative particle and energy fluence to the plasma
boundary and materials reach a level where the long-term evolution of the PFcs, due to erosion,
fuel recycling, macroscopic PFc thickness changes (positive and negative) and the fuel residing
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