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

tical quality of mirrors, polarizers, windows, etc., located close to a burning plasma environment
represents a significant, and perhaps overwhelming, challenge to overcome. in particular, redeposition
and erosion combined with long plasma exposures present a forbidding challenge. in addition,
the bremsstrahlung background emission will be far greater than observed in current devices
and ultimately limit signal to noise. it seems highly likely that the availability and accuracy
of such visible diagnostics will be severely compromised in the iteR environment in comparison
with existing devices. This is especially the case during long pulse (~1000s) exposure to high heat
and neutron loads during high-performance operation. optical-based diagnostics requiring plasma
facing mirrors will be extremely challenging for iteR (area of international concern) and effectively
untenable on a demo.
The variety of challenges posed by a burning plasma environment to measurement capabilities
must be urgently addressed. a primary goal for iteR is to understand the physics of burning plasmas.
This will require a wide range of detailed measurements – ideally superior to current measurement
capabilities. it is critical that immediate work begin to both identify and resolve the
“gaps” in measurement capability so that iteR can prepare us for future burning plasma devices
such as ctF and demo.
rf anTennas and launChers
How can the predictive capability of plasma edge models, including material interaction, be enhanced?
Can these enhanced models incorporate the formation of radiofrequency sheaths produced by radiofrequency
waves transiting between the radiofrequency antennas and absorption in the core plasma?
Can material with greater toughness and improved thermal conductivity be developed with specific materials,
coatings, and heat sinks suitable for high-power radiofrequency components for the fusion environment,
resulting in the handling of higher power density radiofrequency components that are robust and
tolerant of plasma conditions?
Can innovative concepts be developed that move sensitive front-end components far from the plasma edge?
many of the issues concerning the interaction of the near field of the antenna with the plasma in
the sol are not well understood. There is a need to predict, measure, adjust for, and modify this
edge environment since it is the region through which the power is coupled. The formation of the
radiofrequency plasma sheath can lead to focused particle and energy fluxes on the antenna and
on surfaces intersected by the magnetic field lines connected to the antenna, with a dependence
on the antenna structure and phasing. The resulting hot spot formation and enhanced local erosion
serve as local impurity sources and can degrade core confinement. The parasitic radiofrequency
losses in this region include edge modes (shear alfvén and cavity modes), parametric instabilities,
and nonlinear wave-particle interactions.
integration of fully 3-d heating codes with realistic antenna and confinement device geometry is
needed to understand the coupling of radiofrequency power from the antenna to the core, as well
as for fundamental understanding of factors that will limit antenna operation, such as elms, parasitic
losses, breakdown, and nuclear materials issues. to address these challenges, we propose an
133