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

• liquid surface PFc operation in a tokamak environment will be needed for the demonstration
of heat removal, safety and diagnostics, and the coupling with physics performance.
• innovative design for high-thermal performance liquid surface option for demo should
be investigated.
iNtERNal coMpoNENtS
Functional internal components (antennas, sensors, mirrors, control coils, etc.) must meet the
criteria of other plasma facing components in terms of resistance to high heat and particle fluxes
with acceptable levels of impurity production, while performing their functions. most presently
operating high-power toroidal fusion experiments are pulsed and have limited neutron fluxes
(only those resulting from d-d operation). For these experiments, heat loads are handled inertially,
and particle deposition on components can either be: tolerated (as with antennas or limiters),
limited by the use of shadowing or shutters, or periodically removed or replaced (as required for
mirrors, windows, or lenses) to continue functioning. long pulse, nonnuclear experiments like
tore supra add active cooling to all internal components, and development of these techniques
has occurred progressively based on experience more than ~20 years. newer nonnuclear, superconducting
long pulse devices (lhd, kstaR, east [operating] and W7-X, Jt-60sa [under construction])
can be used as future test beds.
measuremenTs
What improvements to diagnostic measurements are needed to validate the PWI models with precision?
Can effective front-end optics used for diagnostics be developed to survive the harsh neutron environment
of a fusion plasma?
burning plasma properties introduce new fundamental measurement limitations to some existing
measurement techniques, as well as present an extremely hostile environment that challenges
a range of diagnostics — particularly those requiring plasma facing, optical quality mirrors.
examples of both of these challenges are provided below.
in a burning plasma, relativistic effects (t e0 ~20kev) degrade the spatial resolution of widely utilized
electron temperature measurement via electron cyclotron emission (ece). analysis by a scientific
team led by the University of texas has indicated an expected spatial resolution of 5 to
10cm in the core of iteR plasmas – almost an order of magnitude worse than currently available.
in addition, edge pedestal temperature (a critical iteR performance parameter) measurements
via ece have a spatial resolution of ~4 cm at an edge pedestal temperature of 4kev, which does
not meet iteR requirements and is likely inadequate to resolve the steep spatial gradient.
The above is an example where plasma parameters of a new regime degrade an existing measurement
capability. The deterioration is not due to hardware limitations, but is fundamental to the
measurement technique itself.
in contrast to such a fundamental limitation, there is also significant concern regarding a broad
array of existing optical diagnostic techniques planned for iteR, such as Thomson scattering,
charge exchange recombination spectroscopy, and motional stark effect. maintenance of the op-
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