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

can enter the plasma core and diminish its performance either through radiation, in the case of
heavy first-wall materials such as tungsten, or through fuel dilution for light impurities such as
beryllium or carbon. Progress is being made both in understanding the physics of radiofrequency
sheath formation and in designing ion cyclotron resonance heating antennas that can reduce
sheath formation. however, given the importance of ion cyclotron resonance heating to the success
of iteR (since it is the only method that heats ions), the research effort needs to be intensified
with dedicated experiments and validated simulations of the antenna-plasma interaction, including
the physics of radiofrequency sheath formation.
Fueling: Fueling efficiency and density profile control require that iteR be fueled well beyond
the separatrix. The means for doing this is pellet injection, where, for maximum efficiency, pellets
are launched from the inner (high field) region of the iteR vessel. Predictive understanding
of the density profiles achieved with this arrangement requires quantitative knowledge of pellet
ablation and penetration physics, in the presence of energetic ions, as well as of particle transport.
The issue is complicated by the burning plasma environment of iteR, which couples the density
produced by pellets to the alpha particle production rate. The effect of pellets on elms (possibly
beneficial if “pellet pacing” is effective in reducing the size of elms) and other mhd activity also
requires further research to make reliable predictions of the efficacy of pellet fueling in iteR.
Fueling by pellets offers the further advantage of reducing the tritium inventory by injecting tritium-rich
pellets, while fueling with gas at the edge with deuterium (and impurities) to maintain
optimal divertor conditions. it then should be possible to provide a degree of control over the burn
in iteR by feedback control of the isotopic mix. The time scale over which the mix can be varied is
an important consideration and this requires additional research in existing pellet-fueled devices.
The overall particle control issue, which includes fueling, is critical for the sustainment of burning
plasmas. Pumping, the introduction of radiating impurities, the introduction of unintentional
impurities, the removal of helium, the fuel mixture and core fueling, and the radiating divertor
and overall power handling all must be simultaneously achieved for the burn to be sustained.
Heat Exhaust: managing the heat load, both during the nominal steady state and transients,
represents one of the biggest challenges to iteR. The largest heat load is on the divertor targets
and, at 5-10 mW/m 2 , is at the limit of what is technologically feasible, especially considering the
difficult plasma environment. transient events such as elms and loss of detachment can severely
limit the target lifetime. to extend the lifetime, research is needed on critical heat removal issues,
including how to control the power flowing to the divertor through feedback. an additional issue
is the viability of the plasma facing components after large-scale surface damage and loss of conforming
surface geometry (relative to the magnetic field) due to repetitive ablation/melt loss from
elms. The physics of radiative divertor solutions — including effects of opacity, impurity radiation
(and effect on the core), and momentum transfer — also needs elaboration to develop confidence
in the projected performance of the divertor design for iteR.
tritium Retention: Retention of tritium in first-wall components had been identified as a critical
issue for iteR since the earliest days of the iteR design. The problem is especially acute for
carbon plasma facing components due to co-deposition, the tendency of carbon to form difficult-
44