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

yond, the economics would dictate that the external power input to a reactor system be minimized.
Thus a tokamak reactor system must operate at high bootstrap current fractions. in such
a system some auxiliary power is still needed for the balance of current drive not provided by the
bootstrap current, for control, and for plasma burn startup. key requirements for fueling and for
each of the potential heating and current drive actuators are summarized below.
Research Requirements for Fueling
other than the limited auxiliary power, a flexible fueling system and efficient pumping system
are all that remains to control and sustain the fusion burn in advanced scenarios. The fueling and
pumping systems must deposit fuel at the required locations, control the density profile to maximize
the bootstrap current fraction, remove helium ash, control the divertor operating density, and
possibly provide a source of external toroidal momentum input to improve plasma stability limits.
additionally, deep core fueling capability for iteR and beyond could possibly improve fusion burn
performance by peaking the density profile. it would also increase the tritium burn-up fraction and
thus potentially reduce tritium retention in the vessel. to meet these needs, four research opportunities
have been identified to develop satisfactory fueling capability for iteR and a demo.
Development of steady-state fueling and pumping technology:
improve present systems and develop new systems capable of steady-state density profile control
and helium ash removal. The primary fueling system for iteR is based on pellet injection. Pellet
fueling technology needs to be extended for reliable high-throughput use with tritium. additionally,
the pellet fueling performance needs to be assessed on iteR. because of the uncertainty of
present pellet fueling systems for deep core fueling, new fueling systems capable of deep fueling,
such as those based on compact toroid (ct) injection or high-speed pellets, need continued development.
These systems also have the potential for localized fueling, which should provide some
control of the density profile.
Fueling efficiency and isotope mixture control:
Quantify the increase in tritium burn-up with localized core fueling. Present transport codes
need to be benchmarked with results from experiments on large tokamaks to improve particle
transport estimates. simulations should then calculate the tritium burn-up fraction as a function
of localized fuel deposition in iteR. simulations and experimental tests of pellet penetration
depth, with velocity and size as a function of the pedestal temperature, are needed. experimental
penetration laws for alternate fueling systems are also needed.
Fueling compatibility with ELMs and MHD:
The compatibility of the fueling system with tolerable-size elms and acceptable plasma mhd behavior
needs to be established. experimental validation of elm pacing with pellets and possibly
compact toroid injection are needed, as is an experimental demonstration of the compatibility
with neoclassical tearing modes.
Advanced scenario modes:
since iteR relies on pellet fueling to fuel advanced operating modes, operating scenarios for pellet-fueled
at and hybrid modes need to be developed on present tokamaks. The potential capability
of ct-based systems for toroidal momentum injection needs experimental validation.
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