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

Demonstrate burn control and control of the operating point thermal stability with sufficient
flexibility to regulate power output to accommodate external demands: With what level
of dynamic performance and flexibility can thermal stability be provided in a high-performance burning
plasma?
a power producing reactor will need to operate at a constant total power output determined by
the external load and economic considerations, but must be sufficiently flexible to vary the operating
point as the external load changes. in addition, thermalized he “ash” needs to be removed
to avoid quenching the reaction. Power control is achieved by controlling the isotopic mix, density
profile, and ion-temperature-dependent fusion cross-section by active fueling and pressure
profile modification. Thermally stable operating points exist in which a positive excursion in the
ion temperature reduces the fusion reactivity, and the plasma control system must be capable of
reaching and smoothly transitioning between these points. however, thermally stable operation
is not necessarily the most efficient, and the option of active feedback control capable of maintaining
the operating point at passively unstable values is desirable. ash removal is a challenge
and methods need to be developed for selective removal of cooled he nuclei. techniques for deep
fueling need to be developed, along with a deeper understanding of particle transport. auxiliary
heating schemes can modify particle transport, including impurity transport and density peaking.
a predictive understanding of these observations, which would quantify the power required
to achieve specific values, requires new research.
Specific Challenges:
Precise control of the total power output requires determination of the currently existing density
and temperature profiles in a burning environment, and actuators for then reaching the desired
operating point. actuators are also required for maintaining passively unstable operation; even for
passively stable operation, externally mandated variation in power output may require transition
through an unstable region. Removal of alpha-particle ash requires new diagnostics for measuring
its density profile and a mechanism for selectively removing the ash, ideally by increasing crossfield
transport of non-hydrogenic ions without degrading overall confinement. Present ideas for
achieving this, using various plasma instabilities, are not much beyond conceptual stages.
Research Plan:
Short-term: Develop understanding of fuel deposition and particle transport in current experiments
sufficient for predictive control of the profile and isotope mix through fuel injection. Develop advanced
fueling techniques, such as pellets or Compact Toroid injection capable of deep fueling in reactor-grade
plasmas. Develop control-level models based on simulations for identifying and controlling the operating
point through profile control and fueling, and develop diagnostics and actuators for controlling alpha
ash.
Medium-term: Test and further optimize in a long pulse D-D device the control models for maintaining
and varying passively stable operating points, and test diagnostics for measuring the alpha particle ash
profile and novel ash removal techniques.
Long-term: Test integrated system and control models for maintaining and varying passively stable and
unstable operating points, ash diagnostics and removal techniques, and deep fueling in ITER. Extend to
higher alpha heating, and high fluence D-T environments.
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