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

Edge localized modes (ELMs): edge localized modes are instabilities that generate explosive fluctuations
in the stored kinetic energy in the plasma edge region, manifesting as the rapid expulsion
of hot dense plasma, which produces impulsive heat loads on divertor plates and other plasma facing
components. elms can also affect plasma rotation and trigger other deleterious modes, including
RWms. With relatively low fields, high-current densities, and broad current profiles, st plasmas
could allow excitation of the underlying instabilities under a wider range of plasma conditions than
conventional tokamaks. as such, st plasmas may offer a unique opportunity to rigorously test theory
with comparisons to detailed experimental data. to produce results that are applicable to conditions
expected in st-ctF discharges, these studies will need to address the role of plasma collisionality,
with plasma flows and equilibrium profiles consistent with global mode stability.
neoclassical tearing modes (ntMs): see later section addressing ntm physics and control.
Control considerations and innovations: significant reduction of disruption probability requires
control innovations that need to be tested in the iteR era. control of instability amplitude,
plasma pressure, plasma rotation, and magnetic field pitch angle (q) profile should be used
to eliminate disruptions with high reliability. Feedback control of multiple instabilities possible
in high-beta st plasmas needs to be addressed using upgraded 3-d control fields and advanced
control algorithms. non-magnetic instability sensors should be tested. The addition of moderate
3-d shaping from applied 3-d control fields should be studied to minimize disruption occurrence.
Real-time assessment of plasma stability based on measured stable RWm behavior and
real-time theoretical stability computation should be considered. transient reduction in plasma
energy transport that increases plasma pressure and creates global instability needs to be controlled
by heating or plasma energy confinement control. Plasma rotation needs to be controlled
to avoid profiles leading to unstable RWms, or rotation reduction caused by stable RWms, saturated
ntms, elms, and error fields. Plasma pressure control is possible via localized heating, fueling,
and transport changes, and q control is possible with localized current drive. control of
plasma rotation and rotation shear is possible by applied 3-d magnetic fields, but depends on
plasma collisionality. This theoretical dependence needs to be experimentally verified at reduced
collisionality approaching st-ctF levels. control systems considered should be compatible with
the continuous operation and high neutron fluence anticipated for st-ctF.
disRUPtions
disruptions terminate plasma operation and must be avoided. Present st experiments have not
achieved the integrated plasma performance conditions needed for an st-based ctF or reactor,
and these conditions could strongly influence the nature and frequency of disruptions. disruption
avoidance for durations progressively up to about five to six orders of magnitude longer than
present st plasma pulse durations and progressively up to about three orders of magnitude longer
than present or planned long-pulse devices (including iteR) will be required.
research requirements
to sustain stable operation near (for baseline st-ctF operation) and above (for improved st-ctF
fusion performance and demo) stability limits for non-rotating plasmas, ctF-relevant plasma
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