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

Thrust 17: Optimize steady-state, disruption-free toroidal
confinement using 3-D magnetic shaping, and emphasizing quasisymmetry
principles
The stellarator concept relies on currents in external coils to confine plasmas magnetically. stellarators
can therefore operate continuously if supplied with heating power and effective means of
heat and particle exhaust (divertor). They have experimentally demonstrated sustained plasmas
with good confinement, high normalized pressure (b), and do not suffer from virulent current or
pressure-driven instabilities that abruptly terminate the plasma.
The magnetic field in a stellarator is not symmetric in the toroidal direction, but has three-dimensional
(3-d) structure. Three-dimensional shaping provides additional control of plasma confinement
not available in the axisymmetric tokamak, and permits designs that are passively stable to
major instabilities, with minimal feedback control. The lack of symmetry in a conventional stellarator,
however, can reduce the confinement of energetic ions, including alpha particles. The application
of innovative quasi-symmetric (QS) shaping is predicted to resolve this issue, leading
to stellarator designs that confine high-energy particles, and permit plasma flows (also favorable
for plasma confinement), while retaining the robust stability of the stellarator. The Qs stellarator
is thus a transformational concept, offering a timely, effective solution to the challenges of severe
transient events and control in steady-state, high-pressure plasmas. in addition, 3-d fields affect
all configurations through self-organization or external perturbations. Understanding of 3-d effects
is thus a core competence, required for the success of all magnetic configurations.
There is a need to understand 3-d shaping in an integrated manner in plasmas with higher levels
of performance.
Key issues:
• simultaneous achievement of Qs confinement at high-pressure without disruptions. Does
quasi-symmetry lead to improved confinement at high plasma pressure? Is there an optimal
type of QS shaping?
• Three-dimensional shaping requires magnets that are more complex than planar coils.
How can optimized magnet design ease construction, reliability, cost and maintenance of 3-D
systems while meeting the physics requirements?
• divertors for 3-d configurations require development. How do we integrate an effective
divertor into the optimization of quasi-symmetric stellarators?
• Three-dimensional fields have application to all toroidal systems. What is the optimum
amount and type of 3-D shaping to effect improvements?
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