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

Proposed actions:
1. conduct two quasi-symmetric experiments spanning a broad range of internal plasma
current, with plasma parameters sufficient to demonstrate low collisionality, disruptionfree
operation at high plasma pressure. examine the merits of completing ncsX as part
of this effort. expand efforts in non-axisymmetric theory and computation to develop
predictive models of Qs confinement. extend the understanding of Qs plasmas to nearburning
conditions.
2. investigate quasi-symmetric configurations with simpler and maintainable magnet
systems.
3. design 3-d divertors compatible with Qs geometry. integrate with 3-d coil simplification.
4. explore the addition of 3-d shaping to other magnetic configurations.
Scientific and technical Research
a fusion power system that operates continuously without frequent destructive off-normal events
is a desirable goal. stellarator devices already demonstrate sustained plasma performance without
abrupt terminations or transients 1 . They also achieve good confinement of high-density plasmas,
typically at temperatures lower than in tokamaks, but not far below that required for fusion
applications.
The geometry of the stellarator is an example of 3-d magnetic shaping. The magnetic fields confining
the plasma do not possess symmetry in the toroidal direction, as nominally exhibited in
tokamaks and most other fusion plasma devices. The 3-d approach produces the necessary helical
magnetic field with magnet coils located outside the plasma. an internal toroidal plasma current
is not required, thus avoiding the need for auxiliary current-drive systems and rapid control
of the current and pressure profiles to maintain stability. in addition to providing rotational
transform (parameter measuring the helicity of the confining field), 3-d shaping also provides
control over global and local shear and curvature of the field, the magnetic well depth, the location
and fraction of trapped particles, shear of the radial electric field, and plasma viscosity. in
short, 3-d shaping may provide control of not only neoclassical transport and energetic particle
confinement, but also the geometrical parameters that affect turbulent transport and global stability.
however, the conventional stellarator’s lack of symmetry can lead to inadequate confinement
of hot fusion plasmas, and of the energetic charged fusion products in particular. Remarkably,
3-d magnetic configurations can be designed to be quasi-symmetric to reduce such losses.
even though the magnetic field of the Qs stellarator plasma remains three-dimensional, the confinement
of plasma particles within it may be similar to that of an axisymmetric device such as a
tokamak. a Qs stellarator fusion system would thus operate steady state, without threat of disruption,
with comparatively few control needs — and with adequate confinement of energetic fusion
products.
1 Presently, stellarators do not exhibit disruptions in normal operation. In contrast to tokamaks,
pressure limiting behavior is not found to terminate the discharge. Nonetheless, stellarator plasmas
can be abnormally terminated by radiative decay of the plasma energy triggered by excessive density
or material falling into the discharge from the wall. In both of these cases, the discharge decays on the
relatively long scale of an energy confinement time.
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