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

Figure 2. Outer surface of a stellarator plasma surrounded by the nonplanar magnet coils that produce the
plasma configuration within (courtesy of P. garabedian 1 ).
although more complex, many stellarator systems have been successfully built and operated.
Plasma parameters achieved to date are second only to tokamaks. large stellarators are pursued
overseas: lhd in Japan (operating) and W7-X in Germany (2014). The 3-d nature of stellarators
can lead to unacceptably large thermal transport in reactor-relevant regimes and poor confinement
of the energetic fusion products. as a result, stellarators must, and can, be optimized
through 3-d shaping to eliminate these losses. The W7-X design was optimized by aligning particle
drift orbits with magnetic surfaces and having minimal plasma currents. The Us program is
focused on optimization through quasi-symmetry (symmetry in the magnitude of the magnetic
field in a particular direction). Quasi-symmetry ensures the same good particle orbits that true
symmetry provides in the tokamak. The Us is a world leader in this concept, and its viability has
been demonstrated in hot electron plasmas in the university-scale helically symmetric experiment
(hsX). Quasi-symmetry allows both lower flow damping (important for reducing turbulent
transport) and smaller plasma aspect ratio than in W7-X, and thus provides a potentially improved
solution for steady-state, stable, thermal plasma containment, with good confinement of
energetic fusion products.
Quasi-symmetry in a torus can be achieved in either the toroidal direction (long way around the
torus; known as quasi-axisymmetry or Qa), the poloidal direction (short way around the torus;
known as quasi-poloidal symmetry or QP), or in a helical direction (known as quasi-helical symmetry
or Qh). all of these quasi-symmetries are predicted to ensure well confined particle orbits.
significant differences among these configurations do, however, exist. as one example, Qa has
a significant toroidal current driven by the plasma pressure gradient (bootstrap current) in contrast
to similar plasmas in Qh or QP configurations. other differences include global and local
magnetic shear, field curvature, location and fraction of trapped particles, magnetic well depth,
and plasma viscosity. a proper selection of type of quasi-symmetry and magnetic field optimization
may provide passive control of not only collisional transport and energetic particle confine-
1 P. Garabedian, Proc. Natl. Acad. Sci. USA 104, 12250 (2007).
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