Proceedings of ITC/ISHW2007

Evolutionary development of an ignited toroidal fusion reactor
M. Zarnstorff, J. H. Harrisa, D. T. Anderson, S. Knowltonc, J. F. Lyona
Princeton Plasma Physics Laboratory, USA
Oak Ridge National Laboratorya, USA
University of Wisconsinb, USAAuburn Universityc
e-mail: zarnstor@pppl.gov

The tokamak, with its axisymmetric field configuration and induced plasma current, has been
the primary vehicle for studying the physics of toroidal plasma confinement, and can be usedeffectively to explore burning fusion plasmas. The stellarator has traditionally been seen as acontrasting alternative to the tokamak, substituting external non-symmetric helical fields forthe plasma current to form nested magnetic surfaces.
However, as we move forward towards a reactor in which a high-pressure plasma must be
stably sustained for weeks or more, the tokamak configuration is evolving. The axiallysymmetric tokamak develops unstable, non-symmetric helical perturbations whosesuppression requires the imposition of multiple helical control fields and/or carefully
programmed local heating. Continuous, toroidally asymmetric sources of current drive andmomentum input are required to maintain the plasma in an enhanced confinement regime: thesteady-state power required for these can exceed that used initially to raise the plasma tofusion temperatures. All these maintenance schemes require active feedback control.
Recent advances in toroidal physics understanding and computation techniques allow theseessential sustainment functions to be carried out using a carefully shaped 3-D stellarator field
in place of the numerous active schemes. The rotational transform required to make magnetic
surfaces together with the shaping required to maintain plasma stability come directly fromthe structure of the DC magnetic field itself. Additional rotational transform required forhigh-beta confinement comes from the self-consistent bootstrap current. The sheared flows
required for improved confinement come from the ambipolar electric field, which arises
naturally in suitably tailored 3-D configurations. The losses of heat and fusion alphas arisingfrom particle orbits in the 3-D field can be reduced to low values by optimization thatminimizes the departure from symmetry in a chosen direction. The ARIES collaboration has
developed a 1 GWe compact stellarator fusion reactor embodiment based on these concepts(ARIES-CS). The design fits within the size envelope of typical tokamak reactor designs,with R/a = 4.5, and yields a comparable cost-of-electricity.
Key questions about the physics features and engineering challenges of these compact
stellarators require experimental assessment. Therefore, the US stellarator program hasembarked on a series of experiments involving university-scale and national laboratorydevices. These include three quasi-symmetric configurations, the Helically Symmetric
Experiment (HSX, Univ. Wisconsin, operating), the National Compact Stellarator Experiment(NCSX , PPPL, under construction) and the Quasi-Poloidal System (ORNL, under design).
These experiments explore the physics properties of quasi-symmetric configurations. An
additional university device, the Compact Toroidal Hybrid (Auburn Univ., operating) is
being used for studies of the diagnosis, stability, and control of current carrying stellarator
plasmas.