Fusion Science and Technology / Volume 50 / Number 2 / August 2006 / Pages 171-176
Technical Paper / Stellarators
Recent results are summarized for the Helically Symmetric Experiment (HSX), which has the capability of running as a quasi-helically symmetric stellarator or as a more conventional, nonsymmetric stellarator. From X-ray measurements, we have demonstrated improved confinement of energetic particles. With central electron cyclotron heating, the density profiles in the quasi-symmetric configuration are peaked, in contrast to the hollow or flat profiles when the symmetry is broken. The difference in profiles is attributed to the lowering of the neoclassical thermodiffusive flux when the symmetry is present. The central electron temperature is ~200 eV higher for the quasi-symmetric configuration over the nonsymmetric case. The power deposition profiles are similar for the two cases, implying that the neoclassical electron thermal conductivity is reduced with quasi-symmetry. Related to the good confinement characteristics in the quasi-symmetric mode of operation, fluctuations in the density and magnetic field, consistent with that of a global Alfvén eigenmode (GAE), are observed. While the neoclassical characteristics of the quasi-symmetric and nonsymmetric configurations are very different, we have yet to find, under present operating conditions, any significant difference (other than the possible GAE mode) in turbulence characteristics or blob formation at the plasma edge.