The Helically Symmetric Perspective

  • The Tokamak possesses desirable physics attributes for a fusion reactor based on its axisymmetric nature:
    • Good single-particle confinement
    • Can be designed for low aspect ratio
    • Simple coil-set
  • The lack of symmetry in the magnetic fields in conventional stellarators results in large deviations of particle orbits from magnetic surfaces and direct loss orbits – enhanced low-collisionality transport and poor low-collisionality confinement
  • Stellarator with helically symmetric field inherently possesses solutions to many problems currently being examined in tokamak research and in conventional stellarator research:
    • Intrinsically steady-state devices
    • No observed density limit
    • No required plasma current for confinement
    • |B| is independent of a helical-like angle variable
    • Neoclassical transport is directly analogous to, and reduced, as compared to an equivalent axisymmetric system (Tokamak)
    • 1/ν regime suppressed
    • No superbanana orbits
    • No direct loss orbits
    • Good collisionless alpha particle confinement

HSX is the only device in the world that has a magnetic field structure that has been termed Quasi-Helically Symmetric (QHS). Quasi-helically symmetric stellarators, to a good approximation, possess a direction of symmetry and are therefore topologically equivalent to a tokamak without plasma current. They achieve this near-symmetry by having virtually no toroidal curvature that a conventional toroidal device would have if it had an aspect ratio greater than 400.