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.

A stellarator with a helically symmetric field inherently possesses solutions to many problems currently being examined in plasma confinement 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 described as 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: HSX, with an aspect ratio of ~8, has a toroidal curvature equivalent to a conventional tokamak device with an aspect ratio of 400.