There are six major components of the HSX program
- Confirmation of Magnetic Geometry:
Fundamental to the successful operation of HSX is a determination of the quality of the
magnetic surface structure and an assessment of the degree to which the quasi-helically
symmetric configuration has been achieved.
- Control of Direct Orbit Losses:
We will investigate three aspects of direct losses in HSX.
- In addition to 1.0 T operation, we will operate a 28 GHz gyrotron at a
magnetic field of 0.5 T and create a high-energy trapped electron population using second
- Steady-state hollow density profiles are routinely observed in stellarators during ECH.
- Direct orbit losses can alter the ambipolarity condition in stellarators and contribute
to large radial electric fields and plasma flow.
Neoclassical Parallel Viscosity:
Two aspects of the parallel viscosity in HSX are unique:
- According to the nonlinear viscosity model of L-H transitions, HSX is the
only experiment in the world that should not exhibit a bifurcation in the radial electric
field at a poloidal Mach number, Mp, of one.
- Plasma rotation damping in stellarators is generally large and dominated by neoclassical
parallel viscosity because of the large variation in the magnetic field in all direction
on a flux surface. HSX has a near-axis of symmetry like a tokamak , hence rotation damping
should be small in this direction. Theoretical models also link large rotation speeds with
peaked density profiles.
Variation of Neoclassical Thermal Conductivity:
One of the major goals of HSX is to demonstrate whether the transport is improved in the
low-collisionality regime as compared to a conventional stellarator. These experiments
will be done with ECH at 1.0 Tesla operation to maximize the electron temperature and
minimize the collisionality. We will determine whether two aspects of the transport,
neoclassical electron thermal conductivity or anomalous thermal conductivity, benefit from
the QHS configuration.
Measurement of Reduced Helical Pfirsch-Schlüter Currents:
The Pfirsch-Schlüter current in HSX is smaller than in a comparable tokamak by a factor of
three because of the quasi-helical symmetry. This results in a higher equilibrium
beta and a relative rigidity of the magnetic field spectrum to finite beta effects. The
equilibrium current is helical, rather than axisymmetric as in tokamaks.
Fluctuation-Induced Transport in a Helical Geometry:
There is some evidence that curvature-driven modes are responsible for anomalous transport
because of the asymmetry in turbulence observed in conventional toroidal devices in the
bad curvature region (outboard side) compared to that in the good curvature region. At one
toroidal location, these good and bad curvature regions correspond to a conventional
tokamak, while at another toroidal angle the good curvature is on the outside and the bad
curvature is on the inside; an inverted tokamak curvature in effect.