Physics Goals

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.

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 harmonic heating.
• 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.

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, M_p, 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.

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.

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.

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.