New results from the Helically Symmetric eXperiment (HSX) at the University of Wisconsin-Madison, USA, published in a recent issue of Physical Review Letters (PRL 98, 085002 (2007)) show that due to the unique design of the HSX, this device in fact loses less energy and by that overcomes a major barrier in stellarator research. „These results demonstrate for the first time reduced particle and heat transport due to low neoclassical transport with quasisymmetry“, the authors (among them Professor David Anderson and research assistant John Canik) summarize their paper.

In a stellarator, currents flowing in external conductors produce the confining magnetic fields. This makes the stellarator an attractive candidate for a fusion reactor, as the lack of large externally driven plasma currents lends itself to steady state, disruption-free operation. However, conventional stellarators have suffered from high neoclassical transport due to asymmetry in the magnetic field, caused by the combination of toroidal and helical curvatures. Following theoretical research at the Max-Planck-Institut für Plasmaphysik, currently three lines of devices overcoming this deficiency are being developed and realized: quasi-helical symmetry (HSX), quasi- axisymmetry (NCSX at Princeton Plasma Physics Laboratories PPPL, under construction) and quasi-isodynamicity (W7-X at Max-Planck-Institut für Plasmaphysik (IPP) in Greifswald, under construction).

The HSX is the first operating stellarator to use a quasi-symmetric magnetic field. While the magnetic field strength is usually a two-dimensional function on the magnetic surfaces traced out by the field lines, quasi-symmetry is achieved by making it one-dimensional in so-called magnetic coordinates. For quasi-helical symmetry this means that the plasma particles experience the device as if it were helically symmetric instead of forming a torus. The HSX main magnetic field is generated by a set of 48 non-planar, modular coils, arranged in four field periods.

The team built the HSX with the motivation that quasisymmetry would reduce transport. And obviously that‘s exactly what it does: The reductions in neoclassical thermodiffusivity and heat conductivity cause the density profile to be centrally peaked and the electron thermal transport to be reduced. „This is the first demonstration that quasisymmetry works, and you can actually measure the reduction in transport that you get,“ Canik is cited on the HSX website.