Experimental Demonstration of Improved Neoclassical Transport with Quasihelical Symmetry

Abstract

Differences in the electron particle and thermal transport are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The thermal diffusivity is reduced in the QHS configuration, resulting in higher electron temperatures than in the nonsymmetric configuration for a fixed power input. The density profile in QHS plasmas is centrally peaked, and in the nonsymmetric configuration the core density profile is hollow. The hollow profile is due to neoclassical thermodiffusion, which is reduced in the QHS configuration.

Benchmarking of the mono-energetic transport coefficients—results from the International Collaboration on Neoclassical Transport in Stellarators (ICNTS)

“…The visible departure from quasi-helical symmetry is instead largely due to the coil ripple produced by the twelve modular coils in each field period. In experiments, the improvement in confinement for thermal and fast particles predicted for quasi-helical symmetry has been demonstrated in HSX by also considering discharges in which the magnetic configuration has been altered to purposely spoil the quasi-symmetry [62,63].…”

Intrinsic Ambipolarity and Rotation in Stellarators

“…In practice it is not possible to create a perfectly quasisymmetric magnetic field, but it is possible to come close. Currently the most quasisymmetric stellarator is the HSX experiment [11], where the "effective helical ripple" is less than 1% over most of the plasma volume.…”

A stellarator reactor based on the optimization criteria of Wendelstein 7-X

“…Consequently, basic reactor properties such as good magnetic surfaces, favourable equilibrium properties and magnetohydrodynamic stability at sufficiently high normalized plasma pressure, β, and satisfactory thermal and fast particle confinement should be achievable. Besides Wendelstein 7-AS, which was partially optimized with respect to reduced equilibrium currents [3], recent experimental results confirm the effectiveness of quasisymmetry regarding improved neoclassical confinement [4].…”