|icc_2006_dta.pdf||2006-03-22 13:43:04||David Anderson|
Transport Improvements in Stellarators with Quasisymmetry
Author: David T. Anderson
Submitted: 2005-12-20 14:33:31
Co-authors: For the HSX Team
Univ. of Wisconsin-Madison
1415 Engineering Dr.
Madison, WI 53706
Advanced stellarators conceptually offer advantages over the tokamak, and also the possibility of improvements to the tokamak concept. Optimized stellarators such as W7-X, NCSX and QPS are under construction or design to address these issues. In the U.S., CTH at Auburn is presently investigating current-driven instabilities and 3-D equilibrium reconstruction. NCSX is an integrated test facility at higher beta, and QPS examines low aspect ratio and high poloidal flow. HSX is an element of this national stellarator PoP Program, and also the ICC Program. It is the first of these optimized stellarators to become operational and is producing results showing transport improvement through quasisymmetry over conventional stellarators. Specifically, good confinement of energetic deeply-trapped particles has been demonstrated, reduced parallel viscous damping of flows, reduced thermodiffusion of particles, and reduced neoclassical electron thermal conductivity.
For the case of quasisymmetry, a reduction of the direct-loss of deeply-trapped energetic particles has been measured directly with a set of collection plates external to the plasma. Hard x-ray fluxes produced by energetic electrons (>100 keV) are a factor of 3 larger and have a factor of 3 longer confinement time with symmetry.
Detailed measurements of flow evolution have demonstrated quasisymmetry leads to reduced parallel viscous damping. The flow in the QHS configuration rises and damps more slowly than with broken symmetry and attains approximately twice the flow velocity for the same drive.
Density profiles in stellarators during ECH are typically flat or hollow. In Wendelstein VII-AS, it was shown this can be explained by a thermodiffusive particle flux. The density profile in HSX is peaked for the case of quasisymmetry, but becomes flat to hollow with broken symmetry. Analysis indicates that for broken symmetry the thermodiffusive term is dominant in the plasma core and accounts for the flatness in the profile, but is too small to hollow out the profile for quasisymmetry.
Measurements of the absorbed power and plasma profiles have been made using the Thomson scattering system. The central temperature in symmetric and non-symmetric operation is 450 eV and 250 eV, respectively, for the same absorbed power; the resulting thermal diffusivities are ~1 m2/s and ~3 m2/s. The results indicate that the reduced neoclassical thermal conductivity for the quasisymmetric configuration has a clear effect on increasing the core temperature profile. MHD fluctuations consistent with an Alfvenic mode have been observed in the presence of energetic electrons, but disappear with broken symmetry. Edge fluctuation data, however, show little difference in the turbulence between symmetric and non-symmetric operation.
Success in neoclassical transport improvement in HSX bolsters confidence in the innovation, science, and concept improvements possible within the US Compact Stellarator Program.