Reduced Neoclassical Particle and Heat Transport with Quasihelical Symmetry in HSX
Author: John M. Canik
Requested Type: Consider for Invited
Submitted: 2006-12-20 13:51:20
Co-authors: D.T. Anderson, F.S.B. Anderson, K.M. Likin, J.N. Talmadge, K. Zhai
Contact Info:
HSX Plasma Laboratory, University of Wisconsin - M
1415 Engineering Dr.
Madison, WI 53706
US
Abstract Text:
The Helically Symmetric Experiment (HSX) has a helical direction of symmetry in the magnetic field strength. As a result of this symmetry, the neoclassical transport is predicted to be reduced to the level of an axisymmetric device. Experimentally, the electron collisionality is in the long mean free path regime so that neoclassical electron transport can be studied as a function of the magnetic field spectrum. Here we report experimental measurements of differences in electron density and temperature profiles between the quasihelically symmetric configuration (QHS) and configurations with the symmetry broken. Also reported are measurements of the particle source rate and absorbed power, which confirm that the profile differences are due to the changed transport properties in plasmas with and without quasisymmetry. For the same launched power, the central electron temperature in the QHS configuration is significantly higher than that in the configuration without symmetry (~1050 vs. ~750 eV). Transport analysis has been performed on discharges in which the power has been adjusted to match the temperature profiles in the two configurations. The same absorbed power profile is used for both configurations, and the total absorbed power is measured using the Thomson scattering system. The resulting electron thermal diffusivity in the core increases from ~2 m2/s for QHS up to ~4 m2/s as the symmetry is broken; this difference is comparable to the difference in neoclassical transport between the configurations. In QHS, the density profile is centrally peaked. Variation of the temperature gradient suggests the flattening of the density profile seen in the non-symmetric configuration with more central heating is connected to the temperature gradient. The experimental particle flux has been inferred using a set of absolutely calibrated Hα detectors coupled with 3D neutral gas modeling. In the core of the plasma the neoclassical particle flux in the non-symmetric configuration is comparable to the experimental flux. This neoclassical flux is dominated by particle flux driven by the temperature gradient, suggesting that the flattening of the density profile in the nonsymmetric configuration is caused by neoclassical thermodiffusion. In QHS, the neoclassical particle flux (including thermodiffusion) is reduced, leading to a peaked density profile. This work is supported by DOE Grant DE-FG02-93ER54222.
Characterization: E1
Comments:
Please group our two abstracts together:
[1] John Canik
[2] Walter Guttenfelder
