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Plasma Flows and Currents in the HSX Stellarator and Future Directions

Author: David T Anderson
Requested Type: Consider for Invited
Submitted: 2011-06-10 16:25:19

Co-authors: HSX Team

Contact Info:
Univ. of Wisconsin-Madison
1415 Engineering Dr
Madison, WI   53706
USA

Abstract Text:
The HSX stellarator at the University of Wisconsin has a quasi-helically symmetric magnetic field resulting in neoclassical transport algebraically equivalent to a tokamak. Low plasma flow damping is predicted in the symmetry direction. Recently, the radial electric field and intrinsic plasma flow parallel to the magnetic field have been determined from charge exchange recombination spectroscopy and compared to neoclassical values calculated using the PENTA code[1]. Intrinsic rotation velocities of up to 20 km/s in the symmetry direction have been measured. The measured radial electric field and flows agree with the neoclassically predicted values in the outer half of the plasma when a momentum conserving collision operator formulation is used in the calculations. Non-momentum conserving calculations (historically used to calculate flows in conventional stellarators), under-predict the parallel flows by an order of magnitude. This momentum conserving approach provides a tool which can be applied across toroidal configurations from pure tokamaks to rippled stellarators.
Traditional axisymmetric toroidal devices determine the MHD equilibrium properties by solving the 2D Grad-Shafranov equation constrained by measured plasma profiles and magnetic diagnostic signals. For stellarators, and tokamaks with asymmetric fields (such as RMP/ELM suppression coils), 3D equilibrium reconstruction is necessary. HSX has been using the V3FIT code[2] to determine the plasma Pfirsch-Schluter (PS) and bootstrap current profiles. Reconstructions confirm that the PS current is helical due to the lack of toroidal curvature and reduced by the high effective transform. Later in the discharge the plasma currents are dominated by the bootstrap current which rises on a timescale comparable to the length of the discharge. The profile is consistent with calculations using the momentum-conserving PENTA code and when the 3D inductive response[3] of the plasma column are included.
HSX has added a second 28 GHz gyrotron for plasma heating, effectively doubling the available heating power. With the first system with central resonance heating we have achieved Te(0) ~2.5 keV, attributed to an internal transport barrier, although over a narrow central region. The launching antenna on the new system is steerable offering the ability to add off-axis heating to try and increase the ITB radius. Power modulation experiments will measure thermal diffusivity by heat-pulse propagation for comparison to previously determined power balance measurements. Impurity transport and accumulations are critical topics for the advancement of stellarators. A laser blow-off impurity injection system has been implemented on HSX and impurity transport characteristics will be determined from spectroscopic measurements and modeling.
[1] D.A. Spong, Phys. Plasmas 12, (2005) 056114.
[2] J.D. Hanson, et al, Nucl. Fusion 49 (2009) 075031.
[3] P.I. Strand and W.A. Houlberg, Phys. Plasmas 8 (2001) 2782.

Characterization: A5

Comments:

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011