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(EX4/5) Confinement in the RFP: Lundquist Number Scaling, Plasma Flow, and Reduced Transport

   
G. Fiksel ¹,A. F. Almagri ¹, J. K. Anderson ¹, T. M. Biewer ¹, D. L. Brower ², C-S. Chiang ¹, B. E. Chapman ¹, J. T. Chapman ¹, D. J. Craig ¹, N. A. Crocker ¹, D. J. Den Hartog ¹, P. W. Fontana ¹, C. B. Forest ¹, Y. Jiang ², A. K. Hansen ¹, D. Holly ¹, N. E. Lanier ¹, K. A. Mirus ¹, S. C. Prager ¹, J. S. Sarff ¹, U. Shah ³, J. C. Sprott ¹, M. R. Stoneking ⁴, and E. Uchimoto ^5
 
1 Department of Physics, University of Wisconsin-Madison, WI, USA
² Department of Electrical Engineering, University of California - Los Angeles, CA, USA
³ Department of Electrical Engineering, Rennselear Polytechnic Institute, Troy, NY, USA
⁴ Department of Physics, Lawrence University, Appleton, WI, USA
⁵ Department of Physics and Astronomy, University of Montana, Missoula, MT, USA

Abstract
Global heat and particle transport in the reversed field pinch  (RFP) result primarily from large-scale, resistive MHD fluctuations which cause the magnetic field in the core of the plasma to become stochastic. Achieving a better understanding of this turbulent transport and identifying ways to reduce it are critical RFP development issues. We report measurements of the Lundquist number scaling ( S-scaling) of magnetic and ion flow velocity fluctuations in the Madison Symmetric Torus (MST ) RFP. The S-scaling of magnetic fluctuations in MST is weaker than previous measurements $\tilde{b}/B \sim S^{-1/2}$ in smaller (lower S) RFP plasmas. Impurity ion flow velocity fluctuations (measured with fast Doppler spectroscopy) have a scaling similar to the magnetic fluctuations, falling in the range $\tilde{V}/V_A \sim S^{-[0.08-0.19]}$. The MHD dynamo $\langle \tilde{V} \times \tilde{b}\rangle$ up to 15 V/m was measured in the plasma core. Interestingly, the scaling of the MHD dynamo $\langle \tilde{V} \times \tilde{b}\rangle \sim S^{-[0.64-0.88]}$ is stronger than for its constituents, a result of decreased coherency between $\tilde{V}$ and $\tilde{b}$ with increasing S. A weak S-scaling of magnetic fluctuations implies fluctuation suppression measures (e.g. current profile control) may be required in higher-S RFP plasmas. Two types of current profile modifications have been examined - inductive and electrostatic. The inductive control halves the amplitude of global magnetic fluctuations and improves the confinement by a factor of 5. The electrostatic current injection, localized in the edge plasma, reduces edge resonant fluctuations and improves the energy confinement . In addition, regimes with confinement improvement associated with the plasma flow profile are attained.

 

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