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RFP Confinement with Varying Degrees of Relaxation
| Author: | Anderson J. K. |
| Coauthor: | J.K. Anderson, A.F. Almagri, T.M. Biewer, A.P. Blair, B.E. Chapman, D. Craig, D.J, Den Hartog, G. Fi |
| Institution : | University of Wisconsin |
| Abstract text: | To be fusion relevant, configurations which rely on magnetic relaxation for the sustainment of the equilibrium field must have access to a relaxation process which does not degrade confinement. In the Madison Symmetric Torus, there are two distinct discharge groups separated by their relaxation characteristics. In standard discharges the equilibrium field is maintained, in part, by turbulent relaxation. Closely spaced resonant surfaces for m=1 resistive tearing modes along with high fluctuation levels create a stochastic magnetic field within most of the plasma. The energy transport is measured to be in good agreement with the prediction of Rechester and Rosenbluth. The second category of discharge is generated by altering the inductive current drive to externally impose a nearly relaxed current density profile. In this case, the RFP configuration is maintained (transiently) by externally applied electric fields, and the dynamo and magnetic relaxation essentially vanish. Although magnetic fluctuation levels are dramatically reduced, the magnetic field remains stochastic over a finite portion of the plasma, presumably due to the close spacing between rational surfaces in the mid-radius region. Nonetheless, energy transport is significantly reduced and a pressure gradient exists even in the region of the stochastic magnetic field. The global energy confinement time increases ten-fold, making it comparable to that expected for a tokamak of similar size and current. |
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