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Title: Role of a continuous MHD dynamo in the formation of 3D equilibria in fusion plasmas

Stationary 3D equilibria can form in fusion plasmas via saturation of magnetohydrodynamic (MHD) instabilities or stimulated by external 3D fields. In these cases the current profile is anomalously broad due to magnetic flux pumping produced by the MHD modes. Flux pumping plays an important role in hybrid tokamak plasmas, maintaining the minimum safety factor above unity and thus removing sawteeth. It also enables steady-state hybrid operation, by redistributing non-inductive current driven near the center by electron cyclotron waves. A validated flux pumping model is not yet available, but it would be necessary to extrapolate hybrid operation to future devices. In this work flux pumping physics is investigated for helical core equilibria stimulated by external 3D fields in DIII-D hybrid plasmas. We show that flux pumping can be produced in a continuous way by an MHD dynamo emf. The same effect maintains helical equilibria in reversed-field pinch (RFP) plasmas. The effective MHD dynamo loop voltage is calculated for experimental 3D equilibrium reconstructions, by balancing Ohm’s law over helical flux surfaces, and is consistent with the expected current redistribution. Similar results are also obtained with more sophisticated nonlinear MHD simulations. The same modelling approach is applied to helical RFP states forming spontaneouslymore » in RFX-mod as the plasma current is raised above 0.8–1 MA. This comparison allows to identify the underlying physics common to tokamak and RFP: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical MHD dynamo flow.« less
Authors:
 [1] ;  [1] ;  [2] ;  [3] ;  [4] ;  [1] ;  [5] ;  [2] ;  [2] ;  [1] ;  [3] ;  [6] ;  [7] ;  [1] ;  [3] ;  [8] ;  [1] ;  [1]
  1. Univ. of Padova (Italy)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  3. General Atomics, San Diego, CA (United States)
  4. Oak Ridge Associated Univ., Oak Ridge, TN (United States)
  5. Columbia Univ., New York, NY (United States)
  6. Aix-Marseille Univ., and CNRS/IN2P3, Marseille (France)
  7. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  8. Culham Science Centre, Abingdon (United Kingdom). Culham Centre for Fusion Energy (CCFE), EURATOM/UKAEA Fusion Association
Publication Date:
Grant/Contract Number:
AC05-00OR22725
Type:
Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 57; Journal Issue: 7; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Contributing Orgs:
The DIII-D; RFX-Mod Teams
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; 3D equilibrium reconstruction; MHD dynamo; hybrid tokamak scenario; helical RFP states
OSTI Identifier:
1376541