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Title: Impact of ideal MHD stability limits on high-beta hybrid operation

Here, the hybrid scenario is a candidate for stationary high-fusion gain tokamak operation in ITER and DEMO. To obtain such performance, the energy confinement and the normalized pressure $${{\beta}_{N}}$$ must be maximized, which requires operating near or above ideal MHD no-wall limits. New experimental findings show how these limits can affect hybrid operation. Even if hybrids are mainly limited by tearing modes, proximity to the no-wall limit leads to 3D field amplification that affects plasma profiles, e.g. rotation braking is observed in ASDEX Upgrade throughout the plasma and peaks in the core. As a result, even the small ASDEX Upgrade error fields are amplified and their effects become visible. To quantify such effects, ASDEX Upgrade measured the response to 3D fields applied by $$8\times 2$$ non-axisymmetric coils as $${{\beta}_{N}}$$ approaches the no-wall limit. The full n = 1 response profile and poloidal structure were measured by a suite of diagnostics and compared with linear MHD simulations, revealing a characteristic feature of hybrids: the n = 1 response is due to a global, marginally-stable n = 1 kink characterized by a large m = 1, n = 1 core harmonic due to q min being just above 1. A helical core distortion of a few cm forms and affects various core quantities, including plasma rotation, electron and ion temperature, and intrinsic W density. In similar experiments, DIII-D also measured the effect of this helical core on the internal current profile, providing information useful to understanding of the physics of magnetic flux pumping, i.e. anomalous current redistribution by MHD modes that keeps $${{q}_{\text{min}}}>1$$ . Thanks to flux pumping, a broad current profile is maintained in DIII-D even with large on-axis current drive, enabling fully non-inductive operation at high $${{\beta}_{N}}$$ up to 3.5–4.
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  1. Consorzio RFX, Padova (Italy)
  2. Max-Planck-Institut fur Plasmaphysik, Garching (Germany)
  3. Columbia Univ., New York, NY (United States)
  4. Univ. of York, York (United Kingdom)
  5. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  6. Culham Science Centre, Abingdon (United Kingdom)
  7. Oak Ridge Associated Univ., Oak Ridge, TN (United States)
  8. DIFFER, Nieuwegein (The Netherlands)
  9. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  10. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  11. General Atomics, San Diego, CA (United States)
Publication Date:
Grant/Contract Number:
FC02-04ER54698; AC05-00OR22725
Accepted Manuscript
Journal Name:
Plasma Physics and Controlled Fusion
Additional Journal Information:
Journal Volume: 59; Journal Issue: 1; Journal ID: ISSN 0741-3335
IOP Science
Research Org:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); General Atomics, San Diego, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Contributing Orgs:
The ASDEX Upgrade Team; The DIII-D Team; The EUROfusion MST1 Team
Country of Publication:
United States
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; hybrid scenario; MHD stability; 3D equilibrium
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1375933; OSTI ID: 1376540; OSTI ID: 1394226