April 15-17

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Author: Mike Kotschenreuther
Requested Type: Pre-Selected Invited
Submitted: 2019-02-22 14:43:54

Co-authors: X. Liu, D.R. Hatch, S. M. Mahajan

Contact Info:
University of Texas
2515 Speedway C1500
Austin, Texas   78712
USA

Abstract Text:
Although electrostatic modes (ITG and TEM) typically dominate core transport, we show there exists a particular physical regime in which these modes are hugely weakened, enabling transport barriers without velocity shear. The passage to this regime has apparently arisen in multiple experimental contexts: high beta poloidal ITB in DIII-D, ITB in JET in pellet injection scenarios, ITB observed in the stellarator LHD, wide pedestal H-mode pedestals (and other H-modes). In all these cases the process that leads to the great weakening of linear electrostatic micro-instabilities is fundamentally similar. Through gyrokinetic simulations in model geometries and actual geometries, and simplified models, we arrive at a clear understanding of the fusion friendly regimes made possible only by the simultaneous presence of 1) geometrical effects (appropriate shaping and/or Shafranov shift) 2) the ITG class of instabilities become slab-like, i.e, parallel velocity resonances dominate; geometry is crucial for this 3) substantial density gradients. The resulting eigenmode structure, which is sometimes highly unusual, can nonetheless be explained rather well by a simple semi-analytic dispersion relation, which shows how only the confluence of geometrical effects and plasma parameters results in near stabilization. Fortunately, also, finite beta effects frequently stabilize other electromagnetic instabilities (e.g. electron modes driven by passing electrons). Performing comprehensive electrostatic and electromagnetic gyrokinetic simulations (using GENE), we find that in plasmas with the characteristics mentioned above, linear modes are weak, and the nonlinear transport can be reduced by about two orders of magnitude compared to characteristic core-like modes for “typical” conditions. We will report our explorations, and indicate how various actuators, including novel ones, might create this regime in fusion relevant conditions, including ones with low velocity shear (as in ITER)

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