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TEM turbulence in stellarators - its simulation and its optimization

Author: Josefine H.E. Proll
Requested Type: Consider for Invited
Submitted: 2015-01-14 02:01:12

Co-authors: B.J. Faber, P. Helander, S.A. Lazerson, H.E. Mynick and P. Xanthopoulos

Contact Info:
Max-Planck/Princeton Center for Plasma Physics
Wendelsteinstr. 1
Greifswald, Mecklenbur   17491
Germany

Abstract Text:
With the advent of neoclassically optimized stellarators like W7-X, turbulent transport becomes relatively more important, and is expected to be dominant in some parts of the plasma. Linear
analytical theory[1] suggests trapped-electron modes (TEM) are stable in large parts of parameter space when all trapped particles experience bounce-averaged “good curvature”, which is
the case in perfectly quasi-isodynamic stellarators with the maximum-J property. It has been found numerically[2] that even configurations like W7-X, where a small but finite fraction
of the trapped orbits experience bad average curvature, benefit from enhanced TEM stability. Two important questions then arise: Does the enhanced linear stability result in reduced
transport? And can this knowledge be used to optimise stellarators not only for neoclassical, but also for turbulent transport? Both questions are addressed here: We present first-of-a-kind density-gradient-driven TEM turbulence simulations demonstrating that this enhanced stability does indeed result in reduced transport compared with a typical tokamak (ITG transport is however comparable). We also present an optimization method for configurations like the TEM-dominated stellarator HSX, where the growth rates of TEMs are significantly higher than in W7-X. A “proxy” function was designed to estimate the TEM growth rate, allowing optimal configurations for TEM stability to be determined with the STELLOPT code[3] without turbulence simulations. The optimized HSX equilibrium has reduced TEM growth rates over a broad
range of wave numbers and density gradients. This method complements the recent success of optimizing stellarators for ion-temperature gradient turbulence[4].
[1] P. Helander, J.H.E. Proll and G.G. Plunk, Phys. Plasmas 20 122505 (2013)
[2] J.H.E. Proll, P. Xanthopoulos and P. Helander, Phys. Plasmas 20 122506 (2013)
[3] D.A. Spong, et al. Nucl. Fusion 41 711 (2001)
[4] P. Xanthopoulos, et al., Phys. Rev. Lett. 113 155

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