(TH/P2-2) Turbulent Transport in Spherical Tokamaks with Transport Barriers

J. W. Connor1), C. M. Roach1), R. J. Hastie1), P. Helander1), T. J. Martin1), D. J. Applegate2), N. Joiner3), M. Reshko4), S. Saarelma1), W. D. Dorland5), S. C. Cowley2)6), S. Newton7), A. R. Field, MAST Team1)
1) Euratom/UKAEA Fusion Association, Culham Science Centre, UK
2) Department of Physics, Imperial College, London, UK
3) Plasma Physics Laboratory, University of Saskatchewan, Canada
4) Department of Physics, University of York, UK
5) Department of Physics, University of Maryland, USA
6) Department of Physics and Astronomy, UCLA, USA
7) H. H. Wills Physics Laboratory, University of Bristol, UK

Abstract.  Spherical tokamaks (STs) such as MAST, particularly at high beta, provide an important test of theoretical models for turbulent transport, which can then be used in a predictive mode for tokamaks in general. A combination of computational studies using the turbulence simulation code GS2 and analytic models has been used to understand both the `numerical experiments’ and the real experimental results. Linear stability studies of L and H mode MAST discharges exhibit a range of dominant instabilities, depending on the wave-number: ion and electron temperature gradient (ITG and ETG, respectively) and trapped electron (TEM) modes, with electromagnetic effects being important even at modest beta values and giving rise to kinetic ballooning modes and micro-tearing modes. However the correspondence between these results for micro-tearing modes and large aspect ratio analytic theories is limited, pointing to the importance of geometry, as treated fully in GS2. While rotation shear in MAST is expected to stabilise longer wavelength modes, non-linear GS2 calculations of the electron thermal diffusivity from ETG modes yield values typical of MAST, ∼5m2s-1. Analytic studies of the collisionality dependence of the TEM reveal a broad, stable spectrum at low collisionality. The value of the relevant critical collisionality parameter is consistent with the appearance of electron ITBs in MAST. The stabilising effect of rotation shear, characteristic of ITBs, has been demonstrated for a generic class of drift wave models. Finally, while this rotation can be generated initially by the Reynolds’ stress from turbulence, we find off-diagonal elements of neoclassical toroidal momentum transport in the presence of impurities can sustain it by transmission from edge to core.
* This work was jointly funded by the United Kingdom Engineering and Physical Sciences Research Council and by EURATOM.

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