Turbulence spreading as a non-local mechanism of global confinement degradation and ion temperature profile stiffness

A new non-local mechanism of the global confinement degradation and ion temperature profile stiffness is proposed based on the results of global gyrokinetic simulations. We find that turbulence spreading into a marginally stable zone can increase turbulent transport to a level exceeding the predictions of the local theories. Also, we present the first quantification of the parametric dependence of turbulence spreading and resulting confinement degradation on toroidal rotation shear and magnetic shear: turbulence spreading is significant for high magnetic shears s > 0.2, while it is slowed for low magnetic shears. The suppression of turbulence spreading by toroidal rotation shear is only effective for the low magnetic shears, which is in a good agreement with the experimental trends of core confinement improvement. Our findings suggest that the non-local mechanism is indispensable for an accurate transport modeling in tokamak plasmas. PACS numbers: 52.35.Mw, 52.35.Ra,52.55.Fa,52.65.Tt Submitted to: Nucl. Fusion Turbulence spreading as a non-local mechanism of global confinement degradation... 2 In tokamak experiments with intensive ion heating, it is commonly observed that ion heat transport becomes turbulent and increases very sharply as ion temperature gradient exceeds the threshold for ion temperature gradient(ITG) instabilities[1, 2, 3]. This so-called profile stiffness phenomenon limits the achievable maximum central ion temperature Ti(0). Therefore, understanding the physical mechanisms of turbulent ion heat transport and Ti profile stiffness is necessary for the enhancement of tokamak performance and the achievement of controlled fusion. The level of turbulent ion heat transport depends on various experimental parameters. The well-known key parameters are magnetic shear and toroidal rotation shear. Both weak or negative magnetic shear and high toroidal rotation shear facilitate the improvement of core ion thermal confinement[3, 4, 5]. Recent experiments on the JET tokamak clarified that strong turbulent transport appears in the low rotation plasmas, while it is significantly reduced by combined high rotation shear and low magnetic shear[5]. The dependence of the core confinement on these two key parameters is interpreted primarily based on local properties of micro-turbulence such as E × B decorrelation of local turbulence. However, the local models are insufficient to explain the parametric dependency of the confinement. In particular, results of the local models often underestimate the level of turbulent transport as compared to the experimental results[6, 7, 8, 9]. The underestimation by the local models implies the existence of other mechanisms which increase ion heat transport. In searching the missing transport mechanisms for the confinement degradation and resulting Ti profile stiffness against external heating, fluctuation measurements during the evolution of internal transport barrier (ITB), which is a prime example of core confinement improvement, provide an important clue. For instance, before the formation of ITB, the radial correlation length of the fluctuation was estimated to be much longer than the typical predictions of the local theories[10]. As the level of turbulent transport decreased with the development of an ITB, the correlation length decreased to the level expected from the local theories [10, 11]. These experimental observations suggest that the core confinement can be improved by the inhibition of turbulence penetration into the ITB region, and that non-local transport mechanisms degrade core confinement and increase the profile stiffness. In this letter, we propose a new physical picture to understand the non-local character of turbulent ion heat transport and Ti profile stiffness based on turbulence spreading[12, 13, 14, 15, 16, 17]. Turbulence spreading is a prime example of the non-local mechanisms for confinement degradation, since fluctuation energy can be directly transferred to distant regions by nonlinear spectral interactions during the spreading. So, fluctuation from a strongly driven region can raise turbulent transport in a marginally stable or weakly turbulent region, and thus degrades the global confinement. We explore the dependence of turbulence spreading on magnetic shear and toroidal rotation shear and its impact on ion heat transport. To this end, we perform a set of carefully designed numerical experiments in which turbulence is triggered by an identical Turbulence spreading as a non-local mechanism of global confinement degradation... 3 0.2 0.4 0.6 0.8 r/a 0 2 4 6 8 10 R0/LTi ηi