Global gyrokinetic simulations of ion temperature gradient (ITG) driven turbulence in an ideal MHD ITER equilibrium plasma are performed with the ORB5 code. The noise control and field-aligned Fourier filtering procedures implemented in ORB5 are essential in obtaining numerically healthy results with a reasonable amount of computational effort: typical simulations require 109 grid points, 109 particles and, despite a particle per cell ratio of unity, achieve a signal to noise ratio larger than 50. As compared with a circular concentric configuration with otherwise similar parameters (same ρ* = 1/720), the effective heat diffusivity is considerably reduced for the ITER MHD equilibrium. A self-organized radial structure appears, with long-lived zonal flows (ZF), modulating turbulence heat transport and resulting in a corrugated temperature gradient profile. The ratio of long-lived ZF to the fluctuating ZF is markedly higher for the ITER MHD equilibrium as compared with circular configurations, thereby producing a more effective ITG turbulence suppression, in spite of a higher linear growth rate. As a result, the nonlinear critical temperature gradient, R/LTcrit,NL, is about twice the linear critical temperature gradient, R/LTcrit,lin. Moreover, the heat transport stiffness above the nonlinear threshold is considerably reduced as compared with circular cases. Plasma elongation is probably one of the essential causes of this behaviour: indeed, undamped ZF residual levels and geodesic acoustic mode damping are both increasing with elongation. Other possible causes of the difference, such as magnetic shear profile effects, are also investigated.
全球回旋动力学模拟使用ORB5代码，在理想MHD ITER平衡等离子体中执行离子温度梯度（ITG）驱动湍流。噪声控制和场对齐傅里叶滤波程序对于以合理的计算工作量获得数值上健康的结果至关重要：典型模拟需要10^9个网格点、10^9个粒子，并且尽管每个粒子比率为1，信噪比仍能达到50以上。与具有相似参数（相同ρ* = 1/720）的圆形同心位形相比，ITER MHD平衡的有效热扩散系数显著降低。出现自组织径向结构，伴随长寿命带状流（ZF），调制湍流热输运，导致温度梯度分布呈波纹状。与圆形位形相比，ITER MHD平衡中长寿命ZF与波动ZF的比率显著更高，从而产生更有效的ITG湍流抑制，尽管线性增长率更高。因此，非线性临界温度梯度R/LTcrit,NL约为线性临界温度梯度R/LTcrit,lin的两倍。此外，非线性阈值以上的热输运刚度与圆形等离子体相比显著降低。等离子体拉长率可能是这种行为的主要原因之一：实际上，无阻尼ZF残余水平和测地声模阻尼均随拉长率增加而增大。还研究了其他可能的原因，如磁剪切轮廓效应。