Synergetic effects between magnetic shear and velocity shear for stability in tokamak and stellarator

在仿星器有限β无电流平衡下，托卡马克负磁剪切位形出现零磁剪切面，从该面开始磁剪切增大。当在这些配置中产生具有速度剪切的等离子体流时，在实验中经常看到约束改善。为了更好地理解约束改善的物理机制，我们发现磁剪切和速度剪切之间存在着协同效应。所得结果总结如下。(l)即使没有速度剪切，也有可能用适当的磁剪切来抑制不稳定性。例如，负磁剪切可以抑制托卡马克中离子温度梯度驱动的漂移模。(2)负磁剪切并不总是比正磁剪切好。例如，在负剪切托卡马克中，电阻交换模变得不稳定。(3)极向流剪切对抑制电阻交换模是有效的。结果表明，无流动时的径向模宽度d小于速度切变的特征长度L，旋转频率大于增长率与L/d的乘积。(4)当极向流的速度切变超过一个临界值时，Kelvin-Helmholtz模变得不稳定。(5)尽管大的速度切变会使Kelvin-Helmholtz模失稳，但重要的是磁场切变会使Kelvin-Helmholtz模稳定。因此，在负磁剪切托卡马克中，电子温度梯度驱动的漂移波湍流只在有限的磁剪切区产生带状流。纬向流中的速度切变有助于产生内部输运障碍。

For the negative magnetic shear configurations in tokamaks on finite beta currentless equilibria in stellarators, there appears zero magnetic shear surface and magnetic shear increases fromn this surface. When plasma flow with velocity shear is generated in these configurations, confinement improvement is often seen in experiments. For understanding physics of the confinement improvement, it is found that synergetic effects exist between the magnetic shear and the velocity shear. Obtained results are summarized in the following.(l) It is possible to suppress instabilities with an appropriate magnetic shear even without the velocity shear. For example negative magnetic shear is suitable to suppress the ion temperature gradient driven drift mode in tokamaks.(2) The negative magnetic shear is not always better than the positive one. For example resistive interchange modes become unstable in the negative shear tokamaks.(3) For suppressing the resistive interchange modes the poloidal flow shear is effective. It is found that radial mode width (d) without the flow is shorter than characteristic length of the velocity shear (L) and rotational frequency is larger than the growth rate multiplied by L/d.(4) It is noted that Kelvin-Helmholtz mode becomes unstable when the velocity shear of poloidal flow exceeds a critical value.(5) It is important that Kelvin-Helmholtz mode is stabilized by magnetic shear, although a large velocity shear is destabilizing. Thus in the negative magnetic shear tokamak, electron temperature gradient driven drift wave turbulence generates zonal flows only in the finite magnetic shear region. The velocity shear in the zonal flow contributes to generate the internal transport barrier.