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

在仿星器有限β无电流平衡态上，托卡马克的负磁切变构型出现零磁切变面，磁切变面从零磁切变面增加。当等离子体在这些结构中产生速度剪切流时，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性质，发现磁剪切和速度剪切之间存在协同效应。所得结果总结如下。(1)即使没有速度剪切，适当的磁剪切也可以抑制不稳定性。例如，负磁剪切适用于抑制离子温度梯度驱动的托卡马克漂移模式。(2)负磁切变并不总是优于正磁切变。例如，电阻交换模式在负剪切托卡马克中变得不稳定。(3)对于抑制阻性交换模式，极向流剪切是有效的。结果表明，无流动时径向模态宽度(d)小于速度剪切特征长度(L)，旋转频率大于增长率乘以L/d。(4)当极向流的速度剪切超过临界值时，开尔文-亥姆霍兹模态变得不稳定。(5)开尔文-亥姆霍兹模式是由磁剪切稳定的，尽管大速度剪切是不稳定的。因此在负磁切变托卡马克中，电子温度梯度驱动的漂移波湍流只在有限磁切变区产生纬向流动。纬向流中的速度切变有助于形成内部输运屏障。

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.