Confinement Improvement with Simultaneous Achievement of Magnetic Well and Stellarator Shear in Helical Plasma

在用NBI加热的CHS螺旋等离子体中发现了新的约束状态。核心区电子温度随着边缘输运势垒的增加而显著升高。边缘输运屏障通常在以Ha信号突然减弱为特征的NBI等离子体中观察到。另一方面，在低电子密度的等离子体中，通常观察到内部输运势垒随核心电子温度的升高。这种新放电的电子密度（3-4x10^<19>m^<-3>）比具有内部输运势垒的电子根等离子体的电子密度（3-4x10^<18>m^<-3>）高约一个数量级。在磁场强度为0.9特斯拉时，通过增加电子密度，等离子体β可以毫无困难地达到0.5%左右。这就造成了地核区域的磁井，尽管地核区域较浅。当施加强烈的气体膨胀时，即使在NBI等离子体中，电子密度分布也会变空。电子温度随时间的变化表现为先上升后下降，时间尺度为几毫秒。即使在有限β等离子体中，CHS也有仿星剪切。试图用磁井和仿星剪切同时实现的漂移反转来解释这一现象，但没有成功。然而，当电子的绝热近似不成立时，可以用中空密度分布下的交换稳定性来解释。

New confinement regime has been found in CHS helical plasma heated with NBI. It is featured with remarkable electron temperature increase in the core region along with the edge transport barrier. The edge transport barrier is usually observed in NBI plasmas characterized with the abrupt reduction of Ha signal. On the other hand, the internal transport barrier with the core electron temperature increase is usually observed in ECH plasmas with low electron density. The electron density (3-4x10^<19>m^<-3>) of this new discharge is higher by about one order than that (3-4x10^<18>m^<-3>) of the electron-root plasma featured with the internal transport barrier. According to the strong gas puff the electron density increases, resulting in an increase in the plasma p. At the magnetic field strength of 0.9 Tesla the plasma β reaches about 0.5% without any difficulty by increasing the electron density. This causes the magnetic well in the core region although it is shallow. When strong gas puff is applied, the electron density profile gets hollow even in the NBI plasma. The temporal evolution of the electron temperature shows a rapid increase followed by a rather rapid decrease with the time scale of a few milliseconds. CHS has a stellarator shear even in the finite β plasma. This phenomenon was tried to explain by the drift reversal that is realized under the simultaneous achievement of the magnetic well and the stellarator shear, but in vain. However, it could be explained with the idea of the interchange stability under the hollow density profile when the adiabatic approximation for electrons does not hold.