Grassy ELM regime at low pedestal collisionality in high-power tokamak plasma
Grassy ELM regime at low pedestal collisionality in high-power tokamak plasma
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DOI：
10.1088/1741-4326/abc59b
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发表时间：
2020-12
期刊：
影响因子：
3.3
通讯作者：
Y.F. Wang;H.Q. Wang;G.S. Xu;G. Jia;F. Turco;C. Petty;J.L. Chen;N. Yan;Q.Q. Yang;
中科院分区：
文献类型：
--
作者：
Y.F. Wang;H.Q. Wang;G.S. Xu;G. Jia;F. Turco;C. Petty;J.L. Chen;N. Yan;Q.Q. Yang;
在DIII-D的高功率（P inj > 13 MW）混合情景中，在低基座碰撞度（νe，ped* φ 0.15）下，已经反复观察到巨型ELMs的自发缓解和草状ELMs的出现。较高的β p和较高的q95似乎有利于实现草状榆树制度。在高中性束流力矩和高堆芯旋转条件下，草状ELM H模等离子体显示出高的能量约束性能（H98 y ~ 2可达1.5以上）。基座宽度似乎超过EPED 1.0模型预测的50%以上。与ELITE代码进行的基座稳定性分析表明，对低n扭结/剥离模式的稳定性得到改善，增加辅助加热功率和操作点在草ELM制度位于附近的气球边界。基于高分辨率和高精度实验测量的等离子体分布，与巨型ELM碰撞相比，研究了长草ELM碰撞过程中的基座稳定性特性。已经发现，所观察到的小振幅ELM崩溃的潜在机制主要是膨胀的稳定性边界的扩展所引起的初始径向局部崩溃的基座，这有助于停止增长的不稳定性和进一步崩溃的基座。通过数值模拟分析了电子密度基座对边缘局域不稳定性的抑制作用，指出具有高ne、sep/ne、ped和低密度梯度的电子密度基座有助于稳定剥离-气球模，这主要是因为基座区的压力梯度较低，而气球边界较低，这主要是因为基座区的离子反磁频率较低，从而引发膨胀的不稳定性并产生内在的草状榆树。采用SOLPS程序对中国聚变工程试验堆（CFETR）进行了数值模拟，结果表明，分界面密度对基座区的电子扩散系数不敏感，随着功率从堆芯流向边缘区而增大。基座稳定性分析表明，在CFETR的高功率等离子体中获得的具有高分界线密度的平坦密度基座将使操作点接近气球边界，这被认为有助于使气球不稳定性不稳定，并有助于进入草状ELM状态。
Spontaneous mitigation of giant ELMs and appearance of grassy ELMs have been observed repeatedly at low pedestal collisionality ( νe,ped*∼0.15 ) in the high-power (P inj > 13 MW) hybrid scenario in DIII-D. Higher β p and higher q 95 appear to be beneficial to achieving the grassy ELM regime. The grassy ELM H-mode plasma shows high energy confinement performance (H 98y2 up to over 1.5) under the conditions of high neutral beam torque and high core rotation. The pedestal width appears to exceed the EPED1.0 model prediction by more than 50%. Pedestal stability analysis performed with the ELITE code indicates that the stability against low-n kink/peeling modes is improved with increased auxiliary heating power and the operational point in the grassy ELM regime is located near the ballooning boundary. The pedestal stability characteristics during the grassy ELM crashes have been investigated in comparison with the giant ELM crashes based on plasma profiles experimentally measured with high resolution and accuracy. It has been found that the underlying mechanism for the observed small-amplitude ELM crashes is mainly the expansion of the ballooning stability boundary induced by an initial radially localized collapse in the pedestal, which helps to stop the growth of instabilities and further collapse of the pedestal. The effect of electron density pedestal on mitigating edge localized instabilities has been analyzed by numerical simulation, suggesting that the electron density pedestal characterized by high n e,sep/n e,ped and low density gradient helps to stabilize peeling-ballooning modes because of a low pressure pedestal gradient and to lower the ballooning boundary mainly because of a low ion diamagnetic frequency in the pedestal region, thus triggering ballooning instabilities and producing the intrinsic grassy ELMs. Numerical simulation of the Chinese fusion engineering test reactor (CFETR) with the SOLPS code indicates that the separatrix density might be insensitive to the electron diffusivity in the pedestal region and increase with the power flowing from the core region to the edge region. Pedestal stability analysis suggests that the flat density pedestal with high separatrix density obtained in the high-power plasma in CFETR would make the operational point close to the ballooning boundary, which is considered to help destabilize ballooning instabilities and facilitate the access to the grassy ELM regime.