EAST不同碰撞率等离子体台基湍流与台基结构关系的研究

H-mode pedestal is a connection between the plasma edge and core confinement region and is especially important for fusion plasma. Understanding physical mechanism of pedestal structure formation is beneficial to further optimization of pedestal parameter and to effective control of ELM. Most of H-mode pedestal in present devices operates in high collisionality regime and so it is questionable whether these experimental results can be generalized to next generation fusion devices (e.g ITER and CFETR). The conventional pedestal model (EPED) predicts that pedestal width is independent of pedestal collisionality. The recent experiments on JT-60U and JET, however, have shown that collisionality has a significant impact on pedestal width. The mechanism for this dependence of width on collisionality is still not yet clear. On the other hand, experimental results have shown that pedestal turbulence plays an important role in pedestal evolution and pedestal structure formation. Since collisionality can largely impact the turbulence characteristics, it is expected that pedestal evolution would be different in plasmas with different collisionality and this could lead to the different pedestal structure. This proposal will explore the relations among pedestal turbulence, pedestal formation and pedestal structure in EAST H-mode plasmas with different collisionality. And also the EPED model and BOUT++ simulation will be utilized to compare with the experimental results, by which we hope that the physical mechanism how collisionality influences the pedestal structure can be uncovered.

高约束模(H-mode)台基连接了等离子体边界和芯部约束区，对于聚变等离子体特别重要。理解台基结构形成的物理机制能够帮助我们进一步优化台基参数以及有效地控制ELM。现今装置大多数H-mode台基处于高碰撞率区，获得的实验结果是否能推广到下一代聚变装置(如ITER和CFETR)仍然存疑。目前常用的台基结构预测模型(EPED)预测的宽度与碰撞率无关。但最近JT-60U和JET的实验表明碰撞率对台基宽度有较大影响，其物理机制仍然不清楚。另一方面，实验表明台基湍流在台基演化和台基结构形成中起着重要作用。由于碰撞率对湍流的特征有很大影响，可能引起台基演化的不同，导致最终的台基结构产生差异。本项目基于EAST平台，研究不同碰撞率H-mode等离子体中台基湍流与台基演化、台基结构的关系，并利用模拟(EPED和BOUT++)与实验比较，理解碰撞率对台基结构产生影响的物理机制。

高约束模台基结构对于磁约束等离子体获得高的约束性能至关重要。现今装置大多数H模台基处于高碰撞率区，获得的实验结果能否推广到下一代聚变装置仍然存疑。本项目基于EAST装置，研究碰撞率对台基湍流和台基结构的影响。实验观察到钼杂质进入分离面后，相干模出现在ρ=0.94处等离子体区。分析表明钼杂质的流入导致台基电子密度/密度梯度增加和台基电子温度降低，这暗示相干模的触发很可能与台基密度梯度和碰撞率的增加相关。进一步分析表明，相干模幅度的增加在很大程度上与碰撞率增加相关。对EAST实验数据进行统计分析，建立起I类ELM放电条件下密度台基宽度演化的数据库。研究发现密度台基宽度不仅会与台基极向比压有关，还与台基碰撞率也存在明显的依赖关系，基本趋势是随着碰撞率增加而增加。当碰撞率小于3时，结果与EPED预测模型一致；而当碰撞率大于3时，台基宽度明显变大且显著偏离EPED模型预测值。通过非线性回归分析得到EAST上密度台基宽度的实验定标律。进一步进行了台基碰撞率扫描实验，获得一组适合目标研究的数据：q95几乎保持不变~4.6，变化范围<3%；台基归一化极向比压~0.3，变化范围<10%；台基归一化拉莫尔半径~0.0041，变化范围<10%；台基碰撞率=1.5-6，具有4倍变化范围。发现密度台基宽度随着碰撞率增加而增加，也同时发现台基湍流的幅度随着碰撞率增加而增加。利用REPED进行理论模拟计算，结果表明台基压强高度与碰撞率的关系与实验一致，但是台基宽度与碰撞率无明显关系。结果表明碰撞率可能通过影响台基湍流和台基输运，进而影响最终的台基结构形成。