Up/Down Impurity Density Asymmetries in C-Mod Plasmas

Brightness profiles of x-ray emission from H-like Ar17+ exhibit a distinct up/down asymmetry under certain operating conditions in C-Mod plasmas, indicating that impurity densities are not constant on flux surfaces with r/a between ~0.8 and ~0.95. In L- and I-mode plasmas, there is an x-ray brightness excess, up to a factor of 8, on the side opposite to the ion BxgradB drift direction. This effect is not observed in H-mode plasmas, presumably due to edge impurity transport being dominated by a strong inward pinch, which is absent in L- and I-mode. The magnitude of the asymmetry in L- and I-mode decreases with increasing plasma current, similar to the observed decrease in radial impurity diffusivity. In I-mode, where the codependence between electron density and temperature can be broken with ICRF heating power, the asymmetry magnitude is found to decrease with increasing density and with increasing edge temperature at fixed density. These measurements exhibit some qualitative features of neo-classical expectations but the observed asymmetry magnitude is much larger than predicted and some scalings with plasma parameters are not seen. The up/down asymmetry appears to be largest when the cross field impurity diffusivity is the highest.

类氢氩17+（H-like Ar17+）的X射线（X-ray）辐射亮度剖面在C-Mod等离子体的特定运行条件下呈现显著的上下不对称性，表明归一化径向位置r/a介于0.8至0.95之间的磁面（flux surfaces）上杂质密度并非恒定。在L模（L-mode）与I模（I-mode）等离子体中，与离子B×∇B漂移方向相反的一侧存在最高可达8倍的X射线亮度过剩现象。该效应在H模（H-mode）等离子体中未被观测到，推测原因是H模等离子体的边缘杂质输运由强烈的向内箍缩效应（inward pinch）主导，而L模与I模中并无该效应。L模和I模中的不对称程度随等离子体电流增大而降低，这与观测到的径向杂质扩散系数（radial impurity diffusivity）随电流升高而下降的趋势一致。在I模等离子体中，电子密度与温度的共变关系可通过离子回旋共振频率（ICRF）加热功率打破，研究发现不对称幅度随密度升高而降低，且在固定密度条件下随边缘温度升高而降低。这些测量结果展现出与新经典（neo-classical）理论预期相符的部分定性特征，但观测到的不对称幅度远大于理论预测值，且部分与等离子体参数相关的标度律并未被观测到。当交叉场杂质扩散系数（cross field impurity diffusivity）达到最高时，上下不对称性的幅度似乎最大。