H-Mode Pedestal Characteristics and MHD Stability of the Edge Plasma in Alcator C-Mod
H-Mode Pedestal Characteristics and MHD Stability of the Edge Plasma in Alcator C-Mod
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DOI：
10.1088/0741-3335/44/4/303
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发表时间：
2002-03
影响因子：
2.2
通讯作者：
D. Mossessian;P. Snyder;M. Greenwald;J. Hughes;Yijun Lin;A. Mazurenko;S. Medvedev;H. Wilson;S. Wolfe
中科院分区：
文献类型：
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作者：
D. Mossessian;P. Snyder;M. Greenwald;J. Hughes;Yijun Lin;A. Mazurenko;S. Medvedev;H. Wilson;S. Wolfe
在Alcator C-Mod的大多数工作条件下，h模式的主导类型是稳态增强Dα (EDA)模式，其特征是良好的能量约束、不断退化的杂质约束和没有规则边缘局域模式（elm）。在此区域，观测到准相干（QC）电磁模式（kθ~5 cm-1, f~100 kHz），定位于密度基座区域。实验证据表明，该模式是粒子输运增强的原因。实验表明，QC模式可以存在于边缘温度安全系数空间中一个明确的区域，有利于高边缘q值（q95≥3.5）和适度的基座温度Teped≤Tc （Tc~400 eV）。随着边缘温度和压力梯度的增大，QC模式被宽带低频波动（f<50 kHz）所取代。在这些排放物中可以观察到小的草状榆树。对C-Mod边的理想气胀稳定性分析表明，如果考虑边缘自举电流，甚至由于高边缘碰撞而减小，则边压力梯度不受无限n个理想气胀模式的限制。在边缘压力和电流梯度共同驱动下，耦合理想剥落/膨胀介质n模态的线性稳定性分析表明，在观察到草状elm的压力梯度范围内（∇P≥1.2×107 Pa Wb-1 rad），模态在C-Mod边缘变得边缘不稳定。这一结果与elm作为中间剥离/膨胀模式的模型一致。另一方面，证明了理想模式在EDA模式下的稳定性，以及其存在所需的低Teped这一事实，支持了显示QC模式电阻特性的理论模型。
Under most operational conditions of Alcator C-Mod, the dominant type of H-mode is the steady state enhanced Dα (EDA) mode, characterized by good energy confinement, continuously degraded impurity confinement and absence of regular edge localized modes (ELMs). In this regime, a quasicoherent (QC) electromagnetic mode (kθ~5 cm-1, f~100 kHz) is observed, localized in the region of the density pedestal. Experimental evidence suggests that the mode is responsible for enhancement of particle transport. It is shown experimentally that the QC mode can exist in a well defined region in edge temperature-safety factor space, favouring high edge q values (q95≥3.5) and requiring moderate pedestal temperature Teped ≤ Tc (Tc~400 eV). As edge temperature and pressure gradient increase, the QC mode is replaced by broadband low frequency fluctuations (f<50 kHz). Small grassy ELMs are observed in these discharges. Analysis of ideal ballooning stability of the C-Mod edge shows that the edge pressure gradient is not limited by infinite n ideal ballooning mode if edge bootstrap current, even reduced by high edge collisionality, is taken into account. Linear stability analysis of coupled ideal peeling/ballooning medium n modes, driven by combination of edge pressure and current gradients, shows that the modes become marginally unstable at C-Mod edge in the range of pressure gradients where the grassy ELMs are observed (∇P≥1.2×107 Pa Wb-1 rad). This result is consistent with a model of the ELMs as intermediate n peeling/ballooning modes. On the other hand, demonstrated stability of the ideal modes in EDA regime, together with the fact that low Teped is required for its existence, supports theoretical models showing resistive character of the QC mode.