The Very High n Rydberg Series of Ar^16+ in Alcator C-Mod Tokamak Plasmas

X-ray transitions of the very high-n Rydberg series in Ar^16+ have been observed from Alcator C-Mod tokamak plasmas. Individual emission lines up to 1s16p - 1s^2 have been resolved and the central chord line brightnesses with principal quantum number n between 7 and 16 are generally found to decay as 1/n^alpha, with alpha slightly larger than 3. In the plasma periphery, emission from 1s9p - 1s^2 and 1s10p - 1s^2 are found to be significantly enhanced relative to this decrease, indicative of selected population of these levels through charge exchange between background neutral deuterium in the ground state and Ar^17+. An unresolved feature between the wavelengths of 1s27p - 1s^2 and 1s30p - 1s^2 is also present, which arises through charge exchange with neutral deuterium in the n^* = 3 excited state. The brightnesses of transitions populated by charge exchange are spatially up/down asymmetric, with an excess on the side of the magnetic surface X-point. The relative brightness of the unresolved very high-n feature compared to 1s7p - 1s^2 is found to increase with electron temperature and decrease with electron density. Simulations of line emission just on the long wavelength side of the Ar^16$ ionization limit indicate that the principal quantum number decay exponent is closer to alpha = 4 at very high n. The brightness dependence on n below 16 is in excellent agreement with calculations from the Flexible Atomic Code package.在阿尔卡特C-Mod托卡马克（Alcator C-Mod tokamak）等离子体中，观测到氩16+离子（Ar¹⁶⁺）极高主量子数里德伯序列（Rydberg series）的X射线跃迁。科研人员已分辨出主量子数最高至16的1s16p → 1s²跃迁对应的单条发射谱线；主量子数n介于7至16之间的中心弦线亮度，总体遵循1/n^α的衰减规律，其中α略大于3。在等离子体边缘区域，1s9p → 1s²与1s10p → 1s²的发射强度相较于该衰减趋势出现显著增强，表明基态背景中性氘与氩17+离子（Ar¹⁷⁺）之间的电荷交换过程，选择性布居了这些能级。在1s27p → 1s²与1s30p → 1s²对应的波长区间内，还存在一处未分辨谱特征，其源于与有效主量子数n^*=3的激发态中性氘的电荷交换过程。由电荷交换过程布居的跃迁谱线亮度呈现空间上下不对称性，在磁面X点一侧存在强度盈余。该未分辨极高主量子数谱特征相对于1s7p → 1s²的相对亮度，随电子温度升高而增强、随电子密度增大而减弱。对氩16+电离极限长波长侧的谱线发射进行模拟后发现，在极高主量子数下，主量子数的衰减指数α更趋近于4。主量子数n<16时，谱线亮度对主量子数的依赖关系，与柔性原子代码包（Flexible Atomic Code, FAC）的计算结果吻合极佳。