The Very High N Rydberg Series Of Ar16+ In Alcator C-Mod Tokamak Plasmas

X-ray transitions of the very high-n Rydberg series in Ar16+ 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 Ar17+. 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 Ar16+ 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.