The Very High n Rydberg Series of Ar^16+ in Alcator C-Mod Tokamak Plasmas
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Author(s)
Rice, John E.
Sciortino, Francesco
Gu, M.
Cao, N.
Hughes, Jerry W.
Irby, J.H.
Marmar, E.S.
Mordijck, S.
Reinke, M.L.
Reksoatmodjo, R.
Date Issued
July 2021
Journal
Journal of Physics B: Atomic, Molecular and Optical Physics
Publisher
IOP
Abstract
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.
Description
Submitted for publication in Journal of Physics B: Atomic, Molecular and Optical Physics
MIT Department
Massachusetts Institute of Technology. Plasma Science and Fusion Center
Persistent DSpace Link
DOI of Published Version
https://doi.org/10.1088/1361-6455/ac2722
