Radial profiles of ground-state transitions of heliumlike argon from the Alcator-Ctokamak

Spectra of the complete series of ground-state transitions in heliumlike ${\mathrm{Ar}}^{16+}$ (1snp-1$s^2$, for 2≤n<50) have been obtained from the Alcator-C tokamak with use of a compact x-ray crystal spectrometer. Radial profiles of these transitions have been measured and have been compared with the results of a transport model which includes the basic atomic processes. In the plasma center, collisional excitation is the most important population mechanism for the upper levels. For the outer regions of the plasma (r/$a_L$>0.5), recombination of ${\mathrm{Ar}}^{17+}$ is the dominant population process for upper levels of most transitions, and preferentially populates the n=2 triplet levels. Impurity transport provides sufficient amounts of hydrogenlike argon at the cooler, outer radii. For the high-n transitions 1s9p-1$s^2$ and 1s10p-1$s^2$, charge-exchange recombination is the dominant population mechanism at the plasma edge under certain operating conditions. The intensities of these lines have been used to determine neutral-hydrogen density profiles. For ground-state transitions with n>13, charge transfer from excited neutral hydrogen is the only important population process for r/$a_L$>0.5.