Precision measurement of the 1s Lamb shift in hydrogenlike argon

Precision measurements of the absolute wavelengths of the Lyman- doublet (2p SP/sub 3/2,1/2/ 1s SS/sub 1/2/) in hydrogenlike argon (Ar X ), as well as those of some strong dielectronic satellites, are reported. The Alcator C tokamak plasma was seeded with trace amounts of argon and the emission spectra were taken with a high-resolution, crystal x-ray spectrometer. In situ wavelength calibration was achieved by utilizing the potassium K lines from a KCl fluorescence source. For the Ly 2 line, a precision of 11 ppm was achieved. Comparisons of these data with QED predicted wavelengths show good agreement, yielding a test of the 1s Lamb shift at the 3% level. The precision of these measurements was limited by uncertainties in the wavelength calibration, with the uncertainties due to the influence of unresolved satellites being of secondary importance.