X-ray observations of Ca^{19 +}, Ca^{18 +} and satellites from Alcator C-Mod tokamak plasmas

X-ray spectra of n = 2 to 1 transitions in hydrogen-like Ca^{19 +}, helium-like Ca^{18 +} and nearby satellites have been obtained from Alcator C-Mod tokamak plasmas using a spatially imaging high resolution x-ray spectrometer system. For Ca^{19 +}, the intensity ratio of Ly ₂(1s¹S₁/2-2p ²P₁/2) to Ly₁ (1s ¹S₁/2-2p ²P₃/2) was found to be ∼0.531 ± 0.005 over a range of plasma parameters, which is somewhat greater than the ratio of the statistical weights of the upper n = 2 levels, 1/2. This difference is mainly due to interaction with the ²S₁/2 fine structure sub-level. Experimental results are compared to calculations from COLRAD, a collisional-radiative modelling code, and good agreement is shown. For Ca^{18 +}, the intensity ratio of the dielectronic satellite k (1s²2p²P₁/2-1s2p ^{2 2}D₃/2) to the resonance line w (1s ²¹S₀-1s2p ¹P₁) is sensitive only to the electron temperature. The observed brightness ratio scaling with T_e is in good agreement with the calculated ratio of the respective dielectronic recombination to the collisional excitation rates.