Measurements of the ionisation rates of lithium-like ions

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Abstract
The ionisation rates of lithium-like nitrogen (N V) and oxygen (O VI) were measured using a 60 kJ theta pinch as a plasma light source. The time evolutions of strong VUV lines were observed with a 2.2 m grazing-incidence monochromator. Two different plasmas were used (2.5 mT N2 and 20 mT (He+1% O2)) and diagnosed with 90 degrees Thomson scattering (Te) and holographic interferometry (Ne). The plasma was modelled using the coupled rate equations with the semiempirical formula of Kunze (1971) for the ionisation rates. The measured ionisation rates for the lithium-like ions were found to be consistent with those calculated by Kunze's semiempirical formula to within 10%. Corrections were made in the model for end losses, temperature dependence on the excitation rates, radial electron density and temperature distributions, lowering of the ionisation limit due to collisions, and overlap of the 2s and 2p levels in the ground-state population.

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