EXPERIMENTAL ELECTRON-IMPACT EXCITATION RATE COEFFICIENTS FOR LITHIUM-LIKE SI-XII

Absolute excitation rate coefficients have been determined experimenta lly for all excited states up to 1s(2)5f of the lithium-like ion Si XI I using a well diagnosed theta-pinch plasma. A new method has been dev eloped to determine the most crucial quantity of these measurements, t he absolute concentration of the silicon ions in the plasma. For this we made use of the fact that sufficiently high lying levels are in par tial local thermodynamic equilibrium (PLTE) with the ground state of t he next higher ionization stage. A theoretical population model has be en developed to study the influence of the different processes that mi ght contribute to the population of the different levels at our plasma parameters. For most levels the theoretical excitation rate coefficie nts were found to be in fair agreement with the theoretical ones. The error of the experimentally determined excitation rate coefficients is of the order of +/- 50%.