Assessment of Laser Induced Ablation Spectroscopy (LIAS) as a method for quantitative in situ surface diagnostic in plasma environments

In this work Laser Induced Ablation Spectroscopy (LIAS) is investigated as an in situ plasma surface interaction diagnostic for fusion reactors and fusion experiments. In LIAS an intensive laser pulse is used to ablate the material under investigation during plasma operation. Ablation products penetrate into the edge region of the plasma and are excited and ionized. In case of molecules and clusters additionally dissociation occurs. The emitted line radiation is observed by radiometric calibrated spectroscopy. Results from LIAS of W/C/Al/D–mixed layers and amorphous hydrocarbon layers are presented. Using a fast camera system time resolved measurements of the LIAS–process could be performed, allowing investigation of the temporal behavior of excitation, dissociation and ionization processes. For Tungsten, 90% of the LIAS light is observed within 10±3 μs after the laser pulse. In case of carbon within 20± 3 μs. Additionally separation in time of LIAS emission and the LIBS emission caused by the laser pulse at the surface within single measurements was demonstrated. This allows the separate analysis of both processes in a coaxial setup which is foreseen for future experiments. The inverse photon efficiency of the Balmer Dα –emission from LIAS of a-C: D– layers was found to be D XB a-C: DL→IASD Dα = 71 ± 7. The plasma perturbation due to LIAS was investigated by laser energy density variation when ablating W/C/Al/D–mixed layers. Local plasma perturbation is found to increase with laser energy density. Balmer Hγ/Hδ – line intensity ratio measurements only show for ohmic discharges and the case of the lowest central density signs of local plasma perturbation in LIAS of graphite samples. A simple analytical model for local plasma perturbation during LIAS is introduced and evaluated. Qualitative agreement between the model and the above reported experimental observations is found; a stronger influence on local conditions is found by tungsten than by carbon ablation, with ohmic discharges more susceptible to perturbation than neutral beam injector heated ones. Limitations and possible improvements of the model are discussed. A Monte Carlo code developed in the framework of this thesis is used for modeling the measured neutral atom emission profiles. The model is in good agreement with the analytical solution in case of a homogeneous plasma. With the best estimate input parameters no agreement between observed and modeled emission profiles is found. Thus, a three dimensional parameter space describing the plasma profile is defined by density and temperature at the last closed flux surface and the density decay length. In this parameter space the surface on which measured and simulated profile emission maxima agree is found for both Tungsten and Carbon. In case of Tungsten, agreement between measured and simulated emission profile shape is found for λne = 13mm. In contrast, for carbon no match for the emission shapes can be found. Taken together with the spectroscopic observation this suggests that non-atomic species significantly contribute to the observed light emission, creating the need for extension of the model. In the concluding discussion the results are discussed and further investigations are proposed.