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As a non-contact process laser beam melt ablation offers several advantages compared to conventional processing mechanisms. During ablation the surface of the workpiece is molten by the energy of a CO2-laser beam, this melt is then driven out by the impulse of an additional process gas. Although the idea behind laser beam melt ablation is rather simple, the process itself has a major limitation in practical applications: with increasing ablation rate surface quality of the workpiece processed declines rapidly. With different ablation rates different surface structures can be distinguished, which can be characterised by suitable surface parameters. The corresponding regimes of pattern formation are found in linear and non-linear statistical properties of the recorded process emissions as well. While the ablation rate can be represented in terms of the line-energy, this parameter does not provide sufficient information about the full behaviour of the system. The dynamics of the system is dominated by oscillations due to the laser cycle but includes some periodically driven non-linear processes as well. Upon the basis of the measured time series, a corresponding model is developed. The deeper understanding of the process can be used to develop strategies for a process control.
The non-LTE radiative transfer problem requires the consistent solution of two sets of equations: the radiative transfer equations, which couple the spatial points, and the equations of the statistical equilibrium, which couple the frequencies. The "Accelerated Lambda Iteration" (ALI) method allows for an iterative scheme, in which both sets of equations are solved in turn. For moving atmospheres the radiative transfer is preferably formulated in the co-moving frame-of-reference, which leads to a partial differential equation. "Classical" numerical solution methods are based on differencing schemes. For better numerical stability, we prefer "short characteristics" integration methods. Iron line blanketing is accounted for by means of the "superlevel" concept. In contrast to static atmospheres, the frequencies can not be re-ordered in the moving case because of the frequency coupling from Doppler shifts. One of our future aims is the coupling of elaborated radiative transfer calculations with the hydrodynamical equations in order to understand the driving of strong stellar winds, especially from Wolf-Rayet stars.
Compact white-light source with an average output power of 2.4 W and 900 nm spectral bandwidth
(2003)
Higher variability in rainfall and river discharge could be of major importance in landslide generation in the north-western Argentine Andes. Annual layered (varved) deposits of a landslide dammed lake in the Santa Maria Basin (26°S, 66°W) with an age of 30,000 14C years provide an archive of precipitation variability during this time. The comparison of these data with present-day rainfall observations tests the hypothesis that increased rainfall variability played a major role in landslide generation. A potential cause of such variability is the El Niño/ Southern Oscillation (ENSO). The causal link between ENSO and local rainfall is quantified by using a new method of nonlinear data analysis, the quantitative analysis of cross recurrence plots (CRP). This method seeks similarities in the dynamics of two different processes, such as an ocean-atmosphere oscillation and local rainfall. Our analysis reveals significant similarities in the statistics of both modern and palaeo-precipitation data. The similarities in the data suggest that an ENSO-like influence on local rainfall was present at around 30,000 14C years ago. Increased rainfall, which was inferred from a lake balance modeling in a previous study, together with ENSO-like cyclicities could help to explain the clustering of landslides at around 30,000 14C years ago.