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 D_alpha-emission from LIAS of a-C:D layers was found to be [D/XB]=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 D_gamma/D_delta - 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 mission profile shape is found for lambda_ne = 13 mm. 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

Analysis and removal of ITER relevant materials and deposits by laser ablation

The analysis of the deposition of eroded wall material on the plasma-facing materials in fusion devices is one of the crucial issues to maintain the plasma performance and to fulfill safety requirements with respect to tritium retention by co-deposition. Laser ablation with minimal damage to the plasma facing material is a promising method for in situ monitoring and removal of the deposition, especially for plasma-shadowed areas which are difficult to reach by other cleaning methods like plasma discharge. It requires the information of ablation process and the ablation threshold for quantitative analysis and effective removal of the different deposits. This paper presents systemic laboratory experimental analysis of the behavior of the ITER relevant materials, graphite, tungsten, aluminum (as a substitution of beryllium) and mixed deposits ablated by a Nd:YAG laser (1064 nm) with different energy densities (1–27 J/cm2, power density 0.3–3.9 GW/cm2). The mixed deposits consisted of W–Al–C layer were deposited on W substrate by magnetron sputtering and arc plasma deposition. The aim was to select the proper parameters for the quantitative analysis and for laser removal of the deposits by investigating the ablation efficiency and ablation threshold for the bulk materials and deposits. The comparison of the ablation and saturation energy thresholds for pure and mixed materials shows that the ablation threshold of the mixed layer depends on the concentration of the components. We propose laser induced breakdown spectroscopy for determination of the elemental composition of deposits and then we select the laser parameters for the layer removal. Comparison of quantitative analysis results from laboratory to that from TEXTOR shows reasonable agreements. The dependence of the spectra on plasma parameters and ambient gas pressure is investigated

Gas-phase detection of HSOD and empirical equilibrium structure of oxadisulfane

Abstract We present the first gas phase spectra of singly deuterated oxadisulfane, HSOD, in its vibrational ground state. More than 100 transitions have been recorded with highest frequency accuracy using the Cologne Terahertz Spectrometer. The molecular parameters derived from a least squares fit analysis proof HSOD to be an almost accidental symmetric prolate top molecule with an asymmetry parameter kZK0.9985. Spectra of c-type and weaker b-type transitions have been recorded in the range from 716 to 772 GHz. The ratio of the dipole moments m c /m b Z2.4(3) has been derived from measured line intensities. The c-type transitions are split by the tunneling motion of a hindered internal rotation, whereas b-type transitions show no splitting within the Doppler limited line profiles. We derived the equilibrium molecular structure of oxadisulfane, HSOH, from experimental values of the rotational constants A 0 , B 0 , and C 0 of HSOH, H 34 SOH, DSOD, and HSOD. The equilibrium rotational constants A e , B e , and C e were derived by taking vibration-rotation interaction constants a r obtained from high-level ab initio calculations into account.

Ablation mass features in multi-pulses femtosecond laser ablate molybdenum target

In this study, the ablation mass features related to reflectivity of bulk Molybdenum (Mo) were investigated by a Ti: Sa 6 fs laser pulse at central wavelength 790 nm. The ablated mass removal was determined using Confocal Microscopy (CM) technique. The surface reflectivity was calibrated and measured by a Lambda 950 spectrophotometer as well as a CCD camera during laser ablation. The ablation mass loss per pulse increase with the increasing of laser shots, meanwhile the surface reflectivity decrease. The multi-pulses (100 shots) ablation threshold of Mo was determined to be 0.15 J/cm2. The incubation coefficient was estimated as 0.835. The reflectivity change of the Mo target surface following multi-pulses laser ablation were studied as a function of laser ablation shots at various laser fluences from 1.07 J/cm2 to 36.23 J/cm2. The results of measured reflectivity indicate that surface reflectivity of Mo target has a significant decline in the first 3-laser pulses at the various fluences. These results are important for developing a quantitative analysis model for laser induced ablation and laser induced breakdown spectroscopy for the first wall diagnosis of EAST tokamak

Quartz Crystal Microbalances for quantitative picosecond laser-material-interaction investigations – Part I: Technical considerations

In this work the technical suitability of Quartz Crystal Microbalances (QMBs) for in situ, pulse resolved mass removal measurements is demonstrated for picosecond laser ablation of magnetron sputtered coatings. The QMBs show a linear characteristic of the sensitivity for layer thickness of different metals up to several microns. Laser pulse resolved measurements of the mass ablated from the metal layer were performed. About 400 ng of chromium was ablated during the first laser pulse while in subsequent pulses < 220 ng were removed. This is compared with previous findings. The sensitivity for ablation of the QMBs is found to be larger than for deposition, which is explained by the radial sensitivity of the QMBs. Future refinements of the setup and the benefits of the pulse resolved mass loss measurements for laser based methods like LIBS and LIAS are discussed and will be presented in part II currently in preparation

Entwicklung und Test von Prototypkomponenten für ITER

Das Bundesministerium für Bildung und und Forschung (BMBF) stellte in 2007/2008 Mittel imRahmen der Projektförderung zur Verfügung, mit der Zielsetzung eine stärker sichtbareBeteiligung der deutschen Fusionsinstitute am Aufbau von ITER zu erreichen, sowie dieChancen deutscher Unternehmen auf die Übernahme von Aufträgen für den Aufbau vonITER zu stärken.Eine wichtige Zielsetzung des hier beschriebenen Forschungsvorhabens (Projektnummer03FUS0007) war es demnach, kritische Prototyp-Komponenten für ITER zu entwickeln undzu testen, sowie entsprechende Mess- und Prüfeinrichtungen aufzubauen. Gleichzeitigwurde in der Projektbearbeitung sehr eng mit einer ganzen Reihe von Unternehmenzusammengearbeitet, um so einen intensiven Know-How Transfer in beiden Richtungen imHinblick auf die Entwicklung von Komponenten für Fusionsanlagen zu erreiche

Laser induced ablation spectroscopy for in situ characterization of the first wall on EAST tokamak

Plasma-wall interaction (PWI) is a crucial issue for high-performance operation and first wall lifetime of future fusion reactors. An in situ measurement and the control of fuel retention in the vacuum-vessel will be an important aspect for the safe operation of nuclear fusion devices. Laser-Induced Ablation Spectroscopy (LIAS) is an attractive option with promising capabilities for in situ characterization of fuel retention and co-deposition during plasma discharges. In this work, an in situ LIAS wall diagnostic system is developed for real-time during discharge diagnosis of the first wall on the EAST superconducting tokamak. A dedicated timing system, which offers the function of multiple temporal settings with high time resolution for LIAS measurement during long-pulse discharges, is successfully commissioned. The experimental setup of the LIAS system for the characterization of the first wall on EAST is presented in detail. First LIAS results obtained in EAST during high power discharges are presented. The results indicate that in situ measurements of deposition, co-deposition and dynamic retention are possible

Development of laser-based technology for the routine first wall diagnostic on the tokamak EAST: LIBS and LIAS

A laser based method combined with spectroscopy, such as laser-induced breakdown spectroscopy (LIBS) and laser-induced ablation spectroscopy (LIAS), is a promising technology for plasma-wall interaction studies. In this work, we report the development of in situ laser-based diagnostics (LIBS and LIAS) for the assessment of static and dynamic fuel retention on the first wall without removing the tiles between and during plasma discharges in the Experimental Advanced Superconducting Tokamak (EAST). The fuel retention on the first wall was measured after different wall conditioning methods and daily plasma discharges by in situ LIBS. The result indicates that the LIBS can be a useful tool to predict the wall condition in EAST. With the successful commissioning of a refined timing system for LIAS, an in situ approach to investigate fuel retention is proposed