Abstract
Coated glass targets are a key component of the Wendelstein 7-X laser blow-off system that is used for impurity transport studies. The preparation and analysis of these glass targets as well as their performance is examined in this paper. The glass targets have a high laser damage threshold and are coated via physical vapor deposition with µm thick films. In addition, nm-thin layers of Ti are used as an interface layer for improved ablation efficiency and reduced coating stress. Hence, the metallic or ceramic coating has a lateral homogeneity within 2% and contaminants less than 5%, being optimal for laser ablation processing. With this method, a short (few ms) and well defined pulse of impurities with about 1017 particles can be injected close to the last closed flux surface of Wendelstein 7-X. In particular, a significant amount of atoms with a velocity of about 1 km/s enters the plasma within 1 ms. The atoms are followed by a negligible concentration of slower clusters and macro-particles. This qualifies the use of the targets and applied laser settings for impurity transport studies with the laser blow-off system in Wendelstein 7-X.
Highlights
The removal of thin absorbing layers, e.g., metals, from transparent substrates such as glasses by means of laser irradiation impact is called “laser ablation” or “laser blow-off” (LBO)
Scitation.org/journal/rsi system, as previously described by Wegner et al.,[7] consists of a high energy laser, optical components, and a glass target holder that is mounted on a manipulator to move the targets into the plasma vessel 600 mm away from the last closed flux surface (LCFS) being the boundary of the confined plasma region
Glass targets coated with Fe and Ti are exemplarily investigated with respect to their applicability for the impurity injection in Wendelstein 7-X (W7-X)
Summary
The removal of thin absorbing layers, e.g., metals, from transparent substrates such as glasses by means of laser irradiation impact is called “laser ablation” or “laser blow-off” (LBO) This method can be divided into different cases depending on the side of irradiation, thicknesses of the layers and substrates, and laser properties such as the pulse duration and wavelength.[1] These different ablation processes enable a wide-spread field of applications. The laser blow-off technique has been applied in fusion experiments for impurity transport studies,[2–7] for electron heat transport investigation,[8,9] and for plasma temperature and density measurements at the plasma edge.[10–13] It has been used for studying atomic processes,[14–18] patterning of thin film solar cells,[19,20] and laser induced metal deposition.[21].
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