Investigation of boron distribution and material migration on the W7-X divertor by picosecond LIBS

One set of horizontal target elements of the Test Divertor Units (TDU), retrieved from the Wendelstein 7-X (W7-X) vessel after the end of second divertor Operation Phase (OP1.2B) in Hydrogen (H), were investigated by picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS). The Boron (B) distribution, H pattern and the material erosion/deposition pattern on these target elements were analyzed with high depth resolution and mapped in the poloidal direction of W7-X. From the spectroscopic analysis, B, H, Carbon (C) and Molybdenum (Mo) were clearly identified. A non-uniformly distributed B pattern on these divertor target elements was determined by the combination of B layer deposition during the three boronizations and W7-X plasma operation with multiple erosion and deposition steps of B. Like the TDU, the analyzed target elements are made of fine grain graphite, but have two marker layers which allow us to determine the material migration via the ps-LIBS technique. Two net erosion zones including one main erosion zone with a peak erosion depth of 6.5 μm and one weak erosion with a peak erosion of 1.3 μm were determined. Between two net erosion zones, a net deposition zone with width of 135 mm and a thickness up to 3.5 μm at the peak deposition location was determined by the ps-LIBS technique. The B distributions are correlated with the erosion/deposition pattern and the operational time in standard magnetic configuration of W7-X in the phases after the boronizations. The thickness of the containing B layer on these target elements also correlates with the erosion/deposition depth, in which the thickness of the containing B layer varies spatially in poloidal direction between 0.1 μm and 6 μm. Complementary, Focused Ion Beam combined with Scanning Electron Microscopy (FIB-SEM) was employed also to verify and investigate the deposition layer thicknesses at typical net erosion and net deposition zones as well as to identify the three boronizations in depth.

Effects of Laser Surface Texturing and Lubrication on the Vibrational and Tribological Performance of Sliding Contact

Various textures are fabricated by a picosecond laser machine on the surfaces of circular stainless steel specimens. Vibrational and tribological effects of laser surface textures are investigated by means of a tribometer and a data acquisition and signal processing (DASP) system. Experimental results show that surface textures can reduce the coefficients of friction (COFs), enhance the wear resistance, and improve the dynamical performance of frictional surfaces. In this study, the surface with micro circular dimples in diameter of 150 μm or textured area density of 25% has the best tribological and dynamical performance. Compared with the non-textured surface, the surface with circular dimples in diameter of 150 μm and 15% textured area density has 27% reduction of COFs, 95% reduction of frictional vibrations, and 66% reduction of frictional noise. The frictional vibrations and noise in the sliding contacts can be effectively reduced by adding graphene to the lubrication oil, and the surface textures enhance the frictional noise reduction performance of lubrication.

Generation of Cold Magnetized Relativistic Plasmas at the Rear of Thin Foils Irradiated by Ultra-High-Intensity Laser Pulses

A scheme to generate magnetized relativistic plasmas in a laboratory setting is proposed. It is based on the interaction of ultra-high-intensity sub-picosecond laser pulses with few-micron-thick foils or films. By means of Particle-In-Cell simulations, it is shown that energetic electrons produced by the laser and evacuated at the rear of the target trigger an expansion of the target, building up a strong azimuthal magnetic field. It is shown that in the expanding plasma sheath, a ratio of the magnetic pressure and the electron rest-mass energy density exceeds unity, whereas the plasma pressure is lower than the magnetic pressure and the electron gyroradius is lower than the plasma dimension. This scheme can be utilized to study astrophysical extreme phenomena such as relativistic magnetic reconnection in laboratory.

Texture optimization and verification for the thrust bearing used in rotary compressors based on a transient tribo-dynamics model

Rotary compressors are designed more and more compact and the compressor cylinder's ambient pressure is designed very high to facilitate oil separation and improve efficiency. However, these designs cause the working condition of the thrust bearing becoming harsher, and severe wear may occur. The present study is aimed at mitigating its wear condition through surface texturing. Based on a transient tribo-dynamics model considering the coupling effect of the journal and thrust bearings, a texture optimization study for the thrust bearing is conducted, in which three different stochastic optimization algorithms are utilized. The results show that thrust bearings with optimized textures have significantly reduced contact forces and wear under a high working frequency due to an extra hydrodynamic support around the texture dimples. The optimized texture designs are fabricated on the thrust bearing surfaces by a high-accurate picosecond laser machine and their performance is assessed through experiments using a compressor performance test platform. The experiment results confirm that the textured thrust bearing has a lower wear depth. Moreover, the coefficient of performance (COP) of the testing compressor with textured thrust bearing is increased while its input power decreases, which implies a reduced friction force and a higher energy efficiency.

Adhesive Joints with Laser Shaped Surface Microstructures

One of the most commonly applied methods of joining dissimilar materials is gluing. This could be mainly attributed to the applicability of this technique in various industries. The article presents a method of material surface treatment, which increases the shear strength of adhesive joints for lightweight metals such as aluminum with plastics. For this purpose, laser surface microstructuring was performed on each of the selected construction materials. As a result of the performed treatment, the active surface of the glued area was increased, which increased the adhesive strength. The picosecond laser with UV radiation used in the research is TruMicro 5325c with which material can be removed as a result of the cold ablation phenomenon. The applied parameters of the laser device did not cause thermal damage to the surface of the microstructured materials, which was confirmed by microscopic examination. Laser micromachining did not deteriorate the degree of wetting of the tested materials, either, as was confirmed by the contact angle and surface energy measurements with the use of water as the measuring liquid. In investigated cases of microstructure types, the presented method significantly increased the shear strength of the joints formed, as demonstrated by the presented strength test results. Research has shown that created joints with microstructure made according to the described method, are characterized by a significant increase in strength, up to 376%, compared to materials without microstructure. The presented results are part of a series of tests aimed at selecting the operating laser parameters for the implementation of geometric shapes of microstructures which will increase the strength of adhesive joints in selected materials.

Machine Learning Analysis of Quasi-Stationary Magnetic Fields Optically-Driven by Short Laser Pulses

Optical generation of kilo-tesla scale magnetic fields enables prospective technologies and fundamental studies with unprecedentedly high magnetic field energy density. A question is the optimal configuration of proposed setups, where plenty of physical phenomena accompany the generation and complicate both theoretical studies and experimental realizations. Short laser drivers seem more suitable in many applications, though the process is tangled by an intrinsic transient nature. In this work, an artificial neural network is engaged for unravelling main features of the magnetic field excited with a picosecond laser pulse. The trained neural network acquires an ability to read the magnetic field values from experimental data, extremely facilitating interpretation of the experimental results. The conclusion is that the short sub-picosecond laser pulse may generate a quasi-stationary magnetic field structure living on a hundred picosecond time scale, when the induced current forms a closed circuit.