Vacuum Laser Research Articles

Vacuum laser welding of SA508 steel

Vacuum laser welding was employed to manufacture 80 mm thick welds in SA508 Grade 3 steel in two weld passes, using a 16 kW laser, while travelling at 150 mm/min. The motivation was to explore the potential for the application of the process to the joining of large, safety-critical nuclear components, such as the steam generators or the pressuriser in a pressurised water reactor (PWR). The advantages of vacuum laser welding are first reviewed, and compared to those of electron beam welding, in terms of the process physics. Preliminary development work is then summarised, together with an evaluation of weld quality, mechanical properties and residual stresses. Vacuum laser welding warrants further development, as it offers significant promise for future nuclear build programmes.

Well collimated MeV electron beam generation in the plasma channel from relativistic laser-solid interaction

Efficient direct electron acceleration in the plasma channel with injection through the breaking of plasma waves generated by parametric instabilities was demonstrated experimentally and reproduced in the 2D3V PIC simulations. The electron bunch was produced using the specific plasma profile containing arbitrary sharp, ∼0.5λ, gradient at the vicinity of 0.1–0.5 critical density and a long tail of a tenuous preplasma. Such a preplasma profile was formed by an additional nanosecond laser pulse with intensity of 5 × 1012 W cm−2. In the case of optimal preplasma parameters femtosecond laser pulse with an intensity of 5 × 1018 W cm−2 and an energy of 50 mJ generates a collimated electron bunch having divergence of 50 mrad, exponential spectrum with the slope of ∼2 MeV and charge of tens of pC. The charge was confirmed measuring neutron yield from Be(g, n) photonuclear reaction with threshold of 1.7 MeV. By the contrast, a ring-like electron beam with divergency of 300 mrad and significantly lower charge is generated if the prepulse intensity drops to 5 × 1011 W cm−2. The 2D PIC simulations confirmed beamed electron’s acceleration in the plasma channel (so-called direct laser acceleration). This channel is formed in a long tail of teneous preplasma by the laser pulse specularly reflected from the arbitrary sharp gradient. The ring-like electron beam was attributed to the longer gradient case enlarging divergence of the reflected laser beam, preventing channel’s formation and electron acceleration by the so-called vacuum laser acceleration, or VLA. We also showed that injected electrons appeared from the wave breaking of plasma waves of hybrid SRS-TPD instability for the both gradients. Electrons received an initial momentum from this breaking to be effectively injected into the plasma channel.

Magnetic field assisted enhanced electron acceleration due to a chirped echelon phase modulated laser in vacuum

Electron acceleration employing chirped echelon phase modulated (EPM) linearly polarised (LP) laser pulse in vacuum under the influence of axial magnetic field has been explored. Echelon supported laser pulse with phase modulation sustains the trapping of pre-accelerated electron in the most favourable phase. Furthermore, the addition of linear frequency chirp enlarges the electron laser interaction duration. Here, application of axial magnetic field serves a critical role in boosting the electron acceleration upto larger distances in the laser propagation direction. So, few MeV electron can be expedited up to GeV energy and retains it upto longer distances due to the collective influence of the frequency chirp and phase modulation. Effect of laser initial intensity, pulse duration, beam waist and electron injection is taken into consideration and optimized for enhanced energy gain. It is observed that the electron of initial energy ˜0.5 MeV can achieve energy of ˜2.5 GeV with I∼1.38 × 1020 W/cm2 due to chirped LP EPM laser pulse in the existence of suitable axial magnetic field (˜41.7 MG) at most favourable values of electron injection angle and EPM gradient parameter.

Deposition of carbon coatings by PVD-methods on polyurethane

Abstract In this article are compared the physico-mechanical properties of the specimens surfaces with deposited carbon nanostructured coatings by using PVD methods (vacuum laser assisted deposition and the plasma ion-assisted deposition methods and finely dispersed and colloidal solutions deposition) on polyurethane substrate. Laser assisted deposition occurred using two laser systems: continuous wave and pulsed laser action. The surface properties of the base substrate and the carbon coatings which were deposited on the polyurethane by using different methods have been investigated. The studies of the microstructure, topology, coatings thickness, roughness and the contact wetting angle were carried out by using the scanning electron, atomic force and optical microscopies.

Increasing effectiveness of conservative treatment of Peyronie’s disease

The study objectiveis to investigate the effectiveness of conservative treatment of Peyronie’s disease using antifibrotic agents.Materials and methods. The study included 32 patients aged 44–65 years with diagnosed Peyronie’s disease. The patients were examined using the International Index of Erectile Function, Visual Analog Scale, tunica albuginea of the penis elasticity testing, ultrasound examination (with intracavernous pharmacological testing and dynamic spiral tomography if necessary). The patients were divided into 2 groups. The patients in the treatment group (n = 16) received magnetic laser therapy at the area of the plaque with the “Mustang-urolog” device, as well as vacuum laser treatment with the “Yarovit” device and bovhyaluronidase azoximer as injections and later suppositories. In the comparison group (n = 16), similar therapy was used without bovhyaluronidase azoximer. Control examination was performed 3 months later. If conservative treatment was ineffective, the patients were forwarded to surgical treatment.Results. As a result of conservative treatment, density of the tunica albuginea and cavernous bodies, structure of the cavernous bodies improved; the International Index of Erectile Function increased, and pain intensity decreased. In the treatment group, a stronger tendency toward improvement was observed compared to the control group. Decrease in the plaque size was observed in 63.3 % of the patients in the treatment group and in 43.7 % of the patients in the control group. At the control examination, plaques weren’t detected in 6 and 3 patients, respectively.Conclusion. Conservative treatment is effective in patients with early stage of Peyronie’s disease with moderate curvature of the penis (<30°) and plaque size <2 cm. We recommend including bovhyaluronidase azoximer in the combination treatment regimen of these patients for increased effectiveness.

Effect of temporal asymmetry of the laser pulse on electron acceleration in vacuum

Abstract We explore the electron dynamics in a Gaussian laser pulse to assess the contribution of the temporal parameters to the mechanism of vacuum laser acceleration. Our numerical study reveals that a considerable nonzero energy gain can be achieved through a spatio-temporally controlled electron injection. Moreover, three-dimensional optimization of the electron’s injection angle suggests that the highest energy gain is obtained by using a sideways injection scheme. We also present the dependence of the electron energy gain on the laser polarization by comprehensively comparing all possible polarization states. It is found that the temporal sensitivity of the electron injection is determined by the polarization of the laser pulse. In addition, it is shown that although the energy gain is maximized in linearly polarized short laser pulses, circularly polarized laser pulses with longer duration would also lead to comparable energy gains but with much less temporal sensitivity for electron injection. To address the technical feasibility of the acceleration procedure under the mentioned optimized conditions, an experimental setup is proposed.

Ultrahigh-charge electron beams from laser-irradiated solid surface

Compact acceleration of a tightly collimated relativistic electron beam with high charge from a laser-plasma interaction has many unique applications. However, currently the well-known schemes, including laser wakefield acceleration from gases and vacuum laser acceleration from solids, often produce electron beams either with low charge or with large divergence angles. In this work, we report the generation of highly collimated electron beams with a divergence angle of a few degrees, nonthermal spectra peaked at the megaelectronvolt level, and extremely high charge (∼100 nC) via a powerful subpicosecond laser pulse interacting with a solid target in grazing incidence. Particle-in-cell simulations illustrate a direct laser acceleration scenario, in which the self-filamentation is triggered in a large-scale near-critical-density plasma and electron bunches are accelerated periodically and collimated by the ultraintense electromagnetic field. The energy density of such electron beams in high-Z materials reaches to [Formula: see text], making it a promising tool to drive warm or even hot dense matter states.

Surface modification of single-crystalline silicon carbide by laser irradiation for microtribological applications

Carbide-derived carbon layers were locally formed on the surface of 4H-SiC single crystalline wafers by irradiation with an infrared laser (λ = 9.3 μm) in a low vacuum. Longer laser irradiation time produced thicker carbon layers and improved the bonding continuity. For an irradiation duration of 4.5 s, the modified layer was formed without surface damage and identified as amorphous carbon containing gradient amount of Si with a layer thickness of several nm. The results of friction tests conducted on the modified layer under micro-loads using a friction force microscope revealed the improvement of lubricity of the modified layer with a friction coefficient of 0.01 that is lower than that of the original SiC surface. No wear tracks were detected after scanning the Si probe over the layer surface 10,000 times with maintaining lubricity. The layer also preserved its lubricity after heating up to 550 °C in an air atmosphere; however, the lubricity disappeared at 600 °C owing to decomposition of the lubricating layer caused by oxidation.

Full penetration welding of thick high tensile strength steel plate with high-power disk laser in low vacuum

ABSTRACTThis study was undertaken in order to investigate the effect of reduced ambient pressure from an atmospheric pressure (101 kPa) to 0.1 kPa on one-pass full penetration welding of thick high-tensile strength steel plate of 23 mm thickness. A 16 kW disk laser of 1030 nm in wavelength was employed to weld HT980 grade plates at the speed of 5–25 mm/s. In partial penetration welding, it was revealed that humping phenomena occurred easily. Full penetration welding of the high-tensile strength steel plates could not be achieved at 101 kPa. On the other hand, full penetration welding was obtained at the welding speed of less than 20 mm/s at the pressure of less than 10 kPa. Especially, at 0.1 kPa, and 17 and 20 mm/s, sound weld joints without defects were obtained. According to the observation results of a keyhole inlet and a surface molten pool during welding with a high-speed video camera, the melt in front of a keyhole was smaller and the behaviour of a keyhole and a plume was much more stable at 0.1 kPa than at 101 kPa. Moreover, in the full penetration welding, spattering was suppressed under the proper conditions. Such phenomena became more stable in fast welding. It was revealed in laser welding of thick high-tensile strength steel plates that the formation of narrow I-shaped weld beads by achieving full-penetration welding in low vacuum was essential for the production of sound welds without defect.

Gas-lens effect in kW-class thin-disk lasers.

We unveil a gas-lens effect in kW-class thin-disk lasers, which accounts in our experiments for 33% of the overall disk thermal lensing. By operating the laser in vacuum, the gas lens vanishes. This leads to a lower overall thermal lensing and hence to a significantly extended power range of optimal beam quality. In our high-power continuous-wave (cw) thin-disk laser, we obtain single-transverse-mode operation, i.e. M2 < 1.1, in a helium or vacuum environment over an output-power range from 300 W to 800 W, which is 70% broader than in an air environment. In order to predict the magnitude of the gas-lens effect in different thin-disk laser systems and gain a deeper understanding of the effect of the heated gas in front of the disk, we develop a new numerical model. It takes into account the heat transfer between the thin disk and the surrounding gas and calculates the lensing effect of the heated gas. Using this model, we accurately reproduce our experimental results and additionally predict, for the first time by means of a theoretical tool, the existence of the known gas-wedge effect due to gas convection. The gas-lens and gas-wedge effects are relevant to all high-power thin-disk systems, both oscillators and amplifiers, operating in cw as well as pulsed mode. Specifically, canceling the gas-lens effect becomes crucial for kW power scaling of thin-disk oscillators because of the larger mode area on the disk and the resulting higher sensitivity to the disk thermal lens.

Gold chloride cluster ions generated by vacuum laser ablation

In this work, a simple way for study the possibility of formation a vapor cluster species of tetrachloroauric acid (HAuCl4), using the laser ablation in the absence of a buffer or reactive atmosphere, and without a postablation supersonic expansion on a commercial matrix assisted laser desorption/ionization time-of-flight mass spectrometer, is reported. Tetrachloroauric acid is known as precursor for the synthesis of gold nanostructures and the complex salts; therefore it is an important task to discover and quantify the species arising from HAuCl4, in order to understand their role in the gold assisted reactions. Mass spectrum of HAuCl4 in a reflector negative-ion mode contains the hydrated mono- and dinuclear gold clusters in the m/z range 286–436, and gold chloride clusters in the m/z range 447–795. In the first part of spectrum, m/z range 286–436, the hydrated gold cluster species of type Au n − (H2O)m (n = 1–2; m = 1, 2, 5, 7, 8) and [Aun(OH)k]−(H2O)m (n = 1–2; k = 1–2; m = 1, 4–8) were found. Besides that, there are gold chloride clusters with general formula [AuHr(HCl)2]−(H2O)m (m = 1–5; 8–9; r = 0–2) in this part of spectrum. In the second part of spectrum, the m/z range 447–795, only gold chloride clusters were obtained. Their general formulae can be written as [AuClt(HCl)v]−(H2O)m (t = 1–4; v = 5–8; m = 2–4, 6–8) and [Aun(HCl)v]−(H2O)m (n = 1–2, v = 4–5, m = 1–2, 5, 7). The analysis of concentration effects on the LDI mass spectra of gold clusters reveals that the relative intensities of signals for the mono- and dinuclear Au clusters increase with decreasing the concentration of water HAuCl4 solutions.

Development of low vacuum laser welding technique using an aerodynamic window

Development of low vacuum laser welding technique using an aerodynamic window

Surface oxide and roughness on test samples for the Ultra High Vacuum section of the Laser Heater for the European XFEL

The European X-ray Free Electron Laser has recently started with operation for users. An approximately 3 m long ultra high vacuum laser heater section is implemented to overcome possible electron bunch instabilities. We describe the process of determining the oxide layer thickness and surface roughness on test samples of the internal surface material in the laser heater vacuum chambers using elastic recoil detection analysis and optical surface profiling. The results are compared to specified values and show that surface roughness on the samples is larger than the requested maximum, with RMS deviations from a mean plane of up to 1.76 μm for 0.60 × 0.45 square millimeter scans. The maximum oxide layer thickness is 5.5 nm on non-electropolished surfaces assuming cuprous oxide with density 6.0 g per cubic centimeter and 4.0 nm on electropolished surfaces.

Opto-Electron Eng, 2017, 44(10)] Research status and development prospects of laser welding under vacuum

Compared with conventional laser welding, the welding quality was improved significantly while the laser welding was conducted under vacuum. The penetration depth of the weld seam increased sharply. The welding formation was improved and the porosity defects were suppressed effectively. In recent years, numerous researches on the mechanism of welding process, low vacuum and local subatmospheric pressure laser welding equipments have been undertaken. The laser welding under vacuum exhibits the wonderful application prospects to weld the thick plates in the shipbuilding, nuclear instrument and pressure vessel industries. The influences of ambient pressure on the laser welding penetration depth, surface formation and porosity defect were summarized. The domestic and overseas research findings on mechanism of laser welding under vacuum were elaborated from the aspects of plasma plume, keyhole and molten pool behaviors. In addition, the applications of laser welding under vacuum in the industry were introduced. Finally, the problems of reported researches were analyzed and the prospects of the technology were discussed.

Selective Thermochemical Growth of Hierarchical ZnO Nanowire Branches on Silver Nanowire Backbone Percolation Network Heaters

The development of hydrothermal growth enabled facile growth of ZnO nanowire over a large area at mild environments, yet its usage in actual electronic devices has been restricted due to poor spatial selectivity in the growth and a difficulty in the integration to other components. We introduce a local thermochemical growth of ZnO nanowire branches on Ag nanowire backbone percolation networks by electrical current induced local resistive heating in liquid environment for easy fabrication of metal–semiconductor hierarchical nanowire structure. Through vacuum filtration transfer and laser ablation process, patterned conductive network composed of ZnO nanoparticle functionalized Ag nanowire percolation network was prepared as a conductive network for localized resistive heating. Upon the application of proper bias voltage, highly localized temperature field was generated in the vicinity of the patterned Ag nanowire network heater to induce the selective growth of ZnO nanowire from the Ag nanowire backbone. As the temperature rise is related to the electrical current flow, ZnO nanowire was selectively synthesized at the area subject to the maximum current density, which was defined by the laser ablation technique. It was further confirmed that the characteristics of grown ZnO nanowires could be controlled by changing the growth condition.

Light-Induced Degradation of CH3NH3PbI3 Hybrid Perovskite Thin Film

The stability of CH3NH3PbI3 was investigated by observing the degradation in a coevaporated film irradiated by a blue laser in ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were employed to investigate the effects of irradiation on the surface. The core levels of CH3NH3PbI3 were observed to shift toward a higher binding energy (BE) during the irradiation, suggesting that the surface became more n-type. A new metallic Pb component in the XPS spectrum appeared after 120 min of irradiation, indicating that the film had started to decompose. The decomposition saturated after about 480 min of irradiation when the ratio of metallic Pb to total Pb was about 33%. Furthermore, the film was no longer continuous after irradiation, as the elements gold and oxygen from the substrate were detected by XPS. SEM images also show a roughened surface after irradiation. The results strongly indicate that CH3NH3PbI3 is sensitive to the laser irradiation and that the light indu...

Oxidation and thermal shock behavior of thermal barrier coated 18/10CrNi alloy with coating modifications

In this study, substrates of 18/10CrNi alloy plates were initially sprayed with a Ni-21Cr-10Al-1Y bond coat and then with an yttria stabilized zirconia top coat by plasma spraying. Subsequently, plasma-sprayed Thermal barrier coatings (TBCs) were treated with two different modification methods, namely, vacuum heat treatment and laser glazing. The effects of modifications on the oxidation and thermal shock behavior of the coatings were evaluated. The effect of coat thickness on the bond strength of the coats was also investigated. Results showed enhancement of the oxidation resistance and thermal shock resistance of TBCs following modifications. Although vacuum heat treatment and laser glazing exhibited comparable results as per oxidation resistance, the former generated the best improvement in the thermal shock resistance of the TBCs. Bond strength also decreased as coat thickness increased.

Effect of Cr2AlC content on the properties of a Cu-Cr2AlC composite

A Cu-Cr2AlC composite material is prepared by low-temperature hot-press sintering and the electrical properties of the composite material are discussed in this paper. The phase composition and microstructures of the synthesized composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Brinell hardness and conductivity were tested by Brinell and conductivity testing machines. The arc erosion properties and surface morphologies were tested by a vacuum electric breakdown testing machine and laser scanning confocal microscopy. The results show the Cu-Cr2AlC composite performs best with a Cr2AlC volume fraction of 20%, at this volume fraction, the hardness and conduction rate are 94.3 HB and 41.93%, respectively, and arc erosion ability of the composite has the best. It also has the lowest ablation crater depth, a small area, and a low breakdown-class ablation pit density.

Vacuum laser acceleration of relativistic electrons using plasma mirror injectors

Accelerating particles to relativistic energies over very short distances using lasers has been a long standing goal in physics. Among the various schemes proposed for electrons, vacuum laser acceleration has attracted considerable interest and has been extensively studied theoretically because of its appealing simplicity: electrons interact with an intense laser field in vacuum and can be continuously accelerated, provided they remain at a given phase of the field until they escape the laser beam. But demonstrating this effect experimentally has proved extremely challenging, as it imposes stringent requirements on the conditions of injection of electrons in the laser field. Here, we solve this long-standing experimental problem for the first time by using a plasma mirror to inject electrons in an ultraintense laser field, and obtain clear evidence of vacuum laser acceleration. With the advent of PetaWatt class lasers, this scheme could provide a competitive source of very high charge (nC) and ultrashort relativistic electron beams.

Relativistic ponderomotive Hamiltonian of a Dirac particle in a vacuum laser field

We report a point-particle ponderomotive model of a Dirac electron oscillating in a high-frequency field. Starting from the Dirac Lagrangian density, we derive a reduced phase-space Lagrangian that describes the relativistic time-averaged dynamics of such a particle in a geometrical-optics laser pulse propagating in vacuum. The pulse is allowed to have an arbitrarily large amplitude (provided radiation damping and pair production are negligible) and a wavelength comparable to the particle de Broglie wavelength. The model captures the Bargmann-Michel-Telegdi (BMT) spin dynamics, the Stern-Gerlach spin-orbital coupling, the conventional ponderomotive forces, and the interaction with large-scale background fields. Agreement with the BMT spin precesison equation is shown numerically. The commonly known theory in which ponderomotive effects are incorporated in the particle effective mass is reproduced as a special case when the spin-orbital coupling is negligible. This model could be useful for studying laser-plasma interactions in relativistic spin-$1/2$ plasmas.