Coupling Efficiency Of Laser Energy Research Articles

Formation and mechanical properties of AZ31B magnesium alloy welds by power-modulated oscillating laser

an energy-space modulated laser was proposed in welding workpieces with AZ31B magnesium alloy to enhance the efficiency of heat transfer and reduce defects of welds, and the proposed laser was power-modulated oscillating laser (PMOL). To validate the performance of the proposed technique, non-penetration welding and butt welding were conducted on the workpieces with a thickness of 5 mm. The impact of power-modulation parameters was investigated at the aspects of micro-structures, appearance, and mechanical properties of welds. The results showed that in comparison with a conventional laser (CL) or circular oscillation laser (COL), an energy-space modulated laser led to (1) an increased depth of weld penetration and (2) an enhanced coupling efficiency of laser energy. The parameters of power modulation could be optimized to reduce the defects of welds such as spatter, underfill, and thick fish scales significantly reduced; it refined the equiaxed grains in the center area of weld. When the frequency and amplitude of power-modulation were set as 50 Hz and 500 W, respectively, the mechanical properties of the weld were improved with the ultimate tensile strength of 238 MPa and an elongation ratio of 13.9 % which were 96.1 % and 58.7 % of those of the base material (BM), respectively. In comparison with the results welded by CL and COL, the ultimate tensile strength by PMOL was increased by 13.9 % and 4.4 %, and the elongation was increased by 54.4 % and 8.6 %, respectively.

Design, Fabrication, and Characterization of a Laser-Controlled Explosion-Initiating Device with Integrated Safe-and-Arm, EMP-Resistant, and Fast-Acting Technology Based on Photovoltaic Power Converter.

To augment the intelligence and safety of a rocket or ammunition engine start, an intelligent initiation system needs to be included in the data link. A laser-controlled intelligent initiation system with inherent safety and a laser-controlled explosion-initiating device (LCEID) incorporating electromagnetic pulse (EMP) resistant, safe-and-arms fast-acting modular device based on photovoltaic power converter technology is designed and fabricated in this work. LCEID is an integrated multi-function module consisting of the optical beam expander, GaAs photovoltaic (PV) array, safe-and-arms integrated circuit, and low-energy initiator. These components contribute to EMP resistance, fast-acting, safe-and-arm, and reliable firing, respectively. To achieve intelligent initiation, each LCEID has a unique “identification information” and a “broadcast address” embedded in integrated-circuit read-only memory (ROM), which is controlled by encoded laser addressing. The GaAs PV array was investigated to meet the low-energy initiator firing voltage requirements. Experimental results show that the open-circuit voltage, short-circuit current, and maximum power output of the four-junction GaAs PV array illuminated by a 5.5 W/cm2 laser beam were 220 mA, 21.5 V, and 3.70 W, respectively. When the voltage of the 22 μF energy storage capacitor exceeds 20 V, the laser charging time is found to be shorter than 2.5 s. Other aspects of LCEID, such as laser energy coupling efficiency, the firing process, and the energy-boosting mechanism, were explored. Measurements show that the coupling efficiency of the micro lens with a radius of curvature D = 20 μm and size of r = 50 μm reaches a maximum of 93.5%. Furthermore, for more than 18 V charge voltage, the LCEID is found to perform reliably. The fabricated LCEID demonstrated a high level of integration and intrinsic safety, as well as a finely tailored initiation performance that could be useful in military applications.

Effects of confining layer and ablating layer on laser-induced shock wave characteristics during laser shock processing by PVDF gauge

The PVDF sensor is used to obtain the time variation curves of the shock wave pressure in the four states of the laser shock peening with or without confining layer and with or without ablating layer, the effects of confining layer and ablating layer on shock wave characteristics are analyzed. The results show that the confining layer limits the plasma expansion area, and the inverse bremsstrahlung absorption effect is enhanced, so that the peak pressure and pulse width of the shock wave are increased to 10 times and 2 times that under the condition of the without confining layer; The ablating layer significantly increases the peak pressure of the shock wave by improving the laser energy coupling efficiency. However, the influence of the ablating layer on the peak pressure tends to saturate with the increase of power density, the difference of the peak pressure is only 8.5% when the laser power density is 4.53GW/cm2. In addition, the ablating layer complicates the shock wave propagation process, and the shock wave curve presents a multi-peak structure.

An investigation on physical phenomena of water-jet assisted underwater wet laser welding technique under continuous and pulsed mode operation

This work presents the development and demonstration of a novel underwater wet laser welding technique, where unlike the conventional underwater wet laser welding, a water-jet parallel to the weld pool surface was introduced inside the water environment. The water flow was found to enhance the removal of water vapour formed at the laser-water-workpiece interface without creating hindrance to the molten weld bead, resulting in lower energy loss due to absorption and scattering, thus enhancing the laser energy coupling efficiency to the material. The process was studied for bead-on-plate welding on 4 mm stainless steel (SS) sheets using a 2 kW Yb-Fiber laser (wavelength 1.07 µm), with water column height of 5–15 mm, and a parallel water jet of speed 2–3 m/s, under continuous (CW) and pulsed mode (modulated power) laser operation. The major objective of the work was to study the effect of water flow and mode of laser operation on the formation and growth of vapour layer at the processing zone and investigate its effect on laser energy coupling efficiency. Further, their effect on weld bead geometry and microstructure was investigated. Welding operation with modulated laser power was found to produce higher weld depth compared to CW mode with same average laser power due to lower scattering loss and better energy coupling under pulsed mode operation. The micro-structural investigation has showed the formation of finer grains and oxide or carbide precipitation in case of underwater welding, indicating the enhanced cooling phenomena in this technique.

Experimental progress of inertial confinement fusion based at the ShenGuang-III laser facility in China

The ShenGuang-III (SG-III) laser facility was developed by the laser fusion research center (LFRC) for inertial confinement fusion (ICF) studies in China. Over 80 diagnostics have been installed at the SG-III laser facility, including optical diagnostics, x-ray imaging diagnostics, x-ray spectrum diagnostics, fusion product diagnostics and general diagnostics assistant systems, as well as central control and data acquisition systems. Various ICF experiments have also been performed at the SG-III laser facility. The first experiment explored the laser-target coupling process, including investigations of hohlraum radiation flux and laser energy coupling efficiency. The second experiment explored ablation and implosion physics, including shell asymmetry and implosion trajectory. The third experiment explored stagnation, hotspot dynamics and the nuclear phase of the implosion.

Combining two-photon-polymerization with UV-lithography for laser particle acceleration targets

In the field of laser driven particle acceleration, many experiments with plain foil targets have been performed to investigate ion acceleration mechanisms such as TNSA and BOA. A more complex target geometry, for which the front and rear surface can be structured in a deterministic way, will lead to a better understanding of e.g. the actual source size, laser contrast related effect to the coupling efficiency of laser energy into the target of specifically shaped target surfaces or means to reduce the particle beam divergence such as ballistic focusing. A promising technology to manufacture complex 3D freeform targets is the two-photon-polymerization. With resolutions better than 200 nm, it is possible to create small targets with microstructured surfaces. We implemented the combination of two-photon-polymerization and UV-lithography to facilitate the handling of such small structures.

Enhancing the electron acceleration by a circularly polarized laser interaction with a cone-target with an external longitudinal magnetic field

The propagation of left-hand (LH-) and right-hand (RH-) circularly polarized (CP) lasers and the accompanying generation of fast electrons in a magnetized cone-target with pre-formed plasmas are investigated. In this work, the strength of external magnetic field is comparable to that of the incident laser. Theoretical analyses indicate that the cut-off density of LH-CP laser is larger than that without an external magnetic field. When the external magnetic field normalized by the laser magnetic field is larger than the relativistic factor, the RH-CP laser will keep on propagating till the laser energy is depleted. The theoretical predictions are confirmed by two-dimensional particle-in-cell simulations. Simulation results show that in the presence of external longitudinal magnetic field, the energies and yields of fast electrons are greatly enhanced for RH-CP laser. Besides, the coupling efficiency of laser energy to energetic electrons for RH-CP laser is much higher than that for LH-CP laser and without external magnetic field. Furthermore, detailed simulation results perform an enhancement of the incident laser absorption with increasing external magnetic field.

Laser irradiation uniformity for polar direct drive on ShenGuang III facility

Inertial confinement fusion utilizes sufficient laser beams to directly illuminate a spherical capsule, or convert the laser into thermal X-rays inside a high Z hohlraum to drive capsule implosion. The direct drive implosion is one of ways toward central ignition and similar to the indirect drive implosion, but has higher laser energy coupling efficiency and the potential for higher-gain implosion than indirect drive, and needs stringent laser condition. In order to develop and execute the direct drive experiment on the laser facility, which is configured initially for indirect drive, the polar direct drive has been proposed and validated on the Omega laser facility and the National Ignition Facility. The polar direct drive repoints some of the beams toward the polar and equator of the target, thus increasing the drive energy on the polar and equator of capsule and achieving the most uniform irradiation. The present article focuses on the laser irradiation uniformity of the target in polar direct drive on ShenGuangIII (SGIII) facility. Firstly, the laser beam configuration of the SGIII, the characteristics of laser spots, the laser beam repointing strategy and the principle of optimization are introduced. The 48 laser beams are distributed over four cones per hemisphere and the beam centerlines are repointed in polar direct drive. The continuous phase plates (CPPs) of the SGIII are designed to have unique shape to make the laser beam with a 250 m-radius circular section at the laser entrance hole in indirect drive, and thus the laser beams have ellipse cross sections with fixed major axis and different minor axes in different cones. Then, the irradiation uniformity of 540 m target is optimized by the three-dimensional (3D) view factor method on the assumption that the laser intensity distribution is super-Gaussian with three and five orders, and the energy deposition distributions are expressed as cos2 and cos . The irradiation nonuniformity of less than 5% on the polar direct drive capsule of 540 m in diameter is achieved. The pressure distribution of the hot spot at the neutron bang time with the optimized parameter is also simulated by Multi2D, and the results of 2D hydrodynamics simulation indicate that the hot spot under the assumption of cos distribution is more symmetric. Finally, the effects on irradiation uniformity of the beam-to-beam power imbalance, the repointing error and the target pointing error are estimated by the Monte Carlo method. According to the simulation results, the laser root mean square nonuniformity on the target will not become worse observably when the maximal beam-to-beam power imbalance is limited to a value of 5%, and the repointing error and the target pointing error are better than 7 m.

Numerical study of the effect of laser-arc distance on laser energy coupling in pulsed Nd: YAG laser/TIG hybrid welding

A three-dimensional (3-d) computational fluid dynamics (CFD) based model in which a ray-tracing algorithm was combined with the volume of fluid (VOF) method was used to simulate a pulsed Nd: YAG laser /tungsten inert gas (TIG) hybrid welding process. The calculated weld cross sections in pure laser welding and hybrid welding process were validated by the experimental results, respectively. Then, the variations of keyhole shape and laser energy coupling behaviour in the laser-arc hybrid welding process were calculated at three different laser-arc distances. It was found that the laser-arc distance would affected the shape of keyhole generated in the laser-arc hybrid welding process, thus influencing the laser energy coupling efficiency. In the initial stage of each laser pulse, small laser-arc distances favoured the generation of keyholes and multiple reflections of laser beam within keyhole. However, as the laser irradiation time increased, the smaller the laser-arc distance, the more liquid metal were found around the keyholes, which was more favourable to the expansion of keyhole in radial direction. As a result, the aspect ratio of keyhole, the reflection time of laser beams in keyhole, and the laser energy absorbed by keyhole wall decreased. Given that the interaction between laser and arc was neglected, the maximum keyhole depth increased with reducing laser-arc distances, while the total laser energy coupling efficiency decreased as the laser-arc distance was reduced.

Modulated fiber laser welding of high reflective AZ31

Power sine modulation fiber laser welding (FLW) of AZ31 magnesium alloy was conducted. When the diameter of transmission fiber and focal spot was 400 μm and 0.4 mm, respectively, the average laser powers for penetrating a 2.7-mm-thick AZ31 plate at a welding speed of 5 m/min were reduced by about 33 % by using modulated FLW. The relationship between modulation parameters (i.e., amplitude, frequency, and average laser power) and weld depth which was closely related to the transfer efficiency of laser energy was studied through partial penetration laser welding test on 8-mm-thick AZ31 plate based on quadratic regression orthogonal design. It was found that influence of power modulation on laser welding of AZ31 alloy was highly dependent on laser power density. In the low power density range, laser energy coupling efficiency could be significantly improved by combining low amplitude with high frequency or high amplitude with low frequency. With the increasing of laser power density, the optimum frequency corresponding to maximum weld depth decreased, and the positive effect of the favorable combination of high amplitude and low frequency on laser energy coupling continuously weakened. When laser power density was high enough, power modulation had hardly positive effect on weld depth and energy coupling efficiency. It was argued that improvement of energy coupling efficiency in laser welding of AZ31 by using power modulation was due to the reduction in the portion of energy lost into surroundings. Finally, laser butt welding was conducted on 2.7-mm-thick AZ31 under the condition of high beam quality and the tensile strength of both butt-welded joint and base metal that was tested.

Weld outline comparison between various pulsed Nd:YAG laser welding and pulsed Nd:YAG laser–TIG arc welding

In this study, four welding methods, pulsed laser welding, defocused pulsed laser welding, laser–arc tandem welding, and laser–arc hybrid welding, were tested on 304 stainless steel sheets, and the differences between the weld cross-section outlines, especially the weld depth, were compared under different welding conditions. The author suggests that during the hybrid welding process, the upper part of the keyhole was surrounded by a large amount of melt such that the evaporation recoil pressure caused the gas–liquid interface to move towards the solid–liquid interface near weld pool surface. This flow behavior promoted both heat transfer from the central region of the weld pool to the solid–liquid interface near the weld pool surface, which resulted in an increased weld width, and increased the diameter of the upper part of the keyhole, which reduced the number of times the laser beam is reflected inside the keyhole and the coupling efficiency of laser energy. During pulse Nd:YAG laser–TIG arc tandem welding, there was a smaller amount of melt around the keyhole, which made it easier to form a U-shaped keyhole, which benefited the multiple reflection of laser beam in the keyhole and promoted the coupling of laser energy. The weld depths obtained from tandem welding and laser–arc hybrid welding were similar, which may be related to the difference in the keyhole shapes produced from the two welding processes.

Effect of defocusing on picosecond laser-coupling into gold cones

Here, we show that defocusing of the laser in the interaction of a picosecond duration, 1.053 μm wavelength, high energy pulse with a cone-wire target does not significantly affect the laser energy coupling efficiency, but does result in a drop in the fast electron effective temperature. This may be beneficial for fast ignition, since not only were more electrons with lower energies seen in the experiment but also the lower prepulse intensity will reduce the amount of preplasma present on arrival of the main pulse, reducing the distance the hot electrons have to travel. We used the Vulcan Petawatt Laser at the Rutherford Appleton Laboratory and gold cone targets with approximately 1 mm long, 40 μm diameter copper wires attached to their tip. Diagnostics included a quartz crystal imager, a pair of highly oriented pyrolytic graphite crystal spectrometers and a calibrated CCD operating in the single photon counting regime, all of which looked at the copper Kα emission from the wire. A short pulse optical probe, delayed 400 ps relative to the main pulse was employed to diagnose the extent of plasma expansion around the wire. A ray-tracing code modeled the change in intensity on the interior surface of the cone with laser defocusing. Using a model for the wire copper Kα emission coupled to a hybrid Vlasov-Fokker-Planck code, we ran a series of simulations, holding the total energy in electrons constant whilst varying the electron temperature, which support the experimental conclusions.

Surface structuring of fused silica with asymmetric femtosecond laser pulse bursts

Fused silica surface structuring has been performed using temporally shaped femtosecond laser pulses. For this purpose we have designed pulse bursts with a triangular intensity envelope and different slope sign and interpulse separation that were experimentally generated using a home-made temporal pulse shaper. We have found that pulse bursts with decreasing intensity envelopes are remarkably more efficient in terms of surface ablation than bursts with increasing intensity envelopes. The results reveal that laser energy coupling in the material is enhanced as the interpulse spacing decreases. A study of the ablation depth using stretched single pulses was carried out and compared to results obtained for pulse bursts with different interpulse spacing. We find that the deepest crater was achieved with bursts of 0.5 ps interpulse separation and decreasing envelope. This pulse form also induced the largest change of the surface reflectivity after irradiation. The results are discussed in terms of how the laser energy coupling efficiency is linked to the temporal pulse shape.

Simulation on the Effects of Misaligned Coupling on the Output Intensity Distribution in Water-Jet Guided Laser

Water-jet guided laser machining is a kind of material processing technology using water optical waveguide which is formed by coupling a high energy laser beam into variable-length water jet. In order to design the coupling unit and form the effective energy-jet, the research on the distribution of output intensity is beneficial to understand the structure of the coupling unit and improve the coupling efficiency of laser energy. This paper lists the different coupling misalignments in the coupling unit when laser couplings into water-jet. In this paper, the distribution of energy output intensity in water-jet guided laser is simulated with the ray trace theory under several different types of coupling misalignments with ZEMAX software, the results show that misaligned coupling provide various morphology of energy output intensity distribution: center peak morphology, ring peak morphology, and uniform peak morphology, which provides a method to optimize the energy output intensity distribution of water-jet guided laser.

Optimum Hot Electron Production with Low-Density Foams for Laser Fusion by Fast Ignition

We propose a foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition. A thin low-density foam is proposed to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.