Divergence angle consideration in energy spread measurement for high-quality relativistic electron beam in laser wakefield acceleration

The thorough exploration of the transverse quality represented by divergence angle has been lacking yet in the energy spread measurement of the relativistic electron beam for laser wakefield acceleration(LWFA). In this work, we fill this gap by numerical simulations based on the experimental data, which indicate that in a C-shape magnet, magnetic field possesses the beam focusing effect, considering that the divergence angle will result in an increase in the full width at half maxima(FWHM) of the electron density distribution in a uniformly isotropic manner, while the length-to-width ratio decreases. This indicates that the energy spread obtained from the electron deflection distance is smaller than the actual value, regardless of the divergence angle. A promising and efficient way to accurately correct the value is presented by considering the divergence angle(for instance, for an electron beam with a length-to-width ratio of 1.12, the energy spread correct from 1.2% to 1.5%), providing a reference for developing the high-quality electron beam source.

Laser shaping and optical power limiting of pulsed Laguerre–Gaussian laser beams of high-order radial modes in fullerene C60

We study the strong nonlinear optical dynamics of nanosecond pulsed Laguerre–Gaussian laser beams of high-order radial modes with zero orbital angular momentum propagating in the fullerene C60molecular medium. It is found that the spatiotemporal profile of the incident pulsed Laguerre–Gaussian laser beam is strongly reshaped during its propagation in the C60molecular medium. The centrosymmetric temporal profile of the incident pulse gradually evolves into a noncentrosymmetric meniscus shape, and the on-axis pulse duration is clearly depressed. Furthermore, the field intensity is distinctly attenuated due to the field-intensity-dependent reverse saturable absorption, and clear optical power limiting behavior is observed for different orders of the input pulsed Laguerre–Gaussian laser beams before the takeover of the saturation effect;the lower the order of the Laguerre–Gaussian beam, the lower the energy transmittance.

Internal phase control of fiber laser array based on photodetector array

Coherent beam combining(CBC) of fiber laser array is a promising technique to realize high output power while maintaining near diffraction-limited beam quality. To implement CBC, an appropriate phase control feedback structure should be established to realize phase-locking. In this paper, an innovative internal active phase control CBC fiber laser array based on photodetector array is proposed. The dynamic phase noises of the laser amplifiers are compensated before being emitted into free space. And the static phase difference compensation of emitting laser array is realized by interference measurement based on photodetector array. The principle of the technique is illustrated and corresponding simulations are carried out, and a CBC system with four laser channels is built to verify the technique. When the phase controllers are turned on, the phase deviation of the laser array is less than λ/20, and ~ 95% fringe contrast of the irradiation distribution is obtained. The technique proposed in this paper could provide a reference for the system design of a massive high-power CBC system.

Relativistic Self-Focusing of Hermite-cosine-Gaussian Laser Beam in Collisionless Plasma with Exponential Density Transition

This work reveals an exploration of self-focusing of Hermite-cosine-Gaussian laser beam in a collisionless plasma under relativistic nonlinearity. Self-focusing along with self-trapping of Hermite-cosine-Gaussian laser beam are analyzed for different values of laser intensity, plasma density, and decentered parameters. Mathematical analysis displays that these parameters play a major role in achieving the stronger and earlier self-focusing. Further, a comparative study between self-focusing of Hermite-cosine-Gaussian laser beam with and without exponential density ramp profile is introduced. Plasma density transition with exponential profile is found to be more effective in order to have stronger self-focusing. The present analysis may lead to very useful applications in the field of efficient harmonic generation, laser driven fusion etc.

Development of a high-repetition-rate lumped-inductance kicker magnet prototype for the beam switchyard of SHINE

The Shanghai high-repetition-rate X-ray free-electron laser and extreme light facility(SHINE)operates at a maximum repetition rate of 1 MHz.Kicker magnets are key components that distribute electron bunches into three different undulator lines in a bunch-by-bunch mode.The kicker field width must be less than the time interval between bunches.A lumpedinductance kicker prototype was developed using a vacuum chamber with a single-turn coil.The full magnetic field strength was 0.005 T.This paper presents the requirements,design considerations,design parameters,magnetic field calculations,and measurements of the kicker magnets.The relevant experimental results are also presented.The pulse width of the magnetic field was approximately 600 ns,and the maximum operation repetition rate was 1 MHz.The developed kicker satisfies the requirements for the SHINE project.Finally,numerous recommendations for the future optimization of kicker magnets are provided.

Effect of process parameters on the morphology of aluminum/copper alloy lap joints by red and blue hybrid laser welding

In order to overcome the problems of many pores,large deformation and unstable weld quality of traditional laser welded aluminumcopper alloy joints,a red-blue dual-beam laser source and a swinging laser were introduced for welding.T2 copper and 6063 aluminum thin plates were lap welded by coaxial dual-beam laser welding.The morphology of weld cross section was compared to explore the influence of process parameters on the formation of lap joints.The microstructure characteristics of the weld zone were observed and compared by optical microscope.The results show that the addition of laser beam swing can eliminate the internal pores of the weld.With the increase of the swing width,the weld depth decreases,and the weld width increases first and then decreases.The influence of welding speed on the weld cross section morphology is similar to that of swing width.With the increase of welding speed,the weld width increases first and then decreases,while the weld depth decreases all the time.This is because that the red laser is used as the main heat source to melt the base metals,with the increase of red laser power,the weld depth increases.As an auxiliary laser source,blue laser reduces the total energy consumption,consequently,the effective heat input increases and the spatter is restrained effectively.As a result,the increase of red laser power has an enhancement effect on the weld width and weld depth.When the swing width is 1.2 mm,the red laser power is 550 W,the blue laser power is 500 W,and the welding speed is 35 mm/s,the weld forming is the best.The lap joint of T2 copper and 6063 aluminum alloy thin plate can be connected stably with the hybrid of blue laser.The effect rules of laser beam swing on the weld formation were obtained,which improved the quality of the joints.

Formation, microstructure and mechanical properties of double-sided laser beam welded Ti-6Al-4V T-joint

The T-joints of Ti?6Al?4V alloy were manufactured by double-sided synchronized laser beam welding with the homologous filler wire. The formation, microstructure and mechanical properties of welded joints as well as the correlations of each other were investigated. The results indicate that the quality of weld seams is good without defects such as discontinuity, beading, visible cracks or porosity, which is linked to the steady molten pool behavior and droplet transition. The morphologies of the heat affected zone (HAZ) located on the skin and stringer are disparate. The microstructure of the HAZ and fusion zone (FZ) is mainly comprised of acicular martensiticα′ phases. The microhardness of the HAZ and FZ is higher than that of the base metal (BM) and reaches a maximum value at the HAZ near FZ on the stringer. The tensile specimens along the skin and stringer fractured at the BM with ductile fracture surfaces.

Highly-Strained InGaAs/GaAs Single-Quantum-Well Lasers Grown by Molecular Beam Epitaxy

Highly stained InGaAs/GaAs Quantum Wells (QW) are grown by using molecular beam epitaxy.The room-temperature photoluminescence (PL) peak wavelength as long as 1160nm is obtained from QW with the In composition of 38% and the well width of 6 8nm.The full-width at half-maximum of the PL peak is 22meV,indicating a good quality.InGaAs/GaAs QW ridge-waveguide lasers with emission wavelength of 1120nm are demonstrated.For 100-μm-wide ridge-waveguide lasers with a cavity length of 800μm,the kink-free output power up to 200mW is achieved with the slope efficiency of 0 84mW/mA under the continue-wave operation.For 10μm-wide ridge-waveguide lasers,the lowest threshold current density of 450A/cm2 and the characteristic temperature of 90K are obtained.

High-Power and High-Efficiency 650nm-Band AlGaInP Visible Laser Diodes Fabricated by Ion Beam Etching

High power and high-slope efficiency 650nm band real-refractive-index ridge w aveguide AlGaInP laser diodes with compressive strained MQW active layer are for med by pure Ar ion beam etching process.Symmetric laser mesas with high perpendi cularity,which are impossible to obtain by traditional wet etching method due to the use of a 15°-misoriented substrate,are obtained by this dry etching metho d.Laser diodes with 4μm wide,600μm long and 10%/90% coat are fabricated.Th e typical threshold current of these devices is 46mA at room temperature,and a s table fundamental-mode operation over 40mW is obtained.Very high slope efficien cy of 1.4W/A at 10mW and 1.1W/A at 40mW are realized.

Optimum Design of the Control System for a Laser Beam Riding Guidance Anti-Tank Missile

The optimum theory and methods were adopted to design the laser beam riding guidance anti tank missile's control system in the short run. Through building the mathematical model of system, selecting a proper method and taking advantage of computer's high speed calculation and logic traits, an optimal controller was designed. Simulation results showed that the designed control system has fair performance and it satisfies the tactical and technical requirements. The results also demonstrate that by the combination of the optimizing methods and the computer the control system could be designed as soon as possible.

Liquation cracking in laser beam welded joint of ZK60 magnesium alloy

The ZK60 magnesium alloy plates were welded by laser beam welding (LBW) and the microstructures in the partially melted zone (PMZ) of welded joints were investigated. For the as-cast alloy, the eutectoid mixtures along grain boundaries (GBs) in the PMZ are liquefied during welding, and their re-solidified materials present hypoeutectic characters, which lead to more severe segregation of the Zn element along GBs, and thus enhance the cracking tendency of the PMZ. The main reasons for liquation cracking of PMZ are described as that the absence of liquid at the terminal stage of solidification leads to the occurrence of shrinkage cavities in PMZ, from which liquation cracking initiates, and propagates along the weakened GBs under the tensile stress originating from solidification shrinkage and thermal contraction. Lower heat input can reduce the cracking tendency, and the plastic processing such as rolling also contributes to the mitigation of PMZ liquation cracking by reducing the size of eutectoid phases and changing their distribution in the base metal.

Novel Method for Spatial Angle Measurement Based on Rotating Planar Laser Beams

Spatial angle measurement, especially the measurement of horizontal and vertical angle, is a basic method used for industrial large-scale coordinate measurement. As main equipments in use, both theodolites and laser trackers can provide very high accuracy for spatial angle measurement. However, their industrial applications are limited by low level of automation and poor parallelism. For the purpose of improving measurement efficiency, a lot of studies have been conducted and several alternative methods have been proposed. Unfortunately, all these means are either low precision or too expensive. In this paper, a novel method of spatial angle measurement based on two rotating planar laser beams is proposed and demonstrated. Photoelectric receivers placed on measured points are used to receive the rotating planner laser signals transmitted by laser transmitters. The scanning time intervals of laser planes were measured, and then measured point＇s horizontal/vertical angles can be calculated. Laser plane＇s angle parameters are utilized to establish the abstract geometric model of transmitter. Calculating formulas of receiver＇s horizontal/vertical angles have been derived. Measurement equations＇ solvability conditions and judgment method of imaginary solutions are also presented after analyzing. Proposed method for spatial angle measurement is experimentally verified through a platform consisting of one laser transmitter and one optical receiver. The transmitters used in new method are only responsible for providing rotating light plane signals carrying angle information. Receivers automatically measure scanning time of laser planes and upload data to the workstation to calculate horizontal angle and vertical angle. Simultaneous measurement of multiple receivers can be realized since there is no human intervention in measurement process .Spatial angle measurement result indicates that the repeatable accuracy of new method is better than 10＂. Proposed method can improve measurement＇s automation degree and speed while ensuring measurement accuracy.

INVESTIGATION OF LASER BEAM WELDING PROCESS OF AZ61 MAGNESIUM-BASED ALLOY

Laser welling process of AZ61 magnesium alloys is investigated using a special CO2 laser experimental system. The effect of processing parameters including laser power, welling speed, and protection gas flow at the top and bottom is researched. The results show that an ideal well bead can be formed by choosing the processing parameters properly. An optimized parameter range is obtained by a large number of experiments. Among them, laser power and welling speed are the two main parameters that determine the well width and dimensions. The protect gas flow rate has a slight effect on the well width, but it directly effects the surface color of the well. The test results for typical welds indicate that the microhardness and tensile strength of the well zone are better than that of the base metal A fine-grained well region has been observed and no obvious heat-affected zone is found. The well zone mainly consists of small α-Mg phase, （α ＋ Al12Mg17）, and other eutectic phases. The small grains and the eutectic phases in the joint are believed to play an important role in the increase of the strength of wells for AZ61 magnesium alloys.

Effects of Powder Feeding Rate on Interaction between Laser Beam and Powder Streamin Laser Cladding Process

A theoretical model was presented to calculate the laser intensity distribution and the particle temperatures at different sites of the workpiece in the laser cladding process. By using this model, the effects of the powder feeding rate on the laser intensity distribution and the particle temperatures were investigated, the calculated results under the condition of different injection angles were also plotted. It is shown that with increasing the injection angle, the laser intensity distributions are similar but the peak value of the laser intensity decreases. Simultaneously, the peak value of the particle temperature increases and the distribution of the particle temperatures gets central symmetrical gradually. These tests results should be considered in model of laser cladding due to their subtle effects on the dynamic processes in laser molten pool.

EXPERIMENTAL STUDY OF FRETTING WEAR OF TITANIUM ALLOY BEFORE AND AFTER LASER BEAM QUENCHING

The effects of amplitudes, normal loads and laser beam quenching on the fretting wear of titanium alloy (TC11) were experimentally investigated on SRV fretting wear test machine in air, at room temperature and without lubrication conditions. The purpose of this study is to learn the rules of fretting wear in a disk blades dovetail joint of an aircraft turbine so the test parameters are determined based on the relative movement and load in the joint. The wear depths are measured by a profilometer, the worn areas are observed and measured by an optical microscopy, and the microtopography of the worn scar is studied by scanning electron microscopy (SEM) .The tests and observations state clearly that fretting wear rate (FWR) is heavily influenced by sliding amplitude(SA) and load. In this experiment, if SA is greater than 60 μm at Hertz contact stress 105 MPa, the FWR is much higher, and the SEM makes it known that the wear mechanism is the combination of adhesive and contact fatigue in the above test conditions. In contrast, if SA smaller, the FWR lower too, and the SEM suggests that the major wear mechanism is contact fatigue. The experiments also reveal that the laser beam quenching greatly improve the fretting wear resistance of titanium alloy, especially at heavy load and large amplitude.

Impact of atmospheric turbulence on coherent beam combining for laser weapon systems

The performance of a laser weapon system based on coherent beam combining(CBC)depends on its propagation properties in the atmosphere.In this study,an analytical model based on partial coherent beam combining(PCBC)for assumed coherence coefficients between beams in a CBC lattice was developed.The Kolmogorov model of atmospheric turbulence and the Hufnagel-Valley model of C^(2)_(n) dependence on atmospheric parameters were implemented.Novel simplified metrics were proposed to assess the CBC performance.Several beam profiles(super-Gaussian,truncated Gaussian,etc.)and geometries were analyzed in terms of maximal intensity in the far field.An approximate formula for PCBC efficiency dependent on the Fried radius was proposed.The results of CBC modeling were compared to those of the Gaussian beam propagation model in a turbulent atmosphere.The dependence of CBC performance on the C^(2)_(n) parameter,range,and elevation angle was analyzed.It could be concluded that the application of CBC for medium and long range propagation is impractical without an effective adaptive optics system.

Effect of shielding gas and defocusing on porosity during laser beam welding of 7A52 alloy

Laser beam welding is used to fabricate the 7A52 aluminum alloy plates.Effects of shielding gas and defocusing on porosities are investigated.Porosities are divided into hydrogen porosities and keyhole-induced porosities.With the increasing flow rate of the front shielding gas,the porosity ratio firstly decreases,then increases.The porosity ratio is lowest under the flow rate of 25 L/min.When the flow rate is 30 L/min,the porosity ratio is higher because the large flow rate can affect the stability of the keyhole.The porosity ratio is also higher when the flow rate is less than 25 L/min because the protection is weakened.With the increase of the defocusing,the porosity ratio firstly decreases,then increases.When the defocusing is-2 mm,the porosity ratio is lowest.When the defocusing is more than 0 mm or less than-4 mm,the porosity ratio is higher due to the movement of the instable keyhole.

Microstructure and high-temperature mechanical properties of laser beam welded TC4/TA15 dissimilar titanium alloy joints

The microstructure evolution and high-temperature mechanical properties of laser beam welded TC4/TA15 dissimilar titanium alloy joints under different welding parameters were studied.The results show that the weld fusion zone of TC4/TA15 dissimilar welded joints consists of coarsenedβcolumnar crystals that contain mainly acicularα’martensite.The heat affected zone is composed of the initialαphase and the transformedβstructure,and the width of heat affected zone on the TA15 side is narrower than that on the TC4 side.With increasing temperature,the yield strength and ultimate tensile strength of the TC4/TA15 dissimilar welded joints decrease and the highest plastic deformation is obtained at 800°C.The tensile strength of the dissimilar joints with different welding parameters and base material satisfies the following relation(from high to low):TA15 base material>dissimilar joints>TC4 base material.The microhardness of a cross-section of the TC4/TA15 dissimilar joints reaches a maximum at the centre of the weld and is reduced globally after heat treatment,but the microhardness distribution is not changed.An elevated temperature tensile fracture of the dissimilar joints is located on the side of the TC4 base material.Necking occurs during the tensile tests and the fracture characteristics are typical when ductility is present in the material.

Joint performance of CO_2 laser beam welding 5083-H321 aluminum alloy

Laser beam welding of aluminum alloys is expected to offer good mechanical properties of welded joints. In this experimental work reported, CO2 laser beam autogenoas welding and wire feed welding are conducted on 4 mm thick 5083- H321 aluminum alloy sheets at different welding variables. The mechanical properties and microstructure characteristics of the welds are evaluated through tensile tests, micro-hardness tests, optical microscopy and scanning electron microscopy （SEM）. Experimental results indicate that both the tensile strength and hardness of laser beam welds are affected by the constitution of filler material, except the yield strength. The soften region of laser beam welds is not in the heat-affected zone （ HAZ ）. The tensile fracture of laser beam welded specimens takes place in the weld zone and close to the weld boundary because of different filler materials. Some pores are found on the fracture face, including hydrogen porosities and blow holes, but these pores have no influence on the tensile strength of laser beam welds. Tensile strength values of laser beam welds with filler wire are up to 345.57 MPa, 93% of base material values, and yield strengths of laser beam welds are equivalent to those of base metal （264. 50 MPa）.

Effects of laser beam welding parameters on mechanical properties and microstructure of AZ31B magnesium alloy

The effects of laser beam welding process parameters such as laser power,welding speed and focal position on mechanical properties and microstructure of AZ31B magnesium alloy were studied.Nine joints were fabricated using different levels of laser power,welding speed and focal position.Tensile properties of the welded joints were evaluated and correlated with the weld zone microstructure and hardness.It is found that the joints fabricated using a laser power of 2 500 W,welding speed of 5.5 m/min and focal position of-1.5 mm yield superior tensile properties compared with the other joints.The formation of very fine grains in weld region,higher fusion zone hardness and uniformly distributed finer precipitates are the main reasons for superior tensile properties of these joints.