Numerical simulation for pulsed laser-gas tungsten arc hybrid welding of magnesium alloy

Based on the extended application of COMSOL multiphysics, a novel dual heat source model for pulsed laser-gas tungsten arc （GTA） hybrid welding was established. This model successfully solved the problem of simulation inaccuracy caused by energy superposition effect between laser and arc due to their different physical characteristics. Numerical simulation for pulsed laser-GTA hybrid welding of magnesium alloy process was conducted, and the simulation indicated good agree- ments with the measured thermal cycle curve and the shape of weld beads. Effects of pulse laser parameters （laser-excited current, pulse duration, and pulse frequency） on the temperature field and weld pool morphology were investigated. The experimental and simulation results suggest that when the laser pulse energy keeps constant, welding efficiency of the hybrid heat source is increased by increasing laser current or decreasing pulse duration due to the increased ratio of the weld bead depth to width. With large laser currents, severe spatters tend to occur. For optimized welding process, the laser current should be controlled in the range of 150-175 A, the pulse duration should be longer than 1 ms, and the pulse frequency should be equal to or slightly greater than 20 Hz.

Clean source of soft X-ray radiation formed in supersonic Ar gas jets by high-contrast femtosecond laser pulses of relativistic intensity

In this work,we optimized a clean,versatile,compact source of soft X-ray radiation(Ex-ray∼3 keV)with an yield per shot up to 7×10^11 photons/shot in a plasma generated by the interaction of high-contrast femtosecond laser pulses of relativistic intensity(Ilas∼10^18-10^19 W/cm^2)with supersonic argon gas jets.Using high-resolution X-ray spectroscopy approaches,the dependence of main characteristics(temperature,density and ionization composition)and the emission efficiency of the X-ray source on laser pulse parameters and properties of the gas medium was studied.The optimal conditions,when the X-ray photon yield reached a maximum value,have been found when the argon plasma has an electron temperature of Te∼185 eV,an electron density of Ne∼7×10^20 cm^-3 and an average charge of Z∼14.In such a plasma,a coefficient of conversion to soft X-ray radiation with energies Ex-ray∼3.1(±0.2)keV reaches 8.57×10^-5,and no processes leading to the acceleration of electrons to MeV energies occur.It was found that the efficiency of the X-ray emission of this plasma source is mainly determined by the focusing geometry.We confirmed experimentally that the angular distribution of the X-ray radiation is isotropic,and its intensity linearly depends on the energy of the laser pulse,which was varied in the range of 50-280 mJ.We also found that the yield of X-ray photons can be notably increased by,for example,choosing the optimal laser pulse duration and the inlet pressure of the gas jet.

The WASP Model:A Micro-Macro System of Wave-Schrödinger-Plasma Equations for Filamentation

In this paper,we model laser-gas interactions and propagation in some extreme regimes.After a mathematical study of a micro-macro Maxwell-Schrödinger model[1]for short,high-frequency and intense laser-gas interactions,we propose to improve this model by adding a plasma equation in order to precisely take into account free electron effects.We examine if such a model can predict and explain complex structures such as filaments,on a physical and numerical basis.In particular,we present in this paper a first numerical observation of nonlinear focusing effects using an ab-initio gas representation and linking our results with existing nonlinear models.