We report on the effect of transverse magnetic field on laser ablation of copper and aluminum targets both experimentally and numerically. The ablation depth is found to increase with magnetic field from 0 to 0.3 T and decreases at a higher magnetic field (0.5 T). It is demonstrated that the nanosecond laser ablation is mainly due to melt ejection and it solely depends on the thermo-physical parameters of the material. The increase in ablation depth with magnetic field is attributed to the increase in heat transfer from the plasma to the target, vapor pressure, and shock pressure. The ablation due to melt ejection is also calculated using vapor pressure through simulation and compared with the experimentally measured depth. In the presence of magnetic field, we introduce the magnetic pressure in Clausius–Clapeyron vapor pressure equation to account for the combined effect of magnetic field and atmospheric pressure on the vapor pressure of plasma. The ratio of calculated ablation depth at 0.3 T with respect to the absence of magnetic field is close to the corresponding experimental depth ratios indicating that the laser ablation modeling in the present work is validated. As the magnetic field increases, we observed the scattered mass at the center and around the crater. The size of deposited mass at the center is found to decrease at higher magnetic field which is attributed to breaking of large droplets into smaller ones due to increase in instability at higher magnetic field.
We report on the spatially resolved optical emission spectroscopic study of laser-produced copper plasma in the presence of static uniform magnetic field in air ambient at atmospheric pressure. The response of copper atomic/ionic lines to magnetic field along the axial direction of plasma is different. It is attributed to the difference in populating process (electron impact excitation and recombination) of each transition. In the present work, we introduced air pressure to calculate the stopping radius and found it to be around the distance at which the intensity is pronounced. The electron density varied as ne = 9.2z−0.33 without magnetic field and in the presence of 0.3 T magnetic field, it varied as ne = 7.9z−0.27. The electron temperature variation with distance from the target in the absence and presence of magnetic field is found to be Te = 1.1z−0.23 and Te = 0.9z−0.18. The electron density and temperature decay slowly along the plasma expansion direction in the presence of magnetic field. It is due to magnetic confinement of plasma. We demonstrated that the thermal conductivity of plasma is enhanced in the presence of magnetic field. From the spatial evolution of the electron density and temperature, we estimated the approximate dimension of the core and tail region of the plasma and found an increase in the core dimension in the presence of magnetic field. The increase in core dimension is in agreement with the intensity variation of ionic line. It is attributed to an increase in heat transfer due to an increase in thermal conductivity in the presence of magnetic field. The present work may help optimize the distance from target to enhance spectral line intensity in optical emission spectroscopy in the presence of magnetic field.
Laser-produced copper plasma in the presence of variable transverse external magnetic field in air is investigated using optical emission spectroscopy. As the magnetic field increases from 0 to 0.5 T, the intensity of Cu I lines initially increases and then decreases slightly at a 0.5 T. The maximum intensity enhancement of all five Cu I lines occurs at a magnetic field of 0.3 T. The increase in intensity is attributed to an increase in the electron impact excitation of Cu. With increase in magnetic field, the electron density and temperature were found to increase due to increase in the confinement of plasma. The difference in intensity enhancement factor is due to the difference in excitation rate coefficients. The surface morphology of irradiated copper target is also analyzed at 0.3 T magnetic field at which the density is maximum and reveals the formation of Cu/Cu2O/CuO nanoparticles (NPs). More NPs are formed at the peripheral region than at the central region of the ablated crater and is due to the oxidation of Cu atom in the plasma–ambient interface. The larger grain size of nanostructures in the presence of magnetic field is due to an increase in the inverse pulsed laser deposition. The intensity of Raman peak of Cu2O decreases in the presence of magnetic field and that of CuO increases which is more likely due to conversion of Cu2O to CuO. The photoluminescence intensity of CuO increases in the presence of magnetic field due to the phase transformation of Cu2O to CuO in agreement with the result of Raman spectroscopy.
For a nanosecond laser ablation of metals, the key physical phenomena involved are thermal evaporation, melt ejection, instability of the molten metal, etc., which depend on the initial temperature evolution in the metal. Understanding the evolution of temperature of the metal needs an effective simulation. In the present paper, we report on the finite element method-based simulation of nanosecond laser ablation of copper in the absence and presence of the magnetic field. Our studies showed that the effective thermal conductivity of the melted layer on the copper surface in the presence of the magnetic field affects the viscosity of the layer, mass ablation rate, instability, and then particle formation. The calculations showed that the condensed nuclei of large critical size are produced in the magnetic field. It is attributed to an increase in the collision rate of plasma particles in the magnetically confined plasma. The simulations are in good agreement with the experimentally measured values.
The authors report on the study of the crater generated using a nanosecond laser on a copper target in air in the presence of uniform and nonuniform magnetic fields. The analysis of particles deposited inside and around the crater revealed that the generation of large particles (≥0.68 μm) is due to the melt ejection and instability in the liquid layer. The presence of a nonuniform magnetic field causes an additional drift to molten liquid which in turn increases the Kelvin–Helmholtz instability. The percentage of large particles increased due to the enhancement in the Kelvin–Helmholtz instability and mass ejection. The intensity of copper atomic transitions was enhanced in the presence of a uniform magnetic field compared to a nonuniform magnetic field. This is more likely due to an increase in melt ejected mass in the plasma in the presence of a nonuniform magnetic field which may scatter or absorb laser light which in turn decreases laser–matter interaction. The energy-dispersive x-ray spectroscopy and Raman spectroscopy showed the deposited particles are Cu2O. In the presence of a nonuniform magnetic field, the intensity of Raman Cu2O was enhanced, which is attributed to an increase in the number of Cu2O particles.
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