Behavior Of Laser-induced Plasma Research Articles

Improvement of the Laser-Induced Breakdown Spectroscopy method sensitivity by the usage of combination of Ag-nanoparticles and vacuum conditions

In this work we studied the effect of vacuum (low pressure) conditions on the behavior of laser-induced plasma (LIP) created on a sample surface covered with silver nanoparticles (Ag-NPs), i.e. Nanoparticles-Enhanced Laser-Induced Breakdown Spectroscopy (NELIBS) experiment in a vacuum. The focus was put on the step by step optimization of the measurement parameters, such as energy of the laser pulse, temporally resolved detection, ambient pressure, and different content of Ag-NPs applied on the sample surface. The measurement parameters were optimized in order to achieve the greatest enhancement represented as the signal-to-noise ratio (SNR) of NELIBS signal to the SNR of LIBS signal. The presence of NPs involved in the ablation process enhances LIP intensity; hence the improvement in the analytical sensitivity was yielded. A leaded brass standard was analyzed with the emphasis on the signal enhancement of Pb traces. We gained enhancement by a factor of four. Although the low pressure had no significant influence on the LIP signal enhancement compared to that under ambient conditions, the SNR values were noticeably improved with the implementation of the NPs.

Laser-induced plasma generation and evolution in a transient spray.

The behaviors of laser-induced plasma and fuel spray were investigated by visualizing images with an ultra-high-speed camera. Time-series images of laser-induced plasma in a transient spray were visualized using a high-speed color camera. The effects of a shockwave generated from the laser-induced plasma on the evaporated spray behavior were investigated. The interaction between a single droplet and the laser-induced plasma was investigated using a single droplet levitated by an ultrasonic levitator. Two main conclusions were drawn from these experiments: (1) the fuel droplets in the spray were dispersed by the shockwave generated from the laser-induced plasma; and (2) the plasma position may have shifted due to breakdown of the droplet surface and the lens effect of droplets.

Appearance of measured signals with changes in basic welding conditions: Features of monitoring methods for laser welding and their application (2)

Summary The authors are engaged in a research programme intended to clarify the behaviour of laser-induced plasma during laser welding and to establish its applicability in monitoring laser welding parameters. This paper describes the appearance of the measured signals in response to changes in the basic welding conditions. The main results obtained are as follows: 1. The average amplitude (Vamp) of both the spectrum emission intensity and plasma potential is far better correlated with the penetration depth than the average output (Vave). 2. The good correlation found between the average amplitude and spectrum emission is due to the fact that a strong effect is exerted by any change in the configuration of metal plasma fluctuation due to the effects of the penetration shape, such as the keyhole and molten pool shape. This also suggests the need for consideration to be given, regarding the signal output changes that occur in response to laser power changes, to the correlation factors of the spectrum emission intensity and laser power as the source of energy for plasma formation. 3. Vamp of the measured signals of the spectrum emission and plasma potential can be applied as monitoring parameters for the process speed and process power in laser welding.

Correlation between process parameters and measured signals during laser welding of plates with artificial defects: Features of monitoring methods for laser welding and their application (3)

Summary The authors are engaged in a research programme intended to clarify the behaviour of laser-induced plasma during laser welding and to establish its applicability in monitoring laser welding parameters. The first paper describes direct observations of laser welding by CCD camera and clarification of shielding gas plasma by measurements of the spectrum emission, acoustic emission, and plasma potential signals. The second paper examines the appearances of the measured signals in response to changes in the basic welding conditions. The present paper seeks to establish the responsiveness of the signal behaviour to welding conditions, such as irregularities or defects occurring during laser welding of plates with artificial defects. The main results obtained are as follows: 1. In the shear cut identification test, the shear cut can be identified by the spectrum emission intensity and plasma potential. Through simultaneous measurement of the spectrum emission intensity and plasma potential signals, poor melting due to abnormal beam energy absorption by the Ar plasma can be identified. 2. In the vertical hole identification test, the vertical holes can be identified by the spectrum emission, acoustic emission, and plasma potential. The molten pool is perturbed by the hole before the laser beam reaches the hole, so that the keyhole is perturbed, and the output waveforms of the acoustic emission, spectrum emission, and plasma potential change before the hole is reached. 3. In Ni powder packed and Cu wire packed horizontal hole tests, data can be obtained on the fusion behaviour in the bead longitudinal section direction through observations being made of the fusion pattern in the Ni powder concentrated layer. Although a flow of molten metal can be clearly identified in this cross-section, the molten metal flows in a direction opposite to that of laser welding progression. If large defects or holes are present in front of the direction of laser welding progression, the flow is perturbed, and the keyhole behaviour is affected. Differences in the identification pattern of these cross-sectional signals can be determined by comparison of the spectrum emission and acoustic emission. 4. A comparison of the characteristics of the spectrum emission acoustic emission and plasma potential signal behaviour for identification of artificial defects reveals that the plasma potential signal responds highly sensitively to the presence of plasma and that the spectrum emission signal responds highly sensitively to migration of the plasma, including the keyhole.