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Using cavity confinement to enhance the plasma emission has been proved to be an effective way in LIBS technique while no direct visual evidence has been made to illustrate the physical mechanism of this enhancing effect. In this work, both laser-induced plasma plume images and shockwave images were obtained and synchronized for both flat surface case and rectangular cavity case. Phenomena of shockwave reflection, plasma compression by the reflected shockwave and merge of the reflected shockwave into plasma were observed. Plasma emission intensities recorded by ICCD in both cases were compared and the enhancement effect in the cavity case was identified in the comparison. The enhancement effect could be explained as reflected shockwave “compressing” effect, that is, the reflected shockwave would compress the plasma and result in a more condensed plasma core area with higher plasma temperature. Reflected shockwave also possibly contributed to plasma core position stabilization, which indicated the potential of better plasma signal reproducibility for the cavity case. Both plasma emission enhancement and plasma core position stabilization only exist within a certain temporal window, which indicates that the delay time of spectra acquisition is essential while using cavity confinement as a way to improve LIBS performance.
© 2016 Optical Society of America
1. Introduction
The relatively low sensitivity and repeatability of Laser-induced breakdown spectroscopy (LIBS) have always been the Achilles heel of this technology for wide commercialization and industrialization [1–4 ]. Recently, spatial confinement has been extensively investigated and applied to LIBS to enhance the plasma signal intensity [5–7 ], as well as improve the pulse-to-pulse repeatability [8, 9 ]. Researchers have used different configurations of spatial confinement to investigate the enhancing effect, such as cylindrical cavity [5, 6, 8, 10 ], hemispherical cavity [11, 12 ] and parallel wall cavity [7, 13 ]. Combination effects of spatial confinement and other physical effects, such as the combination of cylindrical confinement and spark discharge [9] and the combination of spatial and magnetic confinement [14], were also studied.
So far the major efforts of studying the spatial confinement effects have been put on the spectroscopic perspective, that is, using spectroscopic experimental results to clarify the spatial confinement effect [15]. Although some researchers have discussed the physical nature of spatial confinement effect, which mainly contains the idea of reflected shockwave from the cavity wall compressing the expanding plasma and increasing the collision rate among particles [16], much more effort is still needed to better illustrate the physical nature of spatial confinement effect. From the experimentalists’ perspective, it is very important to know how the laser-induced plasma and shockwave expand and interact over a certain amount of time, what is the best time to obtain the spectra from the compressed plasma, and which part of plasma is most greatly compressed etc. By understanding the physical behavior of both laser-induced plasma and shockwave in the spatial confinement, it would be possible to perform better temporal and spatial resolved experiments to obtain the spectra with larger intensity and better pulse-to-pulse repeatability.
In the work, the synchronous behaviors of both laser-induced plasma and shockwave on a flat surface and in a rectangular groove cavity were systematically investigated, respectively. ICCD fast photography and focused shadowgraph technique were applied to obtain the laser-induced plasma image and shockwave image in temporal sequence, respectively. Efforts were made to combine the plasma and shockwave pictures together to reveal the plasma and shockwave behaviors simultaneously. Direct visual evidence of reflective shockwave compressing the expanding plasma was obtained for the first time.
2. Experimental setup
Two separate experimental setups were used to obtain the laser-induced plasma and shockwave in both cavity confinement case and flat surface case, respectively, shown in Figs. 1(a) and 1(b) (Only the cavity case was shown). An integrated LIBS system (Applied Spectra, USA) was utilized to acquire the laser-induced plasma images. A Q-switched Nd:YAG laser (Quantel, France) with wavelength of 1064nm and pulse width of 8ns was applied and the spot size was estimated to be 60 μm in diameter. An ICCD camera (Andor, UK) was used to take the plasma plume images. Different ICCD settings, including the gate widths, gains, pre-amplifier gains and optical filter types, were used to obtain the plasma plume images with optimal image quality at different delay times, which were summarized in Table 1 . Another integrated LIBS system (Applied Spectra, USA) was utilized to acquire the shockwave images. The plasma was produced with a Q-switched Nd:YAG laser (Beamtech, China) with a wavelength of 1064nm and pulse width of 8ns. Another Q-switched Nd:YAG laser (Quantel, France) with a wavelength of 532nm and pulse width of 500ps was used as the probe laser providing the light source to the formation of the shadowgraph. A synchronized CCD camera was utilized to capture the shockwave images in the shadowgraph. The delay time between the laser spark initiation and the shadowgraph acquisition was varied to record the shockwave expansion at different time. The ablation laser pulse energy for each shot in both setups was controlled as 30mJ/pulse.
Fig. 1 Diagrams of experimental set-up. (a) shadowgraph set-up capturing the shockwave images, and (b) ICCD set-up capturing the plasma plume images.
Table 1. ICCD settings at different delay times.
Cubic brass samples (20mmx20mmx20mm) were used in both flat surface case and cavity confinement case. For the flat surface case, the laser pulses were directly shot on the sample surface. For the cavity confinement case, one surface of the sample was grooved to have a straight slot with 3mm width and 4mm depth. Laser pulses were shot on the central line of the groove bottom. The samples were mounted on a 3-dimensional mobile platform. Each laser pulse was shot on a fresh spot. In each case, both shadowgraph and ICCD recorded the signals in the horizontal direction. The ICCD and shadowgraph settings in both cases were exactly the same. Figure 2 shows the examples of plasma and shockwave images obtained by ICCD and shadowgraph in both cases.
Fig. 2 Examples of images obtained from the experiment. (a) plasma image from flat surface; (b) shockwave image from flat surface; (c) plasma image from cavity confinement; (d) shockwave image from cavity confinement. Note: all images were taken with a delay time of 2000ns.
3. Results and discussion
In the following sections, both laser-induced plasma and shockwave expansion dynamics will be discussed for both confinement case and flat surface case. The plasma intensity enhancement effect and plasma core stability will also be presented by comparing the ICCD counts recorded in both confinement case and flat surface case.
3.1 Flat surface case
Figure 3 shows the evolution of laser-induced plasma and shockwave under the same time scale. The images of shockwave obtained by shadowgraph and the images of laser-induced plasma obtained by ICCD were scaled to the same size and overlapped to reveal the expansion dynamics at the same time. To scale the images to the same sizes, a piece of glass carved with scale grids was put at the same position where the laser ablation took place, then both shadowgraph and ICCD images of this glass were acquired. In this way the real size of the plasma and shockwave would be calculated and the images could be adjusted to the same size.
At early time (~50ns), both the shockwave and plasma are greatly elongated towards the laser incidence direction. Specially, at extreme early time (~10ns), the tip of the shockwave is opaque and distorted, which could be explained by the theory of laser supported detonation wave (LSD wave) theory [17]. By continuously absorbing the energy from the trailing laser pulse, the region where LSD occupies has a high temperature and electron density, which results in the non-uniform heating of the shockwave and plasma. Even though the LSD wave stops propagating after the termination of the laser pulse, the non-uniform heating still makes the tip of the shockwave move faster and the irregular structure on the top of the shockwave can still be observed even after the termination of the laser pulse [18]. The shadowgraph probe laser finds it difficult in penetrating the area with high electron density so a dark opaque tip of the shockwave can be observed from the image [18]. As the delay time increases, the shockwave rapidly expands as a hemispherical shape and leave the edge of the plasma behind. The spherical expansion could be modelled and estimated with the classic Taylor-Sedov theory [19], which describes the shockwave expansion from a point explosion:
where is the delay time from the initiation of laser illumination, is the density of the ambient gas, is the energy released during the explosion process, is a constant depending on the specific heat capacity ratio. For spherical shockwave, is equal to 3, thus the equation can be simply written as the following form: . In Fig. 4 , the spherical shockwave radii at different delay time were measured in the experiment and the data were fitted using a power function, which corresponded well with the Taylor-Sedov model. The decreasing of shockwave expansion velocity along with the delay time was also shown in Fig. 4. In the first 100ns, the shockwave velocity decreased rapidly from 20km/s to 3km/s, and then gradually approached to the local sound speed due to the energy dissipation.The expansion and evolution of laser-induced plasma obtained by ICCD camera could also be observed in Fig. 3. The sizes of shockwave and plasma obtained from two separate experiments were scaled so the overlapped pictures are able to show the relative sizes of shockwave and plasma. Unlike the shockwave boundaries, the plasma boundaries were found not so sharp and clear because an ambient plasma due to the ionization of ambient gas atoms could be generated [20]. At the early stage of the expansion, both the shockwave and the plasma expand with an approximately same velocity, corresponding to the Taylor-Sedov blast wave model. Due to the counter-pressure of the ambient gas, the plasma loses its kinetic energy very quickly, slows down the expansion, and finally detaches from the shockwave [21](at ~400ns in this experiment). After detaching from the shockwave, the plasma ceases to expand and finally reaches a maximum expansion radius, which could also be seen in Fig. 4. A drag model can be used to describe the plasma plume expansion in a highly viscous background medium, given by the following equation [18]:
where is the maximum expansion radius of plasma plume, and is a coefficient defining the slowing speed. In this case, the maximum expansion radius equals to 1.7 mm and the slowing coefficient equals to 0.004.3.2 Cavity confinement case
A study about spatially and temporally resolved laser-induced plasma emission under cylindrical confinement has been systematically investigated in our previous work [15]. Plasma emission enhancement was observed. An assumption of reflected shockwave compressing and reheating plasma plume was proposed but no direct visual evidence was made as a solid proof.
The shockwave expansion and reflection behaviors in the rectangular groove cavity are shown in Fig. 5 by overlapping multiple shockwave images together in a certain time sequence. Here we used rectangular groove cavity to replace the cylindrical cavity simply because using cylindrical cavity the shockwave images were almost impossible to obtain due to the strong refraction of the probe laser beam while penetrating the cavity wall. Although the spatial reflection in a rectangular groove cavity is different from that in a cylindrical cavity, it should still be able to explain the shockwave reflection and plasma compression effect to some extent. To better clarify the shockwave expansion and reflection, the shockwave size data were also measured from the shadowgraph pictures and plotted in Fig. 5 over a delay time range from 100ns to 3500ns. After reaching the cavity wall, the shockwave was reflected and then propagated upward obliquely, as shown with the arrow in Fig. 5. Reflected shockwave propagating velocity was also calculated, which was in the same order of magnitude of original shockwave, around 700 m/s at delay time of 1500ns. The reflected shockwave was seen to eventually contact the original shockwave and merge together in the near-wall area, where a nearly horizontal shockwave emerged in the late delay time (i.e. 3500ns). An interesting find was that from 1500ns, the reflected shockwave disappeared within a certain distance towards the cavity center, such as the area marked with red dots in Fig. 5. It is reasonable to speculate that the reflected shockwave started to interact the expanding plasma where the shockwave disappeared in the shadowgraph image, based on two reasons: 1) shadowgraph detects the drastic refractive index. Before the shockwave reached the expanding plasma, the large refractive index difference between the shockwave front and the ambient gas made the shockwave clearly visible in the shadowgraph images; when the shockwave reached the plasma both the shockwave front and the plasma plume had high temperature and pressure, of which the refractive index difference was not so obvious, thus the shockwave was hardly seen in the shadowgraph images; 2) the intensive energy transferring process drastically weakens the kinetic energy of the shockwave, leading to the disappearance of the shockwave. Again, we overlapped the shockwave images and plasma plume images obtained in the rectangular groove cavity under the same size scale, as shown in Fig. 6 .
Fig. 6 The evolution of shockwave and plasma in the cavity confinement case. The shockwave reflection and plasma compression effect were observed.
Before the shockwave reached the cavity wall, the shockwave and plasma plume expansion behaviors were the same with that in the flat surface case. The spherical shockwave reached the cavity wall at ~800ns from the laser initiation, observed from Fig. 6. A clear reflected shockwave front could be observed after shockwave reached the cavity wall, and the shape of the plasma plume was simultaneously modified. From Fig. 6, it is clear that the plasma plume started to experience compression somewhere between 1100ns and 1600ns. Being compressed by the reflected shockwave, the plasma core was restricted within a more condensed area with a higher plasma emission intensity, and according to our previous study [15], this area corresponded to a region with higher plasma temperature and electron density, which will be discussed later.
3.3 Plasma intensity enhancement and plasma core stabilization
From the previous section the reflected shockwave compressing plasma plume was discussed and some simple physical explanations were also proposed. Our previous study also demonstrated the plasma emission enhancement in the cylindrical cavity confinement. The most direct way to evaluate the enhancement effect of the plasma emission is to compare the plasma emission intensities in both confinement and flat surface cases, thus in Fig. 7 , the ICCD counts of plasma in both cases were compared as a function of time. It is necessary to note that because the ICCD settings were different at different delay times as shown in Table 1, including different gate widths, gain levels, optical filter types, it is important to normalize the ICCD counts obtained from different settings to the same standard. The intensity normalization method is based on an idea that under different ICCD settings, the intensities of plasma at the same delay time are different, thus by recording the plasma at the same delay time but different ICCD settings and comparing the plasma intensities from different ICCD settings, normalization factors between different settings can be calculated and the plamsa intensity can be normalized to the same standard. The ICCD counts shown in Fig. 7 have been normalized already, showing the plasma intensities under the same criterion.
Fig. 7 The temporal evolutions of plasma emission intensity in flat surface case and cavity confinement case.
It is clear that from the delay time of ~1400ns, the plasma intensity in the cavity case was prominently higher than that in the flat surface case. This intensity advantage of the cavity case lasted for approximately 2 μs, which could account for the intensity enhancement effect of the cavity confinement. Also we could see from Fig. 7 that in late delay time the plasma intensity from the cavity case fell more rapidly and finally the intensity was lower than that from the flat surface case. This could possibly be explained as in the cavity case, the shockwave reflection and plasma being compressed leads to more violent convection process, which accelerates the energy dissipation in the later stage.
By recording the spatial coordinates of plasma cores where the plasma intensities reached the maximum in both flat surface and cavity confinement cases, which is shown in Fig. 8 , we could see that the hot plasma core was restrained spatially in the cavity confinement case and this restraint lasted for approximately 5μs before the plasma finally dissipated due to the fast cooling of the plasma, while without the cavity confinement the position of hot plasma core experienced a slight rise, then immediately bounced back to the near-surface and only lasted for approximately 3μs before dissipation. From previous study, it was demonstrated that the kinetic energy of the ambient gas behind the shockwave front was much larger than that in the plasma plume, thus the reflected shockwave could “reheat” the plasma to some extent by heat and mass transfer process [22]. However, by calculating and comparing the kinetic energy of ambient gas atoms in the reflected shockwave (Ek = 1/2mv2 = 0.074eV) and excitation energy of Cu or Zn atoms (~3 eV), it was found that the kinetic energy of reflected shockwave was actually one order lower than the excitation energy, thus the effect of “reheating” from reflected shockwave to enhance the plasma emission should be minor. The compressing effect of the reflected shockwave on the plasma plume to create a denser and hotter plasma core was considered to be the main reason of the plasma signal enhancement. Moreover, during the mass transfer process, the reflected shockwave would inevitably bring some cold materials from ambient air into the plasma, thus it is believed that the shockwave compressing plasma to a more condensed area is the dominant factor for the plasma emission intensity enhancement. Because in the cavity the plasma was confined and hot plasma core moved in a more regular path, by properly adjusting the position of the collecting fiber system, better signal stability might also be achieved in the cavity case.
Fig. 8 Temporal evolution of maximum plasma intensity positions (a) with cavity confinement and (b) without cavity confinement
To further demonstrate the possible signal stabilizing effect in the cavity case, the plasma core sizes and positions in the horizontal direction (x direction) were again plotted with the function of delay time. According to our previous study [23], the plasma core contributes to most of its emission intensity, and and due to high temperature and electron density of the core part, the core part is more homogeneous, therefore, if the plasma core position is more stable spatially, the emission collected by the optic imstruments would also be more stable. The plasma core boundary was defined as the intensity of which reached 80% of the maximum ICCD counts of the same plasma. By comparing the plasma core sizes of both cavity and flat surface case, shown in Figs. 9(a) and 9(b) , it was clear that the plasma core was more confined in the cavity case starting from 1400ns, when the reflective shockwave started to compress the expanding plasma, till 3500ns, when the plasma finally dissipated during fast cooling process in the cavity.
Fig. 9 Plasma core horizontal positions (a) in the cavity case and (b) flat surface case. (c) Plasma core center position evolution, and (d) absolute difference of plasma core center positions in adjacent delay times.
The fluctuation of plasma core is always a big factor of plasma signal stability, thus the center position of plasma core in each delay time was also plotted, shown in Fig. 9(c). Three temporal regions were defined, namely early, middle and late, which stand for “before reflective shockwave effect period”, “reflective shockwave effecting period”, “plasma dissipation period”, respectively. In the “early” period, the plasma core position evolution should be approximately the same in the two cases because of no reflective shockwave effect in the cavity. In the “middle” period, the plasma core moved toward a certain direction in the cavity case. This could be explained as the laser focusing position was not precisely at the center position of the cavity, thus the reflective shockwave from one side of the cavity wall reached the plasma earlier than that from the other side, leading to the plasma core moving towards a particular direction. In the “late” period, because of the faster plasma cooling in the cavity case, the plasma dissipated greatly and the plasma core center positions did not make much sense anymore. Also during this period the plasma intensity was so weak that its contribution to the plasma emission could be neglected. In Fig. 9(d), the absolute differences of the plasma core center positions in adjacent delay time were calculated and plotted with the function of delay time, i.e. , where is the absolute difference of the plasma core positions of delay time and delay time, and is the plasma core center position at delay time. Only the “middle” period was shown because our intention was to compare the plasma core fluctuation in both cases. From Fig. 9(d) we could find a smaller plasma core position fluctuation in the cavity case especially between 1400 ns to 2400 ns, when the reflective shockwave compressing effect was the most obvious. This indicated a better plasma signal stability might be achieved with this rectangular cavity confinement.
Lastly, It is worthwhile to note that according to our previous experience, using a cylindrical confinement would actually have better plasma signal stabilization effect due to its more symmetrical geometry [8]. Also, due to the strong dependence of plasma emission enhancement effect and plasma core stabilization effect on the delay time, it gives us a hint that the delay time of spectra acquisition is essential while using cavity confinement to improve the LIBS performance.
4. Conclusions
The expansion dynamics of laser-induced plasma and shockwave in both flat surface case and rectangular groove confinement case were investigated by means of ICCD fast photography and focused shadowgraph technique, respectively. In the flat surface case, the shockwave expansion could be described with a point explosion blast wave model and the plasma expansion approximately matched a drag model. In the confinement case, the shockwave reflection from the cavity wall was observed in the shadowgraph images. The phenomenon of reflected shockwave compressing the expanding plasma was also observed, which gave the explanation of the plasma emission intensity advantage in the cavity confinement case over the flat surface case. The enhancement effect could be explained as reflected shockwave “compressing” effect, that is, the reflected shockwave would compress the plasma to have a more condensed plasma core area with higher plasma temperature, emitting stronger spectral signal collected by LIBS system. Apart from making the plasma more compacted, the reflected shockwave also contributed to a better plasma core position stability, which indicated a better signal stability would be obtained. The plasma emission intensity enhancement and plasma core stabilization effect of cavity confinement highly depend on the delay time of the acquisition system, which gives a hint that the delay time of spectra acquisition system is a key factor while using cavity confinement as a way to improve the LIBS performance.
Acknowledgments
The authors are greatful for the financial support from National Natural Science Foundation of China (No. 51276110) and the National Basic Research Program (973 Program) of China (No. 2013CB228501).
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