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Abstract
This paper investigated how microwaves affect the temperature of laser-generated air plasma. The air breakdown threshold was experimentally characterized by focusing the 1064 nm YAG laser on varied condensing lens focal lengths. Increase in focal lengths increases the focused spot diameter of the laser and decreases the laser fluence. Large spot diameter required large amount of laser fluence for breakdown. However, the plasma generated with small spot sizes found to absorb higher laser energy in compared to the plasma generated with large spot size condition. In terms of energy density, the experimental threshold breakdown was generated between 2.6∼4.9 × 1011 W/cm2. The plasma formation was then observed under a high-speed camera. The area of intensity distribution increased with the input of microwaves owing to re-excitation and microwave absorption. This led to emission intensity measurements of the elusive stable electronically excited molecular nitrogen (N2 2nd positive system) and hydroxyl radical (OH). Without the input of microwave, these molecular and radical emissions were not observed. The OH and N2 2nd positive system emission intensities were then used to measure the rovibrational temperature using the synthetic spectrum method by SPECAIR. The rotational and vibrational temperatures were not found to be equal indicating non-equilibrium plasma. The nonequilibrium and nonthermal plasma was observed from after the initial laser air breakdown using the 2.6 × 1011 W/cm2, 1.0 kW microwave power, and 1.0 ms microwave pulse width. The microwaves were not found to affect the temporal changes in the rotational temperatures, demonstrating that the intensity enhancements and plasma sustainment were caused by re-excitation and not by microwave absorption.
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1. Introduction
The air breakdown may be generated by either electric discharge or lasers [1]. In electric discharge, an avalanche of electrons occurs between two electrodes of high voltage difference owing to the repeated impact ionization effect [2,3]. The impact ionization effect describes the collision between the free electrons and the surrounding molecules caused by the forced movement of the electrons driven by the electric field [3]. For combustion applications, the air breakdown by electrical discharge is typically generated by spark plugs. In laser-induced spark discharge, the air breakdown is initiated by multiple photon absorption (MPA) and inverse bremsstrahlung absorption (IBA) [4–7]. MPA describes the absorption of photons by atoms or molecules, whereas IBA describes the absorption of photons by electrons. These processes lead to atomic and molecular ionization, electron cascade, and gas breakdown [4]. The plasma produced by the laser breakdown has a smaller volume and shorter duration than the plasma generated by the spark plug. However, the laser ignition produced no energy losses induced by the presence of spark plugs [5]. Although the cost of laser ignition in automobiles hinders commercialization, its cost-efficiency in aircrafts has been favorable [6,7]. Both breakdown processes are used not only in combustion applications but also in many industrial and practical applications such as surface processing [8,9], radical-generating systems [10,11], and spectroscopy [12–16].
The spark-induced breakdown spectroscopy (SIBS) and the laser-induced breakdown spectroscopy (LIBS) are extensively used in spectrochemical analysis of gas mixtures. SIBS uses spark plugs or coaxial electrodes to produce the plasma source for optical emission spectroscopy. SIBS was successfully used in many types of target analysis including mercury in soils and hard chromium plates [12,13]. Direct and in situ measurements of solids and many other types of targets have been thoroughly investigated using LIBS [14–16]. The versatility of LIBS has expanded its applications to mars exploration, plasma fusion, and remote elemental analysis inside nuclear power stations [17,18]. In remote elemental analysis, potential issues related to weak intensity emissions and background continuum signals have been reported [19]. Microwave-assisted ignition was proposed to tackle these problems, which causes the microwaves to increase the intensity emissions of the LIBS plasma [20–25]. Figure 1 illustrates how the microwave affects the plasma volume of the laser plasma and spark plasma [26,27]. Larger volumes are observed for both types of plasmas when the microwave antenna is turned on. The emission intensity and plasma lifetime are also enhanced by the microwaves.
The microwave-enhanced spark plasma has been successfully used for limit of detection measurements of metal powders and aqueous jet solutions [28,29]. The combination of microwaves and spark ignition measurements was also called plasma ball spectroscopy since the measurements dealt with detection limits of powders and metals in aqueous solutions. In microwave-assisted LIBS (MA-LIBS), we succeeded in expanding the size and duration of the plasma with 2.45 GHz microwave radiation introduced using a capacitor-like antenna and a spiral-shaped antenna [20,21]. The emission intensity of the plasma increased by 2–3 orders of magnitude, improving the signal-to-noise ratio.
The MA-LIBS holds great promise for direct measurements of radioactive fuel debris in the Fukushima Daiichi Nuclear Power Plant [20]. As optical fibers experience low power transmission owing to nuclear radiation, the laser ablation becomes weak and produces low-intensity emissions. The MA-LIBS increases the intensity emissions of the initially weak ablation through re-excitations and possible microwave absorption by the electrons [20–25]. The microwave may also influence the temperature of the plasma, which was deemed to eliminate the sputtered Cu emission line of the microwave antenna [20].
In this study, we investigated the influence of the microwaves on the temperature characteristics of the laser-generated air plasma at atmospheric pressure. The temporal and spatial profiles of the plasma emissions through a high-speed camera and optical emission spectrometry were measured. The molecular emissions of hydroxyl radicals (OH: 306–327 nm) and N2 second positive system (N2PS: 306–337 nm) were used to calculate the rotational and vibrational temperatures using the synthetic spectra method using the SPECAIR software.
2. Materials and methods
Figure 2 shows the experimental measurement device for generating the laser air breakdown and acquiring the spatially and temporally resolved spectrum and plasma emission distribution. The 1064 nm YAG laser beam (Dawa 200, Beamtech, China) produces an initial beam diameter of 6.0 mm and maximum energy density input of 1.01 × 108 W/cm2 (200 mJ, 7.0 ns) and repetition rate of 10 Hz. The laser pulse width of 7.0 ns was focused on a circular spot (22∼44 µm) in the open ambient air using various condensing lenses of increasing focal lengths (59∼100 mm). The initial diameter profile was provided by the laser supplier which was used to calculate the focused spot sizes.
To measure the incident and transmitted laser energy, two laser power meters (843-R, Newport, UK) were placed in the reflected beam of the beam splitter and the transmitted beam of the plasma. The reflected beam energy of the beam splitter represents the incident beam energy of the laser.
In the case of plasma enhancement via microwaves, the 2.45 GHz microwave source with programmable pulsed operations transmitted the microwave energy via the helical antenna with a cross-plate reflector. The microwave oscillation generates the microwave energy of 100–1000 mJ for the varied duration. A coupler was used to monitor the incident and the reflected microwave power. A tuner stub was also used to minimize the reflected power and optimize the transmitted microwave energy.
The high-speed camera (Phantom V2511, Dantec, UK) and the spectroscope Echelle spectrograph (MS257, Oriel, Andor, UK) were used to measure the plasma emission intensities. The synchronization of measurements was controlled by a pulse generator with picosecond resolution (Model-577, Berkeley Nucleonics, USA).
3. Results
The transmitted laser energy measurements for varied incident energies and focal lengths are shown in Fig. 3. The circular symbol mark denotes the presence of air breakdown, while the cross symbol denotes its absence. Without the air breakdown, the incident and transmitted energies demonstrate linear tendencies. The transmitted laser energy demonstrated a decreasing logarithmic trend after the air breakdown threshold owing to the absorption of the laser energy by the air plasma. The laser breakdown threshold for air at atmospheric pressure was observed at 12.6 mJ (f = 50 mm, Øspot = 21.63 µm), 18.2 mJ (f = 80, Øspot = 34.62 µm), and 26.9 mJ (f = 100 mm, Øspot = 43.1 µm). The values of the air breakdown threshold increased with longer focal lengths, caused by the decrease in energy densities. The equivalent threshold breakdown energy densities are 4.9 × 1011 W/cm2 (f = 50 mm), 2.8 × 1011 W/cm2 (f = 80), and 2.6 × 1011 W/cm2 (f = 100 mm). As the focal length increases, the minimum transmitted laser energy that passed through the plasma also increases. However, the transmitted energy reaches a constant value or plateau at high incident energies, suggesting that more energy is being absorbed by the plasma as discussed in the next section. Lower laser breakdown energy densities have been observed for secondary harmonic generation (532 nm) at 1.27∼31.9 ×1010 W/cm2 [30,31].
Fig. 3. Transmitted energy measurements for the increasing incident laser energies and varied condensing lenses. Different marks are used to denote the presence or absence of air breakdown. The breakdown threshold energy is shown for each condensing lens.
The absorbed energy measurements for each focal length as the incident energy increases are shown in Fig. 4. The absorbed energy increases in linear proportion with the incident laser energy. The same linear increase in the absorbed energy is observed for each focal length used, but the 50 mm focal length resulted in higher absorbed energy. A shorter focal length produces a smaller spot size, thus, absorbs higher laser energy compared to the larger spot diameters.
The temporal profiles of the plasma formation for laser air plasma without and with microwave are shown in Fig. 5. The synchronized laser pulse and the microwave pulse are also shown in the figures. The circular tip of the antenna is also denoted by a circle with broken lines. When the microwave is turned off, the air breakdown disappears between 0 and 10 µs. The microwaves enhance the plasma dimensions even during the initial breakdown process. With the 1.0 kW microwave with 1000 µs pulse width turned on, the plasma continuously expands and lasts until the end of the microwave pulse width. The pea-shaped initial air plasma becomes irregular after 10 µs. The transverse expansion of the ns-induced air by a few microseconds has also been previously reported by others [32–34]. The plasma is known to have asymmetric expansion towards the direction of the incident laser beam. Therefore, the antenna has been placed at the trailing edge of the air plasma to control the plasma expansion.
Fig. 5. The temporal profile of the plasma plume of the laser-generated air plasma with and without the microwave.
After the initial breakdown, only the microwave energy provides the source for air breakdown, where impact ionization takes place because of the presence of a highly localized electric field. The microwave structure has been designed to sustain and expand the plasma initiated by lasers. Using the helical antenna as a stand-alone device to induce air breakdown is however not possible.
The forced motion of the free electrons coming from the initial air breakdown by the laser results to collisions with excited species and molecules leading to re-excitation. The absorption of the microwave energy by the electrons may also occur, which is evident in the temperature measurements.
The volume of the plasma images was approximated by fitting the pixel area measurements in a circle and taking the radius, and finally creating a sphere out of the radius. The image analysis in extracting the pixel area of the plasma was conducted using the imageJ software. The volume measurements are shown in Fig. 6. The 0.80 mm3 laser-induced air plasma is maintained for less than 10 µs. The exact lifetime is unknown and may need a higher resolution camera to exactly pinpoint it. But in the case of the microwave-enhanced laser-induced air plasma, the volume of the plasma expanded to 4.92 mm3 and a lifetime of more than 1800 µs was observed. The maximum volume of 50.8 mm3 was observed at 1000 µs, which is the end of the microwave pulse duration. The expansion rate is logarithmic owing to the continuous microwave radiation of a constant power of 1000 W for 1000 µs. After that, the plasma disappeared at the end of the microwave irradiation. The plasma expanded more than 10 times in this 1000 µs period.
Figure 7 shows the intensity emissions of the laser-induced air plasma with and without the addition of microwave energy. The initial laser breakdown generates a high-intensity air plasma. This causes the intensity emissions captured by the high-speed camera at zero microseconds. This stage has been labeled Stage I: Laser Breakdown Plasma. When the microwave is turned on, the laser breakdown plasma provides the electron seed for the re-excitation in the microwave enhancements. After that initial spark, the succeeding intensity was sustained by the microwave energy. In the Stage II: Microwave-Sustained Plasma, the nonthermal plasma may appear, which is controlled by the microwave parameters, such as the microwave power and pulse width. The presence of the nonthermal plasma was investigated by measurements of the rotational and vibrational temperatures, where unequal values indicate nonequilibrium or nonthermal plasma.
Fig. 7. Schematic diagram of the intensity sustainment of the microwave-enhanced laser-induced air plasma.
Figure 8 shows the spectroscopic measurements of the air plasma between 300 and 340 nm. The OH and N2 (2PS) are observed from 0 to 1800 µs, with the maximum intensity emissions increasing with time. The maximum emissions are generated at the end of the microwave pulse width. The N2 (2PS) stemmed from the nitrogen-rich air gas which was expected to be emitted at higher concentrations compared to OH. The band from 302 to 320 has a strong contribution to the N2 (2PS) where the OH spectrum is also observed. The OH radicals may have been formed from the water vapor in the ambient air through dissociation during collisions with the electrons. The peak intensity of OH radicals during the microwave input duration is almost constant and gradually diminishes afterward. The spectrum of N2(2PS) slowly rises and then quickly disappears. Similar to OH radicals, the intensity of the N2(2PS) dropped, starting at 1000 µs.
Fig. 8. The temporal profile of the (a) emission spectra and (b) emission intensity peaks of the OH radicals and N2 second positive system in microwave-enhanced laser-generated air plasma [35].
The OH and N2 (2PS) temporal profiles are shown in Fig. 9. The microwave pulse width changed values: 200, 400, 600, 800, and 1000 µs. When the microwave source was turned off, the emission intensity of OH and N2 (2PS) were not observed. The OH radical was also not observed when using a microwave pulse width of 100 µs. This indicates that 200 mJ (100 µs × 1000 W) of microwave energy is needed to produce the OH radical. When using a pulse width ranging from 200 to 1000 µs pulse, the OH peak was observed instantaneously at the beginning of the laser air breakdown. The exposure time used was 99 µs and the acquisition of each spectrum was taken every 313 µs. During this spectrum acquisition, the dissociation of the water vapor produced the OH radicals. Except for the 200 µs pulse width, the OH intensity emission peaks were observed after the laser air breakdown. The OH and N2 (2PS) intensity peaks were supposed to be observed at the end of each microwave pulse width based on the plasma formation. Therefore, the OH and N2 (2PS) measurements decreased after the laser air breakdown using the 100–200 µs microwave pulse width.
Using the N2 2PS spectrum, the rotational and the vibrational temperatures were approximated by the synthetic method using the SPECAIR 3.0 software [36–42]. Figure 10 shows the simulated intensity of 313 and 337 peaks at constant rotational temperature, electron temperature and varying vibrational temperatures. With the constant electron and rotational temperatures, the N2 (C 3Πu,1–B 3Πg,0) transition at 315 nm increased as the vibrational temperature increased. Therefore, the N2 (C 3Πu,0–B 3Πg,0) transition at 337 nm was used to measure the rotational temperatures.
Figure 11 shows the rotational and vibrational temperature measurements. The measurements were carried out for varied microwave pulse widths, namely from 200 µs to 1000 µs. The rotational temperature is low for low microwave pulse widths, namely from 200 µs to 800 µs. Higher rotational temperatures at the initial laser breakdown were observed using the microwave pulse width of 1000 µs, which has a constant value until the spectral measurements of 1500 µs. The microwave-sustained plasma after the initial laser air breakdown had the same constant rotational temperature as the initial temperature since it was mentioned earlier that the acquisition window and exposure time of 99 µs was used for each spectrum. Within this period of spectral acquisition, microwave absorption also started. Therefore, the additional microwave absorption did not affect the temporal profile of the rotational temperatures of the microwave-sustained plasma.
Fig. 11. The (a) rotational and (b) vibrational temperature measurements of the microwave-enhanced laser-induced air plasma.
The vibrational temperature values remained almost constant at the laser breakdown stage for every microwave pulse width used. The vibrational temperature did not change under the initial air breakdown for each pulse withs used (200 µs, 400 µs, 600 µs, 800 µs, 1000 µs) for a constant microwave power of 1000 W. The vibrational temperature increased during the microwave-sustained plasma stage. Increased vibrational temperatures may signify higher molecular dissociation [38].
The rotational temperature and the vibrational temperature had different values, indicating that the microwave-sustained plasma is not in equilibrium.
4. Conclusions
Under atmospheric pressure and open ambient air conditions, air breakdown was generated by a pulsed 1064 nm YAG laser with 7.0 ns pulse width at threshold energy density of 2.6∼4.9 × 1011 W/cm2. The air breakdown generated the 0.80 mm3 laser-induced plasma with less than 10 µs lifetime. A 2.45 GHz microwave power of 1000 W was introduced on the plasma using a spiral antenna. As a result, the air plasma generated by the breakdown is sustained in space and time by 1000 W microwave energy of various pulse widths. From the high-speed image measurement of microwave-enhanced laser-induced air plasma, the initial volume of 4.92 mm3 expanded 10 times and reached 50 mm3 at the end of the microwave pulse width. The elusive emission spectra of molecular nitrogen and OH radicals in the laser induced-air plasma were easily observed in the microwave-enhanced laser-induced air plasma. The evolution of the emission intensities of OH radicals and molecular N2 2nd positive system shows high emissions within the 99 µs acquisition time. The molecular nitrogen and OH radical were used to calculate the rotational and vibrational temperatures by synthetic spectra method using the SPECAIR software. During the microwave energy input period, the rotational temperature remained constant. However, the rotational temperature increased when longer microwave pulse width was applied. The vibrational temperature reached a maximum value when the microwave energy was lengthened. We also concluded that the microwave-enhanced plasma maintained under atmospheric pressure is in a nonequilibrium state.
Acknowledgments
This work was supported by JAEA Nuclear Energy S&T and Human Resource Development Project through concentrating wisdom Grant Number JPJA20P20337946.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in Fig. 9 are available in Dataset 1, Ref. [35].
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