| Citation: |
Yan JIANG, Bangfa PENG, Zhengyan LIU, Nan JIANG, Na LU, Kefeng SHANG, Jie LI. Characteristic studies on positive and negative streamers of double-sided pulsed surface dielectric barrier discharge[J]. Plasma Science and Technology, 2022, 24(4): 044005. DOI: 10.1088/2058-6272/ac58ed
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The mechanisms of streamer generation and propagation in double-sided pulsed surface dielectric barrier discharge (SDBD) on both sides have been analyzed and investigated by experiment and numerical simulation. The fully exposed asymmetric SDBD has two discharge processes located on the high voltage electrode (HVE) side and the ground electrode (GE) side. Discharge images of the HVE side and GE side are taken by a digital camera under continuous pulse and ICCD (Intensified Charge Coupled Device) is utilized to diagnose the generation and propagation of streamers in single pulse discharge. In order to understand the physical mechanisms of streamer evolution more deeply, we establish a 2D simulation model and analyze it from the aspects of electron density, ion density, reduced electric field and electron impact ionization source term. The results show that the primary and secondary discharges on the HVE side and the GE side of the double-sided SDBD are composed of positive streamer and negative streamer, respectively. On the HVE side, the accumulation of positive charges on the dielectric surface causes the direction of the electric field to reverse, which is the principal factor for the polarity reversal of the streamer. On the GE side, both the negative charges accumulated on the dielectric surface and the falling voltage are the key factors for the streamer polarity switch.
Fused silica is a commonly used material in laser systems and plays a crucial role in high-power laser devices [1, 2]. However, the interaction between the laser and fused silica often leads to surface damage, which can negatively affect the normal functioning of the laser system. This damage is characterized by the generation of plasma [3–5]. The main causes of damage resulting from high-power laser irradiation of fused silica are the plasma and combustion wave that are produced. These waves are formed as a result of laser-supported absorption, which includes subsonic combustion wave and supersonic detonation wave [6–10]. In order to better understand this phenomenon, Li et al [11] conducted a study on the propagation behavior and acceleration mechanism of shock waves induced by combined millisecond-nanosecond pulse lasers on the surface of silicon. Similarly, Wang et al [12, 13] examined how different focal positions affect the propagations of plasma and combustion wave, as well as the expansion dynamics of high-energy infrared lasers. Geng et al [14] focused on studying the temperature field of fused silica induced by a combination of millisecond and nanosecond pulse lasers. The research also explored the speed and pressure of plasma, as well as the absorption wave generated by fused silica. Wang et al [15] investigated how different experimental conditions affect laser-induced plasma. This study focuses on analyzing how side-blown airflow at varying flow rates impacts the propagations of plasma and combustion wave induced by combined pulse lasers.
Extensive research has been conducted on the damage characteristics of laser-induced fused silica in the natural convection environment of air. Wu et al [16] and Wang et al [17] investigated the probability of damage and the morphology of optical components caused by millisecond pulse lasers. The damage occurred simultaneously on both the front and rear surfaces of the component. The combined effect of temperature and stress caused damage to the front surface, while the rear surface was primarily affected by stress. Xia et al [18] investigated the temperature and stress fields of fused silica when exposed to a combination of millisecond-nanosecond pulse laser irradiation. Additionally, Lyu et al [19], Yu et al [20], Qiu et al [21] and Jiang et al [22] explored the dynamic process and morphological damage of fused silica under varying laser parameters. Their findings suggest that the damage occurs due to two mechanisms: self-focusing and point defect absorption. Point defect absorption emerges as the primary mechanism, leading to material damage over time. However, there are limited studies on the combined pulse laser irradiation of fused silica in the external airflow environment. Currently, most of the relevant research focuses on single-pulse lasers, and the experimental materials mainly consist of steel plates, aluminum plates, and carbon fiber materials. Chen et al [23, 24] examined how continuous pulse laser irradiation affects carbon fiber composites when exposed to tangential forced airflow. Their research showed that higher laser intensity decreased the shielding effect of gaseous pyrolysis products on the laser beam, while the airflow introduced oxygen. Seidel et al [25], Fowler and Smith [26], Pirri, Schlier and Northam [27] and Poueyo et al [28] summarized plasma behavior and the effects of plasma and combustion wave on laser beam propagation during laser welding. Their study demonstrated that modifying laser power levels can reduce plasma shielding, leading to enhanced laser energy delivery to the surface. Zhang et al [29] and Zhang and Huang [30] conducted simulation studies to investigate the impact of temperature fields on target materials when exposed to external airflow environments. They examined the complex interaction of various factors influencing the effects of laser irradiation under different conditions.
The impact of side-blown airflow on the combined pulse laser-induced fused silica fusion process is substantial. It can modify the heat conduction and mass transfer properties of the fused silica surface, leading to improved laser energy absorption, plasma generation, and combustion wave formation. This plays a crucial role in enhancing material processing efficiency and optimizing processing quality. Investigating the effects of laser processing can enhance accuracy and speed, reduce the heat-affected area, prevent material damage and deformation, and facilitate the application of laser processing technology in industrial manufacturing, micro-nano processing, and other fields.
This work presents a simulation model of plasma and combustion wave generated by a combined pulse laser-induced fused silica. The model takes into account the impact of varying side-blown airflow speed. Furthermore, an experimental platform is constructed to analyze the expansion speeds of fused silica plasma and combustion wave under different airflow speeds and conditions. The paper also discusses the rules and mechanism of pulse delay and how they change.
When a laser interacts with fused silica, the state of the material changes from solid to molten and then to a gaseous state. As the laser interacts with the target, the vapor becomes ionized and breaks down, resulting in the formation of a high-temperature and high-pressure plasma. This plasma creates an absorption zone where subsequent laser pulses are rapidly absorbed. The temperature of the plasma increases rapidly, causing it to expand and compress the surrounding air, which leads to the formation of a shock wave. This shock wave exerts a recoil pressure on the surrounding gas, resulting in gas ionization and the production of free electrons. These free electrons, located near the laser source, absorb laser energy through inverse Bremsstrahlung, forming a new layer of plasma absorption. This layer shields the absorption of laser energy by the plasma layer behind it, creating a laser-supported absorption wave in the opposite direction of the laser beam. The laser-supported absorption wave includes both subsonic combustion wave and supersonic detonation wave.
Figure 1(a) presents a two-dimensional model illustrating the irradiation of fused silica by combined pulse lasers while considering varying side-blown airflow conditions. Figure 1(b) depicts the model grid section. The simulation calculations employ a two-dimensional model, with the calculation area including ambient gas (air) and fused silica. The corresponding thermodynamic parameters of the air and fused silica materials are input. To enhance accuracy in crucial areas, it is recommended to utilize an ultra-fine meshing method near the laser heat source. For non-key calculation areas located far from the heat source, a conventional fine meshing method should be employed to ensure overall accuracy and efficiency of the calculations.
During the simulation process, the following assumptions were made. (1) Plasma propagates at a subsonic laminar flow, as its speed is much lower than that of sound. (2) Plasma generation and expansion occur rapidly, and heat exchange with the surroundings is at a local thermal balance.
The total mass of the material remains constant throughout the physical process. To determine the dynamic structure of the gas in the plasma region under different side-blowing gas flow conditions, we utilize the conservation of mass, the Navier-Stokes equation, and the energy conservation equation.
Conservation of mass equation:
| \frac{\partial \rho}{\partial t}+\nabla \cdot \left(\rho {\boldsymbol{V}}\right)=0. | (1) |
Among them, \rho is the density of the fluid and {\boldsymbol{V}} is the velocity vector of the fluid.
Treating the substrate as an incompressible fluid, the mass conservation equation can be simplified to:
| \nabla \cdot{\boldsymbol{ V}}=0. | (2) |
If the volume corresponding to any surface is {{U}} , then the mass of liquid flowing out of the volume per unit time satisfies the following equation:
| \frac{\mathrm{d}M}{\mathrm{d}t}=-\int _{v}\;\rho {\boldsymbol{V}}\cdot {\boldsymbol{n}}\mathrm{d}U. | (3) |
In the formula, \dfrac{\mathrm{d}M}{\mathrm{d}t} represents the mass of fluid flowing out per unit time. \rho represents the density of the fluid, {\boldsymbol{V}} represents the velocity vector of the fluid, {\boldsymbol{n}} represents the normal vector of the surface, and \mathrm{d}U represents the micro-element area on the surface.
| {\boldsymbol{V}}=\left\{\begin{split}&{{\boldsymbol{V}}}_{\mathrm{m}}& t < {\text{Δ}} t\\& {{\boldsymbol{V}}}_{\mathrm{m}}+{{\boldsymbol{V}}}_{\mathrm{n}}& t\geqslant {\text{Δ}} t\end{split}\right. . | (4) |
{\boldsymbol{V}} is the velocity vector of the gas flow generated by the combined pulse laser, while {{\boldsymbol{V}}}_{\mathrm{m}} and {{\boldsymbol{V}}}_{\mathrm{n}} represent the velocity vectors of the gas flow generated by the millisecond pulse laser and nanosecond pulse laser, respectively.
Navier-Stokes equation:
| \rho \left({\boldsymbol{V}}\cdot \nabla \right)=-\nabla \left(p+\frac{2}{3}\eta \nabla \cdot {\boldsymbol{V}}\right)+\nabla \cdot \left(\eta \left({\nabla }_{{\boldsymbol{V}}}+{\nabla }_{\tilde{{\boldsymbol{V}}}}\right)\right)+\left({\rho }_{0}-\rho \right)g | (5) |
Conservation of energy equation:
| \begin{split}\rho {c}_{\mathrm{p}}\frac{\partial T}{\partial t}+\rho {c}_{\mathrm{p}}{\boldsymbol{V}}\cdot \nabla T=&\nabla \cdot \left(\lambda \nabla T\right)+{\sum }_{m-1}^{{N}_{m}}c{X}_{m}\left({U}_{m}-{U}_{\text{eq,}{m}}\right)+\\&\mu \frac{1}{\tau }\rm{exp}\left(-\frac{{r}^{2}}{{r}_{0}^{2}}\right)\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\int }_{0}^{z}\mu \mathrm{d}z\right).\end{split} | (6) |
In the above formulas, {\boldsymbol{V}} is the gas velocity, T is the temperature, p is the deviation relative to the standard atmospheric pressure, g is the gravity acceleration, {c}_{\mathrm{p}} , \eta and \lambda are the specific heat capacity, viscosity coefficient, and thermal conductivity respectively, {U}_{m} and {U}_{\mathrm{e}\mathrm{q},m} respectively represent the body absorption coefficient and medium thermal radiation energy density of the m-th group of radiation. {N}_{m} represents the number of groups of thermal radiation in the multi-group diffusion approximation. \mu is the inverse bremsstrahlung absorption coefficient of laser energy by the plasma.
Transient heat transfer equation in air and substrate regions is:
| {\rho }_{i}{c}_{\mathrm{p},i}\frac{\partial T}{\partial t}+{\rho }_{i}{c}_{\mathrm{p},i}(\overrightarrow{v}\cdot \overrightarrow{\nabla }T)=\overrightarrow{\nabla }\cdot \left({k}_{i}\overrightarrow{\nabla }T\right) | (7) |
In the formula, {c}_{\mathrm{p},i} , {k}_{i} , {\rho }_{i} are the specific heat, thermal conductivity, and density of the fluid respectively, \overrightarrow{v} is the phase change speed of the fluid, T is the fluid temperature, i=1, 2, 3,..., 7 respectively represent each layer of dielectric material in the model. Table 1 displays the material parameters.
| Feature parameter name | Symbol | Dimension | Numerical value |
| Plasma density | \rho | \mathrm{g}{\mathrm{c}\mathrm{m}}^{-3} | 3.49/T×10−6 |
| Plasma thermal conductivity | K | {\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1} | −0.002 + 1.5 × 10−4 × T − 7.9 × 10−8 × T2 + 4.12 × 10−11 × T3 − 7.44 × 10−15 × T4 |
| Plasma heat capacity | C | {\mathrm{J}}\cdot \left(\mathrm{k}\mathrm{g}\cdot\mathrm{K}\right)^{-1} | 1047.27 + 9.45 × 10−4 × T2 −6.02 × 10−7 × T3 + 1.29 × 10−10 × T4 |
| Plasma viscosity coefficient | \eta | \mathrm{P}\mathrm{a}\mathrm{ }\mathrm{S} | −8.38 × 10−7 + 8.36 × 10−8 ×T − 7.69 × 10−11× T2 + 4.64 × 10−14 × T3 − 1.07 × 10−17 × T4 |
| Specific heat rate | {\gamma }_{\mathrm{r}} | 1 | 1 |
| {\gamma }_{\mathrm{a}} | 1 | 1.4 | |
| Relative permittivity | \epsilon_{\mathrm{a}} | C² N−1 m−2 | 1 |
| Reflectivity | {R}_{\mathrm{r}} | 1 | 0.3 |
| Melting point | Tm | K | 1533 |
| Boiling point | Tv | K | 3190 |
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The total power of the combined laser is:
| p={p}_{\mathrm{n}}+{p}_{\mathrm{m}}-mLv. | (8) |
Millisecond pulse laser power:
| {p}_{\mathrm{m}}=\frac{{J}_{\mathrm{m}}}{{\tau }_{\mathrm{m}}}f\left(r\right){g}_{\mathrm{m}}\left(t\right). | (9) |
Speed of sound and Mach number:
| \mathrm{M}\mathrm{a}=\frac{v}{c}. | (10) |
Ma is a crucial factor in the experimental research process due to its positive correlation with compressibility. Airflow is classified as supersonic when \mathrm{M}\mathrm{a} > 1 and low-speed when \mathrm{M}\mathrm{a} < 1 , with sonic airflow being the default value for v . Corresponding experiments should be conducted based on these results to achieve optimal experimental conditions.
Convection heat transfer is the result of fluid displacement and heat conduction between fluid molecules. It can be calculated using the Newton cooling formula.
| q=ha\left({T}_{\mathrm{s}}-{T}_{\mathrm{b}}\right) . | (11) |
The formula defines {T}_{\mathrm{s}} and {T}_{\mathrm{b}} as the solid surface temperature and the temperature of the surrounding fluid, respectively. The variable a represents the wall surface area in contact with the fluid, while h represents the convection heat transfer coefficient, which determines the heat transfer capacity of the fluid.
The ultrafast time-resolved optical shadow method is employed in experimental research to investigate the evolution processes of plasma and combustion wave generated by combined pulse laser irradiation of fused silica under side-blown airflow conditions. The expansion process of the combustion wave is observed and recorded in real time using a high-speed camera. The influence of the side-blown airflow velocities on the plasma and combustion wave generated by the combined pulse laser-induced fused silica is analyzed. The experimental setup is depicted in figure 2.
As shown in figure 2, the experimental device system mainly consists of three parts. The laser energy output system consists of a 1064 nm millisecond laser and a 1064 nm nanosecond laser. The pulse widths of the two lasers are 1 ms and 10 ns, respectively. Both lasers pass through a focusing lens and converge at the same point on the target. To record the motion evolution of plasma, the ultrafast time-resolved optical shadowing method is employed. The plasma and combustion wave detection system comprises a 532 nm continuous green laser, a high-speed camera, a beam expander, an attenuator, and a focusing lens (f = 50 mm). The high-speed camera has a frame rate of 210000 fps and an exposure time of 1/6300000 s. The laser energy output system and the plasma and combustion wave detection system are triggered by the DG645 delayed pulse generator. The DG645 sends the trigger signal to the nanosecond laser. The pulse delay (Δt) of the millisecond laser and the nanosecond laser is controlled by the DG645. The pulse delay is measured using an oscilloscope, which records the time difference from the starting time of the millisecond laser signal to the maximum value of the nanosecond laser signal on the target. The airflow device system consists of an air compressor, pressure gauge, and rotameter. The air compressor has a maximum displacement of 75 L/min and a rated pressure of 0.8 MPa. The airflow monitoring device is a VA/WA30S DN15 rotor flowmeter with a flowmeter accuracy level of 1.5. It has a diameter of Φ = 15 mm and a measurement range of 0.8–8 m³/h. After the airflow device system generates the compressed air flow, the pressure gauge and rotameter can be used to monitor the gas pressure and flow rate in the pipe in real-time. The high-speed airflow generated by the air compressor passes through the pressure gauge and gas flow meter and is blown from the airflow nozzle side to the target surface. In order to conduct the millisecond-nanosecond combined pulse laser-induced damage experiment on fused silica, the air compressor is turned on in advance during the experiment to ensure stable airflow conditions.
The experiment aimed to investigate the effects of two lasers on a fused silica target measuring 25 mm × 25 mm ×4 mm. The lasers had different radii, with one measuring 0.4 mm and the other measuring 0.2 mm. Laser radius refers to the radius of the target damage shape caused by laser irradiation. The experiments were conducted under normal air conditions, at room temperature and pressure, and were repeated five times. This article reports the average value obtained from the five experiments.
To gain a deeper understanding of how plasma and combustion wave spread, it is imperative to carefully study their propagation process. One approach to achieve this is by calculating the velocity fields of these waves using a specific equation (12).
| v\left(t\right)=\frac{L\left(t+ {\text{Δ}} t\right)-L\left(t\right)}{ {\text{Δ}} t} . | (12) |
At time t, the expanding plasma front is detected at a distance L\left(t\right) , and {\text{Δ}} t is a small time interval in μs.
| Parameters | Values |
| Laser wavelength | 1064 nm |
| Millisecond laser energy density | 2800 J/cm2 |
| Nanosecond laser energy density | 142 J/cm2 |
| Millisecond laser radius | 0.4 mm |
| Nanosecond laser radius | 0.2 mm |
| Millisecond pulse laser pulse width | 1 ms |
| Nanosecond pulse laser pulse width | 10 ns |
| Nozzle height | 3 cm |
| Nozzle angle | 40° |
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Figure 3 presents the flow field diagrams of plasma and combustion wave velocities with and without side-blown airflow. Subfigure (a) illustrates the flow field diagrams of the expansion velocities of plasma and combustion wave induced by the combined pulse laser-induced fused silica without any airflow. On the other hand, subfigure (b) depicts the flow field diagrams of plasma and combustion wave velocities generated by combined pulse laser-induced fused silica under the condition of side-blown airflow suppression. The experimental parameters for the combined pulse laser are provided in table 2.
The laser Is incident directly in front of the target, with a side blowing angle of 40° between the right side and the incident angle of the laser. The measured results of the plasma expansion velocity curve are presented in figure 4. Figure 3(a) demonstrates that in the absence of added airflow, the plasma and combustion wave rapidly expand in the initial stage, taking on the shape of a bullet hemisphere in space as they expand outward from the target. The plasma exhibits symmetry along the direction of the laser, and its growth continues over time. The speed of plasma expansion increases abruptly between 0 and 0.1 ms and moves in the opposite direction of the laser beam, forming a combustion wave supported by the laser. Once the laser ceases, the plasma area no longer receives energy supply and the laser no longer supports the propagation of the combustion wave. The maximum speed of the combustion wave reaches 117.2 m/s within 0.01 ms and then rapidly decreases, falling below 10 m/s after 0.6 ms. Under the given energy density condition, plasma is primarily generated through the ionization of the nanometer-thick film layer. This film layer quickly ionizes under the influence of a millisecond pulse laser, converting the plasma’s internal energy into kinetic energy to support the combustion wave. The expansion speed of the combustion wave sharply drops within a small range between 0.2 and 0.3 ms due to a reduced availability of target vapor for ionization. Subsequently, the expansion speed gradually diminishes and eventually levels off. Figure 3(b) demonstrates that when a side blow airflow of 0.6 Ma is applied to blow away the plasma above the target, the volume of plasma above the target significantly decrease and the plasma expansion speed increases within 0.01 ms. However, since the airflow is already blowing sideways at a steady rate before the combined pulse laser is incident, the plasma generated by laser irradiation of fused silica is quickly suppressed by the strong side blowing airflow, and the expansion distance begins to shorten gradually. At 405 μs, the nanosecond pulse laser intervenes in the plasma and combustion wave, causing a slight forward expansion. However, the high speed of the airflow at this time leads to turbulence in the plasma, and it soon ceases to expand forward due to the airflow suppression. As time progresses, the plasma gradually dissipates.
The laser energy parameters for the simulation study were selected according to table 1. The energy density of the millisecond pulse laser was set at 2800 J/cm², while the energy density of the nanosecond pulse laser was set at 142 J/cm². The pulse delay was Δt = 0.25 ms, and the side blow angle of the airflow was set at 40°. The expansion velocity flow field distributions of plasma and combustion wave generated by the combined pulse laser-induced fused silica were then studied under different airflow speeds: 0.2 Ma, 0.4 Ma, and 0.6 Ma.
Figure 5 presents a simulation diagram illustrating the plasma flow field under different airflow velocity conditions. The diagram demonstrates that the plasma rapidly expands and generates a combustion wave within 100 μs when the side-blown airflow speeds are 0.2 Ma, 0.4 Ma, and 0.6 Ma, respectively. The maximum expansion speeds are recorded as 106 m/s, 89 m/s, and 85 m/s, correspondingly. With an increase in airflow speed, the plasma expansion is impeded by the airflow, and this inhibition is inversely proportional to the side-blowing airflow speed. The higher movement speed is concentrated near the central axis, and the peak speed is approximately located between the combustion wave front and the front surface of the target. The energy and momentum released during the combustion process are transferred to the surrounding ions and electrons. Inside the plasma, this energy and momentum transfer occurs at a relatively fast speed, causing the ions to rapidly expand and diffuse, pushing the surrounding ions and electrons at a high velocity. As a result, the ions in this area undergo axial displacement and expansion, with a greater speed in the radial direction. The internal thermal energy is converted into kinetic energy, leading to the expansion of the combustion wave. However, the expansion is gradually cooled by the airflow blown from the side. The rapid expansion and increase in particle kinetic energy cause the internal temperature to decrease, resulting in a decrease in the speed of the combustion wave. When a 250 μs nanosecond pulse laser is introduced, a double combustion waves phenomenon is observed. After the laser irradiation ends, the plasma expansion speed decrease rapidly. The side-blown airflow has a greater impact on the laser action center and the downwind point, aiding in the removal of impurities and heat transfer. The convective heat transfer effect of the side-blown airflow reduces the plasma temperature along the path of the incident laser beam, effectively suppressing the plasma generated during the laser irradiation process. Another important factor is that the side-blown airflow plays a ‘suppressing’ role in the plasma cloud induced by the combined pulse laser. This effect reduces the height of the plasma cloud and diminishes its shielding effect on laser energy.
This section investigates the distributions of expansion velocities flow field of plasma and combustion wave generated by combined pulse laser-induced fused silica. The experiments were conducted under different airflow speeds of 0.2 Ma, 0.4 Ma, and 0.6 Ma.
When a high-energy pulse laser irradiates the surface of fused silica, it generates a plasma with high temperature and density. Plasma is a state of highly excited gas consisting of positively charged ions and negatively charged free electrons. The formation and diffusion of plasma are influenced by variations in side-blowing airflow. Intense side-blowing airflow can disperse the plasma during the laser-gas interaction, thereby reducing its density and duration. The presence of instability in the region of laser irradiation can affect the interaction process between the laser and gas.
Figure 6 presents the evolution diagrams of side-blown plasma and combustion wave expansion at different airflow velocities. The experiment was conducted with a pulse delay of Δt = 0.25 ms and an airflow side-blow angle of 40°. The figure shows that the initial expansion speed of plasma generated by a millisecond laser is relatively high. The addition of a nanosecond laser increases the expansion speed of plasma to some extent. However, the growth is limited due to the inhibitory effect of the side-blowing airflow. The plasma rapidly expands in a short period, forming a laser-supported combustion wave. Figures 6(a)–(c) demonstrate that as the airflow velocity increases during the experiment, the airflow increasingly suppresses the plasma. Eventually, the plasma is completely blown away by the airflow. Interestingly, during the experiment, it was observed that double combustion waves appeared when the nanosecond pulse laser intervened, regardless of the varying airflow speeds. The absorption of energy from a nanosecond laser by a millisecond laser-supported combustion wave results in a pre-ionization effect and an energy accumulation effect. The pre-ionization effect occurs when plasma is formed in the laser irradiation area, lowering the ionization energy threshold of the medium and promoting the formation of combustion wave. The phenomenon of energy accumulation causes the earlier intervening laser beam to accumulate some energy in the medium, while the later intervening laser beam takes some time to reach the same energy level. This higher energy input accelerates the formation and expansion of the combustion wave. Therefore, the simultaneous intervention of two laser beams further increases the initial expansion speed of the combustion wave. The figure shows the second combustion wave at 0.25 ms. However, the resulting phenomenon of the double-combustion wave varies depending on the airflow velocities. For example, at an airflow velocity of 0.2 Ma, a strong phenomenon of double combustion waves occurs at 257 μs, with significant plasma expansion. At airflow velocities of 0.4 Ma and 0.6 Ma, the relatively high airflow velocities lead to rapid suppression of the plasma and a decrease in the expansion distance after the occurrence of the double combustion waves phenomenon. The airflow blown from the side reduces the speeds of plasma and combustion wave expansions, ultimately stopping the plasma expansion and preventing it from moving forward. Meanwhile, the combustion wave generated by the laser-irradiated target continue to advance.
During the experiment, it was observed that with a gradual increase in gas flow rate, both the speed and distance of plasma expansion decrease gradually. Additionally, there is a gradual transformation in the plasma state. However, when the gas flow rate becomes excessively large, the flow transitions from laminar to turbulent. As a result, the plasma near the target surface cannot be completely removed. This phenomenon primarily occurs due to the continuous ionization of the target vapor induced by the incident laser, leading to the formation of plasma.
The pulsed laser action above the plasma can be divided into two parts, as depicted in figure 7. In figure 7(a), when there is no airflow, two types of plasma are present simultaneously. In figure 7(b), a portion of the plasma on the side where airflow is blowing can be dispersed, leading to a larger size of plasma that significantly hinders the transfer of laser energy. However, with the appropriate airflow rate, this dispersed plasma can be completely removed, a process referred to as ‘diffusion of the plasma’. In figure 7(c), the other part of the plasma remains stationary close to the surface of the target material, and regardless of the airflow intensity, it cannot be fully dispersed. This stationary plasma is termed ‘stationary plasma’.
This section examines the airflow conditions with an airflow velocity of 0.2 Ma, an airflow side blow angle of 40°, and pulse delays of Δt = 0.2 ms, Δt = 0.4 ms, Δt = 0.6 ms, and Δt = 0.8 ms. It investigates the alterations in the distribution of the expansion velocities flow field caused by combined pulse laser-induced fused silica on plasma and combustion wave.
Figure 8 displays the airflow velocity of 0.2 Ma and the airflow side-blowing angle of 40°. The progressions of the plasma and combustion wave expansion with the presence of side-blown airflow is depicted under various pulse delay conditions (Δt = 0.2 ms, Δt = 0.4 ms, Δt = 0.6 ms, Δt = 0.8 ms). When the millisecond laser generates the combustion wave, it initially exhibits a relatively high expansion speed. The introduction of the nanosecond pulse laser enhances the expansion speed of the combustion wave, leading to rapid plasma expansion and the formation of a laser-supported combustion wave. The expansion speeds and distances of plasma and combustion wave induced by fused silica, under the influence of the millisecond-nanosecond combined pulse laser, also vary with different pulse delay conditions. Additionally, the experiment observed a dual combustion wave phenomenon when the nanosecond pulse laser intervened at different pulse delay moments. It was found that the diffusion process caused the plasma to undergo turbulence due to the side-blowing airflow, thereby suppressing the propagation of the second combustion wave as the pulse delay increased. As depicted in figure 8(d), this phenomenon occurs because the medium undergoes a change after the initial combustion wave is generated. During this process, the temperature of electrons and ions increases, leading to a change in their density and dissipation of their energy. Within a very short timeframe, the combustible substances in the medium are completely depleted or transformed into incombustible substances, rendering them unable to support the propagation of a second combustion wave. The airflow exerts suppression on the plasma, causing it to quickly subside and eventually be dispersed by the airflow.
Figures 9 and 10 depict the comparison between simulation and experimental outcomes for Δt values of 0.2 ms and 0.8 ms, respectively. In figure 9, with a constant side-blown airflow speed and a small pulse delay, the double combustion waves phenomenon is more pronounced during the expansion processes of plasma and combustion wave resulting from the combined pulse laser. In figure 10, as the pulse delay increases, noticeable changes occur in the medium. Both electron and ion temperatures rise as density fluctuates, leading to energy dissipation and rapid alterations in electron and ion density within the medium. This ultimately depletes the combustible material or transforms it into incombustible material, insufficient to sustain the propagation of the second combustion wave. The simulation results presented in the figures align with the experimental findings.
The results of surface damage experiments performed at three different airflow velocities are shown in figure 11. The velocities tested were (a) 0.2 Ma, (b) 0.4 Ma, and (c) 0.6 Ma. The experiment involved using a side-blowing airflow angle of 40° and a time interval (Δt) of 0.25 ms. Figure 10 illustrates that as the airflow rate increased during the experiment, the ablation rate on the fused silica surface also increased, leading to a larger ablation area. The side-blown airflows, which have varying strengths (Mach numbers), are used to remove decomposition products from ablation and erode the exposed matrix of the fused silica target. This process enhances laser ablation. When treating fused silica with a combined millisecond-nanosecond pulse laser under side-blown airflow, the millisecond pulse laser has a longer action time and higher energy density, which primarily causes thermal damage to the fused silica. On the other hand, the nanosecond pulse laser has a shorter duration but higher power density. Both laser energy densities induce thermal stress, resulting in stress-induced spalling and damage to the fused silica. However, the latter case exhibits more prominent stress-induced spalling, with distinct traces of layered ablation observed in the central region of the damaged morphology. Additionally, when the fused silica substrate is irradiated with a millisecond pulse laser, a nanometers-thick optical film on the surface absorbs the laser energy. This film undergoes rapid phase transition within microseconds during the initial laser irradiation. Once the film layer is completely vaporized, the fused silica substrate melts, vaporizes, and generates plasma. Increasing the airflow rate during experimentation worsens the damage to the fused silica target. However, the convective cooling effect of side-blown airflow effectively inhibits plasma formation. The side-blowing airflow suppresses the plasma during laser irradiation, resulting in a decrease in temperature along the laser beam’s propagation path. Airflow has the ability to suppress plasma expansion, which in turn reduces its upward expansion distance. This suppression diminishes the shielding effect of plasma on laser energy, resulting in enhanced laser transmittance. As a result, the laser energy density reaching the irradiation area is elevated. Additionally, the use of side-blown airflow eliminates gas generated from thermal decomposition, ensuring an ample supply of oxygen for the target’s oxidation reaction. This increased oxygen availability enhances oxidation reactions in the ablation area, leading to the increased heat generation and the accelerated target damage.
Figure 12 presents experimental results on the damage process of fused silica caused by a combination of pulse laser and side-blown airflow under different pulse delay conditions. The airflow velocity was set to 0.2 Ma, and the side-blown airflow angle was 40°. The experiment explored pulse delays of Δt = 0.2 ms, Δt = 0.4 ms, Δt = 0.6 ms, and Δt = 0.8 ms. Notable variations in the surface damage morphology of the laser-irradiated target were observed when using side-blown airflow in conjunction with a nanosecond pulse laser at different time intervals. Adding the nanosecond pulse laser early, with a small delay, resulted in a larger damaged area. When the fused silica surface was irradiated with a millisecond pulse laser, a nanometer-thick optical film intrinsically absorbed the laser energy. During the initial stage of millisecond pulse laser action, the film underwent rapid phase change within microseconds. Subsequently, after complete vaporization of the film layer, the fused silica substrate also experienced melting, vaporization, and plasma generation. Thermal melt damage was identified as the primary mode of damage. The side-blown airflow played an active role throughout this process, removing spatter and oxidation products from the ablation area, which could potentially damage the film layer on the target surface. The plasma acted as a shield against incident laser energy, while the airflow provided convection cooling, reducing energy accumulation in the central area of laser irradiation. This phenomenon had an impact on the laser ablation heat-affected zone.
This study investigates the impact of variations in side-blown airflow on plasma and combustion wave induced by combined pulse laser treatment of fused silica. The researchers used numerical simulations and experiments to explore how the combined pulse laser affects plasma and combustion wave under different airflow rates and pulse delays. The findings of the study suggest that the expansion speeds of plasma and combustion wave are inversely related to the airflow rates from side-blowing. Airflows ranging from 0.2 Ma to 0.6 Ma have varying effects on the morphology of plasma and combustion wave. Since the target vapor is ionized to produce plasma during laser treatment, the plasma cannot be completely displaced regardless of the airflow rate. Under different pulse delay conditions, the nanosecond pulse laser, along with the pressure from side-blowing airflow and mechanisms like reverse Bremsstrahlung radiation absorption and target surface absorption, collaboratively facilitate pre-ionization and energy accumulation with the millisecond laser. This lowers the medium ionization energy threshold and promotes the formation of combustion wave. Under delay conditions of Δt = 0.2 ms and Δt = 0.4 ms, intensified fluid velocity fields occur due to accelerated combustion wave expansion. The further enhancement of the fluid velocity field promotes the secondary propagation and expansion of the combustion wave, resulting in the occurrence of a double combustion waves phenomenon. During the process of side-blowing airflow at different rates, it effectively removes the decomposition products that are generated by the combined pulse laser treatment, rinsing, and stripping of the exposed fused silica target matrix. This process accelerates the laser damage and melting of the target. When the airflow velocity is low, the damage area on the fused silica surface increases. Under various pulse delay conditions, the airflow rinses off and removes spatter and oxidation products in the ablation zone, resulting in damage to the film quality on the target surface. The plasma shields the incident laser energy, while the airflow provides convection cooling, which reduces the accumulation of energy in the central laser irradiation area. This, in turn, affects the heat-affected zone caused by laser ablation. As the pulse delay is extended, the surface damage of fused silica gradually decreases.
The research investigates the impact of combined pulse laser treatment on fused silica, with a specific focus on the generations of plasma and combustion wave in an external airflow environment. The study also explores the control mechanisms that govern the morphology and dynamic characteristics of plasma and combustion wave under side-blown airflow conditions. These findings have important implications for understanding plasma control under laser irradiation and the physical characteristics of combustion wave. Furthermore, they provide valuable insights for engineering design in related fields.
Irradiating fused silica with a combined pulse laser under different side-blown airflow conditions involves complex interactions between thermodynamics, fluid mechanics, and optics. This process has important implications for the advancement of laser processing technology. When using lasers for irradiation, it is crucial to take into account the impact of gas flow on the internal structure of fused silica crystals. This consideration includes the stability of crystal structure and the possibility of phase changes. The study and related research have certain limitations that will be addressed in future studies.
This work is supported by National Natural Science Foundation of China (Nos. 51877027 and 52107140) and Project funded by China Postdoctoral Science Foundation (No. 2021M700662).
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Figures(15) / Tables(1)
Supported by: Beijing Renhe Information Technology Co., Ltd. 
| Feature parameter name | Symbol | Dimension | Numerical value |
| Plasma density | \rho | \mathrm{g}{\mathrm{c}\mathrm{m}}^{-3} | 3.49/T×10−6 |
| Plasma thermal conductivity | K | {\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1} | −0.002 + 1.5 × 10−4 × T − 7.9 × 10−8 × T2 + 4.12 × 10−11 × T3 − 7.44 × 10−15 × T4 |
| Plasma heat capacity | C | {\mathrm{J}}\cdot \left(\mathrm{k}\mathrm{g}\cdot\mathrm{K}\right)^{-1} | 1047.27 + 9.45 × 10−4 × T2 −6.02 × 10−7 × T3 + 1.29 × 10−10 × T4 |
| Plasma viscosity coefficient | \eta | \mathrm{P}\mathrm{a}\mathrm{ }\mathrm{S} | −8.38 × 10−7 + 8.36 × 10−8 ×T − 7.69 × 10−11× T2 + 4.64 × 10−14 × T3 − 1.07 × 10−17 × T4 |
| Specific heat rate | {\gamma }_{\mathrm{r}} | 1 | 1 |
| {\gamma }_{\mathrm{a}} | 1 | 1.4 | |
| Relative permittivity | \epsilon_{\mathrm{a}} | C² N−1 m−2 | 1 |
| Reflectivity | {R}_{\mathrm{r}} | 1 | 0.3 |
| Melting point | Tm | K | 1533 |
| Boiling point | Tv | K | 3190 |
DownLoad:
CSV
| Parameters | Values |
| Laser wavelength | 1064 nm |
| Millisecond laser energy density | 2800 J/cm2 |
| Nanosecond laser energy density | 142 J/cm2 |
| Millisecond laser radius | 0.4 mm |
| Nanosecond laser radius | 0.2 mm |
| Millisecond pulse laser pulse width | 1 ms |
| Nanosecond pulse laser pulse width | 10 ns |
| Nozzle height | 3 cm |
| Nozzle angle | 40° |
DownLoad:
CSV
