A comparative study on the characteristics of nanosecond laser ablation zinc and acrylonitrile butadiene styrene targets

In this study, the influence of laser energy and pressure on propulsion performance of zinc and acrylonitrile butadiene styrene (ABS) is investigated by impulse measurement, fast exposure images, spectral diagnostics and target ablation. A Q-switched Nd:YAG laser with the wavelength of 1064 nm and pulse width of 6 ns is employed. The impulse and coupling coefficient generated by laser ablation ABS are greater than that of Zn, and they exhibit a similar variation trend with pressure. However, at higher pressure levels, the change in impulse versus laser energy is not completely coincident between Zn and ABS samples. The target property plays a significant role in the generation and propagation of plume related to the plasma parameters such as electron density and temperature. The temporal evolution images indicate that the plasma plume of laser-induced Zn presents a faster decay in comparison with that of ABS, which is ascribed to the fact that the gas temperature of ABS is higher than the electron temperature of Zn plasma in the local thermodynamical equilibrium. Also, the electron density is lower for Zn due to the rapid heat diffusion and higher ablation threshold of metal. It is found that the surface absorption is dominant for metal because the ablated crater of Zn performs larger diameter and shallower depth. On the contrary, the shrinkage in diameter but enhancement in depth of crater is observed from ABS surface, and the ablation mass is larger, suggesting the obvious volume absorption for polymer. The results reveal that the target property can engender an important effect on the energy conversion between laser, target and plasma.

In this study, the influence of laser energy and pressure on propulsion performance of zinc and acrylonitrile butadiene styrene (ABS) is investigated by impulse measurement, fast exposure images, spectral diagnostics and target ablation. A Q-switched Nd:YAG laser with the wavelength of 1064 nm and pulse width of 6 ns is employed. The impulse and coupling coefficient generated by laser ablation ABS are greater than that of Zn, and they exhibit a similar variation trend with pressure. However, at higher pressure levels, the change in impulse versus laser energy is not completely coincident between Zn and ABS samples. The target property plays a significant role in the generation and propagation of plume related to the plasma parameters such as electron density and temperature. The temporal evolution images indicate that the plasma plume of laser-induced Zn presents a faster decay in comparison with that of ABS, which is ascribed to the fact that the gas temperature of ABS is higher than the electron temperature of Zn plasma in the local thermodynamical equilibrium. Also, the electron density is lower for Zn due to the rapid heat diffusion and higher ablation threshold of metal. It is found that the surface absorption is dominant for metal because the ablated crater of Zn performs larger diameter and shallower depth. On the contrary, the shrinkage in diameter but enhancement in depth of crater is observed from ABS surface, and the ablation mass is larger, suggesting the obvious volume absorption for polymer. The results reveal that the target property can engender an important effect on the energy conversion between laser, target and plasma.
Keywords: laser propulsion performance, plasma plume propagation, plasma parameters, energy conversion (Some figures may appear in colour only in the online journal) * Author to whom any correspondence should be addressed.
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Introduction
Laser propulsion is one of the most promising propulsion methods, which utilizes an intense laser beam as the remote energy source [1][2][3]. Compared with the conventional chemical propulsion technique, laser propulsion system does not need to carry additional fuels or propellant sources, thus it is likely to improve the specific impulse and payload ratio [4,5]. Moreover, it can realize an adjustable impulse from nN·s to N·s [6]. As a result, laser propulsion is more favored in numerous aerospace engineering aspects of launching objects into low Earth orbit, attitude control or orbit adjustment of satellites, and space debris removal [7][8][9][10][11]. In general, laser propulsion can be divided into air-breathing mode [12] and ablation mode [13] according to various propellants. For the former, air itself acts as the propellant without carrying any working medium whereas it is not effective enough to propel the objects beyond an altitude of 30 km [7]. Therefore, more researchers have paid attention to ablation mode, which can select many materials as the propellants, including polymer [14,15], metal [16][17][18] and so on.
For the case of laser ablation propulsion, the impulse and coupling coefficient C m which is defined as a ratio of impulse to laser energy [18,19], are the key indexes to evaluate propulsion performance. When a high-power laser beam is focused on the target surface, the ablative plasma plume is generated and it can further absorb the incident laser energy [17]. The interaction of laser beam with target and plasma plume involves complex physical processes such as phase change, ionization, laser energy absorption, plasma plume dynamics, heat conduction, radiant heat transfer, etc. In the presence of ambient air, the complexity becomes even greater because laser energy is easily absorbed by gas itself to generate shock wave [19]. In consequence, the propulsion characteristics are influenced by the laser parameters, target material as well as background gas pressure. The dependence of impulse and C m on the different factors has also been investigated extensively [2,5,16,17,[20][21][22][23]. The significant propulsion behaviors for the different metallic pendulums irradiated by nanosecond pulsed Nd:YAG laser were compared [4] and the result showed that the values of exhaust velocity, C m and specific impulse were highest for copper. Pakhomov et al presented that thrust was proportional to atomic mass, but the lighter element produced a higher specific impulse [24,25]. In contrast to metallic materials, the polymeric targets can achieve the higher C m , but most polymers have displayed a lower specific impulse [2,26]. Furthermore, the change in propulsion performance with the various conditions between metal and polymer samples shows different trends. The C m generated by nanosecond laser ablation aluminum target increased monotonically with pressure exceeding 100 Pa [16]. It was different from the report by Tan et al [27], in which the C m increased firstly and then decreased with pressure due to air breakdown near the metal surface. Our previous result also proved that the C m dropped at atmospheric pressure, caused by the plasma plume separation [23]. In addition, the C m generated by laser ablation metal target increased and afterwards decreased with laser fluence, which has been widely reported [17,18,21]. However, for polymer, the influence of laser fluence and ambient pressure on the C m exhibits complicated behaviors. Several researchers found that the C m of polymeric materials had the same trend as that of metal with laser fluence [28,29]. In particular, the effect of ambient pressure on the C m of polyacetal was different at various laser fluences [30]. Even though a large number of explorations about propulsion performance were conducted, the underlying physical mechanism of dynamic characteristics is not completely understood.
During the laser propulsion process, the kinetic energy of plasma plume generated by laser irradiation produces the momentum transferring into the target. As a result, the propulsion performance is directly related to the plume dynamics [23]. A lot of researches have been carried out by using laser-induced breakdown spectroscopy which provides useful information to obtain the plasma parameters such as electron density and temperature. The ambient environment plays a significant role in the evolution of laser-induced plasma. Farid et al found that the electron density and temperature of nanosecond laser-induced copper plasma increased initially and then decreased with the reduction of ambient pressure [31]. However, their previous result demonstrated that the variation of electron density by femtosecond laser irradiation was continuously increasing as the ambient pressure increased [32]. Several research groups had also reported that the electron density showed similar behavior for other metal targets [33,34]. In addition, the effects of surrounding pressure on the emission features of non-metallic materials had been explored, and the plasma parameters of various targets displayed different trends [35,36]. Compared with elementary substance composition, there are relatively few studies on polymer plasma parameters due to its complex structure. Moreover, the comparison of plasma parameters between metal and polymer has rarely been reported, although it is important to reveal the difference in plasma plume dynamics. Therefore, the aim of this paper is to further elucidate the interaction mechanism of laser-target by investigating plasma dynamics and parameters for different materials, then to improve propulsion performance.
In this paper, a comparison of propulsion performance generated by laser ablation zinc and acrylonitrile butadiene styrene (ABS) targets is carried out by measuring the impulse and C m with a ballistic pendulum. It is considered the influence of plasma dynamics and parameters by analyzing the time-resolved plasma plume images, optical emission spectra (OES) as well as target ablation. The underlying physical mechanisms of laser-target interaction and energy conversion are revealed, which provides an insight for optimizing the laser propulsion performance. Figure 1 illustrates the schematic diagram of experimental apparatus which can be separated into three main parts: plasma generation, diagnostic equipment and synchronization system. A Q-switch Nd:YAG laser (Q-smart 850) is used for the laserinduced plasma source, delivering 6 ns pulses at the fundamental wavelength of 1064 nm. In this work, the laser repetition rate is fixed at 1 Hz and the laser pulse energies are set at 120 mJ, 265 mJ and 475 mJ. A power meter (QE25LP-S-MB-D0, Gentec) is employed for measuring the laser pulse energy. The laser beam is focused by a plano-convex quartz lens of 175 mm focus length onto the target. High purity Zn (99.99%) and ABS plates placed on the target holder are selected as the targets. And they are positioned inside the vacuum chamber in which ambient pressure can be accurately changed from 760 Torr to 0.5 Torr. For fresh surface exposures, the target is rotated with the assistance of steeping motor to avoid non-uniform ablation and minimize the error.

Experimental setup
The measurement of plasma plume images, OES and target ablation is given in figure 1(a). The time-resolved images of plasma plume are captured by a high speed motion analyzer (i-SPEED720) equipped with a Nikon lens vertical to the laser beam. An intensified charge coupled device (ICCD, Princeton Instrument) with a gate width of 10 ns is used to collect the fast exposure image. The optical emissions from plasma are transmitted to the spectrometer (Actor Pro, Spectra-Pro 2500i) through a collimating lens and optical fiber at an appropriate angle of 16 • with respect to the laser beam. The calibration of spectrometer is carried out using a He-Ne laser with a specific wavelength of 632.8 nm. A grating groove with 1200 line mm −1 and glancing wavelength of 300 mm is used to record the OES for calculating the electron density and temperature. The delay times between the incident laser pulse and data acquisition system are controlled by a programmable timing generator (DG645). The investigation of ablative crater morphology is performed using laser microscopy (KEYENCE VK-X150) with 400 × magnification. An electronic balance (ES 1205 A) with high precision of 0.01 mg is used to detect the ablation mass of target after laser irradiation. A ballistic pendulum system is employed to measure the impulse and C m generated by laser ablation target. The schematic diagram of ballistic pendulum is shown in figure 1(b). When a laser pulse irradiates the target hanging with a rigid wire of negligible weight, it can induce the generation of plasma and result in the periodic oscillation of ballistic pendulum. It should be noted that air drag and friction are ignored in this case. The swing angle and period are recorded using the above high speed motion analyzer. The detailed principle of impulse and C m measurement is described in [23,27,37]. Based on the energy and angular momentum conservation, the movement of ballistic pendulum is expressed by the following equations: where J, ω and P represent the moment of inertia, the angular velocity and the impulse, respectively. m, l and L denote the mass of pendulum, the distance from mass center of pendulum to the axis and the length of force arm, respectively. g is the gravitational acceleration, θ is the angle at which the target deviates from equilibrium, and T is the oscillation period. The impulse can be obtained from equations (1)- (3): Moreover, the C m is calculated by the impulse according to the definition of C m which is a ratio of impulse to the incident laser energy.

Impulse measurement
A comparison of impulse between Zn and ABS samples is given in figure 2. Figures 2(a) and (b) represent the variation of impulses generated by laser ablation Zn and ABS, respectively. It can be seen from figure 2 that the impulse of ABS target is greater than that of Zn. Based on the fact that the impulse is the time integration result of thrust generated by laser-induced plasma, it is considered that the change in impulse is ascribed to the difference in dynamics and duration of plasma caused by the various material properties. On the one hand, the metal with fast heat diffusion is characterized by a shallow laser penetration depth, therefore the action of laser on the target can be considered as planar, which is called the surface absorption [26,38,39]. Yet the polymer absorbs laser energy by the volume absorption, resulting in a comparable crater depth relative to the diameter of thermal diffusion  [26]. On the other hand, the low bond energy of polymer leads to a lower ablation threshold. The earlier part of laser pulse can remove a large amount of material, and the surface will continue to be ablated by the plasma irradiation and transmitted energy of residual pulse. In order to verify the accuracy of analysis, the plasma characteristics are carried out through temporal evolution images, spectral diagnosis and target ablation in the following part. Figure 2 also shows that the effect of laser energy on the impulses of Zn and ABS is different at high pressure levels (760 Torr and 150 Torr). The impulse produced by laser ablation Zn target increases firstly and then decreases with improving laser energy. Nevertheless, the impulse is increased monotonically with laser energy for ABS sample. Different from high pressure, the change in impulse of laser-induced Zn target versus laser energy is consistent with that of ABS below 22 Torr, presenting that the impulses rise with the enhancement of laser energy. Besides, there is an analogous tendency of impulse with pressure for different materials. The impulse initially rises before the maxima and begins to decline with the further decrease in pressure. It should be noted that the increase in impulse is not severe at laser energy of 120 mJ. A detailed analysis of impulse variation associated with plasma behavior will be given below.
The C m is one of the significant criteria to evaluate propulsion performance, and the result of C m for various materials is shown in figure 3. Figures 3(a) and (b) give the dependence of C m generated by irradiating Zn and ABS targets on laser energy and pressure, respectively. It is observed from figure 3 that the C m for ABS material is higher than that of Zn under the same condition. Besides, the variation of C m with pressure is same as that of impulse for different samples, i.e. increasing firstly and then decreasing with the reduction of pressure. However, the trend of C m produced by laser ablation ABS target versus laser energy shows a diversion from that of impulse. The result indicates that there is an initial increase in C m as the laser energy increases, and then the C m exhibits a decline by further increasing laser energy. Such a similar tendency has also been noticed previously by several authors [17,21,28,40]. As analyzed in our previous report [23], the nonlinearity in C m with laser energy and pressure is due to the shielding effect of plasma. The time scale of plasma generation is much smaller compared to the laser pulse width on the order of nanosecond. The target is melted, vaporized and further ionized by initial laser energy to form plasma which in turn absorbs the laser energy and prevents the remaining pulse from reaching the target surface. The plasma parameters affected by laser energy and pressure determine the inverse bremsstrahlung (IB) absorption coefficient [21,41,42]. The absorption capacity of plasma to laser may be strengthened as the laser energy and pressure increase. This means that the proportion of transmitted laser energy will decrease sharply upon increasing laser energy and pressure, thus affecting the change in C m .

Temporal evolution images of plasma plume
The temporal evolution images of plasma plume from laserinduced Zn and ABS targets are captured by high speed motion analyzer with an exposure time of 1 µs, as shown in figures 4(a) and (b). The delay times relative to laser pulse are marked in the images and the position of target is expressed as the red line. Figure 4 indicates that the plasma plume for ABS target presents a longer duration in comparison with that of Zn plasma. This is also the reason that the overall values of impulse and C m produced by laser ablation ABS are higher than that generated by Zn plasma. Because the initial plume is mixed with heavier Zn atoms, more energy of electrons will be exhausted to collide with Zn atoms. Besides, the fast heat diffusion [39] and high ablation threshold of metal lead to the reduction of electron density, which in turn affects the electron temperature. It can be demonstrated by the plasma parameters obtained from spectral measurement in the following part. In consequence, the difference in plasma lifetime between metal and polymer is ascribed to the change in electron temperature caused by the diversity of collision and recombination processes. In addition, it can also be seen from figure 4 that the plasma plume by irradiating Zn target with laser energy of 265 mJ manifests in the separation at atmospheric pressure, while there is no plume splitting for ABS sample. As the laser energy is increased to 475 mJ, the plume splitting of Zn plasma is still observed at 150 Torr. However, the separation of plasma plume generated by laser ablation polymer is visible only at atmospheric pressure. It means that the plume separation of Zn and Al in our previous work [23] is prior to that of polymer. A few investigations had also reported this phenomenon that the plasma plume showed a peculiar feature with the separating into several parts [43][44][45]. As mentioned in [23,43], the ambient gas in front of target was preferentially broken down with increasing laser energy, which was called the fast portion. Plasma generated in the initial stage of pulse can absorb the residual laser energy, and then emits strong radiation and heat to the surroundings [44]. As a result, the surface layer of target is secondarily vaporized to produce atomic vapor, corresponding to the slow portion. The transmitted laser energy and heat radiated by plasma are easy to continuously ablate the polymer target due to its low ablation threshold and weak thermal conductance, leading to the difference in hydrodynamics of plasma plume between metal and polymer. Furthermore, it is found that the plasma plume is rapidly quenched at atmospheric pressure. Since electrons collide with background gas more frequently during propagation, neutral particles cannot be effectively excited and ionized. Accordingly, the plasma separation and fast fading at atmospheric pressure are responsible for the low impulse and C m .
When the ambient pressure drops to 150 Torr or 22 Torr, the luminosity of plasma lasts for a longer time. Although the confinement effect of background gas is weakened in comparison with that at atmospheric pressure, the energy loss of electrons is decreased due to the reduction of collision frequency, which results in neutral particles can be excited and ionized more easily. Hence, the plasma plume lifetime is prolonged and the increasing impulse and C m in this pressure range are obtained. As the pressure is further decreased to 0.5 Torr, the plasma plume expands severely and the duration of plasma is significantly shortened. Even for the polymer material, the duration of plasma is difficult to exceed 10 µs. It has been illustrated previously that the confinement effect of background gas is minimal in near vacuum, thus the directional kinetic energy is rapidly converted into random thermal expansion [23]. Owing to the lower energy conversion efficiency and shorter interaction time between plasma and target, the impulse and C m drop rapidly at 0.5 Torr pressure.
To observe the dynamic characteristics of plasma plume more clearly, the fast exposure images recorded by ICCD camera are given in figure 5. Similar to the behaviors of plasma plume shown in figure 4, at atmospheric pressure, the plasma plume exhibits separation with increasing laser energy. When the ambient pressure decreases to 0.5 Torr, there is a severe expansion and fast quenching of plasma plume, indicating that the electron energy is not effectively converted into the kinetic energy of target. The fast time resolution images further confirm that the initial laser-induced plasma production under different conditions can affect the propagation process and energy conversion.

Time-resolved OES.
Visible spectral diagnostics has been carried out to analyze the hydrodynamics of plasma plume. Typical time-resolved OES of Zn plasma at various laser energies and pressures are given in figure 6. The entrance slit of spectrometer is set to 0.5 mm and the exposure time is 100 ns. By comparing the National Institute of Standards and Technology (NIST) database [46], it is determined that the spectral transition lines in the wavelength range from 465 nm to 495 nm are composed of atomic and ionic lines of Zn, as well as ionic lines of N and O, and their corresponding wavelengths are marked in figure 6. It can be seen that the continuum emission is stronger in the early time at higher ambient pressure, which is caused by electron bremsstrahlung and electron-ion recombination [47][48][49]. At atmospheric pressure, the emission lines of Zn atom, N and O ions are clearly observed, but the ionic line of Zn is faint. It has been mentioned that the background gas is broken down prior to the target when the laser energies are increased to 265 mJ and 475 mJ. The initial plasma can absorb the remaining laser pulse, leading to the fact that most of laser energy is injected into the plasma plume rather than the target. Furthermore, the temporal evolution line intensity of Zn atom shows a growing trend when the pressure changes from 760 Torr to 22 Torr. On the contrary, the ionic line intensities of Zn, N and O decrease constantly with the development of time. As mentioned in [23,[50][51][52], ions with the higher ionized state dominated in the early time, while the atomic lines were prominent in the later time. The three-body recombination of Zn ions and electrons contributes to the generation of Zn atoms as shown below [23,[50][51][52]: where M represents the neutral particles. Therefore, an increase in atomic line intensity of Zn is dependent upon the three-body recombination. When the pressure is further reduced to 0.5 Torr, the atomic and ionic line intensities of Zn manifest in a rapid decrease and the ionic lines of N and O vanish. It is based on the fact that the plasma plume expands adiabatically without laser energy injection in near vacuum and undergoes free expansion due to the weak confinement effect of background gas, resulting in the severe energy dissipation of plume. In addition, the atomic and ionic line intensities of Zn decline with time. The threebody recombination of Zn ions and electrons at high pressure levels is replaced by the radiation recombination owing to the low number density of neutral particles and electrons in near vacuum, which is given as follows [23,53,54]: Different from the three-body recombination mechanism, the transition of Zn ions to excited state leads to the generation of photons rather than Zn atoms. As a consequence, the change in atomic line intensity of Zn with time is opposite to that at high pressure levels.

Plasma density.
The plasma parameters are not only relevant to the various line intensities by changing laser-target and laser-plasma couplings, but also determine the dynamic characteristics of plume affecting propulsion performance. Thus the electron density is discussed by considering the method of line broadening in this part. Here, the atomic line of Zn at 481.1 nm [55][56][57] and H α line at 656.5 nm [58,59] are used to obtain the electron density generated by laser ablation metal and polymer, respectively. The line broadening consists of Stark broadening, Doppler broadening, instrument broadening and resonance pressure broadening, etc [31,60,61]. Doppler broadening resulting from the random thermal motion of emitter can be estimated by the following equation based on the Maxwellian distribution law [31,60,61]: where ∆λ 1/2 is the full-width at half-maximum (FWHM) of line, λ is the wavelength of transition line, and k is the Boltzmann constant. T is the kinetic temperature and its value is equal to the excitation temperature in the local thermodynamical equilibrium (LTE) state. m and c are the atomic mass and light speed. For the Zn I line at 481.1 nm, the estimated width is 0.012 nm. Similarly, the Doppler widths corresponding to Zn I at wavelengths of 334.5 nm and 481.1 nm are 0.0031 nm and 0.0014 nm, respectively [56,60]. Hence, the contribution from Doppler broadening can be ignored in comparison with the measured broadening given in figure 7. Lorentzian fit of Zn I at pressure of 150 Torr and laser energy of 475 mJ is shown in figure 7(a) when the entrance slit is set to 0.5 mm. In order to improve the ratio of signal to background, H α line is collected at an entrance slit of 0.625 mm and its Lorentzian fit is presented in figure 7(b). The FWHMs for Zn I and H α lines under the same condition are 0.971 nm and 7.261 nm, respectively. In reference [62], the Doppler width is estimated to ∼0.05 nm by H α line and thus the contribution to line width can be neglected. Resonance pressure broadening depending on the ground state number density and transition oscillator strength is small and can also be negligible [56,63]. Fast moving electrons and slow ions will generate a perturbing electric field. Plasma species under a perturbing electric field can shift the energy levels, which results in Stark broadening is formed [31,34,64]. Stark broadening and instrument broadening are the dominating mechanisms, and Stark broadening can be extracted by the measured line width subtracting the instrument width [60,63]. Instrument broadening is determined by measuring the FWHM of He-Ne laser at 632.8 nm. For Stark broadening, it is considered that the contribution of ion broadening is much smaller than that of electron broadening and is reasonable to be neglected due to the major mass difference between electron and ion. Thus, for the non-H line of metal material, the electron density N e is related to the FWHM of Stark broadening by the following equation [60,65]: ω is the electron impact width parameter. The impact parameter can be obtained from [66]. Besides, the electron density for polymer can be deduced from the H-line width according to the following formula [58,67]: The time-averaged electron densities for Zn and ABS targets are given in figures 8 and 9, respectively. Because the integration time of spectrometer is 1 ms, the entrance slit is set to 0.125 nm to avoid the line intensity exceeding the maximum range. The variations in time-resolved electron densities of Zn and ABS are shown in figures 10 and 11, respectively. It can be seen from figures 8-11 that the electron density produced by laser ablation ABS target is significantly higher than that of laser-induced Zn plasma. As analyzed above, due to the fast heat diffusion of metal, more laser energy is required to compensate for the energy loss from heat conduction. The result is that laser can only penetrate the thin thickness of metal surface [26,38,39]. Moreover, the ablation threshold corresponding  to evaporation and vaporization of metal is higher, which is confirmed by measuring the ablation morphology and mass of target in the following. The fast heat diffusion and high ablation threshold for metal limit the laser energy utilization efficiency, leading to a lower electron density for Zn than that of ABS.
Besides, there is a decrease in electron density with the reduction of laser energy and pressure, which is consistent with our previous work [23]. The coupling of laser-target  is weakened because of the enhanced shielding effect with increasing laser energy, more incident energy is absorbed by electrons, promoting ionization through collision with background neutral particles. And the addition of ambient pressure will enhance the confinement of background gas to plasma plume [31,68], resulting in the increasing electron density. Free expansion in near vacuum rather than confinement of ambient gas is a dominant effect of the decrease in electron density. Figures 10 and 11 also imply that the temporal evolution electron density displays a rapid decay rate in the early time at atmospheric pressure, while there is a slight decrease in electron density with time at lower pressures. Plasma exhibits isothermal expansion during the laser pulse, but in the absence of laser injection, the electron density and temperature decrease through adiabatic expansion [47]. With the improvement of pressure, more energy is transferred from plasma to ambient environment due to the frequent collisions with background gas [69]. As analyzed in [47,69], the thermal energy will fall to much less than the ionization energy at higher pressures and hence the three-body recombination is dominated, resulting in a fast electron loss rate. On the contrary, a small three-body recombination rate in near vacuum will change the electron temperature decay rather than the electron density decay.

Plasma temperature.
The electron temperature is necessary for understanding the ionization and ablation processes, which is correlated to the emission intensity [63]. It is assumed that plasma is in an LTE state, the electron temperature can be evaluated from the measuring OES using the ratio of line intensities. The population of excited state is estimated by using the following Boltzmann formula [60,63]: where λ mn , g m and A mn represent the wavelength, statistical weight and transition probability, respectively. E m , T e , N (T) and U (T) denote the upper level energy, electron temperature, total number density and partition function, respectively. A plot of ln (λ mn I mn / g m A mn ) versus E m yields a straight line having a slope of −1/ kT e . Here, the atomic emission lines of Zn I at 334.5 nm, 472.2 nm and 481.1 nm are used to determine the electron temperature [57,60,70]. The relevant spectroscopic parameters for the transitions of laser-induced Zn plasma used in the calculation have been taken from the NIST database [46] and are listed in table 1. Figure 12 gives an example of Boltzmann plot at pressure of 150 Torr and laser energy of 475 mJ. A sufficiently high electron density should be considered to ensure the dominance of collision process over radiation if plasma is in LTE state [42,63]. In general, the assumption for the validity of LTE is satisfied by using Mc Whirter criteria [42,49,63,71]. The lowest limit of electron density for which plasma is in LTE can be achieved according to the Mc Whirter equation. For polymer, the non-metallic elements (C and N) are involved in the form of molecular species. The OES of CN violent band ( are recorded to obtain the rotational temperature [59,[72][73][74] which is approximately equal to gas temperature due to the fast equilibrium between rotational and transitional motions achieved by frequent collisions of heavy particles. The gas temperature can also represent the electron temperature if   in the LTE state. The rotational temperature of CN violent band ( is evaluated by comparing the experimental spectrum and the best fitting spectrum from LIF-BASE program [74], as shown in figure 13. Figures 14(a)-(c) give a comparison between the electron temperature of Zn plasma and gas temperature of laser ablation ABS at various laser energies of 120 mJ, 265 mJ and 475 mJ when the exposure time is 10 µs. It can be observed from figure 14 that the gas temperature is larger compared with the electron temperature of Zn plasma, which is ascribed to the difference in IB mechanism. Plasma generated by laser-induced ABS has stronger laser absorption due to more electron density, resulting in higher electron energy. Moreover, the existence of heavier Zn atoms also causes the electron energy of laser-induced Zn plasma is consumed more severely than that for ABS during the collision process. Accordingly, the ionization capacity of electrons is enhanced, supporting the fact that the duration of plasma plume for ABS is prolonged as shown in figure 4. It is noted that the electron energies for both Zn and ABS at 0.5 Torr are lower than that at higher pressure levels. In near vacuum, photons are released by the radiation recombination of electrons and ions, but plasma expands seriously due to the weak confinement effect of background gas, causing the energy dissipation of plasma. Meanwhile, the absorption of laser energy by plasma is weakest owing to the low IB coefficient depending on the smallest electron density, forming the minimal electron energy. It is also responsible for the minimums of plasma duration, impulse and C m at 0.5 Torr. The electron and gas temperature measurements demonstrate that the electron energy can directly affect plasma duration in the propagation, and play a significant role in the hydrodynamics.

Target ablative morphology and mass
The dimensions of ablation crater in terms of diameter and depth are dependent on the material property and laser-target interaction due to the difference in plasma parameters, and thus it is necessary to investigate the target ablation morphology and mass. Figure 15 presents the ablation morphology of target surface and information about the crater dimensions using single laser shot at laser energy of 475 mJ and pressure of 0.5 Torr. It is observed from figure 15(a) that the intense ablation region in the center is surrounded by the resolidified ridges of molten material. Owing to the Gaussian distribution of laser intensity and fast thermal conduction of metal, a temperature gradient is generated, which causes molten material splashing along radial direction to form the resolidified ridges [31]. By comparing figures 15(a 1 ) and (b 1 ), it is found that the crater diameter of ablated Zn target is larger than that of ABS. Besides, figures 15(a 2 )-(b 3 ) reveal that the crater depth of Zn is shallower than that of ABS. The difference in crater dimensions indicates that the laser absorption mechanism of metal is surface absorption, while the polymer absorbs the laser energy by the volume absorption. Also,  the crater profile shown in figure 15(a 2 ) is conical whereas figure 15(b 2 ) presents an approximate cylindrical outline of crater. It means that the metal material is characterized by rapid heat dissipation. Figures 16(a) and (b) give the ablation masses of Zn and ABS using 90 laser shots, respectively. It can be seen from figure 16 that the ablation mass of ABS is larger than that of Zn, which is mainly ascribed to the different absorption mechanisms and ablation thresholds between metal and polymer. Combined with figures 8, 9 and 16, it is found that both the ablation mass and electron density for ABS are higher than that of Zn, which further verifies the accuracy of above analysis about the difference in ablation process. Furthermore, the ablation mass increases with the decrease in pressure for two different materials. It should be noted that the ablation mass of Zn target decreases with the enhancement of laser energy at relatively high pressure levels, which is opposite to that of ABS. Because the IB absorption coefficient is proportional to the electron density [31,75], the addition of background gas will result in a stronger confinement effect and the increase in electron density, which in turn weakens the laser-target interaction and decreases the ablation mass at high pressure. However, as mentioned above, the transmitted laser energy and thermal radiation of plasma also cause the target to be ablated for polymer due to the low ablation threshold. At high pressure levels, although the shielding effect is enhanced with the increase in laser energy, there is a more significant increase in thermal radiation from plasma. Consequently, the ablation mass of ABS represents an increase rather than a decrease with increasing laser energy.

Conclusion
In conclusion, a comparison of laser propulsion performance between Zn and ABS at various laser energies and pressures is investigated. The results indicate that the impulse and C m generated by laser ablation Zn target are lower than that of ABS target. At laser energy of 120 mJ, the impulse and C m are hardly dependent on pressure except for 0.5 Torr. When the laser energy is increased, the impulse and C m increase firstly and afterwards decrease with pressure. The effects of laser energy and pressure on propulsion performance are achieved by changing the hydrodynamic characteristics of plasma plume. By observing the temporal evolution images of plume, the duration of Zn plasma is shorter in comparison with that of laser ablation ABS target. More importantly, the plasma plume manifests in the separation at atmospheric pressure and severe expansion in near vacuum, and both of them are quenched rapidly, which are considered to be the factors for the low impulse and C m . By contrary, the fact that the plasma plume lasts longer in the intermediate pressures is responsible for the increasing impulse and C m .
In addition,at higher pressure levels, the impulse generated by laser ablation Zn increases firstly and then decreases with increasing laser energy. However, the impulse shows a monotonous variation trend with laser energy for ABS target. The plasma parameters and target ablation which are dependent on the material property determine the different tendencies of impulse with laser energy. The measured OES represent that the electron density of ABS material is higher than that of Zn. It is ascribed to the surface absorption mechanism due to the fast heat diffusion and higher ablation threshold of metal. It can be proved by the result of target ablation that the crater morphology is characterized by a larger diameter and shallower depth. Meanwhile, there is a lower ablation mass for Zn target. Besides, the electron density is increased at higher laser energies and pressures, leading to the stronger shielding effect of plasma on laser pulse through IB absorption mechanism. Nevertheless, the thermal radiation from plasma and transmitted laser energy also cause the ABS target to be ablated owing to its lower ablation threshold. As a result, the ablation mass of ABS increases with the enhancement of laser energy, which is opposite to that of Zn at high pressures.
Furthermore, compared with the gas temperature of laser ablation ABS, the electron temperature of Zn plasma is lower. The IB absorption of Zn plasma is weakened due to lower electron density, and electrons lose more energy by colliding with heavier Zn atoms. The result is that the ionization ability of electrons is inevitably suppressed. Accordingly, the duration of plasma plume is shortened, which is responsible for the lower impulse and C m . In near vacuum, energy dissipation due to severe plasma plume expansion determines the reduction of impulse and C m . A series of measurement results suggest that the laser propulsion performance depends on the target property and plasma characteristics, and provide the underlying energy conversion mechanism between laser, plasma and target.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).