State Key Laboratory of Laser Propulsion & Application, Department of Aerospace Science and Technology, Space Engineering University, Beijing 101416, People’s Republic of China
2.
Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, People’s Republic of China
The efficacy of spacecraft propulsion systems significantly depends on the choice of propellant. This study utilized laser-induced breakdown spectroscopy (LIBS) to investigate the impact of different fuel types, fuel ratios, and laser energies on the plasma parameters of ammonium dinitramide (ADN)-based liquid propellants. Our findings suggest that 1-allyl-3-methylimidazolium dicyanamide (AMIMDCA) as a fuel choice led to higher plasma temperatures compared to methanol (CH3OH) and hydroxyethyl hydrazine nitrate (HEHN) under the same experimental conditions. Optimization of the fuel ratio proved critical, and when the AMIMDCA ratio was 21wt.% the propellants could achieve the best propulsion performance. Increasing the incident laser energy not only enhanced the emission spectral intensity but also elevated the plasma temperature and electron density, thereby improving ablation efficiency. Notably, a combination of 100 mJ laser energy and 21wt.% AMIMDCA fuel produced a strong and stable plasma signal. This study contributes to our knowledge of pulsed laser micro-ablation in ADN-based liquid propellants, providing a useful optical diagnostic approach that can help refine the design and enhance the performance of spacecraft propulsion systems.
With the rapid advancement of micro-nano satellite technology, the demand for efficient and reliable space propulsion systems has intensified to support crucial attitude and orbit control operations [1]. Pulsed laser ablation micro-propulsion technology has emerged as a leading method in space propulsion, attracting significant attention in recent scientific research [2, 3]. A critical aspect of this technology is the selection of propellant [4]. Liquid propellants are considered ideal due to their high combustion efficiency and the absence of additional angular momentum contributions [5]. Notably, ammonium dinitramide (ADN)-based liquid propellants have gained prominence over the traditional hydrazine variants owing to their non-toxic nature, low characteristic signal, and environmental sustainability [6, 7]. ADN, synthesized initially in 1971 [8, 9], and chemically denoted as NH4N(NO2)2, is highly soluble in water [10]. The Swedish Space Research Center and the Swedish Defense Research Institute have successfully utilized a mixture of ADN and methanol (CH3OH) in water to create an effective liquid monopropellant [11]. Extensive research has been conducted on ADN-based liquid propellants. For instance, Hou et al explored the feasibility of microwave ignition of propellant droplets, investigating the combustion characteristics and noted a reduction in ignition delay and enhanced chemical reactions with the addition of aluminum oxide nanoparticles [12‒14]. Similarly, Li et al assessed electric ignition methods for these propellants, highlighting how ignition voltage and electrode structure significantly influence the ignition efficacy [15, 16]. Recent studies by Cao et al and Du et al on the ablation characteristics of pulse laser ablation of ADN-based liquid propellant revealed that a lower absorption depth can markedly enhance ablation efficiency and increase the specific impulse [17, 18]. However, while existing research has predominantly focused on ablation performance, studies concerning the spectral information of the plasma generated during laser micro-ablation of liquid propellant are scarce. Consequently, the development of effective optical diagnostic techniques to intricately analyze the spectral data during this process is vital for the optimization of such micro-propulsion systems.
Laser-induced breakdown spectroscopy (LIBS) is an analytical technique that uses a high-powered laser to vaporize a small area of the material surface, thereby creating plasma. This process allows for the examination of the atomic emission spectrum to determine the elemental composition of the material [19]. LIBS offers several distinct advantages including rapid analysis, capability for simultaneous multi-element detection, no requirement for sample pretreatment, and versatility across different sample states [20‒22]. These features have propelled its adoption across a wide range of applications, from geological exploration [23] and combustion diagnostics [24] to environmental monitoring [25], space exploration [26], and the preservation of cultural artifacts [27]. In the field of solid samples, LIBS has proven exceptionally adept. It not only facilitates precise detection of heavy metals in soils [28, 29] but has also been integrated with machine learning techniques to identify the provenance of ginseng [30] and classify types of aluminum alloys [31]. Challenges such as liquid splashing, surface fluctuations, and inefficient coupling of laser energy to the sample have historically complicated the application of LIBS to liquid samples [32]. However, recent advances in the understanding of the underlying mechanisms and methodological improvements have largely overcome these obstacles. Significant contributions in this area include the work of Tian et al [33], who investigated the physical processes and temporal dynamics of underwater LIBS under varying pressure conditions, offering insights into the pressure-induced effects on LIBS detection through the study of cavitation bubble dynamics. Xue et al explored LIBS performance near the gas-liquid interface, demonstrating that optimizing the focus position of the laser can enhance plasma signal intensity [34, 35]. Further, Chen et al employed techniques such as surface-enhanced LIBS and dry droplet pretreatment to precisely quantify elements like chromium and lead in aqueous solutions [36, 37]. Additionally, Sun et al combined LIBS with glow discharge to lower the detection limits for heavy metals in water [38]. Lastly, Wang et al successfully quantified the mixture ratios in ADN-based liquid propellants using LIBS, showcasing the technique’s efficacy in determining the plasma characteristics of liquid samples and facilitating accurate sample analysis [39].
At present, the efficacy of micro-thrusters in space propulsion technology critically hinges on the selection and optimization of propellants. Currently, research into enhancing propellant performance specifically for pulsed laser ablation micro-thruster systems is sparse, particularly in terms of understanding how the plasma characteristics generated during the laser-propellant interaction affect propulsion performance. Addressing this gap, our study employs LIBS to comprehensively analyze the impact of different fuel types, fuel ratios, and laser energies on plasma parameters during the laser micro-ablation of ADN-based liquid propellants. The research begins by varying the fuel type within the propellant formulation to identify the fuel that delivers optimal performance. Once the best fuel type is established, we further manipulate the mass fraction ratios of the propellant components. This step involves detailed analyses of how these variations influence plasma temperature, with the aim of determining the most effective propellant composition. In the final phase of the study, with the fuel type and optimal mass fraction ratio held constant, we explore how different laser energy levels affect critical plasma parameters of the ADN-based liquid propellant.
2.
Experimental setup and method
2.1
Sample preparation
The ADN-based liquid propellant used in our experiments consists primarily of ADN (the molecular formula is N4H4O4) as the oxidant, water (H2O) as the solvent, and a variety of fuels. For these experiments, methanol (CH3OH), 1-allyl-3-methylimidazolium dicyanamide (AMIMDCA, its molecular formula is C9H11N5), and hydroxyethyl hydrazine nitrate (HEHN, its molecular formula is C2H9N3O4) were chosen as fuels. Table 1 details the specific components, their proportions, and the laser energy parameters utilized throughout the experimental phases.
Table
1.
Experimental propellants components, ratios, and laser energy.
Initially, to identify the optimal fuel, we varied the type of fuel while keeping other parameters constant (conditions 1‒3). Subsequent experiments then determined the ideal type and composition of the propellant, exploring the effects of different content ratios on plasma temperature (conditions 4‒6). Finally, with a fixed fuel type and optimized ratio, we extensively studied how varying levels of laser energy influence the plasma parameters (conditions 7‒9).
The propellant components, namely ADN, AMIMDCA, and HEHN, were supplied by the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences. Methanol was acquired from J&K Scientific. To enhance the absorption of the propellant at a 1064 nm laser wavelength, 0.5wt.% of an infrared dye (IR1036) was added. This addition enabled achieving an absorption depth of less than 200 μm, without compromising the overall performance of the propellant. IR1036 was procured from Changchun Xuancai New Materials Co., Ltd.
2.2
Experimental system
The LIBS setup designed to analyze ADN-based liquid propellants is depicted in schematic figure 1(a). All experiments were conducted in atmospheric conditions. An Nd:YAG laser (Continuum, Surelite III), characterized by a 10 ns pulse width and a 1064 nm wavelength, served as the primary light source. The laser beam was precisely focused onto the sample using a plano-convex lens (BK7, 100 mm focal length), achieving a spot diameter of approximately 300 μm.
Figure
1.
(a) Schematic diagram of LIBS system, (b) schematic diagram of chip, (c) single pit, (d) timing diagram.
The samples, corresponding to conditions 1‒9, were dispensed into uniformly spaced pits on an aluminum chip, with each pit having a diameter of 300 μm and a depth of 200 μm, as illustrated in figure 1(b). This chip was crafted by Kunshan Chuangqiu Precision Technology Co., Ltd. A confocal laser scanning image of a single pit is displayed in figure 1(c). For consistent and repeatable ablation, the chip was mounted on an electric three-dimensional translation stage (Thorlabs, PT3/Z8M), which systematically positioned a fresh, non-ablated pit for each laser pulse.
The plasma optical signal generated by each ablation was captured by a convex lens (BK7, 75 mm focal length) and channeled into an optical fiber connected to a spectrometer (Princeton Instruments, Acton, Spectra Pro 500i). An intensified charge-coupled device (ICCD, Princeton Instruments, PIMAX4, 1024×1024 pixels) detected the spectrometer-dispersed spectrum. To synchronize the laser pulse with the ICCD gate, a digital pulse delay generator DG535 (Stanford™ Research Systems) was employed, ensuring optimal timing alignment for plasma signal acquisition. The timing diagram in figure 1(d) further clarifies the synchronization between the DG535, laser pulse, plasma generation, and ICCD acquisition.
Additionally, a digital camera was integrated into the system to monitor the focusing light path, enabling real-time adjustments of the ablation site. Typically, each data point was averaged over 40 laser shots. Given the transient nature of liquid plasma, the ICCD gate width was set to 20 ns to obtain temporal resolution during data acquisition.
2.3
Calculation method
Under the assumption of local thermodynamic equilibrium (LTE) [40], the plasma temperature can be determined using the Boltzmann plot method [41], which is described by the following equation:
ln(λIgkAki)=−EkkBTe+C,
(1)
where λ represents the wavelength, I denotes the spectral line intensity, k and i are the upper and lower levels of the transition, respectively, kB is the Boltzmann constant, Te is the plasma temperature, Aki is the relative transition probability, gk is the statistical weight of the upper level, Ek is the energy of the upper level, and C is the constant. The necessary parameters for this calculation can be obtained from the standard spectral database, such as National Institute of Standards and Technology (NIST). Given the abundant presence of oxygen in the samples, its spectral lines are used as characteristic markers for these calculations. The specific parameters utilized are detailed in table 2.
In addition to the plasma temperature, the electron density is also an important indicator of plasma. In LIBS, other broadening mechanisms (such as Doppler broadening and natural broadening) besides Stark broadening usually have a relatively small impact on the spectral line profile and can therefore be ignored. So Stark broadening is the main broadening process of the spectral line profile [42]. Due to the extremely rich content of H in the liquid sample, a large number of H atomic spectral lines are generated after laser excitation, and the Hα line is the strongest spectral line among the H atomic spectral lines. Therefore, the Hα line (656.28 nm) can be used for electron density calculation. The relationship between electron density and Hα spectral line width is as follows [43, 44]:
Ne=8.02×1012(Δλ1/2/Δλ1/2α1/2α1/2)1.5,
(2)
where Δλ1/2 represents the full width at half maxima (FWHM) of spectral line, Ne is the electron number density, and α1/2 is the collision coefficient. This coefficient reflects the impact of collisions on the spectral line width, and it is only weakly dependent on environmental factors such as pressure and temperature. By incorporating a value of α1/2 = 1.86×10−3 nm [45], the equation can be simplified to directly calculate the electron number density, as shown below:
Ne=1×1017Δλ1.51/2.
(3)
3.
Results and discussion
3.1
Influence of fuel type
Figure 2(a) presents the time-resolved LIBS spectrum in the 760–840 nm range under experimental condition 1. Prior to 180 ns, the spectrum was predominantly characterized by a high-intensity continuous background. As the observation time extended, this continuous background diminished, allowing the atomic spectral lines to become more pronounced and thereby improving the signal-to-noise ratio. Notably, the spectral lines of oxygen atoms (O I) at 777.54 nm and 822.18 nm became increasingly evident. Over the course of the measurement, however, the absolute intensity of these spectral lines progressively decreased, approaching near zero intensity around 600 ns. This rapid decline in spectral intensity was significantly shorter than the plasma spectral signal durations typically observed in air-based measurements [46]. The decay rate of plasma in this context is attributed to the relatively weak plasma generated by the laser ablation of liquid propellants. The high density of the liquid medium facilitates quicker and more frequent energy transfer processes, leading to a faster decay of plasma emission.
Figure
2.
(a) Time-resolved spectra in a range of 760‒840 nm at condition 1, (b) evolutions of plasma temperatures for different fuels, (c) comparison of spectral intensities at 777.54 nm for different fuels, (d) comparison of spectral intensities at 822.18 nm for different fuels.
Figures 2(c) and (d) illustrate the variation curves of spectral intensity for three different fuels at the spectral wavelengths of 777.54 nm and 822.18 nm, respectively, after background subtraction. Upon analyzing the results from conditions 1‒3, it is evident that the overall trends in spectral intensity changes are consistent across the three fuels. Notably, methanol (CH3OH) demonstrated a relatively higher spectral intensity compared to 1-allyl-3-methylimidazolium dicyanamide (AMIMDCA) and hydroxyethyl hydrazine nitrate (HEHN), which both showed lower and comparable spectral intensities. Furthermore, before 200 ns, the spectral intensities of all three fuels exhibited an increasing trend, coinciding with the reduction of continuous spectral intensities and the enhanced visibility of atomic spectral lines as shown in figure 2(c). This observation indicates that during the initial stages, the plasma emission characteristics are predominantly influenced by atomic emissions.
Figure 2(b) illustrates the temporal evolution of plasma temperatures calculated for different fuels. Across all samples, the temperature profiles exhibited a monotonically decreasing trend, with a particularly rapid decline observed within the first 200 ns. This pattern aligns with the typical behavior of plasma expansion and decay in air [47]. Among the fuels tested, HEHN initiated with the lowest plasma temperature, which remained below that of the other fuels throughout the measurement period. In contrast, the use of AMIMDCA resulted in a relatively higher plasma temperature, while methanol (CH3OH) demonstrated intermediate performance. The variation in plasma temperature is indicative of the thermal motion of electrons, atoms, and ions within the plasma. A higher temperature corresponds to more intense collisions among these particles. Therefore, selecting AMIMDCA as a fuel component for the propellant enhances the plasma temperature, which could potentially improve the propulsion efficiency and overall performance of space micro-thrusters. This choice reflects a strategic optimization based on the comprehensive evaluation of the thermodynamic properties of the propellant plasma, aimed at enhancing the functionality of space propulsion systems.
3.2
Influence of fuel ratio
Once the fuel type was established as AMIMDCA, further experiments were conducted to assess the impact of different fuel ratios on the emission spectrum (conditions 4‒6). Figure 3(a) displays the comparison of spectral intensities at a wavelength of 822.18 nm for varying fuel ratios. The spectral line intensities across the three ratios exhibited minimal variance, prompting a deeper examination into which ratio yielded the most favorable plasma temperature. Data shown in figure 3(b) trace the variation of plasma temperature over time for different fuel ratios. It was observed that increasing the AMIMDCA ratio corresponded with a rise in plasma temperature at equivalent delay times. This trend suggests that a higher fuel ratio enhances the kinetic activity within the plasma, which is intrinsically linked to the augmented chemical energy resultant from increased fuel content. In the context of ADN-based propellants, the process of laser ablation accelerates the molecular dissociation and recombination of chemical bonds between the fuel and oxidizer, thereby liberating additional energy [48]. Consequently, as the fuel ratio is increased, more energy is released, elevating the plasma temperature. The experimental results indicate that an AMIMDCA mass fraction of 21wt.% optimizes plasma temperature and chemical reaction efficiency, marking it as an ideal ratio. This finding offers experimental support for enhancing the performance of ADN-based propellants. However, it is critical to avoid excessively high AMIMDCA ratios, as this can lead to oxygen-deficient combustion [49], potentially resulting in fuel wastage and incomplete combustion. This situation should be avoided in propellant design to prevent performance loss and toxic gas generation. Thus, a 21wt.% ratio of AMIMDCA provides a balanced approach in this experimental framework.
Figure
3.
(a) Comparison of spectral intensities at 822.18 nm for different AMIMDCA ratios, (b) evolutions of the plasma temperatures for different AMIMDCA ratios.
With the propellant composition set to 21wt.% AMIMDCA, the study proceeded to investigate the influence of varying laser energies on critical parameters of the ADN-based propellant plasma (conditions 7‒9). Focusing on a spectral wavelength of 822.18 nm, figure 4(a) outlines the impact of different pulsed laser energies on spectral signal intensity. It is evident from the figure that an increase in laser energy leads to a marked enhancement in spectral signal intensity, illustrating the significant role of laser energy in improving these metrics. For a more detailed comparison, spectral intensities at a delay time of 240 ns were analyzed, as depicted in figure 4(b). This analysis revealed that higher laser energies not only intensified the atomic spectral intensity of oxygen (O I) but also amplified the overall spectral intensity across the band. Spectral intensity is directly proportional to both the transition probability and the number of atoms in the excited state [50]. Given that the transition probability for a specific spectral line remains constant, the observed increase in spectral intensity is attributable to a higher number of atoms being excited. This enhancement is a direct consequence of increased excitation efficiency of the sample, driven by the elevated laser energies. Furthermore, the relationship between plasma temperature and delay time under various laser energies is presented in figure 4(c). Distinct variations in plasma temperature are apparent across different laser energies. Specifically, higher laser energies correlate with elevated plasma temperatures at the same delay times. Notably, at a laser energy of 20 mJ, the plasma temperature initially exhibits a slight increase before decreasing, suggesting an asynchronous release of chemical energy at lower laser energies.
Figure
4.
(a) Comparison of spectral intensities at 822.18 nm for different laser energies, (b) comparison of spectral intensities at a delay of 240 ns, (c) evolutions of plasma temperatures for different laser energies, (d) evolutions of electron densities for different laser energies.
Figure 4(d) presents the changes in electron density as a function of delay time across different laser energies. Consistently, the electron density exhibited a decline with increasing delay time. Notably, at a given delay time, a higher laser energy corresponded to a greater electron density, affirming a positive correlation between plasma intensity and laser energy. This observation aligns with findings from figure 4(b), where higher laser energies resulted in increased spectral intensities. Interestingly, the electron densities at laser energies of 20 mJ and 60 mJ were more similar to each other compared to those between 60 mJ and 100 mJ. This pattern was distinct from that observed in the evolutions of plasma temperatures shown in figure 4(c), where increasing laser energy was uniformly associated with increased temperatures. This discrepancy suggests that higher laser energies are particularly effective in enhancing electron density rather than just elevating plasma temperature. A laser energy of 100 mJ was found to be most suitable; however, this does not imply that higher energies are always preferable. Excessively high laser energies can lead to the unnecessary breakdown of the aluminum chip substrate, thus squandering energy and counteracting the efficiency objectives of liquid laser excitation. Additionally, at 20 mJ, unlike the plasma temperature, the electron density did not show an initial increase followed by a decrease. This indicates that the release of chemical energy does not significantly impact electron density, or the effect is insufficient to counteract the natural decay processes of the plasma.
4.
Conclusions
This study employed LIBS to investigate the influence of different propellant fuel types, fuel ratios, and laser energies on the plasma parameters within a micro-propulsion system using pulse laser ablation of ADN-based liquid propellants. Our findings reveal that using AMIMDCA as the propellant fuel consistently results in higher plasma temperatures compared to CH3OH and HEHN. Notably, the optimal performance of the propellant was achieved with an AMIMDCA ratio of 21wt.%. Further investigations demonstrated a positive correlation between laser energy and the key plasma parameters. Increases in laser energy significantly enhanced both plasma temperature and electron density, thereby improving the ablation effect. Specifically, the most stable and strong plasma signals were observed under the conditions of 100 mJ laser energy and a 21wt.% AMIMDCA ratio. These results provide a reference for the optimization of ADN-based liquid propellants in pulsed laser micro-ablation applications. This study contributes to the understanding of LIBS technology in the diagnostic field of micro-nano satellite propulsion systems and provides practical insights that may assist in enhancing the efficiency and reliability of pulse laser ablation micro-propulsion technology.
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