Keeping in view the weightage of electric propulsion over chemical propulsion, materials from metals to polymers and liquid (water) have been tested as propellants in ablative laser propulsion. This emerging propulsion technique can be widely used for aerospace applications like debris removal in the range of cm, pointing micro and nano satellites, laser micro thrusters for spacecraft attitude, and orbit control. Laser propulsion can become a less expensive alternative to chemical propulsion. In this review, we compile the work done in ablative laser propulsion and different modes of propulsion along with the efficiency of different propellants. We summarize the optimized propulsive parameters with solid propellants and liquid propellants along with the efficiencies and theories of laser thrusters with optimized specific impulses. The article provides precise developments done in the field of ablative laser propulsion and deep insights into the analysis done between the different propellants used recently in ablative laser propulsion.

Low earth orbit (LEO) is becoming dense with approximately 11 000 tons of space objects in it due to growing use of satellites. According to the European Space Agency, out of 15 880 satellites launched in space, only 8500 are still functioning. As a result of this increasing orbital debris, a threat of collision seems imminent. The removal of space debris without a complex design and onboard mass can be achieved by an attractive theme of propulsion by means of radiation, which earned a lot of attention by scientists and researchers. This new kind of scheme was named by Kantrowitz when he suggested “propelling” the thrust engine of space ships using ablative laser propulsion (ALP).1 This technique depends on the irradiation provided by the active laser beam on the target materials, also termed as propellants. After irradiation, the direction of movement of the material becomes opposite to the direction of the laser beam. All electric propulsion techniques, except photonic propulsion, obey Newton's law of motion. A jet of vapors or plasma after ablation of materials provides an essential thrust to propel objects. In general, a pulsed laser beam is focused onto the surface of a metal target. Due to strong absorption of electromagnetic radiation by a solid surface, the irradiated materials are dissociated and ablated toward the incident beam. The ablated material consists of excited and ionized species and it creates a plasma plume immediately. A domain of high pressure and temperature is generated in front of the target. This process induces a blast wave into the air, which generates the thrust to propel the target.2 Various thermodynamic states are involved in the propulsion and in determining the applicability of ALP. Momentum of a body is the parameter that designates the body's translational motion in space. For a specific application or mission, optimized parameters can be achieved. It is important to note that choice of laser energy is independent of mass of propellant (targeted material) although thrust is greatly dependent upon the geometry of the propellant. The variation in specific impulse and other key features directly corresponds to the propellants showing a strong dependence on the material used.3 

Laser propulsion (LP) played an important role by suggesting a new way to explore space applications and in launching space engines.4,5 As the laser-based thruster does not require burning of solid or liquid propellant onboard, it is a promising technique for eco-friendly and compact satellite design. Moreover, regulating the thrust by controlling the pulse energy level is simple and a proven concept. Laser propulsion can provide a possible alternative to conventional chemical propulsion.6 One of the core benefits of this technique is its ability to propel remote objects without any physical connection between the object and propelling system. Each laser pulse removes only certain grams of the material depending upon factors like type of material, laser fluence, etc. As a result of this ablation, propulsion is produced. Hence, a large jet can transfer sufficient momentum to the target material. Phipps introduced the concept of the laser-propelled rocket based on aforesaid estimates.7 Compared to conventional propulsion, LP resulted in simpler designs and structure of thrusters and there is a detailed comparison available with conventional propulsion in terms of various aspects like performance, cost-effectiveness, environmental impact, scalability, and technological challenges available. When compared to chemical propulsion, laser propulsion systems can achieve a higher specific impulse, which means they can use propellant and energy more efficiently. Conventional chemical systems, on the other hand, offer a high thrust and a comparatively lower specific impulse due to their established nature. The requisite sophisticated laser technology and accurate aiming might result in substantial upfront development and deployment expenses for laser propulsion systems. Nevertheless, chemical propulsion systems are more often established technologies with lower initial costs. They do, however, incur operating costs due to the continuous generation and storage of propellants. In terms of the environmental effect, LP is approachable because laser propulsion does not require chemical combustion; it may have a smaller environmental impact. However, high-powered laser effects on the environment must be carefully considered. Nonetheless, during combustion, chemical propulsion can release fumes and contaminants that have an adverse effect on the environment. For the scalability of larger space missions, challenges with power scalability, laser beam divergence, long-range precision targeting, and efficiency maintenance arise with scaling up LP for larger missions. Although chemical systems have been effectively expanded for more extensive missions, they still have obstacles in attaining greater specific impulses.

In conventional thrusters, due to complex designing and structure, space vehicles become overloaded, making it difficult for the space crafts to maintain flight missions. LP has the capability to maintain high payloads with desirable flight interest. Moreover, it is an environmentally friendly technique, providing a lesser cost matrix and a high ratio of performance. But for larger thrust considering the availability of the present technologies, chemical propulsion is still the best choice for rockets. For ground-based laser applications, the thrust to weight ratio is higher in electric propulsion than in chemical propulsion because the power source remains on the ground. Ground-based installation of a laser source and power transmission systems seek a considerable investment without any space qualification. A space-based practical demonstration was conducted using micro laser plasma thrusters and debris removal.8,9 Though it is still in the conceptual and developmental stages, ablative laser propulsion shows promise for dealing with space debris. More study and testing will be required to determine its viability, safety, and usefulness for larger-scale space debris removal. The freedom to control power density carried by propellants while controlling laser parameters makes laser propulsion much more productive for space missions. Energy efficiency or mass efficiency is the key indicator to assess propulsion system performance. In this review article, detailed discussions on the ablation mechanism, optimized propulsion parameters, especially the coupling coefficient and specific impulse achieved through various experimental configurations and with different propellants, are discussed, which would give an exhaustive insight into the manufacturing process of thrusters, real world application requirements, and a thoughtful analysis of data as compared to previous review articles. This review article has been designed to understand and get insightful analysis for technologists and researchers to get data and information form one frame, which is rarely available in the literature for not only the technologist but also the researchers on the early stages. This article gives new insights and traditions for readers to understand the ALP from definition to future implementations. Figure 1 gives a layout of this review article. In Sec. I, the modes of ablation with scientific explanation have been discussed, and the various kinds of propellant with their optimized values are given in Sec. II. Section III summarizes the propulsive parameters and the various techniques used to measure these propulsive parameters along with different techniques used by researchers to confine the ablation mechanism. Section IV provides the compilation of data regarding the practical demonstrations and developments from the perspective of future implementations and challenges faced along with the challenges available for aerospace applications. LP has the potential to be very efficient and to rely less on chemical propellants, which makes it a promising technology for future space propulsion. However, for LP to be a competitive and feasible option for large space missions, technological obstacles pertaining to power scalability, beam control, safety, and cost-effectiveness must be addressed.

FIG. 1.

Layout of the review article.

FIG. 1.

Layout of the review article.

Close modal

In ALP, an energetic beam of laser is shined on the surface of the material to offer an extensive momentum transfer. Laser energy penetrates to the surface of the sample, generating free electrons that collide with the sample of the surface, transferring energy. The surface of the sample heats up due to these collisions and vaporization occurs. Laser ablation results in material removal, yielding plume ejection from the surface. That plume may be the mixture of liquid or solid clusters of material. The ejected species become ionized at high intensities and produce plasma. This plasma absorbs a part of incident laser energy when shining the target with laser pulses having a pulse duration greater than picoseconds. Materials can go through different stages of ablation, which depend upon the surface geometry as well as the properties of the propellants. Metals yield excellent results for ablation with nanosecond lasers, which provide significant “ablation” for the materials to generate the plasma cloud above the surface of the sample. Threshold fluence above which the process of laser ablation starts generally depends on the properties of the material, morphology, microstructures, energy absorption mechanism, surface defects, and laser parameters. Threshold fluence varies from material to material. Its values are 1–10, 0.5–2, and 0.1–1 J/cm2 for metals, inorganic insulators, and organic materials, respectively.

FIG. 2.

Schematic diagram of ablation (Ref. 10). [Melendez, Fabrication of an integrated PCB-MEMS dielectric sensor node for liquid characterization (2017), 10.13140/RG.2.2.13476.48003. Copyright 2017 Author(s), licensed under Creative Commons Attribution (CC BY) 4.0.].

FIG. 2.

Schematic diagram of ablation (Ref. 10). [Melendez, Fabrication of an integrated PCB-MEMS dielectric sensor node for liquid characterization (2017), 10.13140/RG.2.2.13476.48003. Copyright 2017 Author(s), licensed under Creative Commons Attribution (CC BY) 4.0.].

Close modal

For fluences on a large scale, normal boiling occurs due to heterogeneous nucleation. Rapid heating of a material at critical temperatures results in phase explosion.11 For ultrafast laser pulses, photochemical ablation results in Coulomb's explosion.12 Biological materials and polymers have large thermalization time as compared to metals. However, for non-metals, photochemical ablation provides small HAZs when ablated with nanosecond lasers of small wavelengths.13 In metals and semiconductors, excitation energy is thermalized enough and the laser source can be considered a heat source to excite electrons in the conduction band, but the situation would be changed with a high power ultra-short pulse of laser. Many of the materials change their structure electronically and vibrationally when they are melted by the nanosecond laser pulses.

The overall ablation rate of the mechanism becomes more significant in thermal evaporation in which the material surface is melting due to heat and temperature. The whole process is the result of radiation absorption and going to excitation. After excitation, when optical properties change with the laser light, the excited energy suddenly is utilized to form heat and finally plasma is generated. Enough high temperature is used to evaporate the material with or without melting its surface and this process is called thermal material ablation. Plastic deformation has been observed in materials as a consequence of shock waves produced by the plume recoil.14 These shock waves are also responsible for molten material expulsion. Resolidification of ablated material in the surroundings of irradiated surfaces alters the topography of the surface. For laser pulses with a shorter pulse duration, more energy is deposited onto the surface, resulting in increased ejection of the ablated material. In situations like this, the ejected material can be described as a function of laser penetration depth and spatial profile.15 Optimum laser fluence is suggested to match laser ablation threshold multiplied by 5–10, which in most cases results in 2–20 J/cm2 for nanosecond pulses.16 

There are two modes of ablation (Fig. 2). One is called direct ablation in which a laser beam is directly focused on the target. In the other mode, plasma plumes are confined by different media; it only covers either the target or the entire plasma region. Moreover, an experiment shows that confinement enhances the propulsion parameters. In this mode of propulsion, more energy is absorbed by the propellants and less by the environment. This energy could be in a continuous beam or can be pulsed.

Surface irradiation provides a brief understanding of all parameters in propulsion whether it is direct ablation or confined ablation of a material. For studies, different types of confinements were used by various researchers. Doping is also one way of confinement in ablative laser propulsion. Carbon-doped glycerol yields enhanced values of coupling coefficient and specific impulse.17,18 Several propellants with confinement methods were used to increase the efficiency according to desire.19–28 Recent developments in the optimization of propulsion parameters achieved by the confinement of the plasma plume have been discussed in detail in Sec. III B and Table III.

In this section, the optimized parameters of ALP with different propellants are discussed in detail. Throughout this section, we will focus on the classification of the propellants, which yield the optimized momentum transfer, specific impulse, and the propulsion efficiency that could be utilized to obtain the best results in the ALP. Along with the propulsive parameters’ optimization, the best choice of the propellant also has earned great attention in the recent era. Focusing on the properties of the materials used as propellants, a deep understanding of the phenomenon of the ablation and its efficiency plays a significant role to choose the propellant. The choice of a suitable propellant significantly impacts the propulsive parameters, especially specific impulse and overall efficiency of propulsion. The combustion process and energy release are influenced by the unique chemical compositions of various propellant components. The amount of energy released during combustion is determined by the propellant's energy content. A higher specific impulse is produced by higher energy content in the result of high-energy processes and so on. Beside this, some propellants can achieve high temperature; as a result, higher combustion temperature can lead to high exhaust velocity and specific impulse while the complete combustion of the propellant results in better enhancement. ALP is dependent on ablative materials capable of producing high-velocity exhaust and absorbing laser energy. Materials like carbon-based materials or polymeric composites are frequently utilized but the decision is based on variables like thermal characteristics, structural integrity, and absorption efficiency. Although the choice of propellant affects ALP's efficiency, in general, its specific impulse is less than that of chemical rocket engines. ALP is more concerned with transferring momentum to objects than with reaching the highest possible specific impulse. However, for the applications of ablative laser propulsion, the properties of propellants were affected as the characteristics of laser absorption and the effectiveness of momentum transfer to debris are influenced by the propellant material selection. The actual implementation of ALP for debris removal is confronted with obstacles related to power needs, scalability, precision aiming, and range. The environmental impact of the propellant materials used in ALP must be taken into account, particularly when operating in space. It is critical to reduce pollution and debris production in orbital surroundings. In order to ensure effective momentum transfer for debris removal, propellant materials for ALP must be optimized by balancing factors like energy absorption, mass-to-thrust conversion, thermal resistance, and structural integrity. This requires taking into account practical issues like scalability and environmental impact.

Metals, polymers, plastics, ferrites, and nanoparticles are the best propellants considered due to their significance and properties.28,35–38 Metals have an excess number of free electrons, which provides high temperature plasma and hence, large ablation velocities are achieved. ALP technique involves heating of gas with a single wavelength of radiation and transforming the heated propellant into a significant thrust. Another important factor of an efficient propellant is that thrust depends upon the geometry and morphology of the material used for ablation because specific impulse measurements would help in understanding the mechanism of ablation. It is said that ALP is strictly material dependent and that it should be a highly absorbing material so that the energy of the laser beam would be absorbed completely by the propellant. A transfer of momentum gives direction to the material to propel or accelerate. There are several methods to calculate the coupling coefficient. Momentum coupling coefficient, represented as Cm, is the most explored concept that determines the magnitude of energy for a propulsion system.

Cm is defined as the ratio of thrust to laser power or it is a measure of energy by laser pulse used by the propellants: For pulsed laser, Cm can be calculated as in Ref. 38. Momentum transfer defines the efficiency of propellant to produce thrust. The coupling coefficient for cobalt nickel ferrite was reported to be about 0.147 N s/J.38 Solid targets including aluminum, copper, iron, and graphite were also ablated using a 50 fs laser pulse. The maximum coupling coefficient for copper was achieved which was ranged in 4 dyn/W for a maximum fluence of 4 J/cm2. For aluminum target, optimum fluence was 9 J/cm2 and coupling coefficient was 4.5 dyn/W. Similarly, for iron and graphite, maximum Cm was recorded as 2.9 and 4.3 dyne/W, respectively. Fluence used for this experiment was from 0.2 to 4 J/cm2. An aluminum target with a piezoelectric sensor was made in the sandwich structure to enhance its response sensitivity. With a maximum radiation energy of 12.7 mJ, the coupling coefficient obtained was 47.1 × 10−5 kg m/J and thrust generated with the aluminum foil with a thickness of 12.5 μm showed maximum thrust generation of 15 μN with a laser power of 20 W or 0.75 μN/W by Zhang et al., Guo et al., and Horisawa et al., respectively.36–39 

Cohen et al. used two pulsed techniques to find the specific impulse in the range of 2000–4000 s by using materials such as carbon, aluminum, iron, silicon, copper, zinc, and gold. The specific impulse was achieved with an efficiency of 20%. Those measurements were based on the function of pulse time separation. Graphical representation of the optimized coupling coefficient for different propellants over the past years is summarized in Figs. 3(a) and 3(b).

FIG. 3.

Year-wise graphical representation of coupling coefficient achieved using an (a) ND:YAG laser and (b) various other lasers, for different propellants.

FIG. 3.

Year-wise graphical representation of coupling coefficient achieved using an (a) ND:YAG laser and (b) various other lasers, for different propellants.

Close modal

The maximum ISP was achieved for aluminum and carbon, while lead gave the maximum value for the coupling coefficient, which was recorded to be 6.50 N s/J and the maximum specific impulse was found to be 950 s for Cu. Cm for the aluminum target was 1.54 × 103 N s/J, for copper Cm = 1.88 × 103 N s/J, and for gold foils, the coupling coefficient was 1.08 × 103 N s/J, which were studied by Pakhomov et al., Zheng et al., and Bhatti et al., respectively. Results showed that copper was the best among all the target materials as it gave the optimum values of the specific impulse and thrust.40–42 

In accordance with earlier findings, it is discovered that target materials with low atomic mass and high hardness, melting temperatures, and thermal conductivities produce higher specific impulses. For every metal, the mass removal rate is quite important. While Cu has the lowest mass removal rate, Au has a mass removal rate that is almost three times higher than Al's. The copper metal's hardness could be the cause of this.

Target momentum and coupling coefficient have been explored in the study42 based on their respective mass removal rates. To demonstrate how these parameters depend on the physical characteristics of metals, they are presented to study in relation to their respective atomic masses.42 Apart from metals, another class of materials that are studied as propellants are ferrites. Ferrites are particles in the nanometer range, which, according to the studies, gives enhanced propulsion parameters.43 Propulsion parameters with aluminum, cobalt nickel ferrites, and polymers were investigated for the first time in the literature. The fluence range used in the experiment was 1.02 × 106 to 5.79 × 106, with a maximum laser energy of 102.3 m J. Results showed that Cm for ferrites was 0.147, for polymers Cm = 0.109, and for aluminum Cm = 0.0642 N s/J. The maximum specific impulse was noted for cobalt nickel ferrite, which was recorded as 143 s. After using ferrites as propellants, it was decided to confine the generated plasma with transparent glasses on the surface of materials; enhanced results were recorded as the maximum coupling coefficient at different fluences in the range of 4–6 × 105 J/m2. The obtained coupling coefficient was in the range of 4.4 × 105 to 3.306 × 105 at the fluence of 4 × 105 N s/J, for a fluence of 5 × 105 Cm was 3.984 × 10−4 to 6.218 × 10−4 N s/J, for fluence 6 × 105 J/m2, the coupling coefficient was 1.880 × 10−4 to 9.098 × 10−4 N s/J by Jamil et al. and Ahmad et al., respectively.20,38

In a vacuum environment, for solid propellants, Cm can also be determined as
C m = M V E laser = m V p E laser ,
(1)
where m, M, Vp, P , and Elaser are mass of the exhaust propellant, mass of the projectile, velocity of the projectile, velocity of the exhaust particles, and energy of the incident laser, respectively. For a continuous laser, it can be calculated as
C m = F P laser ,
(2)
where F is the generated thrust and Plaser is the incident power of laser. Matsubara et al. investigated laser assisted plasma thrusters with a short pulse of laser. The maximum specific impulse of 7200 s was achieved with an energy of 8.6 J with the dimensions of 10–50 mm thrusters. To improve the thrust performance, a thrust efficiency of about 15% was achieved with a specific impulse of 4000 s.44 Zhang et al. investigated different propellants for plasma thrusters that were further accelerated electromagnetically. They used aluminum and PTFE and found a specific impulse in the range of 8000 s with a thrust efficiency of 90%, while with PTFE, the specific impulse was 2400 s with a thrust efficiency of 16%. These results proved aluminum to be the best propellant.27 The optimized amounts of specific impulse for different propellants at 1064 nm wavelength lasers have been graphically represented in Fig. 4.
FIG. 4.

A graphical representation of the achieved specific impulse from propellants using 1064 nm lasers.

FIG. 4.

A graphical representation of the achieved specific impulse from propellants using 1064 nm lasers.

Close modal

Laser propulsion in water is also studied to investigate the optimized propulsive parameters. As propulsion from liquids for space applications was not a good choice for researchers recently due to splashing, various improvements were done to get experimental results. Some of them have been elaborated in this section to investigate the momentum coupling coefficient and specific impulse achieved from liquid propellants. It is shown that in the atmosphere, the coupling coefficient was 3–10 dyn/W, while for water, it was in the range of 60–150 dyn/W. The optimum fluence was obtained as 158.53 kJ/m2 by Chen et al.45 but to improve the propulsion, targets are specially designed for ablation, and Qiang et al. found the best propellant for underwater propulsion by using aluminum, copper, and tin for the experiment. The optimum fluence for all three samples was found, which was 155.32 kJ/m2 for aluminum, 156.24 kJ/m2 for tin, and 160.26 kJ/m2 for copper. It was also found that the coupling coefficient was greater for tin, Cm = 100.11 dyn/W and proved it to be a better propellant for underwater propulsion. Furthermore, the effect of laser energy density on the coupling coefficient in a water propellant was also investigated, and for a laser density of 1.28 × 105 J/m2, a coupling coefficient equal to 906.88 N/MW was achieved.46 In the experiment,47 the focus area was changed by varying the distance between the liquid surface and focal point because it affected the value of the coupling coefficient. In order to enhance the coupling coefficient with liquid propellants by using confined geometries, different geometries of water shells as the confining medium were used and results were compared with unconfined planes. The maximum coupling coefficient was obtained for the hemispherical shell that was 4.1 × 10−2 N s/J, while the 90-degree conical shell exhibited the best propulsion. Results showed that the maximum Cm observed was 1785.8 dyn/W and specific impulse was 19.3 s. The total energy conversion efficiency was more than 50% as observed by Han et al. and Zhang et al.48,49 Recently, a mixed solution of ammonium dinitramide and ionic liquid-1-allyl-3-methylimidazolium dicyandiamide was used as the ablation target for propulsion. The propellant sample doped with 0.6 wt. % infrared dye yielded the highest impulse bit of 114.32 μ N  s, largest specific impulse of 84.14 s, highest impulse coupling coefficient of 1.07 mN s J−1, and the maximal ablation efficiency of 44% at the laser energy density of 21.51 J cm−2.50 Experimental results revealed that improving the absorption performance of propellants can enhance the energy release of the energetic propellants.

Novelistic propellants having properties between liquids and solids have been discussed by Zheng et al.18 They investigated the propulsive parameters of a carbon glycerol gel—a solidlike material. To enhance the specific impulse with such type of the propellants, carbon contents with high percentage were investigated. Results showed that Cm and ISP, as well as thrust, have a strong effect on the percentage of the carbon content. The maximum value of ISP of about 148 s was achieved with a significant decrease in the Cm from 72 to 17 dyn/W. When the ablation efficiency was kept constant, Cm and the specific impulse showed a direct relationship.

In order to achieve the desired results in ablative laser propulsion, it is imperative to comprehend and optimize the interaction between laser parameters, material properties, and propulsive parameters. For some materials and applications, it is frequently necessary to do modeling, experiments, and iterative refining to determine the optimized parameters. For certain materials, there exists a complex and intricate interaction between propulsive parameters and laser parameters.

Table I provides a detailed description of the materials' response to ablation in the form of optimal propulsive parameters for various laser wavelengths and pulse durations. The material's properties related to absorption are influenced by the laser's wavelength. The efficiency of energy transfer from the laser to the material varies depending on the material since different materials have varying absorption coefficients at different wavelengths. On the other hand, the heat input and the material's thermal reaction are influenced by the duration of the laser pulses. Less heat-affected zones and improved control over material removal could arise from shorter pulses.

TABLE I.

Propellants and propulsive parameters with laser parameters over past years.

Propellant Specific impulse Momentum coupling coefficient Laser parameters Reference
Carbon, aluminum, lead  4.0 × 103 s for C and Al  Cm = 8 dyn/W for lead
Carbon = 2.6 dyn/W,
Cm = 4.3 dyn/W for Al 
Nd:YAG laser with 532 nm  40  
Transparent tape with absorbing layer  1000 s  Cm = 6 dyn/W  Multimode diode laser with 1–5 W  52  
Solid targets  500 s  Cm = 100 μN/W  Diode laser  51  
Tape with transparent layer  1800 s  Cm = 50 dyn/W  Rep-pulsed diode laser with 1–5 W  60  
Microbeads  —  Cm = 5.0 dyn/W  Milli joule femtosecond laser  61  
Teflon, Bunan  1.8 × 10−5 N s for Teflon 7.0e -6 N s for Bunan  Cm = 14 dyn/W for Teflon
Cm = 20 dyn/W for Bunan 
Nd:YAG laser (532 nm)  63  
Parabolic craft  —  Cm = 10.5 × 10−5 to 8.5 × 10−5 N/W  TEA (CO2) laser Single pulse energy
13 mJ 
64  
Aluminum plates  —  Enhanced with confinement
Cm = 18.09 × 10−5 N s/J 
Pulsed laser with
532 nm 
30  
Lead, aluminum, copper, graphite  Lead = 280 s Al = 400 s graphite = 520 s
copper = 950 s 
Lead Cm = 6.5 dyn/W  Pulsed laser with
532 nm 
41  
Carbon ink  19.3 s  Cm = 1785.8 dyn/W  Pulsed laser with
532 nm 
47  
Target for prepulse method  —  Cm = 5.8 × 10−5 N/W  Pulsed laser beam with 532 nm  28  
Solid materials (aluminum and copper)  104 s  Cm = nearly 3.5 dyn/W for Al
Cm = nearly 4 dyn/W for copper 
Fs laser with 50 fs pulse  62  
Metal/salt pellets  —  Cm = 10 dyn/W at IR μm
Cm = 8 dyn/W at UV 355 nm 
TEA CO2 Laser with two different wavelengths  22  
Aluminum, copper, gold  Max 1.37 × 107 s for copper, 7.16 × 105 s for Au,
3.14 × 106 s for Al 
Max Cm = 1.88 × 103 N s/J for copper,
1.08 × 103 Ns/J for Au, and
1.54 × 103 for Al 
Nd:YAG laser at 1064 nm  42  
Liquid propellants  —  Cm = 906.88 N/MW  (TEA) CO2 laser operated at 10.6 μ 47  
Water environment  —  Cm = 4.1 × 10−2 N s/J for hemispherical shell  Nd:YAG Laser  48  
Aluminum, graphite, iron, copper  —  Cm = 4 dyn/W for copper target
Cm = 4.5 dyn/W for AL
Cm = 4.3 dyn/W for graphite
Cm = 2.9 dyn/W for iron 
Fs laser with central wavelength 800 nm  65  
In water and atm  Impulse = 10−5–10−4 Ns  Cm in atm = 3–10 dyn/W
Cm in water = 60–150 dyn/W 
Nd:YAG laser at 1064 nm  54  
Aluminum, cobalt nickel ferrites, polymer  62.20 s for Al 143.15 s for ferrites 105.51 s for polymer  Cm = 0.0642 N s/J for AL
Cm = 0.147 for ferrites
Cm = 0.109 for polymer 
Nd:YAG laser at 1064 nm  38  
Double base propellant  Max 448.9 s for round beam  Max Cm = 120 dyn/W for
round beam shape 
Diode laser with
808 nm 
66  
Aluminum, titanium, copper underwater  —  Cm = 80 dyn/W for AL
Cm = 100 dyn/W for titanium
Cm = 90 dyn/W for copper 
Nd:YAG laser at
1.06 μ
46  
Aluminum    Cm = 47.1 × 10−5 kg m s−1 J−1  YAG (LS-2137) laser with 532 nm  36  
Nano structured ZnO  Max 560 s  Max Cm = 2.5 × 10−4 N s/J  Nd:YAG laser with 1064 nm  19  
Ferrites  —  Cm = 9.098 × 10−4 N s/J
after confinement 
Nd:YAG laser with 532 nm  21  
POM  820 s  Cm = 108.5 dyn/W  CO2 laser with
10.6 μm wavelength 
53  
Aluminum, PTFE  8000 s for Al 2400 s for PTFE  —  Nd:YAG laser with 1064 nm  67  
Carbon doped glycerol gel  228 s  Cm = 70–80 dyn/W  Pulsed laser with 1064 nm  18  
Aluminum target  3000 s  —  Nd:YAG pulsed laser with 1 μm wavelength  59  
Carbon-doped glycerol gel  10.7 s  Cm = 145 dyn/W  Nd:YAG laser with 1064 nm  17  
Carbonized PTFE  400 s  Cm = 4 × 10−4 N/W  Fiber coupled diode laser with 10 W  55  
Aluminum  930 ± 8 s  Cm = 20.6 μN s/J  Krf excimer laser with 248 nm  58  
PTFE with different dopants  9648.81 s  Cm = 16.43 N/MW Cm = 4.74 N/MW  ms pulsed fiber laser with 1080 nm wavelength  57  
Metal foils  2175 s for Fe
1835 s for Cu
1767 s for Al at 532 nm
at 1064 nm Fe = 41 675 s
Cu = 18 000 s
Al = 13 000 s 
At 532 nm Cm for Cu = 1.26 × 10−5 at 1064 Cm for Cu = 2.27 × 10−4 For Al, at 532 nm Cm = 1.82 × 10−5, at 1064 nm Cm = 1.83 × 10−5 For iron, at 532 nm Cm = 1.5 × 10−5 at 1064 nm Cm = 1.5 × 10−5  Nd:YAG laser at 1064 and 532 nm  56  
Propellant Specific impulse Momentum coupling coefficient Laser parameters Reference
Carbon, aluminum, lead  4.0 × 103 s for C and Al  Cm = 8 dyn/W for lead
Carbon = 2.6 dyn/W,
Cm = 4.3 dyn/W for Al 
Nd:YAG laser with 532 nm  40  
Transparent tape with absorbing layer  1000 s  Cm = 6 dyn/W  Multimode diode laser with 1–5 W  52  
Solid targets  500 s  Cm = 100 μN/W  Diode laser  51  
Tape with transparent layer  1800 s  Cm = 50 dyn/W  Rep-pulsed diode laser with 1–5 W  60  
Microbeads  —  Cm = 5.0 dyn/W  Milli joule femtosecond laser  61  
Teflon, Bunan  1.8 × 10−5 N s for Teflon 7.0e -6 N s for Bunan  Cm = 14 dyn/W for Teflon
Cm = 20 dyn/W for Bunan 
Nd:YAG laser (532 nm)  63  
Parabolic craft  —  Cm = 10.5 × 10−5 to 8.5 × 10−5 N/W  TEA (CO2) laser Single pulse energy
13 mJ 
64  
Aluminum plates  —  Enhanced with confinement
Cm = 18.09 × 10−5 N s/J 
Pulsed laser with
532 nm 
30  
Lead, aluminum, copper, graphite  Lead = 280 s Al = 400 s graphite = 520 s
copper = 950 s 
Lead Cm = 6.5 dyn/W  Pulsed laser with
532 nm 
41  
Carbon ink  19.3 s  Cm = 1785.8 dyn/W  Pulsed laser with
532 nm 
47  
Target for prepulse method  —  Cm = 5.8 × 10−5 N/W  Pulsed laser beam with 532 nm  28  
Solid materials (aluminum and copper)  104 s  Cm = nearly 3.5 dyn/W for Al
Cm = nearly 4 dyn/W for copper 
Fs laser with 50 fs pulse  62  
Metal/salt pellets  —  Cm = 10 dyn/W at IR μm
Cm = 8 dyn/W at UV 355 nm 
TEA CO2 Laser with two different wavelengths  22  
Aluminum, copper, gold  Max 1.37 × 107 s for copper, 7.16 × 105 s for Au,
3.14 × 106 s for Al 
Max Cm = 1.88 × 103 N s/J for copper,
1.08 × 103 Ns/J for Au, and
1.54 × 103 for Al 
Nd:YAG laser at 1064 nm  42  
Liquid propellants  —  Cm = 906.88 N/MW  (TEA) CO2 laser operated at 10.6 μ 47  
Water environment  —  Cm = 4.1 × 10−2 N s/J for hemispherical shell  Nd:YAG Laser  48  
Aluminum, graphite, iron, copper  —  Cm = 4 dyn/W for copper target
Cm = 4.5 dyn/W for AL
Cm = 4.3 dyn/W for graphite
Cm = 2.9 dyn/W for iron 
Fs laser with central wavelength 800 nm  65  
In water and atm  Impulse = 10−5–10−4 Ns  Cm in atm = 3–10 dyn/W
Cm in water = 60–150 dyn/W 
Nd:YAG laser at 1064 nm  54  
Aluminum, cobalt nickel ferrites, polymer  62.20 s for Al 143.15 s for ferrites 105.51 s for polymer  Cm = 0.0642 N s/J for AL
Cm = 0.147 for ferrites
Cm = 0.109 for polymer 
Nd:YAG laser at 1064 nm  38  
Double base propellant  Max 448.9 s for round beam  Max Cm = 120 dyn/W for
round beam shape 
Diode laser with
808 nm 
66  
Aluminum, titanium, copper underwater  —  Cm = 80 dyn/W for AL
Cm = 100 dyn/W for titanium
Cm = 90 dyn/W for copper 
Nd:YAG laser at
1.06 μ
46  
Aluminum    Cm = 47.1 × 10−5 kg m s−1 J−1  YAG (LS-2137) laser with 532 nm  36  
Nano structured ZnO  Max 560 s  Max Cm = 2.5 × 10−4 N s/J  Nd:YAG laser with 1064 nm  19  
Ferrites  —  Cm = 9.098 × 10−4 N s/J
after confinement 
Nd:YAG laser with 532 nm  21  
POM  820 s  Cm = 108.5 dyn/W  CO2 laser with
10.6 μm wavelength 
53  
Aluminum, PTFE  8000 s for Al 2400 s for PTFE  —  Nd:YAG laser with 1064 nm  67  
Carbon doped glycerol gel  228 s  Cm = 70–80 dyn/W  Pulsed laser with 1064 nm  18  
Aluminum target  3000 s  —  Nd:YAG pulsed laser with 1 μm wavelength  59  
Carbon-doped glycerol gel  10.7 s  Cm = 145 dyn/W  Nd:YAG laser with 1064 nm  17  
Carbonized PTFE  400 s  Cm = 4 × 10−4 N/W  Fiber coupled diode laser with 10 W  55  
Aluminum  930 ± 8 s  Cm = 20.6 μN s/J  Krf excimer laser with 248 nm  58  
PTFE with different dopants  9648.81 s  Cm = 16.43 N/MW Cm = 4.74 N/MW  ms pulsed fiber laser with 1080 nm wavelength  57  
Metal foils  2175 s for Fe
1835 s for Cu
1767 s for Al at 532 nm
at 1064 nm Fe = 41 675 s
Cu = 18 000 s
Al = 13 000 s 
At 532 nm Cm for Cu = 1.26 × 10−5 at 1064 Cm for Cu = 2.27 × 10−4 For Al, at 532 nm Cm = 1.82 × 10−5, at 1064 nm Cm = 1.83 × 10−5 For iron, at 532 nm Cm = 1.5 × 10−5 at 1064 nm Cm = 1.5 × 10−5  Nd:YAG laser at 1064 and 532 nm  56  

From early 2000 to the present, various materials have been studied by different researchers and the propulsive parameters Cm and Isp are reported. Figure 5 shows the materials used to study the parameters over the years. However, the experiment involves so many parameters that can be adjusted to obtain better results, for instance, the phase of material used, temperature of material, and whether the experiment is performed in air or vacuum are some examples of parameters that are changed for different experiments, apart from measurable parameters like the wavelength of the laser, pulse duration, pulse energy, pulse repetition rate, average power, etc., which can be changed precisely for each experiment. Cm and Isp in vacuum for pulses longer than 100 ps can be predicted for the majority of materials within a factor of 2 just using intensity, wavelength, pulse duration, atomic number, and ionization state of the plasma formed. However, for majority of the studies, the parameters chosen are not clearly mentioned, which decreases the chance of finding one organizing principle behind the obtained values. To find a formula that can be used to find the ablative properties for the majority of the materials, some basic parameters should be adjusted and defined so a comparison between different outcomes can be justified. Like, for instance, in Refs. 68 and 69, relationships among Cm, Isp, and ablation efficiency were developed but that is only possible if some basic parameters are kept the same. In most of the studies reported here, important parameters like the laser pulse, pulse energy, light intensity, pulse duration, etc. are not defined, which makes it hard to develop a link between various studies.

FIG. 5.

Graphical representation of the work done in ALP over the years.

FIG. 5.

Graphical representation of the work done in ALP over the years.

Close modal

Propellants are not limited to metals or liquids. Phipps and Luke developed a miniature jet for pointing micro satellites.51 For this purpose, multimode diode lasers were used with a brightness of 100% and thrust was produced by ablation of material, which was specially made with one layer of transparent tape and one layer of absorbing material. The specific impulse was in the range of 1000 s with a maximum coupling coefficient of 6000 dyn/W.51 After developing microsatellites, a laser driven microthruster was developed for pointing nanosatellites. Any solid material other than the tape was used for creating plasma jets, which gave thrust with a 1 W laser power, and a specific impulse from 200 to 500 s would be achieved with a five orders high magnitude in the dynamic range of the impulse. The coupling coefficient achieved was in the range of 50–100 μN/W.52 A femto second laser was used to study ablation of metals and organic materials and resulting in the formation of laser plasma. Results showed that for metal targets, the maximum coupling coefficient achieved was 42 μN/W at an optimum fluence of 21 kJ/m2. For organic materials, the maximum coupling coefficient achieved was 80 μN/W at an optimum fluence of 10 kJ/m2. The ablation efficiency was about 100% for this plasma thruster;5 then a new prototype device was developed, which gave a specific impulse in the range of 200–3800 s with ablation of gold. The maximum Cm was obtained in the range of 70–70 μN/optical watt with an ablation efficiency of about 100% at the optimum fluence of MJK/m2.52 Horisawa et al. investigated the thrust performance of a plasma jet structure with rectangular nozzle elements with dimensions of 0.5 mm. Results showed 20% improvements in thrust and specific impulse at a laser power of 6.3 W. A specific impulse of 77 s was achieved in the experiment.37 Thompson and Moeller71 investigated computational models for improvements for micro laser plasma thrusters. Results showed 36% improvements in the coupling coefficient and 50% in the specific impulse achieved. Matsubara et al. investigated laser assisted plasma thrusters with a short pulse of the laser. The maximum specific impulse of 7200 s was achieved with an energy of 8.6 J with the dimensions of 10–50 mm thrusters. To improve thrust performance, a thrust efficiency of about 15% was achieved with a specific impulse of 4000 s,44 Zhang et al. investigated different propellants for plasma thrusters that were further accelerated electromagnetically. Aluminum and PTFE were used and a specific impulse in the range of 8000 s with a thrust efficiency of 90% achieved, while with PTFE, the specific impulse was 2400 s with a thrust efficiency of 16%. These results proved aluminum as the best propellant.27 Thrust performance with POM in the thruster was studied. The effect of nozzle geometry was studied and the results were recorded after the irradiation with two lasers. With laser 2, the coupling coefficient was 108.5 dyn/W and specific impulse reached 820 s, while with laser 3, the Cm was 103 dyn/W and ISP in the range of 642 s was recorded.71 A compilation of data of specific impulse for different laser plasma thrusters is given in Table II.

TABLE II.

Maximum specific impulse achieved in different laser plasma thrusters.

Propellants/Thrusters Specific impulse (s) Reference
Aluminum, PTFE  8000  25  
Laser-assisted plasma thruster  7200  42  
Miniature jet  1000  49  
Laser driven microthruster  500  50  
Prototype with gold propellants  3800  70  
Thruster with POM  820  71  
Propellants/Thrusters Specific impulse (s) Reference
Aluminum, PTFE  8000  25  
Laser-assisted plasma thruster  7200  42  
Miniature jet  1000  49  
Laser driven microthruster  500  50  
Prototype with gold propellants  3800  70  
Thruster with POM  820  71  

When a target is irradiated by a high power laser pulse in air, the blow-off of the target material and breakdown of air occur in front of the target surface. The inverse Bremsstrahlung absorption of laser energy by the plasma forms a laser shock detonation (LSD) wave41 and plasma then expands with supersonic velocity. If the plasma density is lower than the critical density, the laser energy can be directly absorbed by the target material and the plasma expansion plays a main role in the momentum transfer. If the plasma density is higher than the critical density, the laser energy is absorbed by the LSD wave. In this case, the momentum transfer is mainly completed by the LSD wave. It can be seen that for a fixed ambient pressure and laser energy, target momentum is proportional to I−1/3, which implies that when the target approaches the geometrical focal position (the laser's minimum focus), the target momentum diminishes. The experimental findings presented in figure in Ref. 41 confirm this when the target surface is situated ahead of the geometric focal position. Under vacuum, the figure in Ref. 41 displays the particular impulse as a function of target position. It is evident that as the target gets closer to the geometrical focal point, the particular impulse decreases. Numerous causes are connected to this phenomenon. The target material sputtering is a significant one. Targets that are exposed to high strength laser pulses ionize their surface layer, creating plasma. Accompanied by the plasma are many un-ionized particles that can be removed from the target surface with a low velocity. These particles mainly affect the ablation mass and result in a lower specific impulse. This phenomenon is serious for soft target materials ablated with a high laser intensity. Furthermore, the thermal properties of target materials can also affect the specific impulse. A good thermal conductivity can lead to heating and vaporization of the target material in deep layers. This might be the reason why the higher specific impulse is always realized on the target material with a high hardness and a high melting and boiling temperature.41 The formulas show that for long plasma scale length, low temperature, and high atomic number, the absorption of laser energy is considerable.28 It is possible to anticipate a high coupling coefficient because the prepulse produced on the target surface extends the plasma scale length. The interaction mechanism is determined by the prepulse-main beam delay duration. When there is a significant short delay time, meaning that the pulse essentially arrives at the target simultaneously, there is a large incident laser energy contact with the target. It is impossible to demonstrate the pre pulse's role. The prepulse expands away and cannot boost the coupling coefficient for very long delay times; tens of ns. It has been established that high electron densities in plasma can be produced by substantial incident laser energy, leading to high coupling coefficients.

For a 70 mg Al target from a pulse laser beam, the target momentum and coupling coefficient transferred by the nanosecond laser are 1.54 × 101 (kg m/s) and 1.54 × 103 (N s/J), respectively. These values are represented with respect to the mass removal rate and with regard to atomic mass. The targets' tiny, millimeter-scale movement is the only restriction in this instance. The thermal dissipation and reduced conversion of incident energy to ablate the mass are the causes of the energy absorbed in the targets. In the event of a larger pulse width, the target surface's surface reflectivity is also dominant.42 Momentum coupling coefficient for targets 1 (with hemispherical cavity), 2 (with conical cavity), 3 (hemispherical target without cavity), and 4 (conical target without cavity) are represented by the coupling coefficients for the squares, circles, triangles, and stars, respectively. It is evident from Ref. 53 that the momentum coupling coefficient Cm first rises with laser energy before leveling out for each of the four targets. This is due to the fact that it first grows with laser energy, but when the laser energy increases further, the growth tendency eventually slows down. The target with the largest momentum coupling coefficient among the four is the one featuring a hemispherical cavity.

The coupling coefficient and specific impulse as a function of carbon concentration are displayed in Ref. 18 as thrust performance is significantly impacted by the carbon content. In actuality, the coupling coefficient falls while the particular impulse rises quickly as the content of carbon rises from 5% to 31%. The liquid splashing primarily determines the contact procedure rather than the plasma expansion. Splashing raises the intended coupling coefficient while decreasing the specific impulse. In the second phase, where the content is more than 10% and particularly around the maximum content of 31% achieved in the current circumstances, the physical characteristics of the carbon-doped glycerol display certain characteristics of a solid. Both the quantity and size of the droplets diminish to the micrometer level. This indicates that the volume of the splashing starts to decrease. From this angle, the carbon doping can entirely eliminate the splashing. The estimated coupling coefficient values for the polymer, Al, and Co-Ni ferrite at various energies per laser pulse at a wavelength of 1064 nm are displayed in Ref. 38. The varying coupling coefficient values for different targets in relation to fluence demonstrate how Cm is influenced by laser power, spot size, and material composition. The propellant's energy consumption by incident laser pulses is measured in terms of Cm; for 1064 nm, it has been found that the higher the incident laser's energy per pulse, the higher the value of Cm. For higher values of specific impulse, less propellant is required to gain a specific amount of momentum. From Fig. 3(b),38 it is seen that Co–Ni ferrite has much higher values of Isp as compared to Al, which might be due to higher laser induced ionization in Co–Ni plasma. Many un-ionized particles are ejected during the splattering of targets along with ionized particles, which results in a lower specific impulse and also affect the ablated mass.38, Figure 6 displays the impulse coupling coefficient as a function of energy density.36 It is discovered that the impulse coupling coefficient rises quickly in the energy density range of 0.75–2.0 m J cm−2. Upon exceeding 2.0 m J cm−2, the laser impulse coupling coefficient exhibits a gradual increase. The coupling coefficient reaches its greatest value at an energy density of 3.92 m J cm−2. The coupling coefficient will gradually drop if the laser energy density is further increased. The energy density is above the aluminum goal threshold value when it falls between 0.86 and 2.08 m J cm−2. Due to the shielding effect of the target, the laser impulse coupling coefficient grows slowly when the energy density rises above 2.08 m J cm−2.36 

FIG. 6.

The curve of energy density and impulse coupling coefficient. Reproduced with permissions from Guo et al., Mater. Res. Innovations, 19(suppl8), S8-483–S8-485 (2015). Copyright 2015, Taylor and Francis.

FIG. 6.

The curve of energy density and impulse coupling coefficient. Reproduced with permissions from Guo et al., Mater. Res. Innovations, 19(suppl8), S8-483–S8-485 (2015). Copyright 2015, Taylor and Francis.

Close modal

As discussed in Sec. II, solids were being irradiated with an energetic beam of laser in order to get the desired values of the parameters of propulsion. There are many techniques and methods that are used to measure parameters, for example, time of flight (TOF) method, non-optical triangulation method, two-layer method, temperature variation method, plasma imaging method, torsional pendulum, prepulse method, shock wave production, using ablator with high frequency, microchip laser thrusters, piezoelectric sensor method, angle of incidence to calculate impulse, gel propellants with two different techniques, ballistic pendulum method, and pulse duration effect on thrust. All of these above mentioned techniques are examined by different researchers to get the desired propulsive parameter. These techniques are discussed in detail in this section, although the techniques mentioned here do not form an exhaustive list of all applied techniques. The propulsion parameters of cobalt nickel ferrite, aluminum, and polymer with non-optical triangulation method have been discussed with a 1064 nm laser with a pulse width of 5 ns in the atmospheric condition. To focus the beam of the laser, bi-convex lens and a He-Ne laser on the reflecting surface were used. With a high energy laser beam, the coupling coefficient and specific impulse achieved were high for the cobalt nickel ferrites. In cobalt nickel ferrite, the plasma plume has high ionization, which yields a high value of specific impulse. The coupling coefficient for cobalt nickel ferrite was reported to be about 0.147 N s/J.38 A thin film of polyimide (solid propellant) was examined to get high values of ISP and to avoid mechanical losses by using a method other than TOF. A 532 nm laser with a 7 ns pulse width was used for irradiation in the vacuum chamber of 5 Pa pressure with the sample deposited on the aluminum substrate with a hole comparable with a focal spot area of the beam. It was examined that if the laser intensity would exceed the value of 10 × 1012 W/cm2, then a large amount of specific impulse of about 7200 s would be achieved with this technique.39 

After the ablation of aluminum,54 target shock waves were produced, which were amplified and probed to evaluate the relation between shockwave and plasma plume and then investigated. The temperature was reduced to 200 K for the formation of the cluster and varied pressure was applied. At the start of the expansion of shock waves, speed and pressure were inversely related to each other. Speed of the wave was high at the start and after the attenuation of time, shock waves were converted into sound waves. With the passage of time, it showed a different behavior when back ground pressure was kept at low values. Aluminum proved itself as the best propellant due to its coupling coefficient and specific impulse results.54 The performance of propulsion with repetitive pulse ND:YLF laser irradiated aluminum targets was tested.25 A single pulse did not give a significant impulse (an ablator was developed by Phipps et al., in which two layers were developed to ablate and to achieve a significant impulse). A large specific impulse was achieved after changing the ablated spot to a fresh surface for irradiation with a laser beam. Due to this change, one could avoid the generation of a crater of deep depths.25 

To make a spiral path of ablation for propellants, a single drive motor was used,35 which provided rotatory and translator motion. The ablation effluents are thought to be expelled in a direction that is nearly parallel to the target surface normal. The propellant feed mechanism concept has been discussed in detail, which is made up of a motor, a cam gear, a gear box, and a cylindrical solid propellant. More specifically, parameters for an MLT system using an Al propellant and a 10 μJ/pulse microchip laser yielded a thrust range = 0.5 nN–5 μN, system mass = 455 g, Isp = 4900 s, and maximum power used = 6.5 W. Crater volume, shape, and size for single- and multi-pulse affected the propellant feed idea in MLT.35 Propulsion parameters were investigated by the plasma imaging method and the specific impulse was investigated from the angular profiles obtained via digital imaging. As the plumes depict the expansion of the neutral and ionized components of the plasma, it was anticipated that the specific impulse derived from the pictures would validate the information from the previously published force measurements. An Nd-YAG laser with a 532 nm wavelength and pulse energy ∼ 35 mJ and width ∼ 100 ps with a regenerative amplifier was used and the results of specific impulse obtained by three different methods including TOF, plasma imaging, and direct force measurement method were compared.72 

A different method (piezoelectric sensors) was used to measure the parameters of propulsion and for the calibration of the sensors.36 The relationship between the laser's energy density and the impulse coupling coefficient can be deduced from the linear relationship between the impact momentum and the sensor's signal voltage. Experimental results found the impulse coupling coefficient as a function of energy density. To analyze the energy density at the material ablation threshold, a lattice-shaped diaphragm wall (LSDW) model was used to fit at this threshold. The coupling coefficient reaches its maximum value of 47.1 × 10−5 kg m s−1 J−1 when the energy density is 3.92 mJ cm−2. The energy density surpasses the threshold value of the aluminum target when it falls between 0.86 and 2.08 mJ cm−2 as it can be seen from graphical data from Fig. 6 represented in Sec. III.

The study26 presents the relationship of the laser ablation impulse, measured with a torsion pendulum impulse measuring apparatus, on the incident angle at different laser beam fluences. On the basis of the impulse characteristics, optimization of the fluence at oblique incidence is considered. Ablation of Al was done with a ns laser at different fluences by changing the angle of incidence. It was seen that the coupling coefficient is sensitive to the angle of incidence and changes were also observed with different morphologies of the sample. For better performances and enhancement in impulse, angles of incidence were kept at larger distances in order to keep the optical path shorter. ND:YLF laser with the wavelength of 1047 × 10−9 m was used in a vacuum chamber. It was reported that the coupling coefficient was affected with laser fluences. The effect of the laser fluence on different materials in the laser ablation propulsion for low pressure cases was also discussed.23 After this investigation, most of the issues related to the ablation of materials were resolved. Results showed that the impulse generated after ablation is dependent on the beam spot size area. But if the pressure and shock waves effect was neglected, then the coupling coefficient and specific impulse both are independent of beam spot size area. This was a shocking result observed after the theoretical model was developed. Cm was modeled as a roll off function of fluence and studied in the range of fluence of 1–150 J/m2. Plasma transmission in the material at high fluences decreased the transmissivity and increased the condition of roll off. For high values of laser fluences, the internal energy reaches constant values and when the plasma reduction effect is under consideration, then transmissivity decreases.

A Co2 pulsed laser with a 10.6 × 10−6 m wavelength irradiates tin with the ballistic pendulum method.73 The intensity of the beam of laser was in the range of 107–1010 W/cm2. Due to the threshold value of the ablation mechanism, the coupling coefficient was low when the laser intensity was less than 2.3 × 108 W/cm2. Dependency of results on the laser pulse intensity was weak due to the two main factors. In the numerical simulation, Mora's model did not consider the high atomic number elements to describe the properties of plasma when the ideal gas equation was considered for the description. Second, transport energy of thermal radiation of plasma did not account for the high atomic number elements. The emitted radiations of Sn plasma were in the range of 20 nm, which would ablate the material furthermore. As a result, the value of the coupling coefficient was increased further.73 

To date, various procedures have been reported in order to enhance the ablation mechanism to get propulsion parameters exclusively. Some of them are listed in this section, which emphasized how the coupling coefficient and specific impulse can be enhanced via confinement of plasma plumes. To get more momentum and specific impulse, different studies have been conducted enforcing confinement via glass layers or other materials in front of the plasma above the sample surface. After confinement, it was seen that a specific impulse with a value of 1520 s was achieved when polyimide films were irradiated with nanosecond lasers.29 The sample was irradiated with the beam of laser having a wavelength of 532 × 10−9 m with a pulse duration of 10 ns and the maximum energy applied was 200 × 10−3 J. Carbon black was used as a confining media. Zinc oxide nanoparticles19 initially had a different morphology but then are subjected to the nano noodles to examine the effects, and four samples with different morphologies were treated with a laser of 1064 nm wavelength and a pulse duration of 5 ns. Results showed that the sample with a single phase hexagonal structure was considered the best propellant due to its high values of Cm = 2.5 × 10−4 N s/J at an optimum fluence of 2.5 × 106 J/m2 and acted as a phenomenon of the confinement method. The specific impulse of this sample was also found greater than all other samples, which were in the range of 25–560 s.

For the first time, confinement of plasma by using cavity walls to increase the coupling coefficient for ferrites was investigated with a ND:YAG laser of 532 × 10−9 m and 5 × 10−9 s pulse.20 Most of the energy is absorbed and consumed in the atmosphere and the process of inverse bremsstrahlung and shielding effect of plasma decreases the produced thrust in direct ablation. But if the confinement method is used, then it would reduce the strength of the shock wave and increase the values of propulsion parameters. As a result, there was a large pressure difference between the front and back sides of the samples, which produced thrust. If the cavity aspect ratio (CAR) was high, then plasma temperature and electron number density were also high for any depth and diameter. Cylindrical confinement was observed to be better than the rectangular ones, because shock waves are produced in the spherical shape. The value of Cm for a cavity diameter of 5 mm was calculated to be 3.27 × 10−5 N s/J. Optimized values of the coupling coefficient for all the diameter of cavity aspect ratio with laser fluence range from 4 to 6 × 105 J/m2 were reported.

An intense beam of laser with a 532 nm wavelength and a pulse duration of 7 ns was used to investigate the dependence of coupling coefficient and momentum on the cavity with a layer of transparent glass.30 With the LSD waves, the momentum coupling coefficient got enhanced. This showed that Cm also depends on the thickness of the layer covering the sample surface as well on laser intensity and energy. Results showed that when the laser intensity reached a certain value of 11.37 GW/cm2 then the value of Cm was increased for both direct and confined ablations.30 It is investigated that water confinement enhances coupling coefficient many times as compared to planar ablation.41 In the glass layer confinement or with the use of water thin films, most of the energy was observed to be dissipated. On the other hand, coupling coefficients were enhanced around 30 times in water-based confinement methods. This increases the absorption efficiency as well as specific impulse. A laser with 532 nm and pulse duration of 7 ns irradiated aluminum with cavities in which the water layer serves as a layer for confinement of the plasma and the results were observed. At high values of laser fluence, the value of the coupling coefficient decreased due to the process of conversion of more kinetic energy of water.

A prepulse idea was investigated28 in order to get enhancement of propulsion parameters and absorption efficiency by using a laser of 532 nm wavelength and a pulse duration of 7 ns. Delay time was directly related with the coupling coefficient, wavelength, and beam width of the laser and it was controlled by the time section. For a short time, Cm could be enhanced due to greater energy of laser interaction with the matter (with obtained Cm 5.8 × 10−5 N/W) and for the large time scale, the pulse would expand and fewer enhancements in Cm can be observed. In order to get the enhanced parameters from a refined structure of Co0.5Ni0.5 Fe2O4 ferrite in a single phase, an Nd:YAG laser of 532 nm and a pulse duration of 5 ns was used for irradiation.21 Two categories of the prepared sample were used. One sample was used as pellets and the second as a cavity target with 2 mm depth and 2 mm diameter. To enhance the produced thrust, confinement of plasma was achieved by using a transparent glass. It was found from the experiment that without confinement, the coupling coefficient increases from 5.747 × 10−5 to 7.064 × 10−5 N s/J in the fluence range of 4 × 109 to 6 × 109 J/m2 and values change from 1.41 × 10−4 to 2.68 × 10−4 N s/J in the fluence range of 4 × 109 to 6 × 109 J/m2 with the cavity.

Ablative laser propulsion using semi-elliptical cavities gives better results. The plasma was contained by semi-elliptical chambers made of various CARs.32 Shockwaves reflected off the cavity walls compressed the plasma, increasing the Isp and Cm. Variations in the cavity's primary axis were found to have a large impact on the propulsion characteristics. For the 9 mm cavity, the most optimal value of Cm was found to be 1.7 × 10−4 N s/J for aluminum at 7.21 × 106 J cm−2 and 9 × 10−5 N s/J for silver at 3.37 × 106 J cm−2. Also, the 9 mm cavity yielded the highest Isp value. When it came to silver, the fluence value was 3.16 × 103 s for a fluence of 7.21 × 106 J cm−2; however, for aluminum, it was 2.02 × 103 s at the fluence of 9.52 × 106 J cm−2. It has been studied how exterior semi-spherical cavities affect the laser ablation propulsion characteristics for targets made of aluminum, gold, and silver.33 Shockwaves were produced by trapped plasma in semi-spherical cavities with varying sizes. Shockwaves from the cavity were uniformly reflected, which greatly increased the Cm and Isp. It was discovered that the improvement of propulsion characteristics was significantly influenced by the cavity's diameter. As the diameter increased, enhancement diminished. The confinement of plasma with a cavity aspect ratio of different depths and diameters was further studied in Ref. 34 by using semiconductors and the enhanced parameters are listed in Table III.

TABLE III.

Different techniques used by the researchers in order to get the enhanced propulsion parameters.

Energy and wavelength of laser Propellants Enhancement methods Enhanced values of parameters (Cm, Isp) Reference
1064 nm Nd:YAG  Zinc oxide  Different morphological structures  2.5 × 10−4 Ns/J, 560 s  19  
1064 nm Nd:YAG  Ferrites  CAR  3.27 × 10−5 NS/J  20  
532 nm Nd:YAG  Co0.5Ni0.5 Fe2O4  Glass cavities  2.68 × 10−4 N s/J  21  
532 nm laser  —  Prepulse method with controlled delay time  5.8 × 10−5 N/W  28  
200 × 10−3 J laser  Polymide film  Carbon contents  263 dyn/W, 1520 s  29  
532 nm laser  Aluminum  Water layers as confining media in Al cavities  250 dyn/W  30  
532 nm ultrafast laser  Copper  Single, double and triple ultra-short pulses  Thrust enhanced twice with double pulse  31  
Nd:YAG laser operating at 532 nm  Silver and aluminum  External semi-elliptical cavities  5.01 × 10−5 N s/J to 1.75 × 10−4 for silver 3.4 × 10−5 N s/J to 8.9 × 10−5 N s/J for Aluminum 3.16 × 103 s for silver 2.02 × 103 s for Al  32  
Nd:YAG laser operating at 1064 nm  Aluminum, silver, gold in the form of foils  Semi-spherical cavities above sample  39.6 dyn/W for Al, 30.6 (dyn/W) for Ag 34.5 (dyn/W) for Au 40 000 s  33  
Nd:YAG laser operating at 1064 nm  Pure silicon, P-type semiconductor  Cavity aspects ratio with different diameters and depths  Cm for pure si = 6.6292 × 10−4 N s/J Cm for p-type = 4.3268 × 10−4 N s/J Isp for pure silicon = 64.86 s Isp for p-type = 48.76 s  34  
Energy and wavelength of laser Propellants Enhancement methods Enhanced values of parameters (Cm, Isp) Reference
1064 nm Nd:YAG  Zinc oxide  Different morphological structures  2.5 × 10−4 Ns/J, 560 s  19  
1064 nm Nd:YAG  Ferrites  CAR  3.27 × 10−5 NS/J  20  
532 nm Nd:YAG  Co0.5Ni0.5 Fe2O4  Glass cavities  2.68 × 10−4 N s/J  21  
532 nm laser  —  Prepulse method with controlled delay time  5.8 × 10−5 N/W  28  
200 × 10−3 J laser  Polymide film  Carbon contents  263 dyn/W, 1520 s  29  
532 nm laser  Aluminum  Water layers as confining media in Al cavities  250 dyn/W  30  
532 nm ultrafast laser  Copper  Single, double and triple ultra-short pulses  Thrust enhanced twice with double pulse  31  
Nd:YAG laser operating at 532 nm  Silver and aluminum  External semi-elliptical cavities  5.01 × 10−5 N s/J to 1.75 × 10−4 for silver 3.4 × 10−5 N s/J to 8.9 × 10−5 N s/J for Aluminum 3.16 × 103 s for silver 2.02 × 103 s for Al  32  
Nd:YAG laser operating at 1064 nm  Aluminum, silver, gold in the form of foils  Semi-spherical cavities above sample  39.6 dyn/W for Al, 30.6 (dyn/W) for Ag 34.5 (dyn/W) for Au 40 000 s  33  
Nd:YAG laser operating at 1064 nm  Pure silicon, P-type semiconductor  Cavity aspects ratio with different diameters and depths  Cm for pure si = 6.6292 × 10−4 N s/J Cm for p-type = 4.3268 × 10−4 N s/J Isp for pure silicon = 64.86 s Isp for p-type = 48.76 s  34  

Laser ablation propulsion has better electrical efficiency as compared to other propulsion techniques. In ALP, constant momentum and exhaust velocity improve the efficiency of flight with variable specific impulse, which is difficult in chemical jets. The main limitation of this technique is that only 40%–60% of laser electrical efficiency can be utilized meaning the newton level thrust is still not achieved. A micro laser plasma thruster was the first practical demonstration of laser propulsion for small satellites in space technology.51,52,70 Station keeping and reshaping of orbits were important and among major flight missions for the space engines because it was seen that it is more convenient and reliable to manufacture clusters of small satellites as compared to larger thrusters. Numerous usable materials for laser thrusters can be found in space junk, abandoned satellites, and meteorites. In contrast to conventional spacecraft engines, which are typically fed with a highly particular substance in a certain method, laser impact can be carried out on almost any material, resulting in creation of reactive plasma jets. Adjusting the performance of a laser thruster is rather easy by adjusting the pulse repetition rate and focal spot size. Irradiation schedules need to be changed to avoid target destruction and formation of tiny debris. Since mass utilization really works against the addition of more mass, it makes sense to draw on experience that the electric propulsion technique has earned much attraction. The use of DPSS lasers, 3D scanning systems, object capture devices, and other technologies in space ensures technical feasibility.16 One of the best uses of laser propulsion in space is the elimination of space trash. Debris is defined as objects with a mass more than a few kilograms that can move faster than 10 km/s in space and endanger satellites and thrusters. As opposed to ground-based laser systems, onboard laser propulsion systems are more frequently used for debris removal. For aerospace purposes, spaceships must burn all of their propellants; hence, a high amount of particular impulse is required to increase their lifespan. After the introduction of fiber lasers, laser propulsion has a wide variety of applications on the ground too. In laser ablation propulsion for ground-based applications, material is removed from the back surface of vehicles. A laser thruster named LDU-7 was designed for space flight missions and experiments from Bauman Moscow State technical University. The general layout of the LDU-7 is represented in Fig. 7.55 

FIG. 7.

LDU-7 general layout. Fiber coupled laser module: 1.1 Diode laser, 1.2 Peltier cooling of laser diode, 1.3 Laser diode temperature sensor, 1.4 Laser output control photodiode, 1.5 Ball lens, 1.6 Optical fiber, 1.7 Optical fiber plug; 2. Working medium and supply system; 3. Base plate coupled to satellite thermal control; Power supply and conditioning: 4.1 DC-DC converter, 4.2 Laser diode driver; 5 Laser radiation blind photodiode for plasma detection; Plasma chamber: 6.1 Interaction chamber; 6.2 Conical nozzle. [Reproduced with permission from Gurin et al., Opt. Laser Technol., 120(May), 1–6. (2019). Copyright 2019 Elsevier.]

FIG. 7.

LDU-7 general layout. Fiber coupled laser module: 1.1 Diode laser, 1.2 Peltier cooling of laser diode, 1.3 Laser diode temperature sensor, 1.4 Laser output control photodiode, 1.5 Ball lens, 1.6 Optical fiber, 1.7 Optical fiber plug; 2. Working medium and supply system; 3. Base plate coupled to satellite thermal control; Power supply and conditioning: 4.1 DC-DC converter, 4.2 Laser diode driver; 5 Laser radiation blind photodiode for plasma detection; Plasma chamber: 6.1 Interaction chamber; 6.2 Conical nozzle. [Reproduced with permission from Gurin et al., Opt. Laser Technol., 120(May), 1–6. (2019). Copyright 2019 Elsevier.]

Close modal

The following operations were carried out for the design of laser thrusters: (a) study of operational features of optical, mechanical, and electronic elements and systems of laser thrusters in LEO up to 600 km range. (b) Attitude control performance, thrust performance, and evaluation of power. By the end of 2013, LDU-7 had successfully completed all necessary ground testing to be qualified as an experimental payload for a space trip. On 28 November 2017, “Soyuz 2.1b” launched “Baumanets-2” from the brand-new Russian spaceport Vostochny as a piggyback payload. Unfortunately, the “Fregat” upper stage failed after it reached 196 km apogee non-closed orbit, causing all 19 of the payload's satellites to crash into the Atlantic Ocean. The thrusters’ design, performance, and testing plans have been experimentally explained in Ref. 55. The designing of space systems that enable the deorbiting of upper rocket parts and the transfer of large-size satellite remnants from orbits that are commercially and scientifically beneficial to disposal orbits is taken into consideration in order to lessen the number of these large-sized objects. The proposed system relies on strong laser propulsion. The problem of reducing large-scale human-made space trash in LEO is thought to be applied to the laser-orbital transfer vehicle (LOTV) with the proposed aerospace laser propulsion engine. For moving large-size trash objects from LEO to geostationary equatorial orbit (GEO) or toward a “liberation point” in the Earth-Moon system, a LOTV is viewed as a type of space tugboat. Because of this, the vehicle is anticipated to have a significant inertial mass of around 10 T. To provide a continuous mode of the thrust production independent of atmosphere conditions, an air-born laser with a power of 500 kW is assumed to be used.74 To define the laser energy consumption for the LOTV orbital mission, a parameter such as expenditure is defined and the laser energy applied to transfer a unit mass (kilogram) of the LOTV payload up to a high orbit. To analyze the space maneuver of an LOTV, the transfer scenario of the vehicle within a coplanar trajectory was considered.74 A higher specific impulse compared to other traditional propulsion methods is the benefit of laser ablation propulsion; however, a lightweight, high power laser is needed for this. A novel idea for laser-augmented chemical propulsion (LACP) is put forth, which uses a low-power laser to simply turn on and off thrust. The solid propellant combustion under thermal radiation, in which the solid propellant burning rate varies linearly on the radiation strength of a continuous wave (CW) laser laid the basis for the laser-augmented chemical propulsion concept. For the laser-augmented chemical thrusts, certain photosensitive and lower energetic propellants are used, such as carbamide, ammonium nitrate, guanidine nitrate, and 5-aminotetrazole, which burn when exposed to laser radiation but stop burning when the laser is turned off. The propulsion energy comes from chemical reaction heat and laser energy. The feasibility and ballistics of laser-augmented chemical propulsion are discussed in experimental and theoretical analysis.75 One of the advantages of laser ablation propulsions is its high specific impulse, but the bottleneck technology is in light, miniaturized, and high-power lasers. If a ground-based laser is used to launch vehicles, the problems of size, weight, and power for the laser system are easy to overcome, but a small-size, lightweight, and high-power laser is required in space-based laser propulsion and onboard laser propulsion. LACP is a new concept to overcome the disadvantage of laser ablation propulsion. LACP takes the principle of laser-controlled combustion and not the principle of laser ablation, in which propulsion energy comes from chemical energy and laser energy, indicating that LACP does not require a high-power laser and can easily vary thrust.

However, there are challenges available for practical implementations of this technique including several factors. One of the many difficulties in implementing ablative laser propulsion for aircraft applications is the power demand. Large energy sources are needed to produce high-power laser beams that can reach space debris and change its orbit. It is difficult to guarantee such power sources’ availability in space while preserving operational effectiveness. Another technical challenge is precision targeting. Accurate tracking, aiming, and focusing systems are necessary for lasers to target space debris precisely. It is technically challenging to target objects traveling at high speeds in different orbits with the necessary precision. On the other hand, laser beams from ground-based systems may be affected by air interference, such as scattering or absorption, when aiming for space debris. The efficiency and precision of the lasers may be compromised by this interference. Laser beams must have enough coverage and range to reach objects at various altitudes and orbits. A major technical difficulty is ensuring that the lasers have the requisite reach to target a variety of space debris efficiently. High-energy laser use in space necessitates adherence to diplomatic agreements, safety procedures, and international laws. It is vital to prevent interfering with other space activities and to guarantee the security of operating staff and other spacecraft. Furthermore, building and implementing a network of laser systems in orbit or on Earth demands a large infrastructure and financial commitment. One problem is the cost of developing, maintaining, and running such systems. It is a continuous struggle to advance the technology for ablative laser propulsion to be safely and reliably deployed for useful aerospace applications. It takes a lot of research and development to create the required hardware, software, and control systems. To solve these issues and make ablative laser propulsion a practical option for space debris clearance and other aerospace uses, coordinated efforts in research, technological innovation, regulatory frameworks, and international collaboration are needed.

Among the various uses of lasers, one noteworthy application is its ability to propel distant objects via ALP. Ablation depends on various factors like pulse duration, momentum coupling coefficient, fluence, and specific impulse and also varies with the material being ablated. Different techniques can be used to find the value of these parameters. Various research studies are focused on finding the best propellant by using different kinds of materials as propellants like liquid propellants, polymer propellants, etc. In this study, we compiled the results of these research studies, which show that prolusion greatly depends on the pulsed laser and a propellant and can be enhanced if confinement is applied. The study discusses various techniques used over the years and different materials studied for ALP, which can help researchers in the field to choose the best material along with the technique to achieve optimized results.

Yasir Jamil is thankful to the Higher Education Commission of Pakistan for providing financial support under Grant No. HEC-NRPU-6409.

The authors have no conflicts to disclose.

Ayesha Abbas: Conceptualization (equal); Data curation (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Syeda Tehreem Iqbal: Data curation (equal); Validation (equal); Writing – review & editing (equal). Yasir Jamil: Project administration (equal); Resources (equal); Supervision (equal); Validation (equal).

The data that support the findings of this study are available within the article.

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