1. Introduction

Various space applications for laser induced ablation impulse, including the laser induced ablation thruster and the laser thrust rocket launcher [1] have been proposed since the 1980s. The efficiency of laser-induced ablation thrust was discussed by Phipps et al. [2], who compiled various data according to the scaling law and expressed the momentum coupling factor as ${C_m} = ({5.6\sim 6.5} )\times {\left( {I\lambda \sqrt \tau } \right)^{ - ({0.25\sim 0.3} )}},$ where I, $\lambda $, and $\tau $ represent the intensity, wave length, and pulse width of the laser, respectively. ${\; }{C_m}$ is the efficiency of converting laser energy to impulse; ${C_m} = {P_a}/I = {\; }{I_m}/{E_L},{\; }$ where ${P_a}$, ${I_m},,$, and ${E_L}$ represent pressure, impulse, and pulse energy, respectively. Space debris can cause serious problems for space activities as the number of satellites increases over time. Remotely deorbiting debris using laser ablation has been proposed via both ground [3,4] and space borne lasers [5,6]. Illumination by a high-power pulsed laser (of a few hundred kilowatts) between a few ten seconds to several minutes has been shown to be sufficient to deorbit 10 cm pieces of debris.

A recent large low Earth orbit (LEO) constellation program used a few thousands of small satellites. Even a low failure rate could cause dozens of satellites to lose control and become debris. As they are located in an operational orbit, they can pose a serious problem. This system can be used to remove them efficiently.

Neodymium-doped yttrium aluminum garnet (Nd:YAG) is the most promising high-power pulse laser for space applications [7–10]. The average power of these applications ranges in the tens of Watts, which is three to four orders of magnitude lower than expected. However, assuming that ${C_m} = 10\; \mu $Ns/J for an Nd:YAG laser, continuous irradiation from a 100 W laser generates a total impulse of $3.2 \times {10^4}$Ns, which can be used to deorbit a 100 kg piece of debris from an altitude of 1000 km to about 500 km, at which point the orbit will decay naturally due to atmospheric drag, and the debris will re-enter after 25 years. To design a deorbit demonstrator mission on board small satellites, ${C_m}$ in vacuum environment must be characterized for various structural materials used in satellites, including the dependence of laser fluence, incident angle, and shape of the engraved crater. The magnitude of LAII in the atmosphere is several times larger than in vacuum [11], where the atmospheric shock wave produced by the ablation plasma plays an important role. The measurement of LAII in vacuum can be an important issue not only for the application of thrust in space, but also for the study of ablation.

Previously, the laser ablation induced impulse (LAII) in the vacuum environment was measured by the response of a torsion-wire suspended pendulum. A high sensitivity of 0.1 μNs was achieved by the long pendulum arm and the low spring constant, with a lengthy response time of several minutes [12,13]. The torsion wire system had a heavy load capacity of several kilograms, which is sufficient to mount a spacecraft component. However, the size is so large that it requires a vacuum chamber several meters across to measure the LAII. Such a system is useful for evaluating thrusting components and systems but is unsuitable system for investigating the interaction between materials and laser radiation. In order to observe a large number of LAII characteristics in vacuum required for mission design, a new LAII measurement system with more compact dimensions, shorter response times, and higher impulse sensitivity than torsion pendulum devices is required.

2. Impulse measurement system

2.1 Impulse measurement stand

A pendulum with a 10 g bob suspended from a 100 mm wire will have a natural frequency ${f_0} = \sqrt {g/l} /2\pi $ of 1.6 Hz and a response amplitude of approximately 0.3 μm for a ${10^{\; - 7}}$Ns impulse. Figure 1(a) illustrates the impulse measurement stand (IMS) based on the concept of this pendulum system. The four suspension wires restrict the motion of the pendulum mass (PM) to one degree of freedom. A small ablation target is mounted on one side of the PM, while the displacement can be measured at the opposite side. Any contactless displacement sensor, either optical or electrical, with a resolution better than 0.1 μm, is sufficient for observation. In this case, a Shinkawa Electric type VC-020C eddy current-type sensor was used. This IMS is highly compact at 150 mm x 150 mm x 150 mm and can easily be installed in small vacuum chamber.

 

Fig. 1. (a) Impulse sensor based on a simple pendulum. (b) Force generated by Eddy current. current.

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One problem with a pendulum operating in vacuum is damping. When the pendulum has small damping, the oscillation continues for several times its natural period, which prevents frequent measurements [13]. An effective solution to damping in vacuum is to use eddy currents.

Figure 1(b) illustrates the effect of an eddy current damping installed in the IMS. The magnetic fields are arranged to be perpendicular to the motion of the PM. The PM moves across the fields, and the eddy current generated by the motion dissipates kinetic energy and generates a drag force proportional to PM velocity and magnetic flux. The damping is adjusted by tuning the magnitude of magnetic flux. The equation of motion for the pendulum is very simple:

 

Fig. 2. IMS single pulse response for 250 mJ of energy.

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The system described by Eq. (1) is a second order lag system. The transfer function is

The sensitivity of the IMS is calibrated by a simple solenoid actuator. A small permanent magnet attached to the PM generates a force proportional to the current in the coreless solenoid. When the solenoid is driven by a DC current, the generated force constant ${c_i}$ is determined by the displacement of the PM according to $F = Mgx/l = {c_i}{I_s},$ where ${I_s}$ represents the current. If the solenoid is driven by a pulse with a width of ${t_p}$ and a current of ${I_s},$ the calibrated impulse can be found using ${I_{ms}} = $ $F{t_p} = {c_i}{I_s}$, and the response of the IMS is calibrated by this impulse. This so-called self-calibrating system is accurate to 0.5% or more.

2.2 Optical system

The optical layout of the impulse measurement in vacuum is shown in Fig. 3(a). A SPECTRA-PHYSICS GCR-230 pulsed Nd:YAG laser is used for the measurements. It has a maximum energy of 1 J and a maximum repetition frequency of 20 Hz. The nearly Gaussian beam profile has a smooth distribution from zero to the peak; therefore the interaction of that beam is observed as a mixture of various radiation intensities. To investigate the radiation interaction, a top-hat profile is preferable. Accordingly, the laser used in this measurement was tuned to a top-hat rather than a Gaussian shape using an aperture that cuts out the central region of the profile. A variable attenuator using a $\lambda /2$ wave plate and a polarizer tunes the irradiation energy over two orders of magnitude without any change in beam profile. A beam expander is used for reducing the fluence to avoid damaging the vacuum windows and the mirror. The beam spot size on the IMS target is tuned by adjusting the expander focus. The high reflection mirror (5) shown in Fig. 3(a) can set the spot position on the target by turning. A camera (11) focused on the target surface serves as an image monitor and can provide a real-time image of the scattered laser on the target as in Fig. 3(b) or the ablating plasma emission with an additional filter to reject the scattered laser as in Fig. 3(c). The image monitoring path has a poor efficiency, due to the low transmittance of the high reflection mirror (5) and the low reflectance of the high reflection mirror (6) for laser and visible light, respectively. The image monitor can provide useful information on the focused position, but as high quality images cannot be obtained, the spot size is measured by a microscope after removal from the vacuum chamber. Figure 3(d), which is SEM observations of the beam mark on the target with different spot sizes, confirms that the spot shape had small change regardless of the spot size.

 

Fig. 3. (a) Optical layout for thrust measurement. (1) Aperture, (2) l/2 Wave plate, (3) Polarizing beam splitter, (4) Beam splitter, (5) (6) High reflection mirror, (7) Beam dumper, (8) Energy meter, (9) Beam expander and focus adjust, (10) Imaging lens, (11) Image monitor, (12) Vacuum chamber with optical ports, and (13) Impulse measurement stand. (b) Scattered laser image. (c) Visible emission image. (d) SEM image of engraved crater.

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3. Impulse measurement on aluminum 7075

Nd:YAG laser ablation was tested on a 7075 aluminum alloy in vacuum. Figure 4 shows the relationship between ${C_m}$ and the laser fluence (J/cm²). The vacuum is maintained at a pressure below $3 \times {10^{ - 2}}$Pa during measurement. The fluence is defined by the laser pulse energy and spot size at the irradiated surface. The beam expander is focused to tune the spot to a size larger than approximately 1 mm in diameter, at which point the wave optic effect and optical aberration are negligible and the spot profile changes similarly. The fluence was primarily scanned by the variable attenuator and secondly changed by the spot size.

 

Fig. 4. Momentum-coupling factor for a 7075 aluminum target under three different conditions. Each data point corresponds to an individual laser pulse. See Data File 1 for underlying values for individual conditions.

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The impulse is measured directly for 1 Hz operation and is calculated from a measured average force for 20 Hz operation. During 1 Hz operation, the pulse width is controlled by the pumping energy of the oscillator. The pulse energy for 20 ns operation is about half that of 10 ns operation. In the case of 20 Hz operation, the thermal lens effect of the Nd:YAG material becomes significant in oscillation, and the outer region is unexpectedly enhanced in the profile. The top-hat shape on the target is maintained by selecting a smaller aperture size to cut the central part of the beam. The spot sizes were selected to be in a range of 0.8 mm to a few millimeters in order to mitigate the effects of the crater engraved by the laser¹²⁾. The aspect ratio (depth to width) should be less than 0.2 at the end of the measurement process.

As the major source of system noise in the measurements is the mechanical vibration of the chamber generated by a turbo molecular vacuum pump, the minor impulse that corresponds to approximately 20 mJ has a small signal-to-noise ratio of about 10. A single scan of a variable attenuator covers 5 times the fluence range. Therefore, data of a condition is typically obtained by stitching together three data points with different spot sizes that overlap by half. In the 20 Hz case, running the average force through the low pass filter described by Eq. (2) improves the signal-to-noise ratio such that ${C_m}$ is detectable in the lower fluence region down to ∼1 J/cm². Stitched data points are seamless and smooth; the fluence can be controlled by varying the spot size without altering the profile.

The data in Fig. 4 shows that the coupling factor ${C_m}$ plateaus at higher fluence regions between 5 to 50 J/cm² and increases to 50% at a fluence of 2.5 J/cm². The generation of impulses for lasers with pulse widths of 10 to 20 ns is well described with fluence, rather than flux as a parameter. It was reported that ${C_m}$ peaks near 10 J/cm² and decreases with higher fluence values due to the plasma shielding effect [13,14]. However, these results may be affected by the laser profile change and the effects of the engraved crater, particularly the profile change due to tuning the pumping energy of the laser. This data shows that the laser profile on the target and the influence of the craters was mitigated in this measurement, and there is no plasma shielding effect. Thus, the plasma density is not high enough to absorb the laser radiation. The ablation of metals with a nano-second pulse laser is modeled such that the free electrons of the metal are heated by radiation, and the energy is transferred to the lattice to evaporate and ionize it in less than 1 ns. Then, the plasma is heated by the radiation through inverse bremsstrahlung. In this case, the reaction occurs less than 10 μm from the ablated surface because the aluminum plasma velocity is on the order of ${10^3}\textrm{m}/\textrm{s}$ for a temperature of approximately ${10^4}\textrm{K}$. The impulse generated by the plasma with a temperature ${T_p}$ and mass ${m_p}$ is described by the relation, ${C_m} = {I_m}/{E_L} \propto {m_p}\sqrt {{T_p}} /{E_L} = \textrm{constant}$ in the current fluence range of 5 to 50 J/cm² and pulse width of 10 to 20 ns.

The 20 Hz operational data present a slightly higher ${C_m}$ than the 1 Hz cases. The ${C_m}$ may not be saturated near 10 J/cm². The significance of these features is not confirmed; however, one possible cause is the temperature of the target. The aluminum target was mounted using polycarbonate washers to reduce the exchange of energy between the target and the PM. The heat diffusion timescale of thickness L, for a diffusion constant ${D_h}$ is ${\tau _h} = {L^2}/{D_h}\; $[15]. As ${\tau _h}$ is approximately $0.1\; \textrm{s}$ for a 3 mm thick 7075 aluminum alloy, a uniform temperature distribution is achieved at the successive 20 Hz laser pulse. During the 20 Hz measurement, a total of 660 J of energy was irradiated onto the target in 22 s. As the processed aluminum alloy has a reflectance of 75% [16], the 1.25 g target absorbed a maximum of 165 J. The target temperature may be increased to more than 100 Kelvin$,$ if 30% of the absorbed energy is used for ablation. The higher target temperature may decrease the energy required to generate the initial plasma and increase the ablated mass and ${C_m}.$ In the 1 Hz measurement cases, the temperature increased by only a few tens K, and the effect was not detectable because a total energy of 64 J was radiated in 300 s.

4. Conclusion

According to the ${C_m}$ characteristics, the following mission may be designed assuming a satellite primarily constructed of an aluminum alloy. The laser furnished chaser satellite (CS) rendezvouses and continuously irradiates a pulsed laser on the target satellite (TS) for one year. An average laser power of 100 W, e.g., 1 J at 100 Hz, can generate a 2 mN thrust and a total velocity change (${\Delta \textrm{V}}$) of 420 m/s for a 150 kg TS. This ${\Delta \textrm{V}}$ is enough for the orbit of a TS at a 1200 km altitude to decay to 500 km. In order to produce enough thrust, the laser fluence should be kept above 5 J/cm² during operation. Assuming a pulse energy of 1 J, the beam spot should be less than 3.6 mm in diameter. The diameter of the laser transmitting optics D is obtained by the relation $D/d = 3.8 \times {10^{ - 4}}{M^2}$, where d is the distance between CS and TS and ${M^2}$ is the beam quality parameter for the laser. This makes for an optic diameter under 10 cm and a rendezvous distance of 100 m for a ${M^2} = 2,$ 1J, 100 Hz laser, which is a reasonable package to mount onboard a small satellite

The new LAII measurement system has a high resolution of 0.1 μNs is very compact for easy use in a vacuum chamber. It can accurately measure various materials commonly found on spacecrafts such as CFRP, GFRP, polyimide, urethane, and epoxy resin. In addition, the new measurement method provides insights into the modification of existing plasma shielding models for aluminum ablation.