Thrust Measurement for Laser-Detonation Propulsion with a Solid-State Laser

I N LASER-detonation propulsion [1], a thruster is designed to use the atmosphere as propellant during its atmospheric flight, which enables a considerable increase in the payload fraction. High-power laser pulses irradiated from a ground-based laser facility are concentrated to induce explosions at the thruster bottom, thereby producing a propulsive impulse. Selection of the laser is an important issue in laser-detonation propulsion. As estimated, for launching a nanosatellitewith amass of several kilograms into low earth orbit (LEO), a high-powered laser with 1 MW average output must be used. Earlier studies have specifically examined using a transversely excited atmosphericpressure (TEA) CO2 laser as a power source [2,3]. The measured momentum-coupling coefficient Cm, defined as the ratio of the obtained propulsive impulses I to the input laser energy Ei, was around 0.3–0.4 mN · s∕J with various thruster nozzles [2,4,5]. However, recent progress made in high-power solid-state laser technology makes it a possible alternative to achieve mission requirements. As demonstrated at the National Ignition Facility, megajoule-level pulse energy has been achieved [6]. In addition, the working mechanism of solid-state laser makes it more feasible to use a group of lasers for use in combination with good synchronicity in a repetitive pulsed laser-propulsion launch mission than using a TEA-CO2 laser. The major difference between these two lasers is the laser energyabsorption coefficient, which is proportional to the cube of the laser wavelength λ; λ 10.6 μm for TEA-CO2 laser and 1.053 μm for a neodymium glass (Nd:glass) laser. The author’s earlier work [7] demonstrated that, irrespective of the wavelength difference, the energy-conversion efficiency ηbw (equal to Ebw∕Ei) from the irradiated laser energyEi to the induced blast-wave energyEbwwith a solid-state laser approximates that measured in the CO2 laser experiments [8], as shown in Table 1. Here, the blast wave energy is defined as the source energy that is necessary to drive an equivalent blast wave in a calorically perfect gas. This result has laid the foundation for the application of a solid-state laser to the future launch of laser-detonation thrusters. This Note describes laser-propulsion impulse measurements conducted using a solid-state laser. The thrust performance and its dependence on the nozzle scale are discussed in comparison with those in the CO2 laser experiments. The influence of the focus position was also investigated.