Laser-driven In-tube Accelerator Research Articles

Moderate-Acceleration Launch Using Repetitive-Pulse Laser Ablation in a Tube

Projectile launch experiments were conducted using a new laser-driven, in-tube accelerator (LITA) in which the propellant was supplied not as pre-filled gas but from laser ablation. The launch tube had an inner diameter of 25 mm and was 1 m in length, made of acrylic. The 7.1-gram projectile had a parabolic nozzle and an ablator rod made of polyacetal that was located at the focus of the parabola. Laser pulses of 3.1 J to 3.8 were repetitively sent at a frequency of 50 Hz from a Transversely-Excited Atmospheric (TEA) CO2 laser (wavelength; 10.6 μm). Time-modulating the laser-ablative pressure, a large impulse performance was obtained with a moderate acceleration level. A momentum coupling coefficient of 1.5 mNs/J and a gross acceleration of 2.5g were obtained.

Impulse Generation Mechanisms in a Laser-Driven In-Tube Accelerator

To enhance laser-propulsion thrust performance, a unique Laser-driven In-Tube Accelerator (LITA) has been developed. This paper numerically analyzes the impulse generation mechanisms in LITA. For this purpose, a LITA performance experiment was conducted in atmospheric air with a projectile installed on a ballistic pendulum to calibrate the numerical approximations. We conducted experimental flow visualization by framing shadowgraph and computational fluid dynamics solving the axi-symmetric Euler equation applied to an ideal gas. The results show that a laser-driven blast wave is generated by a spherical hot gas core where the supplied laser energy is absorbed first. The effect of confinement by the tube or shroud wall is confirmed. The impulse production is established not only from the interaction between the incident blast wave and projectile, but also from the following repetitive pressure waves. Assuming that about 30% of the input laser energy is absorbed by the working air, both the impulse and peak pressure agrees quantitatively between the experiment and numerical simulation.

Impulse Dependence on Propellant Condition in a Laser-Driven In-Tube Accelerator

The laser-driven in-tube accelerator is a unique laser propulsion device, in which a projectile is accelerated in a launch tube with the power of repetitive laser pulses. The impulse is enhanced owing to a confinement effect, and the type and fill pressure of the propellant gas are tunable to the thrust performance. We have conducted operation experiments using a 25-mm-bore launch tube with a laser energy from 2.4 to 3.5 J/pulse at a laser-pulse repetition frequency of up to 70 Hz. The 3-gram projectile has a centerbody; as a parabolic mirror, its base focuses an incident laser beam to a spot on the center axis. Three monoatomic gases (i.e., argon, krypton and xenon) are examined as the propellant. The momentum coupling coefficient is measured from a condition that the time-averaged impulse is balanced with the gravitational force onto the projectile. The measured impulse characteristics are analyzed by referring to scaling relations that are obtained from dimensional analysis and pressure histories measured for the launch tube. In accordance with the dependence of the impulse generation period, the measured impulse is in inverse proportion to the propellant speed of sound. For a fill pressure up to about 100 kPa, the impulse sharply increases as the fill pressure increases, while it becomes saturated for higher pressures. Those performance characteristics are analyzed by estimating the overpressure level and the duration time of the impulse generated by the laser-driven blast wave.

Impulse-scaling in a laser-driven in-tube accelerator

The laser-driven in-tube accelerator (LITA) is a unique device for laser propulsion. It is characterized by the acceleration of a projectile in a tube. The thrust performance can be improved by exploiting a confinement effect. In the experiment, a 3.0-g projectile is vertically launched, and the momentum coupling coefficient is measured for various monoatomic gases. The measured coupling coefficient is almost proportional to the reciprocal of the speed of sound. The same impulse generation characteristics are obtained in simplified situations that are analyzed based on conservation relations.

Laser-driven in-tube accelerator

A device of accelerating a projectile using laser power, “laser-driven in-tube accelerator,” has been developed. It is characterized by accelerating the projectile in a tube and by supplying the laser beam from the direction opposite to the projectile motion. The laser beam is incident on the nose of the projectile, reflected first on the nose and second on the inner wall of the projectile shroud, then focused behind. Operation performance is measured using a CO2 transversely excited atmospheric laser of a nominal output energy of 5 J/pulse. A coupling coefficient of 73 N/MW is obtained using the atmospheric air as the working fluid.