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

Abstract

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.