Micropropulsion Using a Laser Ablation Jet

Abstract

We investigated exothermic laser ablation "fuels" for the micro Laser Plasma Thruster (<em>μ</em>LPT), a novel type of microthruster. Using ms-duration laser pulses, which are required for multi-mode laser diodes to exceed ablation threshold fluence in the smallest focal spots available with conventional optics, successful target materials were restricted to those of low thermal conductivity, i.e., polymers, not metals. Polymers studied included carbon-doped Polyvinylchloride for a passive target baseline, and several carbon-doped exothermic photopolymers specifically designed for their task, including polyvinylalcohol, a triazene polymer and a proprietary exothermic polymer (EP). In our single-shot impulse test setup, millimetric fuel samples were evaluated using a tiny torsion pendulum. Promising polymers were then made into fuel tapes and tested for continuous thrust under repetitive-pulse excitation. Two-layer fuel tapes consisted of a transparent supporting layer through which the light passed to ignite an absorbing fuel layer which formed a jet on the opposite side of the tape from that illuminated by the laser, an example of confined ablation. Best results were obtained with EP-I up to 680 <em>μ</em>N thrust with 2.1 W average optical power incident and jet velocity of 2-3 km/s. Repeatability of our thrust-measuring torsion pendulum was improved to 1<em> μ</em>N.