Interferometric method for specific impulse determination

Abstract

One of the main figures of merit in laser-ablative propulsion is the specific impulse, Isp, defined as the impulse per unit weight of fuel, and it is related to the exhaust velocity, ve, by the acceleration of gravity, Isp = ve/g. Being a key magnitude, Isp needs to be accurately determined. It is usually inferred from other measurable quantities: the impulse coupling coefficient, Cm, defined as the ratio of the target momentum produced to the incident laser pulse energy, and Q∗, the laser energy consumed per unit weight of ablated target material. Thus, Isp is calculated as Isp = CmQ∗/g. However, single pulse ablated mass leading to Q∗ is in the nanogram scale and cannot be directly measured by weighting the targets. So, mass loss measurements are performed by analyzing the volumes of the craters produced by a large number of laser pulses. These procedures lead to larger than desired uncertainties in the Isp values. On the other hand, more precise measurements of Isp can be carried out from the direct measurement of the exhaust velocity of the ejected particles by interferometric methods. In this work, a system based on a Nomarsky interferometer has been set up for the time-resolved diagnostic in the nanometric scale of laser ablation plumes. The performance of the implemented system was first validated by measuring the Isp produced by aluminum targets and solid propellants based on metal/salt mixtures. The Cm dependence on laser parameters and binary composition of these propellants have been determined in previous works with a torsion pendulum and a piezoelectric sensor. Once the interferometer performance is characterized, the Isp produced by solid propellants composed of metal (Zn) and metal oxides (ZnO) matrices doped with nanoparticles of different materials is determined and compared.