Interferometric method for Isp determination

CARLOS A. RINALDI; CINTHYA TORO; AZCÁRATE, M. LAURA

Santa Fe

Congreso; International High Power Laser Ablation; 2018

Photonic Associates, LLC

Oneof the main figures of merit in laser-ablative propulsion is specificimpulse, Isp,defined as the impulse per unit weight of fuel and is related to theexhaust velocity, <ve>,by the acceleration of gravity, Isp= <ve>/g.Being a key magnitude, Ispneeds to be accurately determined. It is usually inferred from othermeasurable quantities: the impulse coupling coefficient, Cm,defined as the ratio of the target momentum produced to the incidentlaser pulse energy and, Q*, the laser energy consumed per unit weightof ablated target material. Thus, Ispis calculated as Isp =CmQ*/g.However, single pulse ablated mass leading to Q* is in the nanogramscale and cannot be directly measured by weighting the targets. So,mass loss measurements are performed by analyzing the volumes of thecraters produced by a large number of laser pulses. These procedureslead to larger than desired uncertainties in the Ispvalues. On the other hand, more precise measurements of Isp can becarried out from the direct measurement of the exhaust velocity ofthe ejected particles by interferometric methods. Inthis work a system based on a Nomarsky interferometer [1] has beenset up for the time resolved diagnostic in the nanometric scale oflaser ablation plumes. The performance of the implemented system wasfirst validated by measuring the Ispproduced by aluminum targets [2] and solid propellants based onmetal/salt mixtures. The Cmdependence on laser parameters and binary composition of thesepropellants had been determined in previous works with a torsionpendulum and a piezoelectric sensor [3-4]. Once the interferomenterperformance had been characterized, the Ispproduced by solid propellants composed of metal (Zn) and metal oxides(ZnO) matrices doped with nanoparticles of different materials havebeen determined and compared.p { margin-bottom: 0.25cm; direction: ltr; line-height: 115%; text-align: left; background: transparent none repeat scroll 0% 0%; }p.western { }