Laser Ablation Propulsion: Synthesis and Analysis of Materials and Impulse Measurements

Among the many possible applications of laser ablation one of the more recent taken is related to nanosatellites propulsion. The study of Laser Ablation Propulsion (LAP) requires research activity on different fields like high power pulsed lasers, laser ablation itself, because it is still a problem to relate the well known mechanisms to impulse generation, and finally materials that represent the fuel in LAP. This thesis presents a research activity on LAP from its very beginning, with the development of an experimental apparatus to measure laser generated impulse and the first results on metals and polymers that paves the way to the development of future LAP materials. Chapter 1 presents an overview of the actual situation of space economy and its recent fast evolution that led in the last years to the exploitation of space for many different applications, also by private companies. The so called New Space Economy is the background on which LAP develops, as an attractive propulsion technique for nano satellites, nowadays extremely diffused in all kind of space missions, and as a possible solution for the space debris problem. In this Chapter typical results obtained in LAP are also reviewed and compared with other solutions both for space debris and propulsion, in order to obtain a better image of its applicability range. Chapter 2 deals with laser ablation. Initially the parameters that play a role in laser ablation are discussed, in particular those related to the laser source like wavelength, pulse duration and repetition rate, to give an overview of the experimental conditions involved. Then general phenomenological observations on laser ablation are presented and related to the physical mechanisms involved, both in the case of metals and polymers, highlighting the main differences between these two classes of materials. The experimental part of this thesis starts in Chapter 3, with the description of the experimental apparatus developed to measure the laser generated mechanical impulses in the order of uN s. The different strategies to perform this kind of measurements are reviewed and compared to the one adopted in this work, based on a ballistic pendulum, and main advantages and problems are discussed. A technical description of the apparatus is given, focusing in particular on all the precautions that have been taken in order to let the pendulum operate in as ideal as possible conditions. The measurement procedure developed during this work is then described in detail, by discussing data analysis and showing some examples. Chapter 4 also deals with the development of the apparatus, in particular for what concerns the estimation of the laser energy density that reaches the target material (fluence), a fundamental parameter for LAP measurements. Some measurements on metals are also presented here in order to discuss some features related to the measurements of some common LAP parameters. Chapter 5 and 6 deal with LAP using polymers, and in particular with experiments devoted to the understanding of material properties that mainly affect LAP performances. The starting material chosen for these experiments is poly(vinyl chloride)(PVC), a benchmark in LAP experiments. Chapter 5 compares localized or uniform laser absorption by PVC, that can be obtained respectively by including carbon nanoparticles in the polymer matrix or by mixing PVC with an absorbing polymer (poly(styrene sulfonate)). The comparison is carried out from the optical an thermodynamical point of view, along with impulse generation. Specific ablation mechanisms are also discussed, showing that a localized absorption of laser radiation is more energetically efficient for impulse generation. Chapter 6 then continues the work on PVC containing nanoparticles, investigating the role of their size, morphology and concentration in laser ablation and in impulse generation. Both commercial and green produced carbon nanoparticles are used for these experiments showing that, at least in the considered size range, the only parameter that affects laser ablation is the number density of absorption centers in the polymer matrix, and not size or morphology. This points the direction to follow in the development of a polymeric material for LAP applications. Some open problems and future works are presented in Chapter 7. Effects on impulse generated by irradiating multiple times the same region are discussed, showing opposite behaviours between metals and polymers, for which still there is not a clear explanation. Then experimental issues and some results on specific impulse measurements are presented, and difficulties related to this measurement in metals briefly discussed. Finally laser ablation in a confined geometry is considered as an attracting technique to enhance impulse generation. And some results on PVC are shown. As a conclusion, main results obtained in this thesis are highlighted, and possible future research activities, developments and perspectives are discussed.