Validation of a Lagrangian model for laser-induced fluorescence

Abstract

To my parentsii v Since man is not born with knowledge, who can be without doubts? If one has doubts and is not willing to learn from a teacher, his doubts will never be resolved. Those who were born before me must have learned the Tao earlier than me, and I take them as my teachers. Those who were born after me may also have learned the Tao earlier than me, and I take them as my teachers. Since the Tao is what I desire to learn, why should I care if my teachers were born before or after me? Therefore, whether a man is noble or lowly, old or young, where there is the Tao, there is my teacher.vi ABSTRACT Extensive information can be obtained on wave-particle interactions and wave fields by direct measurement of perturbed ion distribution functions using laserinduced fluorescence (LIF). For practical purposes, LIF is frequently performed on metastable states that are produced from neutral gas particles and ions in other electronic states. If the laser intensity is increased to obtain a better LIF signal, then optical pumping can produce systematic effects depending on the collision rates which control metastable population and lifetime. We numerically simulate the ion velocity distribution measurement and wave-detection process using a Lagrangian model for the LIF signal. The simulations show that optical pumping broadening affects the ion velocity distribution function (IVDF) f 0 (v) and its first-order perturbation f 1 (v, t) when laser intensity is increased above a certain level. The results also suggest that ion temperature measurements are only accurate when the metastable ions can live longer than the ion-ion collision mean free time. For the purposes of wave detection, the wave period has to be significantly shorter than the lifetime of metastable ions for a direct interpretation. Experiments are carried out to study the optical pumping broadening and metastable lifetime effects, and the results are compared with the simulation in order to validate the Lagrangian model for LIF. It is more generally true that metastable ions may be viewed as test-particles. As long as an appropriate model is available, LIF can be extended to a range of environments. vii PUBLIC ABSTRACTIn laser-induced fluorescence (LIF), measurements of spontaneous light emission after laser excitation of plasma particles provides useful insight into some its qualities, like its ion temperature and wave-particle interaction dynamics.Usually physicists analyze LIF results with the fixed coordinate-based Eulerian approach. But in this thesis, we apply a Lagrangian approach to LIF analysis using newly written code to follow each individual ion orbit. We found the Lagrangian model, with which we numerically simulated the ion velocity and wave-detection process, is more efficient and revealing than the Eulerian approach. It computationally allows a separation of the quantized description of ionic states from the classical dynamics of the ion center of mass. Also, some key calculations can be done by hand and included readily, leading to large tim...