Measurements of electron-ion collision rates and Rydberg atom populations in ultracold plasmas by using short electric field pulses

2017 Fall.Includes bibliographical references.Ultracold plasmas are good tools for studying fundamental plasma physics. In particular, these plasmas are well-suited to study so-called strong coupling physics the physics of plasmas where nearest-neighbor Coulomb interactions become large enough to cause spatial correlations and break assumptions. An ultracold plasma makes such a good tool because it is it is free of interactions with neutral atoms, and has a well controlled and tunable initial conditions. The UCPs in this work were created from the photoionization of cold 85Rb atoms. The experiments described in this thesis are focused on the measurements of damping of electron center-of-mass oscillations. We developed a method that uses two short electric field pulses to map the temporal profile of the oscillation amplitude. We found that the damping of such oscillations can result from dephasing which is a collisionless mechanism or from electron-ion collisions or a combination of both. Thus, we separate the study of two pulse measurements into two parts. The first part of the two short electric field pulse measurement is about the measurements and modeling of in the collisionless damping regime. The second part will focus on the regime where the damping is dominated by electron-ion collisions where we not only observed strong coupling influence on electron-ion collision rates, but also saw break down of one or more standard assumptions used in plasma physics calculations. Rydberg atoms can be formed in ultracold plasmas through three-body recombination process. Our setup was capable of measuring Rydberg atoms in a energy range above the bottleneck energy. We measured the Rydberg populations at different temperatures, and our preliminary results agree well with a parameter-free calculation. However, there are some unexplained parts of our measurements on early time Rydberg populations. This means more studies are needed in the future in order to interpret our results and make use of them. Future work includes measurements of the strong coupling influence on electron-ion collision rates in a magnetized ultracold plasma, measurement of Rydberg population below the bottleneck energy, a detailed study of evaporations in ultracold plasmas

Imaging reconstruction of a buried imperfect conductor by the genetic algorithm

[[abstract]]The problem of determining the shape of an imperfectly conducting cylinder buried in a half-space by the genetic algorithm is investigated. It ia assumed that a conducting cylinder of unknown shape and conductivity is buried in one half-space and scatters the field incident from another half-space where the scattered field is measured. Based on the boundary condition and the measured scattered field, a set of nonlinear integral equations is derived and the imaging problem is reformulated into an optimization problem.[[notice]]需補地點及國別[[conferencetype]]國際[[conferencedate]]20000815~2000081

Neural Discrete Representation Learning

Learning useful representations without supervision remains a key challenge in machine learning. In this paper, we propose a simple yet powerful generative model that learns such discrete representations. Our model, the Vector Quantised-Variational AutoEncoder (VQ-VAE), differs from VAEs in two key ways: the encoder network outputs discrete, rather than continuous, codes; and the prior is learnt rather than static. In order to learn a discrete latent representation, we incorporate ideas from vector quantisation (VQ). Using the VQ method allows the model to circumvent issues of "posterior collapse" -- where the latents are ignored when they are paired with a powerful autoregressive decoder -- typically observed in the VAE framework. Pairing these representations with an autoregressive prior, the model can generate high quality images, videos, and speech as well as doing high quality speaker conversion and unsupervised learning of phonemes, providing further evidence of the utility of the learnt representations

Colour Design for Carton-Packed Fruit Juice Packages

The present research studies the relationships between observers’ expectations for 7 fruit juice packages and the colour design of the package. To do this, a two-stage experiment was conducted. At the first stage, we studied perceived colours for the fruit images shown on each package. At the second stage, fruit juice packages with 20 package colours were rated using 5 bipolar scales: colour harmony, preference, freshness, naturalness and product quality. The experimental results show that the observers tended to perceive fruit image colours lighter and more saturated than those measured using colour measuring instruments. Using factor analyses, we classified the 5 bipolar scales into 2 factors: Product Preference and Freshness. Package colour design was found to have significant impacts on both factors: similarity in chroma and hue between package colour and perceived fruit colour would lead to high product expectations. Keywords: colour design; colour harmony; product expectation; perceived image colour</p

Inverse scattering of a buried imperfect conductor

[[abstract]]The problem of determining the shape of an imperfectly conducting cylinder buried in a half-space by a genetic algorithm is investigated. Assume that a conducting cylinder of unknown shape and conductivity is buried in one half-space and scatters the field incident from another half-space where the scattered field is measured. Based on the boundary condition and the measured scattered field, a set of nonlinear integral equations is derived and the imaging problem is reformulated into an optimization problem[[conferencetype]]國際[[conferencedate]]20001003~20001006[[conferencelocation]]Tbilisi, Georgi