Photovoltaic power systems workshop

Discussions are presented on apparent deficiencies in NASA planning and technology development relating to a standard power module (25-35 kW) and to future photovoltaic power systems in general. Topics of discussion consider the following: (1) adequate studies on power systems; (2) whether a standard power system module should be developed from a standard spacecraft; (3) identification of proper approaches to cost reduction; (4) energy storage avoidance; (5) attitude control; (6) thermal effects of heat rejection on solar array configuration stability; (7) assembly of large power systems in space; and (8) factoring terrestrial photovoltaic work into space power systems for possible payoff

A Survey on Compiler Autotuning using Machine Learning

Since the mid-1990s, researchers have been trying to use machine-learning based approaches to solve a number of different compiler optimization problems. These techniques primarily enhance the quality of the obtained results and, more importantly, make it feasible to tackle two main compiler optimization problems: optimization selection (choosing which optimizations to apply) and phase-ordering (choosing the order of applying optimizations). The compiler optimization space continues to grow due to the advancement of applications, increasing number of compiler optimizations, and new target architectures. Generic optimization passes in compilers cannot fully leverage newly introduced optimizations and, therefore, cannot keep up with the pace of increasing options. This survey summarizes and classifies the recent advances in using machine learning for the compiler optimization field, particularly on the two major problems of (1) selecting the best optimizations and (2) the phase-ordering of optimizations. The survey highlights the approaches taken so far, the obtained results, the fine-grain classification among different approaches and finally, the influential papers of the field.Comment: version 5.0 (updated on September 2018)- Preprint Version For our Accepted Journal @ ACM CSUR 2018 (42 pages) - This survey will be updated quarterly here (Send me your new published papers to be added in the subsequent version) History: Received November 2016; Revised August 2017; Revised February 2018; Accepted March 2018

Controlling Condensate Collapse and Expansion with an Optical Feshbach Resonance

We demonstrate control of the collapse and expansion of an 88Sr Bose-Einstein condensate using an optical Feshbach resonance (OFR) near the 1S0-3P1 intercombination transition at 689 nm. Significant changes in dynamics are caused by modifications of scattering length by up to +- ?10a_bg, where the background scattering length of 88Sr is a_bg = -2a0 (1a0 = 0.053 nm). Changes in scattering length are monitored through changes in the size of the condensate after a time-of-flight measurement. Because the background scattering length is close to zero, blue detuning of the OFR laser with respect to a photoassociative resonance leads to increased interaction energy and a faster condensate expansion, whereas red detuning triggers a collapse of the condensate. The results are modeled with the time-dependent nonlinear Gross-Pitaevskii equation.Comment: 5 pages, 3 figure

On the asymptotic flux of ultrapermeable seawater reverse osmosis membranes due to concentration polarisation

Just as thermodynamic considerations impose a finite limit on the energy requirements of reverse osmosis, concentration polarisation imposes a finite limit on flux, or equivalently, on system size. In the limit of infinite permeability, we show the limiting flux to be linearly dependent on the mass transfer coefficient and show this to be true for low recovery systems just as well as moderate and high recovery single stage and batch reverse osmosis system designs. At low recovery, the limiting flux depends on the logarithm of the ratio of hydraulic to bulk osmotic pressure and at moderate or higher recovery, the relationship with this pressure ratio is a little more complex but nonetheless can be expressed as an explicit analytical formula. For a single stage seawater reverse osmosis system operating at a hydraulic pressure, recovery ratio, and value of mass transfer coefficient that are typical today, the flux asymptote is roughly 60 L m[superscript −2] h[superscript −1] – roughly four times where average fluxes in seawater reverse osmosis systems currently stand

On the potential of forward osmosis to energetically outperform reverse osmosis desalination

We provide a comparison of the theoretical and actual energy requirements of forward osmosis and reverse osmosis seawater desalination. We argue that reverse osmosis is significantly more energy efficient and that forward osmosis research efforts would best be fully oriented towards alternate applications. The underlying reason for the inefficiency of forward osmosis is the draw-dilution step, which increases the theoretical and actual energy requirements for draw regeneration. As a consequence, for a forward osmosis technology to compete with reverse osmosis, the regeneration process must be significantly more efficient than reverse osmosis. However, even considering the optimisation of the draw solution and the benefits of reduced fouling during regeneration, the efficiency of an optimal draw regeneration process and of reverse osmosis are unlikely to differ significantly, meaning the energy efficiency of direct desalination with reverse osmosis is likely to be superior

Electron Temperature Evolution in Expanding Ultracold Neutral Plasmas

We have used the free expansion of ultracold neutral plasmas as a time-resolved probe of electron temperature. A combination of experimental measurements of the ion expansion velocity and numerical simulations characterize the crossover from an elastic-collision regime at low initial Gamma_e, which is dominated by adiabatic cooling of the electrons, to the regime of high Gamma_e in which inelastic processes drastically heat the electrons. We identify the time scales and relative contributions of various processes, and experimentally show the importance of radiative decay and disorder-induced electron heating for the first time in ultracold neutral plasmas

Ultracold Neutral Plasmas

Ultracold neutral plasmas are formed by photoionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1-1000 K and the ion temperature is around 1 K. The density can approach $10^{11}$ cm$^{-3}$. Fundamental interest stems from the possibility of creating strongly-coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr$^+$ ${^2S\_{1/2}} -> {^2P\_{1/2}}$ transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

High Resolution Ionization of Ultracold Neutral Plasmas

Collective effects, such as waves and instabilities, are integral to our understanding of most plasma phenomena. We have been able to study these in ultracold neutral plasmas by shaping the initial density distribution through spatial modulation of the ionizing laser intensity. We describe a relay imaging system for the photoionization beam that allows us to create higher resolution features and its application to extend the observation of ion acoustic waves to shorter wavelengths. We also describe the formation of sculpted density profiles to create fast expansion of plasma into vacuum and streaming plasmas

THE EFFECT OF VERY HIGH HYDRAULIC PRESSURE ON THE PERMEABILITY AND SALT REJECTION OF REVERSE OSMOSIS MEMBRANES

We employ a stirred-cell reverse osmosis setup to demonstrate that a seawater reverse osmosis membrane can maintain excellent salt rejection at pressures as high as 172 bar. However, we also demonstrate a very significant drop in membrane permeability at high pressures–likely due to membrane compaction. At 172 bar, permeability is more than 50% lower than at a pressure of 34.5 bar. In addition, our results illustrate how flux fluctuates significantly in time when the pressure is removed and then reapplied, even for very short periods, in high pressure reverse osmosis processes–an effect that requires careful consideration from the perspective of process control and operation. From the perspective of membrane performance, RO is feasible at high pressures but distinct challenges are presented by reduced permeability and increased variability in flux