Friends For Good - Wicked: A New Musical and the Idealization of Friendship

This rhetorical analysis explores the relationship between the two main characters, Elphaba and G(a)linda, in the musical Wicked through the interpersonal communication lens of friendship. This article focuses on the role that friendship plays in the musical and suggests that friendship is a relationship that can be stronger than romantic relationships. Through the application of Rawlins’ work on friendship to the relationship between Elphaba and G(a)linda, this analysis suggests that friendship is the most prominent relationship in Wicked. Wicked offers an im-portant message to theatre-goers: Friendship is something to be valued and cherished

Reply to Comment on ‘Issues in the understanding of negative ion extraction for fusion’

International audienceIn this reply we respond to the comments of MochaIskyy et al. on our paper « Issues in the understanding of negative ion extraction for fusion » where we pointed out some errors in previously published papers reporting results of Particle-In-Cell Monte Carlo Collisions (PIC MCC) of negative ion extraction. We stress again that 1) it is always better to solve properly a simplified problem than to present misleading simulation results albeit under conditions closer to the real problem and 2) good comparison of simulation to experimental results does not constitute validation of the simulation if accuracy of the simulation is doubtful unless thorough verification of the simulation has been carried out

Hollow cathode modeling: II. Physical analysis and parametric study

International audienceA numerical emissive hollow cathode model which couples plasma and thermal aspects of the NASA NSTAR cathode has been presented in a companion paper and simulation results obtained using the plasma model were compared to experimental data. We now compare simulation results with measurements using the full coupled model. Inside the cathode, the simulated plasma density profile agrees with the experimental data up to the ±50% experimental uncertainty while the simulated emitter temperature differs from measurements by at most 5 K. We then proceed to an analysis of the cathode discharge both inside the cathode where electron emission is dominant and outside in the near plume where electron transport instabilities are important. As observed previously in the literature, the total emitted electron current is much larger 34 A () than the set discharge current collected at the anode 13 A () while ionization plays a negligible role. Extracted electrons are emitted from a region much shorter than the full emitter (0.9 cm versus 2.5 cm). The influence of an applied axial magnetic field in the plume is also assessed and we observe that it leads to a 10-fold increase of the plasma density 1 cm downstream of the orifice entrance while the simulated discharge potential at the anode is increased from 10 V up to 35.5 V. Lastly, we perform a parametric study on both the operating point (discharge current, mass flow rate) and design (inner radius) of the cathode. The simulated useful operating envelope is shown to be limited at low discharge current mostly because of the probable ion sputtering of the emitter and at high discharge current because of emitter evaporation, plasma oscillations and sputtering of the keeper electrode. The behavior of the cathode is also analyzed w.r.t. its internal radius and simulation results show that the useful emitter length scales linearly with the cathode radius

Hollow cathode modeling: I. A coupled plasma thermal two-dimensional model

International audienceA two dimensional axisymmetric quasi-neutral fluid model of an emissive hollow cathode that includes neutral xenon, single charge ions and electrons has been developed. The gas discharge is coupled with a thermal model of the cathode into a self-consistent generic model applicable to any hollow cathode design. An exhaustive description of the model assumptions and governing equations is given. Boundary conditions for both the gas discharge and thermal model are clearly specified as well. A new emissive sheath model that is valid for any emissive material and in both space charge and thermionic emission limited regimes is introduced. Then, setting the emitter temperature to an experimentally measured profile, we compare simulation results of the plasma model to measurements available in the literature for NASA NSTAR barium oxide cathode. Qualitative discrepancies between simulation results and measurements are noted in the cathode plume regarding the simulated plasma potential. Motivated by experimental evidence supporting the occurrence of ion acoustic instabilities in the cathode plume, an enhanced model of electron transport in the plume is presented and its consequences analyzed. Using the obtained plasma model, simulated quantities in the plume are qualitatively comparable with measurements. Inside the cathode, the simulated plasma density agrees well with measurements and is within the ±50 % experimental uncertainty associated with these measurements. A comparison of simulation results of the full coupled cathode model for the NASA NSTAR cathode with experimental measurements is presented in a companion paper, as well as a physical analysis of the cathode behavior and a parametric study of the influence of the operating point and key design choices

Numerical modeling and incoherent Thomson scattering measurements of a 5A cathode with LaB 6 emitter

International audienceEmissive cathodes serve as the electron source for Hall plasma thrusters, needed to maintain the ionization region, and also to act as neutralizers for the accelerated ions in these devices. As they are currently the lifetime-limiting components of thrusters, reliable and robust numerical modeling of their operation is key. In this paper, results from the application of a recently-developed 2D axisymmetric fluid code to the modeling of a 5A LaB6 emissive cathode are presented. These results are compared with new electron property measurements obtained via a recently-developed incoherent Thomson scattering diagnostic. Good agreement between numerical and experimental results are found, as well as an interesting dependency of the code results to the LaB6 emitter temperature

Modeling of the plasma jet of a stationary plasma thruster

We have developed a two-dimensional hybrid fluid – particle-in-cell Monte Carlo collisions (PIC- MCC) model to study the plume of a stationary plasma thruster. The model is based on a fluid description of the electrons (the electron density follows a Boltzmann distribution) and a particle description of the ion and neutral transport. Collisions between heavy species are taken into account with a Monte Carlo method. The electric field is obtained from Poisson's equation or from the quasineutrality assumption. We first show that the results from the PIC-MCC model are close to the results of a more time-consuming direct simulation Monte Carlo approach. We then compare the model predictions of the plume density and ion energy distribution with experimental measurements. Finally, we present a brief discussion on the assumptions of the model and on its ability to give reliable predictions on important issues such as the flux of ions backscattered to the satellite. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70320/2/JAPIAU-91-12-9521-1.pd

Measurements of electron emission under electron impact on BN sample for incident electron energy between 10 eV and 1000 eV

International audienceElectron emission measurements have been performed on a BN sample by using a new specific protocol and experimental setup , which allows characterizing electron emission under electron impact on resistive material in a short time and with a wide variety of extracted data: total electron emission yield, emitted electron energy distribution, elastically backscattered electron emission yield and energy efficiency of electron-surface interaction. Methodology, calibration, biases corrections and results are presented in this letter. Results are compared to that measured on another material SiO2. As there are few published data on electron emission at low incident electron energy on BN sample, it is expected that these measurements could be useful for numerous studies implying electron emission on BN surface

Pitfalls in Modeling Walls and Neutrals Physics in Gas Discharges Using Parallel Particle-in-Cell Monte Carlo Collision Algorithms

Owing to their ability to model the physics of low-pressure plasmas away from thermodynamical equilibrium, particle-in-cell (PIC) techniques have become one of the tools of choice to simulate the operation of many plasma devices. This trend is reinforced by the growing access to parallel computing resources which enables tackling problems that were previously intractable with PIC techniques. However, accurate modeling of these plasmas often depends critically on the detailed description of a variety of physical phenomena ranging from microscopic to macroscopic scale and from electrons' to neutral particles' timescale. Among those are coupling phenomena between charged particles and neutrals. We illustrate here how the implementation of simplified models for scattering kinematics, neutrals dynamics and particle-wall interaction can affect simulation results. Until the full breadth of these effects can be captured in models, these results underline the importance of using extensive parametric scans to assess the importance of these effects

Space micropropulsion systems for Cubesats and small satellites: From proximate targets to furthermost frontiers

Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec - P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.This work was supported in part by the following funds and organizations: OSTIn-SRP/EDB through National Research Foundation and in part by MoE AcRF (Rp6/16 Xs), Singapore; National Natural Science Foundation of China (Grant Nos. 51777045 and 51477035); National Technical Basic Scientific Research of China, Grant No. JSZL2016203c006; NASA DC Space Grant Consortium; Grant-in-Aid for Scientific Research under Grant S: 21226019 and Grant B: 17H02295 through the Japan Society for the Promotion of Science, and by NIFS budget code NIFS17KLER063, and KAKENHI grant: Grant-in-Aid for Scientific Research (S), No. JP16H06370; S.S. thanks late Professor K. Toki, late Dr. K. P. Shamrai, Dr. Kuwahara, and the HEAT project members for their contribution Y.R. acknowledges the support from the US DOE under Contract No. DE-AC02-09CH11466; I.L. acknowledges the support from the School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology; special thanks to L. Xu, M. Lim, S. Huang, and the entire PSAC/SPCS for their help