Tailored electron bunches with smooth current profiles for enhanced transformer ratios in beam-driven acceleration

Collinear high-gradient ${\cal O} (GV/m)$ beam-driven wakefield methods for charged-particle acceleration could be critical to the realization of compact, cost-efficient, accelerators, e.g., in support of TeV-scale lepton colliders or multiple-user free-electron laser facilities. To make these options viable, the high accelerating fields need to be complemented with large transformer ratios $>2$, a parameter characterizing the efficiency of the energy transfer between a wakefield-exciting "drive" bunch to an accelerated "witness" bunch. While several potential current distributions have been discussed, their practical realization appears challenging due to their often discontinuous nature. In this paper we propose several alternative current profiles which are smooth which also lead to enhanced transformer ratios. We especially explore a laser-shaping method capable of generating one the suggested distributions directly out of a photoinjector and discuss a linac concept that could possible drive a dielectric accelerator

On the usefulness of the directional distance function in analyzing environmental policy on manure management

The purpose of this paper is to model the manure management policy implemented in the European Union, and more specifically the limit imposed on the spreading of organic nitrogen. A theoretical model is defined in such a way that a number of specificities concerning livestock production can be introduced.The theoretical framework is used to investigate how the land can be shared out optimally between the non-productive purpose of spreading manure in a manner compliant with the environmental regulation and the productive function of providing crops.Then,we define an empirical model derived from the previous theoretical model, using the directional distance function.It provides a framework for deriving shadow prices of pollutant, of productive and non productive use of land and of the constraint on organic manure involved by the European environmental regulation.Environmental Economics and Policy,

Magnetic-field-induced Stoner transition in a dilute quantum Hall system

In a recent paper [Phys.Rev.B.\textbf{84}, 161307 (2011)], experimental data on spin splitting in the integer quantum Hall effect has been reported in a high mobility dilute 2D electron gas with electron density as low as 0.2 $\times$ 10$^{11}$ cm $^{-2}$. In this work, we show that an excellent \emph{quantitative} description of these data can be obtained within the model of the magnetic-field-induced Stoner transition in the quantum Hall regime. This provides a powerful tool to probe the non-trivial density dependance of electron-electron interactions in the dilute regime of the 2D electron gas

Is the Bellman residual a bad proxy?

This paper aims at theoretically and empirically comparing two standard optimization criteria for Reinforcement Learning: i) maximization of the mean value and ii) minimization of the Bellman residual. For that purpose, we place ourselves in the framework of policy search algorithms, that are usually designed to maximize the mean value, and derive a method that minimizes the residual $\|T_* v_\pi - v_\pi\|_{1,\nu}$ over policies. A theoretical analysis shows how good this proxy is to policy optimization, and notably that it is better than its value-based counterpart. We also propose experiments on randomly generated generic Markov decision processes, specifically designed for studying the influence of the involved concentrability coefficient. They show that the Bellman residual is generally a bad proxy to policy optimization and that directly maximizing the mean value is much better, despite the current lack of deep theoretical analysis. This might seem obvious, as directly addressing the problem of interest is usually better, but given the prevalence of (projected) Bellman residual minimization in value-based reinforcement learning, we believe that this question is worth to be considered.Comment: Final NIPS 2017 version (title, among other things, changed

Plural Form and Franchise Chains Efficency: A Dea Meta-Frontier Approach applied to French Chains

This paper deals with the performance of franchise chains related to their percentage of company-owned outlets (PCO). This research uses a Data Envelopment Analysis (DEA) to assess franchise chains’ efficiency, and a meta-frontier approach to analyze chains’ efficiency between and across sectors. The sample includes 43 chains of the service and retail sectors, located in the French market. Data are available over the 2005-2007 period allowing a longitudinal analysis. The main findings show that the meta-frontier is built up on retail chains rather than on service chains, and that there is a relationship between the PCO and the chain efficiency. Finally, there is no significant difference between the observed PCOs and the optimal PCOs which means that franchisors in our sample have already reached a PCO that is close to the PCO that optimizes the chain efficiency.Creation-Date: 2012-02Franchising, Plural Form, Percentage of company-owned outlets (PCO), Efficiency, Data Envelopment Analysis, Meta Frontier