Plasma wave undulator for laser-accelerated electrons

Laser-plasma accelerators have become compact sources of ultrashort electron bunches at energies up to the gigaelectronvolt range thanks to the remarkable progress made over the past decade. A direct application of these electron bunches is the production of short pulse x-ray radiation sources. In this letter, we study a fully optically driven x-ray source based on the combination of a laser-plasma accelerator and a plasma wave undulator. The longitudinal electric field of a laser-generated plasma wave is used to wiggle electrons transversally. The period of this plasma undulator being equal to the plasma wavelength, tunable photon energies in the 10 keV range can be achieved with electron energies in the 100-200 MeV range. Considering a 10s TW class femtosecond laser system, undulators with a strength parameter K~0.5 and with about ten periods can be combined with a laser-plasma accelerator, resulting in several 10^-2 emitted x-ray photons per electron.Comment: 6 pages, 4 figure

Alleviating Human-Elephant Conflict Through Deterrent Fences and Environmental Monitoring in Southern Kenya

Human-wildlife conflict is present across the world. In areas where human settlements overlap with elephant habitats, human-elephant conflict can result from crop raiding events, compromising farmers’ food and economic security, and putting humans and elephants in danger through farmer retaliation. Elephants raid crops primarily at night, when detection by humans is lowest, and during the dry season, as crops are developing towards harvest and natural forage quality drops. People living in these areas facing HEC have developed mitigation strategies to lessen the impacts and move towards coexistence. As a team member on the Elephants and Sustainable Agriculture in Kenya project, I conducted my research in the Kasigau Wildlife Corridor of southeastern Kenya. Over the past five years (2017-2022), our international team tested the effectiveness of eight deterrent fence designs, including four modern single deterrents (one line of deterrent strung between fence posts), three modern double deterrents (two strands of single deterrents), and one traditional deterrent (acacia branches). Each fence consisted of one or more negative stimuli to deter elephants, and any deterrent was hypothesized to perform better than the grand control of just fence posts alone. Compared to single deterrents, double deterrent fences were hypothesized to deter elephants better because they stimulate more sensory modalities. We also examined timing within the crop season and moon phase as potential predictors of crop raiding events. Elephant presence around experimental fields was hypothesized to be higher during the end of the crop season and inversely related with lunar light levels. To test these four hypotheses, eight blocks of land were leased from farmers along the boundary between Sasenyi Village and Rukinga Wildlife Sanctuary. Four of the eight blocks were divided into eight fields each around which four experimental deterrent fences and their matching four controls were erected. The other four blocks were each divided in half with one half encompassed by a beehive fence and the other by fake hives. Moon phase and timing within the crop season were determined using a lunar calendar, camera trap evidence, and crop data. During each of the two growing seasons per year, all elephants within 12 m of the deterrent fences were categorized as approaching; an instance of entering a field was termed a breach and not entering a deterrence. Analyses consisted of generalized linear mixed models, Linear Regression, and mixed effect logistic regression models. In support of my first hypothesis, the modern experimental deterrents performed better than the grand control, which had a successful deterrent rate of 27%. The traditional acacia fence (19%), and the cloth fence (66.6%) were the only deterrents tested that did not perform significantly better than the grand control. In contrast to the second hypothesis, the double deterrent fences (68%) did not perform significantly better than single deterrent (62.3%) fence designs. The third hypothesis on elephant presence being positively correlated with progression of the crop season was supported and aligned with past findings in other study sites. However, the fourth hypothesis that presence was inversely correlated with lunar light levels was not upheld, though was impacted by the direction of lunar light level, as more elephants were present during the waning moon phases, as light levels were decreasing. Using these results, we can advise farmers on which deterrents to use, and at what times to be more vigilant due to changes in the probability of crop raiding events. The results of this study are being shared with the farmers living in the KWC and may be useful to others living in high HEC areas by providing additional crop raiding mitigation strategies. Our methods of analysis can be expanded past HEC and applied to areas facing other forms of HWC to promote coexistence

Comment on "Scalings for radiation from plasma bubbles" [Phys. Plasmas 17, 056708 (2010)]

Thomas has recently derived scaling laws for X-ray radiation from electrons accelerated in plasma bubbles, as well as a threshold for the self-injection of background electrons into the bubble [A. G. R. Thomas, Phys. Plasmas 17, 056708 (2010)]. To obtain this threshold, the equations of motion for a test electron are studied within the frame of the bubble model, where the bubble is described by prescribed electromagnetic fields and has a perfectly spherical shape. The author affirms that any elliptical trajectory of the form x'^2/{\gamma}_p^2 + y'^2 = R^2 is solution of the equations of motion (in the bubble frame), within the approximation p'_y^2/p'_x^2 \ll 1. In addition, he highlights that his result is different from the work of Kostyukov et al. [Phys. Rev. Lett. 103, 175003 (2009)], and explains the error committed by Kostyukov-Nerush-Pukhov-Seredov (KNPS). In this comment, we show that numerically integrated trajectories, based on the same equations than the analytical work of Thomas, lead to a completely different result for the self-injection threshold, the result published by KNPS [Phys. Rev. Lett. 103, 175003 (2009)]. We explain why the analytical analysis of Thomas fails and we provide a discussion based on numerical simulations which show exactly where the difference arises. We also show that the arguments of Thomas concerning the error of KNPS do not hold, and that their analysis is mathematically correct. Finally, we emphasize that if the KNPS threshold is found not to be verified in PIC (Particle In Cell) simulations or experiments, it is due to a deficiency of the model itself, and not to an error in the mathematical derivation.Comment: 5 pages, 5 figure

Femtosecond x rays from laser-plasma accelerators

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent and femtosecond radiation. In this article we review, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.Comment: 58 pages, 41 figure

Motion planning control of the airpath of a S.I. engine with Valve Timing Actuators

International audienceWe address the control of the airpath of a turbocharged S.I. engine equipped with Variable Valve Timing actuators (VVT). Compared to standard configurations, the engine does not possess any external EGR (Exhaust Gas Recirculation) loop. Rather, VVT are used to produce internal EGR, providing similar beneficial effects in terms of emissions reduction. The airpath dynamics takes the form of a single mono-dimensional air balance in the intake manifold. In this equation, the VVT act as a disturbance by impacting on the air mass flow through the inlet valves. This impact can be estimated from real-time measurements. We use this information in a motion planning based control strategy by, successively, turning the driver's torque demand into a trajectory generation problem for the air mass contained in the intake manifold, and then deriving an intake manifold pressure trajectory. Supportive simulation results show the relevance of this approach and suggest ways of further improvements

Modeling and control of a Diesel HCCI engine

International audienceThis article focuses on the control of a Diesel engine airpath. We propose a detailed description of the airpath of a Diesel HCCI engine supported by experimental results. Moreover, we propose a simple, yet innovative, motion planning control strategy. At the light of this study, we can finally conclude, with supportive results, that motion planning is indeed an appropriate solution for controlling the airpath dynamics

Observation of longitudinal and transverse self-injections in laser-plasma accelerators

Laser-plasma accelerators can produce high quality electron beams, up to giga-electronvolts in energy, from a centimeter scale device. The properties of the electron beams and the accelerator stability are largely determined by the injection stage of electrons into the accelerator. The simplest mechanism of injection is self-injection, in which the wakefield is strong enough to trap cold plasma electrons into the laser wake. The main drawback of this method is its lack of shot-to-shot stability. Here we present experimental and numerical results that demonstrate the existence of two different self-injection mechanisms. Transverse self-injection is shown to lead to low stability and poor quality electron beams, because of a strong dependence on the intensity profile of the laser pulse. In contrast, longitudinal injection, which is unambiguously observed for the first time, is shown to lead to much more stable acceleration and higher quality electron beams.Comment: 7 pages, 7 figure

Probing electron acceleration and X-ray emission in laser-plasma accelerator

While laser-plasma accelerators have demonstrated a strong potential in the acceleration of electrons up to giga-electronvolt energies, few experimental tools for studying the acceleration physics have been developed. In this paper, we demonstrate a method for probing the acceleration process. A second laser beam, propagating perpendicular to the main beam is focused in the gas jet few nanosecond before the main beam creates the accelerating plasma wave. This second beam is intense enough to ionize the gas and form a density depletion which will locally inhibit the acceleration. The position of the density depletion is scanned along the interaction length to probe the electron injection and acceleration, and the betatron X-ray emission. To illustrate the potential of the method, the variation of the injection position with the plasma density is studied