A bremsstrahlung gamma-ray source based on stable ionization injection of electrons into a laser wakefield accelerator

Laser wakefield acceleration permits the generation of ultra-short, high-brightness relativistic electron beams on a millimeter scale. While those features are of interest for many applications, the source remains constraint by the poor stability of the electron injection process. Here we present results on injection and acceleration of electrons in pure nitrogen and argon. We observe stable, continuous ionization-induced injection of electrons into the wakefield for laser powers exceeding a threshold of 7 TW. The beam charge scales approximately linear with the laser energy and is limited by beam loading. For 40 TW laser pulses we measure a maximum charge of almost 1 nC per shot, originating mostly from electrons of less than 10 MeV energy. The relatively low energy, the high charge and its stability make this source well-suited for applications such as non-destructive testing. Hence, we demonstrate the production of energetic radiation via bremsstrahlung conversion at 1 Hz repetition rate. In accordance with Geant4 Monte-Carlo simulations, we measure a gamma-ray source size of less than 100 microns for a 0.5 mm tantalum converter placed at 2 mm from the accelerator exit. Furthermore we present radiographs of image quality indicators

Simple and clear evidence for positive feedback limitation by bipolar behavior during scanning electrochemical microscopy of unbiased conductors

On the basis of an experimentally validated simple theoretical model, it is demonstrated unambiguously that when an unbiased conductor is probed by a scanning electrochemical tip (scanning electrochemical microscopy, SECM), it performs as a bipolar electrode. Though already envisioned in most recent SECM theories, this phenomenon is generally overlooked in SECM experimental investigations. However, as is shown here, this may alter significantly positive feedback measurements when the probed conductor is not much larger than the ti

Mesure du débit des eaux usées ou fortement chargées en matières solides en vue de déterminer les flux de pollution

The quantity of pollution due to industrial or urban effluent over a given time T is given by P = S T 0 q c dt, where q and c are instantaneous polluant discharge and concentration respectively. Where continous concentration measurement is impracticable, the only theoretically correct procedure is by taking a sample at a discharge proportional to that of the effluent. A common alternative is to take a set number of samples at constant discharge and to multiply the average concentration found for each by the volume flow during the smapling time. The aim in every case is to record discharge. No special methods are available for polluted water discharge measurement. Standard methods are applied, there fore, and are briefly discussed from the point of view of possible effects of pollution or of how the instrumentation is installed on measurement accuracy and sensitivity. Vane-type water meters or discharge measurement orifice plates or nozzles are hardly suitable for pipes containing highly sediment-laden or corrosive water. Dilution methods and electro-magnetic flow measurement instruments are less sensitive to effluent quality, and acoustic instruments seem to hold great promise. Finally, pump running time measurement should not be disregarded altogether, in spite of its not particularly accurate results. Measurements by electro-magnetic pickup are preferable to current meters for free-surface flow. Broad-crested weirs or Venturi flumes producing a hydraulic jump are preferable to sharp-crested weirs, but are more difficult to set up. Dilution methods call for precautions in the choice of a suitable tracer, and it is not always easy to achieve the right mixture. The use of radio-active tracers enables discharge to be recorded. With chemical tracers, the present trend is towards constant-rate injection and the constitution of average samples from which both flow volume and pollution concentration can be established. There is a systematic error with this method which, however, remains within acceptable limits if the discharge does not vary too much. Present research subjects include the use of electro-magnetic pickups and the behaviour of various tracers in polluted water. It is proposed to design special equipment for the use of radio-active tracers

Electrochemical Li-Ion Battery Modelisation for Electric Vehicles

International audienceThe future development of electric vehicles is now strictly linked with their batteries. In parallel of the actual research focused on the development of new materials and increase their performances in terms of energy, power, cost, durability and weight, it is necessary to develop modeling tools. The simulations are helpful for improving the knowledge of both physical and chemical phenomena, optimize the battery design according with the user requirements, and reduce the test/validation phase. In this framework, this articles, contributes to the development on an electrochemical based model for Li-ion batteries [1], using the powerful COMSOL Multiphysics® software, allowing to use custom equation systems. The partial differential equations are resolved coupling a 1D geometry, describing the cell cross section, with a 2D geometry describing the active material particles using the "coefficient form PDE"[2]. This work revisits one of the most used porous electrodes based model to describe the behavior of lithium-ion batteries. Firstly, all the physical quantities are set in a dimensionless form, as commonly used in fluid mechanics: the parameters that act in the same or the opposite ways are regrouped and the total number of simulation parameter is thus reduced. Then the numerical explorations with the limit conditions, allow to understand the effect of each dimensionless parameter, in the overall equation system. The Figure 1 shows the effect of the solid phase diffusion over the practical retained capacity for a galvanostatic discharge, while in Figure 2 shows the voltage drop and relaxation for pulses. The simulations are finally compared with half-cell obtained from commercial 18650 Lithium ion cells for EV. The mid-term perspectives includes the simulation of ageing and temperature [3]

Electrochemical Li-Ion Battery Modelisation for Electric Vehicles

International audienceThe future development of electric vehicles is now strictly linked with their batteries. In parallel of the actual research focused on the development of new materials and increase their performances in terms of energy, power, cost, durability and weight, it is necessary to develop modeling tools. The simulations are helpful for improving the knowledge of both physical and chemical phenomena, optimize the battery design according with the user requirements, and reduce the test/validation phase. In this framework, this articles, contributes to the development on an electrochemical based model for Li-ion batteries [1], using the powerful COMSOL Multiphysics® software, allowing to use custom equation systems. The partial differential equations are resolved coupling a 1D geometry, describing the cell cross section, with a 2D geometry describing the active material particles using the "coefficient form PDE"[2]. This work revisits one of the most used porous electrodes based model to describe the behavior of lithium-ion batteries. Firstly, all the physical quantities are set in a dimensionless form, as commonly used in fluid mechanics: the parameters that act in the same or the opposite ways are regrouped and the total number of simulation parameter is thus reduced. Then the numerical explorations with the limit conditions, allow to understand the effect of each dimensionless parameter, in the overall equation system. The Figure 1 shows the effect of the solid phase diffusion over the practical retained capacity for a galvanostatic discharge, while in Figure 2 shows the voltage drop and relaxation for pulses. The simulations are finally compared with half-cell obtained from commercial 18650 Lithium ion cells for EV. The mid-term perspectives includes the simulation of ageing and temperature [3]