Observation of beam loading in a laser-plasma accelerator

Beam loading is the phenomenon which limits the charge and the beam quality in plasma based accelerators. An experimental study conducted with a laser-plasma accelerator is presented. Beam loading manifests itself through the decrease of the beam energy, the reduction of dark current and the increase of the energy spread for large beam charge. 3D PIC simulations are compared to the experimental results and confirm the effects of beam loading. It is found that, in our experimental conditions, the trapped electron beams generate decelerating fields on the order of 1 GV/m/pC and that beam loading effects are optimized for trapped charges of about 20 pC.Comment: 10 pages,4 figure

Control and optimization of a staged laser-wakefield accelerator

We report results of an experimental study of laser-wakefield acceleration of electrons, using a staged device based on a double-jet gas target that enables independent injection and acceleration stages. This novel scheme is shown to produce stable, quasi-monoenergetic, and tunable electron beams. We show that optimal accelerator performance is achieved by systematic variation of five critical parameters. For the injection stage, we show that the amount of trapped charge is controlled by the gas density, composition, and laser power. For the acceleration stage, the gas density and the length of the jet are found to determine the final electron energy. This independent control over both the injection and acceleration processes enabled independent control over the charge and energy of the accelerated electron beam while preserving the quasi-monoenergetic character of the beam. We show that the charge and energy can be varied in the ranges of 2–45 pC, and 50–450 MeV, respectively. This robust and versatile electron accelerator will find application in the generation of high-brightness and controllable x-rays, and as the injector stage for more conventional devices

Experimental Measurements of Electron-Bunch Trains in a Laser-Plasma Accelerator

International audienceSpectral measurements of visible coherent transition radiation produced by a laser-plasma accelerated electron beam are reported. The significant periodic modulations that are observed in the spectrum result from interference of transition radiation produced by multiple bunches of electrons. A Fourier analysis of the spectral interference fringes reveals that electrons are injected and accelerated in multiple plasma wave periods, up to at least ten periods behind the laser pulse. The bunch separation scales with the plasma wavelength when the plasma density is changed over a wide range. An analysis of the spectral fringe visibility indicates that the first bunch contains most of the charge. In a laser wakefield accelerator (LWFA) [1], an intense and ultrashort laser pulse drives a relativistic plasma wave, or wakefield, which can be used to accelerate electrons to high energies in a short distance. With accelerating gradients in excess of 100 GV/m, electron bunches in the 100 MeV-1 GeV range are now produced in mm distances, with few percent energy spreads and charges of 10's of picocoulombs [2, 3]. These electron bunches also have the characteristic feature of having femtosec-ond duration and kA peak current [4], which make them good candidates as a compact electron source for a Free Electron Laser. This has triggered a large number of experimental studies aiming at characterizing the temporal structure of these laser-plasma produced electron bunches. The technique of choice has been Coherent Transition Radiation (CTR) which can give insights on the temporal spread of the electron bunches [5]. The first studies have focused on measuring CTR in the THz region , showing that the bunches were sub-100 fs [6]. More recently, several experiments have shown that the electron bunches can be as short as a few femtoseconds in duration [4, 7] but these works focussed on the electron bunch contained in the first bucket of the plasma wave. Few studies have considered the fact that the electron beam can be composed of several beamlets although some publications suggest that it can occur in certain cases [8, 9]. Here, we present a detailed experimental study showing that several short electron bunches can be injected and efficiently accelerated in multiple plasma wave periods, thus forming an electron bunch train. We use spectral measurements of CTR to diagnose the temporal distribution of electrons. CTR is emitted when the electron bunch passes an interface between two media , e.g. a metallic foil and a vacuum. For a monoener-getic electron beam, the angular radiation field is a hollow cone with half opening angle θ = 1/γ. The spectral radiation field at frequency ω and observation angle θ, is given by [10–12] d 2

Injection and acceleration of quasimonoenergetic relativistic electron beams using density gradients at the edges of a plasma channel

International audienceThe injection of quasimonoenergetic electron beams into a laser wakefield accelerator is demonstrated experimentally using density gradients at the edges of a plasma channel. In the experiment, two laser pulses are used; the main laser pulse drives a wakefield, while a second countercrossing laser beam produces a plasma whose expansion creates a channel with significant density gradients. Local injection of electrons in the wakefield is triggered by wave breaking in the density ramp. The injection is localized spatially and leads to the generation of collimated and narrow energy spread relativistic electron beams at the 100 MeV level, with charges in the range of 20-100 pC. The stability of this injection process is compared to the stability of the colliding pulse injection process and is found to be inferior for our experimental conditions. On the other hand, it is found that as the electron beam divergence is smaller in the case of gradient injection, the transverse emittance might be better. (C) 2010 American Institute of Physics. [doi:10.1063/1.3469581

Femto-second ultrashort laser wakefield electron bunch-duration measurements: a prism-based dispersion visible-to-IR spectrometer

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Capillary discharge for laser beam guiding

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