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