In this article we present a theoretical investigation on an experimental design of a laser wakefield accelerator in which electron bunches from a photocathode radio frequency linac are injected into a capillary discharge plasma channel just in front of a few tens of terawatt drive laser pulse. The electron bunch, with a kinetic energy of 2.9 MeV and an energy chirp imposed by the linac, is magnetically compressed by a factor of 8 to a duration of 250 fs, and is magnetically focused into the plasma channel where it matches the spot size of the drive laser (≈30 μm). The dynamics of the bunch, starting from the photocathode, through the linac, along the beam transportation line, through the magnetic compressor, and its focusing into the plasma channel are comprehensively simulated with the general particle tracer code. Further, we use our three-dimensional numerical codes to calculate the laser wakefield and to determine and optimize the trapping and acceleration of the injected bunch in the wakefield. We show that, injecting a 5 pC electron bunch of 250 fs duration, the experiment should deliver an electron bunch of approximately 744 MeV energy, with 1.1% relative energy spread, and with an extremely short duration (6 fs), after acceleration in a 5.4 cm long plasma channel.

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In recent experiments ultra-relativistic femtosecond electron bunches were generated by a Laser Wakefield Accelerator (LWFA) in different regimes. Here we predict that even attosecond bunches can be generated by an LWFA due to the fast betatron phase mixing within a femtosecond electron bunch. The attosecond bunches are stable outside the LWFA and can propagate in vacuum many tens of centimeter without significant change in their duration. Our calculations show that evidence for the formation of attosecond bunches can be found in the spectrum of coherent betatron radiation from LWFA's. The generation of attosecond electron bunches would be of great interest to provide a new and unique tool for modern physical research. The potential of such bunches ranges from electron microscopy with attosecond resolution, to the generation of attosecond X-ray beams, for investigating physical, chemical and biological processes on the attosecond timescale.The shortest bunches to date are available from Laser Wakefield Accelerators (LWFA's) [1]. In recent experiments exploiting different parameter regimes [2,3,4,5,6,7,8,9], ultra-relativistic femtosecond (fs) electron bunches were generated by an LWFA. Here we predict that even attosecond bunches can be generated by an LWFA due to the fast betatron phase mixing withing a femtosecond electron bunch. The attosecond bunches are stable outside the LWFA, which means that they can propagate through vacuum over distances of many tens of centimeters, without a significant change of duration. We also predict how such bunches can be identified in the optical spectrum emitted during the acceleration process.Several schemes for the generation of attosecond electron bunches have been proposed so far. Examples are an inverse free-electron-laser process [10], the interaction of high intensity laser pulses with overdense plasma [11], the acceleration of electrons with a short tailored laser pulse [12], the slicing of an electron bunch with a laser pulse [13], the interaction of an ultra-short laser pulse with a nanofilm [14], with a wire or a plasma slice [15], or the interaction of a PW radially-polarized laser pulse with a sub-micron droplet of a high-Z material [16]. However, all these schemes are of limited attractivity, because they either require large accelerator structures [10] or high intensities in the order of 10 20 − 10 22 W cm −2 (normalized amplitude, a 0 , 10-100) [11,12,13,14,15,16], such as available only from rather exclusive (Petawatt) laser systems. A strong disadvantage is that these schemes would deliver bunches of rather low energy (a few to a few tens of MeV's) which would make it difficult to keep the duration of such bunches and apply them due to space charge effects. In some schemes the bunches would suffer from a limited life time (about 10 fs [14]) or a limitation of charge to well below a pC [10,12,16].Our scheme of attosecond bunch generation, presented in this Letter, has the advantage that it is based on the nowadays well-known technique of LWFA, and that the at...

For the purpose of laser wakefield acceleration, it turned out that also the injection of electron bunches longer than a plasma wavelength can generate accelerated femtosecond bunches with relatively low energy spread. This is of high interest because such injecting bunches can be provided, e.g., by state-of-the-art photo cathode RF guns. Here we point out that when an e-bunch is injected in the wakefield it is important to take into account the ponderomotive scattering of the injecting bunch by the laser pulse in the vacuum region located in front of the plasma. At low energies of the injected bunch this scattering results in a significant drop of the collection efficiency. Larger collection efficiency can by reached with lower intensity laser pulses and relatively high injection energies. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch.PACS numbers: 52.38. Kd, 41.75.Jv, 41.85.Ar In Laser Wakefield Accelerator (LWFA) a femtosecond high-power laser pulse generates a strong plasma wave (laser wakefield) which can accelerate charged particles to ultra-relativistic energies [1]. In order to avoid large energy spread in accelerated bunch when a relativistic electron bunch is injected in the laser wakefield, the bunch has to be injected at a suitable position in the wake with a precision of a fraction of the plasma wave period and the duration of the injected bunch has to be similarly short. For the plasma parameters of interest, which involves typical plasma wavelength of a few tens of microns, this requires initial bunches of the order of 10 femtoseconds duration and a similar precision of the synchronization with the wakefield. With current technologies these requirements cannot be fulfilled in practice. As an alternative, in the recently demonstrated "bubble" injection method [2] electrons from the background plasma are trapped in the correct phase of the wake yielding the required ultra-short bunches. This method led to acceleration to energies of

External injection of electron bunches longer than the plasma wavelength in a laser wakefield accelerator can lead to the generation of femtosecond ultrarelativistic bunches with a couple of percent energy spread. Extensive study has been done on external electron bunch (e.g. one generated by a photo-cathode rf linac) injection in a laser wakefield for different configurations. In this paper we investigate a new way of external injection where the electron bunch is injected at a small angle into the wakefield. This way one can avoid the ponderomotive scattering as well as the vacuum-plasma transition region, which tend to destroy the injected bunch. In our simulations, the effect of the laser pulse dynamics is also taken into account. It is shown that injection at an angle can provide compressed and accelerated electron bunches with less than 2% energy spread. Another advantage of this scheme is that it has less stringent requirements in terms of the size of the injected bunch and there is the potential to trap more charge