Effect of the ponderomotive scattering and injection position on electron-bunch injection into a laser wakefield

Abstract

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

Theoretical investigation of external injection schemes for laser wakefield acceleration

“…More recently the shot-to-shot stability was considerably improved by employing a second, counterpropagating laser pulse [30]. It was also proposed that external injection of a long (longer than the plasma wavelength), low energy (typically a few MeV's), electron bunch in the laser wakefield can lead to the generation of good quality femtosecond relativistic bunches [37,38,40,[63][64][65][66][67][68][69][70]. The injected bunch can be generated by a stateof-the-art radio-frequency (rf) photo-cathode linear accelerator (linac).…”

“…First there is the problem of ponderomotive scattering [40]. Before acceleration, the electron bunch propagates more slowly than the laser pulse in the vacuum, which means that, at some distance in front of the plasma channel, the bunch is situated ahead of the laser pulse.…”

“…The following three chapters each discuss one of the external injection schemes. Chapter 3 investigates injection behind the drive laser pulse and discusses two important perturbing effects that have been overlooked so far, namely bunch scattering by the laser pulse in vacuum [40] and in the inhomogeneous laser wakefield in the vacuum-plasma transition at the entrance of the plasma channel [41]. In chapter 4, we introduce a novel external injection scheme for which the electron bunch is injected at a small angle into the laser wakefield [39].…”

Integral design of a laser wakefield accelerator with external bunch injectioon

“…For example, if the injected bunches comprised a length of several plasma wavelengths, they would be sliced into several sub-bunches trapped in several accelerating regions behind the laser pulse. The next problem with injection behind the laser pulse is that the injected bunch would become scattered by the drive laser pulse (in the vacuum region in front of the plasma) [42] if high drive intensities are used. The scattering is caused by the ponderomotive force associated with the intensity gradient of the drive laser pulse and it can significantly reduce the fraction of the injected electrons trapped in the wakefield.…”

“…Recently, it was theoretically shown that the injection problem might be overcome by injecting electron bunches from conventional rf-linacs although it seemed that such bunches would be far too long (much longer than the plasma wavelength) [21][22][23][24][37][38][39][40][41][42]. In order to prepare a corresponding demonstration experiment at the University of Twente, we carried out a more detailed theoretical investigation of a novel bunch injection method.…”