Sub-2-cycle laser-driven wakefield electron acceleration

Summary form only given. The idea of electron acceleration by laser wakefield in plasma has been suggested [1] and proved [2] to give an accelerating gradient up to several orders of magnitude higher than conventional RF based linac. The unique property of laser plasma not only offers the opportunity to build a compact X-ray source [3] but also can be used as an electron diffraction source with high temporal resolution and brightness due to the natural property of laser plasma wave.We report on experimental results of electron acceleration with LWS-20 (Light Wave Synthesizer-20) [4], which is a sub-5 fs, 16 TW OPCPA system. LWS-20 is the world's first multi-TW sub-2-cycle light source. Our target is a supersonic He nozzle combined with shock-front injection [5]. The nozzle gives a plateau-like density profile with gas density on the order of few times 1019 cm 3, and the shock front was generated by a razor blade put into supersonic flow. The target is designed as an energy tuneable electron source by changing injection position with the blade position. The laser pulses are focused by an f/4 off-axis parabolic mirror which generates on target peak intensity of about 1019 Wcm 2. The total charge, beam profile and divergence were detected by a scintillating screen imaged onto a CCD, while the electron spectra were measured by using a carefully calibrated permanent magnet equipped with scintillating screens as detectors.Laser plasma acceleration was achieved after optimizing the most important laser and plasma parameters such as pulse duration, energy, focusing, plasma density, shock front position, etc. In a certain range of these parameters we obtained electron beams with 2.5 Hz repetition rate. We got a few pC of charge per shot within 1/e2 of electron distribution with about 25 mrad divergence in this unique regime. Typical single shot electron spectrum shows a mono-energetic dark-current-free feature with tuneable peak energy about 8-11 MeV and about 30% energy spread as shown in Fig. 1. Such an electron source has high potential for many applications such as time-resolved electron diffraction [6] or external injection source of a cascaded electron accelerator.